Hubble Space Telescope 2018-2015
Astronomy and Telescopes
Hubble status and imagery in the period 2018-2015 as well as the Hubble Servicing Missions & Ground Segment
• 21 December 2018: The bright southern hemisphere star RS Puppis, at the center of the image (Figure 1), is swaddled in a gossamer cocoon of reflective dust illuminated by the glittering star. The super star is ten times more massive than the Sun and 200 times larger. 1)
- RS Puppis rhythmically brightens and dims over a six-week cycle. It is one of the most luminous in the class of so-called Cepheid variable stars. Its average intrinsic brightness is 15,000 times greater than the Sun's luminosity.
- The nebula flickers in brightness as pulses of light from the Cepheid propagate outwards. Hubble took a series of photos of light flashes rippling across the nebula in a phenomenon known as a "light echo." Even though light travels through space fast enough to span the gap between Earth and the Moon in a little over a second, the nebula is so large that reflected light can actually be photographed traversing the nebula.
- By observing the fluctuation of light in RS Puppis itself, as well as recording the faint reflections of light pulses moving across the nebula, astronomers are able to measure these light echoes and pin down a very accurate distance. The distance to RS Puppis has been narrowed down to 6,500 light-years (with a margin of error of only one percent).
• 20 December 2018: Astronomers using data from the NASA/ESA Hubble Space Telescope have employed a revolutionary method to detect dark matter in galaxy clusters. The method allows astronomers to "see" the distribution of dark matter more accurately than any other method used to date and it could possibly be used to explore the ultimate nature of dark matter. The results were published in the journal Monthly Notices of the Royal Astronomical Society. 2)
- In recent decades astronomers have tried to understand the true nature of the mysterious substance that makes up most of the matter in the Universe – dark matter – and to map its distribution in the Universe.
Note 1: Dark matter makes up about 85% of the matter in the Universe, and about a quarter of its total energy density. Dark matter does not emit any kind of electromagnetic radiation – its presence can only be determined via gravitational effects.
Now two astronomers from Australia and Spain have used data from the Frontier Fields program of the NASA/ESA Hubble Space Telescope to accurately study the distribution of dark matter.
Note 2: The Hubble Frontier Fields program was a deep imaging initiative designed to utilize the strong gravitational lensing effects in galaxy clusters to see extremely distant galaxies and thereby gain insight into the early Universe and the evolution of galaxies since that time. The program observed six galaxy clusters over 630 hours of Hubble's time. To receive the new results presented here the data was used in a different way, without using gravitational lensing.
- "We have found a way to 'see' dark matter," explains Mireia Montes (University of New South Wales, Australia), lead author of the study. "We have found that very faint light in galaxy clusters, the intracluster light, maps how dark matter is distributed." 3)
- Intracluster light is a byproduct of interactions between galaxies. In the course of these interactions, individual stars are stripped from their galaxies and float freely within the cluster. Once free from their galaxies, they end up where the majority of the mass of the cluster, mostly dark matter, resides.
- "These stars have an identical distribution to the dark matter, as far as our current technology allows us to study," explained Montes. Both the dark matter and these isolated stars – which form the intracluster light – act as collisionless components. These follow the gravitational potential of the cluster itself. The study showed that the intracluster light is aligned with the dark matter, tracing its distribution more accurately than any other method relying on luminous tracers used so far.
- This method is also more efficient than the more complex method of using gravitational lensing. While the latter requires both accurate lensing reconstruction and time-consuming spectroscopic campaigns, the method presented by Montes utilizes only deep imaging. This means more clusters can be studied with the new method in the same amount of observation time.
- The results of the study introduce the possibility of exploring the ultimate nature of dark matter. "If dark matter is self-interacting we could detect this as tiny departures in the dark matter distribution compared to this very faint stellar glow," highlights Ignacio Trujillo (Instituto de Astrofísica de Canarias, Spain), co-author of the study. Currently, all that is known about dark matter is that it appears to interact with regular matter gravitationally, but not in any other way. To find that it self-interacts would place significant constraints on its identity.
- For now, Montes and Trujillo plan to survey more of the original six clusters to see if their method remains accurate. Another important test of their method will be the observation and analysis of additional galaxy clusters by other research teams, to add to the data set and confirm their findings.
- The team can also look forward to the application of the same techniques using future space-based telescopes like the NASA/ESA/CSA James Webb Space Telescope, which will have even more sensitive instruments able to resolve faint intracluster light in the distant Universe.
- "There are exciting possibilities that we should be able to probe in the upcoming years by studying hundreds of galaxy clusters," concludes Ignacio Trujillo.
• 14 December 2018: The speed and distance at which planets orbit their respective blazing stars can determine each planet's fate - whether the planet remains a longstanding part of its solar system or evaporates into the universe's dark graveyard more quickly. - In their quest to learn more about far-away planets beyond our own solar system, astronomers discovered that a medium-sized planet roughly the size of Neptune, GJ 3470b, is evaporating at a rate 100 times faster than a previously discovered planet of similar size, GJ 436b. 4)
- The findings, published in the journal of Astronomy and Astrophysics, advance astronomers' knowledge about how planets evolve. 5)
- The study is part of the Panchromatic Comparative Exoplanet Treasury (PanCET) program, led by Sing, which aims to measure the atmospheres of 20 exoplanets in ultraviolet, optical and infrared light, as they orbit their stars. PanCET is the largest exoplanet observation program to be run with NASA's Hubble Space Telescope.
- One particular issue of interest to astronomers is how planets lose their mass through evaporation. Planets such as "super" Earths and "hot" Jupiters orbit more closely to their stars and are therefore hotter, causing the outermost layer of their atmospheres to be blown away by evaporation.
- While these larger Jupiter-sized and smaller Earth-sized exoplanets are plentiful, medium Neptune-sized exoplanets (roughly four times larger than Earth) are rare. Researchers hypothesize that these Neptunes get stripped of their atmospheres and ultimately become smaller planets.
- It's difficult, however, to actively witness them doing so because they can only be studied in UV light, which limits researchers to examining nearby stars no greater than 150 light-years away from earth, not obscured by interstellar material. GJ 3470b is 96 light-years away and circles a red dwarf star in the general direction of the constellation Cancer.
- In this study, Hubble found that exoplanet GJ 3470b had lost significantly more mass and had a noticeably smaller exosphere than the first Neptune-sized exoplanet studied, GJ 436b, due to its lower density and receipt of a stronger radiation blast from its host star.
- GJ 3470b's lower density makes it unable to gravitationally hang on to the heated atmosphere, and while the star hosting GJ 436b was between 4 billion and 8 billion years old, the star hosting GJ 3470b is only 2 billion years old; a younger star is more active and powerful, and, therefore, has more radiation to heat the planet's atmosphere.
- Sing's team estimates that GJ 3470b may have already lost up to 35 percent of its total mass and, in a few billion years, all of its gas may be stripped off, leaving behind only a rocky core.
- "We're starting to better understand how planets are shaped and what properties influence their overall makeup," Sing said. "Our goal with this study and the overarching PanCET program is to take a broad look at these planets' atmospheres to determine how each planet is affected by its own environment. By comparing different planets, we can start piecing together the larger picture in how they evolve."
- Looking forward, Sing and the team hope to study more exoplanets by searching for helium in infrared light, which will allow a greater search range than searching for hydrogen in UV light.
- Currently, planets, which are made largely of hydrogen and helium, can only be studied through tracing hydrogen in UV light. Using Hubble, the upcoming NASA James Webb Space Telescope (which will have a greater sensitivity to helium), and a new instrument called Carmenes that Sing recently found can precisely track the trajectory of helium atoms, astronomers will be able to broaden their pursuit of distant planets.
• 4 December 2018: Twenty-five years ago this week, NASA held its collective breath as seven astronauts on space shuttle Endeavour caught up with the Hubble Space Telescope 353 miles (568 kilometers) above Earth. Their mission: to fix a devastating flaw in the telescope's primary mirror. 6)
- Hubble Space Telescope has a primary mirror of 2.4 m in aperture. The largest optical telescope launched into space, where it could observe the universe free from the distorting effects of Earth's atmosphere, Hubble had a lot riding on it. But after the first images were obtained and carefully analyzed following the telescope's deployment on April 25, 1990, it was clear that something was wrong: The images were blurry.
- Astronomers and engineers rallied to study a variety of solutions to the problem, and NASA convened an independent committee to find the source. They all came to the same conclusion: Hubble's primary mirror, which looks like a very shallow bowl, had been polished into the wrong shape. The error was smaller than the width of a human hair, but the effect was significant. If the error went uncorrected, Hubble would never reach its full potential.
- During the week of 6 December 1993, the astronaut crew installed two pieces of hardware intended to fix the error. The Corrective Optics Space Telescope Axial Replacement (COSTAR) was designed and built by a team at NASA's Goddard Spaceflight Center in Greenbelt, Maryland, and would correct for the mirror error in three of the five instruments on Hubble.
- The second instrument was the Wide Field and Planetary Camera 2 (WFPC2), designed and built at NASA's Jet Propulsion Laboratory in Pasadena, California. WFPC2, which actually contains four cameras, would go on to produce many of Hubble's breathtaking images, helping transform our view of the cosmos.
- The size of baby grand piano, the instrument imaged objects and events that occurred in our own solar system - such as comet Shoemaker-Levy 9's crash into Jupiter - to the most distant cosmological images that had ever been taken in visible light. It generated breathtaking snapshots of galaxies, exploded stars and nebulae where new stars are born. During the instrument's tenure, Hubble managers pointed the telescope at a single, black patch of sky for more than a week and found thousands of previously unseen galaxies.
- But WFPC2's success was far from guaranteed. The instrument was built on an incredibly tight timeline, and designing it to correct the flaw was something JPL's John Trauger, principal investigator for WFPC2, would later describe as being akin to "trying to play baseball on the side of a hill."
- "There's a lot of pressure when you're building a space instrument even under normal circumstances," said Dave Gallagher, JPL's associate director for strategic integration, who served as integration and test manager for WFPC2. "But when you're fixing something that will essentially make or break the reputation of the entire agency, the pressure goes through the roof."
A Mirror Image
- In June 1990, NASA announced that the Hubble telescope was not working as expected. WFPC2 team members say they remember that the reaction from the public and the media was often pessimistic or even incredulous. Trauger watched network news anchor Tom Brokaw begin his program that evening by saying, "The Hubble Telescope you've heard so much about - it's broken."
- "The promise of the Hubble program, the application of our best technology to push back the frontiers of astronomy, had been instantly transformed in the public eye to an icon of technical failure," Trauger wrote in an essay in 2007.
- Trauger brought his team together to work the problem. The telescope's primary and secondary mirrors collected light and fed it to the five onboard science instruments. The primary mirror could not be replaced and could not be returned to Earth for repairs. A solution would have to be found for each of Hubble's instruments. The COSTAR device provided corrective optics for three of them, eliminating the need to fully replace those instruments. But the same approach wouldn't work for the telescope's Wide Field and Planetary Camera (WFPC), the predecessor of WFPC2.
- Trauger and his team came up with a potential solution. The primary mirror error caused light striking different parts of the mirror to come into focus at different locations, so the team had to figure out how to redirect it to the appropriate focal point. Their solution was to reverse-engineer the problem: They would place four identical nickel-sized mirrors inside the instrument - one for each of the four cameras inside WFPC2 - with the same error as the flawed primary mirror, but where the primary mirror was too flat, the new mirrors would be curved too deeply. Together, these two errors would cancel each other, producing the equivalent of a single mirror with the correct shape.
- NASA accepted JPL's proposal to build a WFPC replacement. The agency had planned to carry out Hubble repair missions every three years and decided to maintain this schedule. The first repair mission was set for the fall of 1993. JPL would need to deliver the replacement by the winter of 1992 - just over 2 years away. The race to repair Hubble was on.
- Two years was nowhere near enough time to build a new camera instrument from scratch. Thankfully, WFPC2 was already under construction at JPL; NASA had intended to eventually use it as an upgrade for WFPC or a replacement if the instrument ever failed.
- Even with work on WFPC2 already under way, the deadline required an accelerated schedule. Dave Rodgers and Larry Simmons, the WFPC2 project managers, held daily meetings with the leaders of each of WFPC2's several components to help stay on target.
- "The daily meetings kept the pressure on all of us, all the time," said Simmons, who retired from JPL in 2005. "We knew we only had a few years, and we had to get it done."
- While the corrective mirrors were small, they affected nearly every step of the building process and created "an endless string of novel problems," according to Trauger.
- To minimize the chance for error during WFPC2's installation in low-Earth orbit, the seven astronauts who were scheduled to execute the repair mission traveled to JPL to learn about the instrument and be trained on how to install it. They would be inserting WFPC2 into a cavity in the telescope's body, as if sliding it in a drawer. And although they would need to make sure that the electrical connections at the back of the instrument were secure, they had no way of reaching those connections; they could control only how they inserted the instrument.
- Complicating matters further was the weight of WFPC2: At more than 600 pounds (272 kilograms), it was unwieldy even in the microgravity of low-Earth orbit. One of the instrument's mirrors, called the pickoff mirror, was mounted on a short arm located outside the protective casing. Merely bumping the mirror would misalign the system and essentially ruin the entire instrument. During WFPC2's construction, Trauger and colleagues showed a model of the instrument to an astronaut, who bumped the pickoff mirror. Trauger couldn't help but wonder, "Is this an omen?"
Time to Fly
- The leaders of the WFPC2 team traveled to NASA's Kennedy Space Center in Florida for the early morning launch on Dec. 2, 1993. After departing Kennedy and seeking out an early breakfast, Gallagher remembers looking up at the predawn sky to see the space shuttle passing overhead and nearing Hubble; the objects appeared as two faint points of light in the sky as they orbited Earth.
- On the sixth day of the mission, astronauts Jeffrey Hoffman and Story Musgrave conducted a spacewalk to remove WFPC from Hubble and install WFPC2. Everything seemed to go as planned, but the real test was yet to come.
- The astronauts returned to Earth on Dec. 13, and the first raw data from WFPC2 came back on Dec. 18. The team put the data through the image-processing software and watched anxiously as the pictures began to ratchet across the screen. There was instant relief.
- "They were sharp," Trauger said of the images. "And it wasn't just that we had pictures that looked amazing, it was that we were making new discoveries right away. There were things in the images that we'd never seen before."
- NASA released those first images to the public on Jan. 13, 1994. The next day, the WFPC2 team presented the results to an overflow audience at the winter meeting of the American Astronomical Society.
- "When we showed the first images, the room erupted; we got a standing ovation," Trauger said. "You don't usually see that at an astronomy meeting!"
- The WFPC2 instrument operated on Hubble for over 15 years and took more than 135,000 observations of the universe. More than 3,500 science papers were written based on that data before the instrument was retired in 2009, and over 2,000 more have been published since.
- "WFPC2 didn't succeed by magic or luck; it succeeded because we had a competent and hardworking group of people who understood what was at stake and stepped up to the challenge," Gallagher said. "And just like with every project, I wish I could have transported that team with me to the next mission."
- In May of 2009, astronauts removed WFPC2 from Hubble and replaced it with the Wide Field Camera 3 (WFC3), which continues to operate today - 28 years after Hubble first switched on. WFPC2 was later placed on public display at the Smithsonian Air and Space Museum in Washington, D.C.
- The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington.
• 29 November 2018: Gazing across 300 million light-years into a monstrous city of galaxies, astronomers have used NASA's Hubble Space Telescope to do a comprehensive census of some of its most diminutive members: a whopping 22,426 globular star clusters found to date. 7)
- The survey, published in the November 9, 2018, issue of The Astrophysical Journal, will allow for astronomers to use the globular cluster field to map the distribution of matter and dark matter in the Coma galaxy cluster, which holds over 1,000 galaxies that are packed together. 8)
- Because globular clusters are much smaller than entire galaxies — and much more abundant — they are a much better tracer of how the fabric of space is distorted by the Coma cluster's gravity. In fact, the Coma cluster is one of the first places where observed gravitational anomalies were considered to be indicative of a lot of unseen mass in the universe — later to be called “dark matter.”
- Among the earliest homesteaders of the universe, globular star clusters are snow-globe-shaped islands of several hundred thousand ancient stars. They are integral to the birth and growth of a galaxy. About 150 globular clusters zip around our Milky Way galaxy, and, because they contain the oldest known stars in the universe, were present in the early formative years of our galaxy.
- Some of the Milky Way's globular clusters are visible to the naked eye as fuzzy-looking "stars." But at the distance of the Coma cluster, its globulars appear as dots of light even to Hubble's super-sharp vision. The survey found the globular clusters scattered in the space between the galaxies. They have been orphaned from their home galaxy due to galaxy near-collisions inside the traffic-jammed cluster. Hubble revealed that some globular clusters line up along bridge-like patterns. This is telltale evidence for interactions between galaxies where they gravitationally tug on each other like pulling taffy.
- Astronomer Juan Madrid of the Australian Telescope National Facility in Sydney, Australia, first thought about the distribution of globular clusters in Coma when he was examining Hubble images that show the globular clusters extending all the way to the edge of any given photograph of galaxies in the Coma cluster.
- He was looking forward to more data from one of the legacy surveys of Hubble that was designed to obtain data of the entire Coma cluster, called the Coma Cluster Treasury Survey. However, halfway through the program, in 2006, Hubble's powerful Advanced Camera for Surveys (ACS) had an electronics failure. (The ACS was later repaired by astronauts during a 2009 Hubble servicing mission.)
- To fill in the survey gaps, Madrid and his team painstakingly pulled numerous Hubble images of the galaxy cluster taken from different Hubble observing programs. These are stored in the Space Telescope Science Institute's Mikulski Archive for Space Telescopes in Baltimore, Maryland. He assembled a mosaic of the central region of the cluster, working with students from the National Science Foundation's Research Experience for Undergraduates program. "This program gives an opportunity to students enrolled in universities with little or no astronomy to gain experience in the field," Madrid said.
- The team developed algorithms to sift through the Coma mosaic images that contain at least 100,000 potential sources. The program used globular clusters' color (dominated by the glow of aging red stars) and spherical shape to eliminate extraneous objects — mostly background galaxies unassociated with the Coma cluster.
- Though Hubble has superb detectors with unmatched sensitivity and resolution, their main drawback is that they have tiny fields of view. "One of the cool aspects of our research is that it showcases the amazing science that will be possible with NASA's planned Wide Field Infrared Survey Telescope (WFIRST) that will have a much larger field of view than Hubble," said Madrid. "We will be able to image entire galaxy clusters at once."
- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
• 26 November 2018: This dark, tangled web is an object named SNR 0454-67.2. It formed in a very violent fashion — it is a supernova remnant, created after a massive star ended its life in a cataclysmic explosion and threw its constituent material out into surrounding space. This created the messy formation we see in this NASA/ESA Hubble Space Telescope image, with threads of red snaking amidst dark, turbulent clouds. 9)
- SNR 0454-67.2 is situated in the Large Magellanic Cloud, a dwarf spiral galaxy that lies close to the Milky Way. The remnant is likely the result of a Type Ia supernova explosion; this category of supernovae is formed from the death of a white dwarf star, which grows and grows by siphoning material from a stellar companion until it reaches a critical mass and then explodes.
- As they always form via a specific mechanism — when the white dwarf hits a particular mass — these explosions always have a well-known luminosity, and are thus used as markers (standard candles) for scientists to obtain and measure distances throughout the Universe.
• 15 November 2018: Astronomers may have finally uncovered the long-sought progenitor to a specific type of exploding star by sifting through NASA Hubble Space Telescope archival data. The supernova, called a Type Ic, is thought to detonate after its massive star has shed or been stripped of its outer layers of hydrogen and helium. 10) 11)
- These stars could be among the most massive known — at least 30 times heftier than our Sun. Even after shedding some of their material late in life, they are expected to be big and bright. So it was a mystery why astronomers had not been able to nab one of these stars in pre-explosion images.
- Finally, in 2017, astronomers got lucky. A nearby star ended its life as a Type Ic supernova. Two teams of astronomers pored through the archive of Hubble images to uncover the putative precursor star in pre-explosion photos taken in 2007. The supernova, catalogued as SN 2017ein, appeared near the center of the nearby spiral galaxy NGC 3938, located roughly 65 million light-years away.
- This potential discovery could yield insight into stellar evolution, including how the masses of stars are distributed when they are born in batches.
- "Finding a bona fide progenitor of a supernova Ic is a big prize of progenitor searching," said Schuyler Van Dyk of the California Institute of Technology (Caltech) in Pasadena, lead researcher of one of the teams. "We now have for the first time a clearly detected candidate object." His team's paper was published in June in The Astrophysical Journal.
- A paper by a second team, which appeared in the Oct. 21, 2018, issue of the Monthly Notices of the Royal Astronomical Society, is consistent with the earlier team's conclusions.
- "We were fortunate that the supernova was nearby and very bright, about 5 to 10 times brighter than other Type Ic supernovas, which may have made the progenitor easier to find," said Charles Kilpatrick of the University of California, Santa Cruz, leader of the second team. "Astronomers have observed many Type Ic supernovas, but they are all too far away for Hubble to resolve. You need one of these massive, bright stars in a nearby galaxy to go off. It looks like most Type Ic supernovas are less massive and therefore less bright, and that's the reason we haven't been able to find them."
- An analysis of the object's colors shows that it is blue and extremely hot. Based on that assessment, both teams suggest two possibilities for the source's identity. The progenitor could be a single hefty star between 45 and 55 times more massive than our Sun. Another idea is that it could have been a massive binary-star system in which one of the stars weighs between 60 and 80 solar masses and the other roughly 48 suns. In this latter scenario, the stars are orbiting closely and interact with each other. The more massive star is stripped of its hydrogen and helium layers by the close companion, and eventually explodes as a supernova.
- The possibility of a massive double-star system is a surprise. "This is not what we would expect from current models, which call for lower-mass interacting binary progenitor systems," Van Dyk said.
- Expectations on the identity of the progenitors of Type Ic supernovas have been a puzzle. Astronomers have known that the supernovas were deficient in hydrogen and helium, and initially proposed that some hefty stars shed this material in a strong wind (a stream of charged particles) before they exploded. When they didn't find the progenitors stars, which should have been extremely massive and bright, they suggested a second method to produce the exploding stars that involves a pair of close-orbiting, lower-mass binary stars. In this scenario, the heftier star is stripped of its hydrogen and helium by its companion. But the "stripped" star is still massive enough to eventually explode as a Type Ic supernova.
- "Disentangling these two scenarios for producing Type Ic supernovas impacts our understanding of stellar evolution and star formation, including how the masses of stars are distributed when they are born, and how many stars form in interacting binary systems," explained Ori Fox of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, a member of Van Dyk's team. "And those are questions that not just astronomers studying supernovas want to know, but all astronomers are after."
- Type Ic supernovas are just one class of exploding star. They account for about 20 percent of massive stars that explode from the collapse of their cores.
- The teams caution that they won't be able to confirm the source's identity until the supernova fades in about two years. The astronomers hope to use either Hubble or the upcoming NASA James Webb Space Telescope to see whether the candidate progenitor star has disappeared or has significantly dimmed. They also will be able to separate the supernova's light from that of stars in its environment to calculate a more accurate measurement of the object's brightness and mass.
- SN 2017ein was discovered in May 2017 by Tenagra Observatories in Arizona. But it took the sharp resolution of Hubble to pinpoint the exact location of the possible source. Van Dyk's team imaged the young supernova in June 2017 with Hubble's Wide Field Camera 3. The astronomers used that image to pinpoint the candidate progenitor star nestled in one of the host galaxy's spiral arms in archival Hubble photos taken in December 2007 by the Wide Field Planetary Camera 2.
- Kilpatrick's group also observed the supernova in June 2017 in infrared images from one of the 10-meter telescopes at the W. M. Keck Observatory in Hawaii. The team then analyzed the same archival Hubble photos as Van Dyk's team to uncover the possible source.
- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
• 14 November 2018: The SPC (Science Program Committee) of ESA has confirmed the continued operations of ten scientific missions in the Agency's fleet up to 2022. After a comprehensive review of their scientific merits and technical status, the SPC has decided to extend the operation of the five missions led by ESA's Science Program: Cluster, Gaia, INTEGRAL, Mars Express, and XMM-Newton. The SPC also confirmed the Agency's contributions to the extended operations of Hinode, Hubble, IRIS, SOHO, and ExoMars TGO. 12)
- This includes the confirmation of operations for the 2019–2020 cycle for missions that had been given indicative extensions as part of the previous extension process, and indicative extensions for an additional two years, up to 2022.
- The decision was taken during the SPC meeting at ESA/ESAC (European Space Astronomy Center) near Madrid, Spain, on 14 November.
- ESA's science missions have unique capabilities and are prolific in their scientific output. Cluster, for example, is the only mission that, by varying the separation between its four spacecraft, allows multipoint measurements of the magnetosphere in different regions and at different scales, while Gaia is performing the most precise astrometric survey ever realized, enabling unprecedented studies of the distribution and motions of stars in the Milky Way and beyond.
- Many of the science missions are proving to be of great value to pursue investigations that were not foreseen at the time of their launch. Examples include the role of INTEGRAL and XMM-Newton in the follow-up of recent gravitational wave detections, paving the way for the future of multi-messenger astronomy, and the many discoveries of diverse exoplanets by Hubble.
- Collaboration between missions, including those led by partner agencies, is also of great importance. The interplay between solar missions like Hinode, IRIS and SOHO provides an extensive suite of complementary instruments to study our Sun; meanwhile, Mars Express and ExoMars TGO are at the forefront of the international fleet investigating the Red Planet.
- Another compelling factor to support the extension is the introduction of new modes of operation to accommodate the evolving needs of the scientific community, as well as new opportunities for scientists to get involved with the missions.
• 08 November 2018: Blue compact dwarf galaxies take their name from the intensely blue star-forming regions that are often found within their cores. One such region can be seen embedded in ESO 338-4, which is populated with bright young stars voraciously consuming hydrogen. These massive stars are doomed to a short existence, as despite their vast supplies of hydrogen fuel. The nuclear reactions in the cores of these stars will burn through these supplies in only millions of years — a mere blink of an eye in astronomical terms. 13)
- The young blue stars nestled within a cloud of dust and gas in the center of this image are the result of a recent galaxy merger between a wandering galaxy and ESO 388-4. This galactic interaction disrupted the clouds of gas and dust surrounding ESO 338-4 and led to the rapid formation of a new population of stars.
• 05 November 2018: On the first day of the 15th annual European Space Weather Week, this image from the NASA/ESA Hubble Space Telescope fittingly shows a striking occurrence of celestial weather in the outer reaches of the Solar System: an aurora on Uranus. 14)
- Auroras, also known as polar lights, are a relatively familiar type of space weather to Earth-based stargazers, but have also been spied on many other planets in the Solar System.
- Views of the Earth’s Northern and Southern Lights show glowing sheets and rippling waves of bright light painting the sky in striking shades of green and even red, blue, and purple; these breath-taking scenes are created as streams of energetic charged particles hit the upper layers of Earth’s atmosphere at altitudes of up to a few hundreds of kilometers, and interact with resident atoms and molecules of mostly oxygen and nitrogen. These emit photons at specific visible wavelengths or colors – green and red for oxygen, blue and purple for nitrogen – and fill the sky with an eerie auroral glow.
- Hubble has observed auroras on Uranus on various occasions: in 2011, when the telescope became the first to image the phenomenon from the vicinity of Earth, then again in 2012 and 2014, taking extra data beyond visible light.
- By pointing Hubble’s ultraviolet eye on Uranus twice during the same month, from 1 to 5 and 22 to 24 November 2014, scientists were able to determine that the planet’s glimmering auroras rotate along with the planet. The observations also helped to locate Uranus’ magnetic poles, and allowed scientists to track two so-called interplanetary shocks that propagated through the Solar System. These shocks were triggered by two powerful bursts of material flung out by the Sun via the solar wind, an ongoing flow of charged particles constantly emanating from our star, and caused the most intense auroras ever seen on Uranus.
• 31 October 2018: The NASA/ESA Hubble Space Telescope has captured part of the wondrous Serpens Nebula, lit up by the star HBC 672. This young star casts a striking shadow – nicknamed the Bat Shadow – on the nebula behind it, revealing telltale signs of its otherwise invisible protoplanetary disc. 15)
- The Serpens Nebula, located in the tail of the Serpent (Serpens Cauda) about 1300 light-years away, is a reflection nebula that owes most of its sheen to the light emitted by stars like HBC 672 – a young star nestled in its dusty folds. In this image the NASA/ESA Hubble Space Telescope has exposed two vast cone-like shadows emanating from HBC 672.
- These colossal shadows on the Serpens Nebula are cast by the protoplanetary disc surrounding HBC 672. By clinging tightly to the star the disc creates an imposing shadow, much larger than the disc – approximately 200 times the diameter of our own Solar System. The disc's shadow is similar to that produced by a cylindrical lamp shade. Light escapes from the top and bottom of the shade, but along its circumference, dark cones of shadow form.
- The disc itself is so small and far away from Earth that not even Hubble can detect it encircling its host star. However, the shadow feature – nicknamed the Bat Shadow – reveals details of the disc's shape and nature. The presence of a shadow implies that the disc is being viewed nearly edge-on.
- Whilst most of the shadow is completely opaque, scientists can look for color differences along its edges, where some light gets through. Using the shape and color of the shadow, they can determine the size and composition of dust grains in the disc.
- The whole Serpens Nebula, of which this image shows only a tiny part, could host more of these shadow projections. The nebula envelops hundreds of young stars, many of which could also be in the process of forming planets in a protoplanetary disc.
- Although shadow-casting discs are common around young stars, the combination of an edge-on viewing angle and the surrounding nebula is rare. However, in an unlikely coincidence, a similar looking shadow phenomenon can be seen emanating from another young star, in the upper left of the image.
- These precious insights into protoplanetary discs around young stars allow astronomers to study our own past. The planetary system we live in once emerged from a similar protoplanetary disc when the Sun was only a few million years old. By studying these distant discs we get to uncover the formation and evolution of our own cosmic home.
• NASA’s Hubble Space Telescope returned to normal operations late Friday, Oct. 26, and completed its first science observations on Saturday, 27 October 2018 at 2:10 AM EDT. The observations were of the distant, star-forming galaxy DSF2237B-1-IR and were taken in infrared wavelengths with the WFC3 (Wide Field Camera 3) instrument. The return to conducting science comes after successfully recovering a backup gyroscope, that had replaced a failed gyro three weeks earlier. 16)
- One of Hubble’s gyros failed on 5 October, and the spacecraft’s operations team activated a backup gyro the next day. However, the backup incorrectly returned rotation rates that were far in excess of the actual rates.
- Last week the operations team commanded Hubble to perform numerous maneuvers, or turns, and switched the gyro between different operational modes, which successfully cleared what was believed to be blockage between components inside the gyro that produced the excessively high rate values. Next, the team monitored and tested the gyro with additional maneuvers to make sure that the gyro was stable. The team then installed additional safeguards on the spacecraft in case the excessive rate values return, although this is not anticipated.
- On 26 October, the team began the process to restore the scientific instruments to standard operating status. Hubble successfully completed maneuvers to get on target for the first science observations, and the telescope collected its first science data since 5 October.
- Hubble is now back in its normal science operations mode with three fully functional gyros. Originally required to last 15 years, Hubble has now been at the forefront of scientific discovery for more than 28 years. The team expects the telescope will continue to yield amazing discoveries well into the next decade, enabling it to work alongside the James Webb Space Telescope.
• 26 October 2018: The constellation of Cassiopeia, named after a vain queen in Greek mythology, forms the easily recognizable "W" shape in the night sky. The central point of the W is marked by a dramatic star named Gamma Cassiopeiae. 17) 18) 19)
- The remarkable Gamma Cassiopeiae is a blue-white subgiant variable star that is surrounded by a gaseous disc. This star is 19 times more massive and 65,000 times brighter than our Sun. It also rotates at the incredible speed of 1.6 million km/hour – more than 200 times faster than our parent star. This frenzied rotation gives it a squashed appearance. The fast rotation causes eruptions of mass from the star into a surrounding disk. This mass loss is related to the observed brightness variations.
- The radiation of Gamma Cassiopeiae is so powerful that it even affects IC 63, sometimes nicknamed the Ghost Nebula, that lies several light years away from the star. IC 63 is visible in this image taken by the NASA/ESA Hubble Space Telescope.
- The colors in the eerie nebula showcase how the nebula is affected by the powerful radiation from the distant star. The hydrogen within IC 63 is being bombarded with ultraviolet radiation from Gamma Cassiopeiae, causing its electrons to gain energy which they later release as hydrogen-alpha radiation – visible in red in this image.
- This hydrogen-alpha radiation makes IC 63 an emission nebula, but we also see blue light in this image. This is light from Gamma Cassiopeiae that has been reflected by dust particles in the nebula, meaning that IC 63 is also a reflection nebula.
- This colorful and ghostly nebula is slowly dissipating under the influence of ultraviolet radiation from Gamma Cassiopeiae. However, IC 63 is not the only object under the influence of the mighty star. It is part of a much larger nebulous region surrounding Gamma Cassiopeiae that measures approximately two degrees on the sky – roughly four times as wide as the full Moon.
- The region is best seen from the Northern Hemisphere during autumn and winter. Though it is high in the sky and visible all year round from Europe, it is very dim, so observing it requires a fairly large telescope and dark skies.
• 23 October 2018: An international team of astronomers have discovered two stars in a binary pair that complete an orbit around each other in a little over three hours, residing in the planetary nebula M3-1. Remarkably, the stars could drive a nova explosion, an entirely unexpected event based on our current understanding of binary star evolution. The team, led by David Jones of the IAC (Instituto Astrofisica de Canarias) and the Universidad de La Laguna, report their findings in Monthly Notices of the Royal Astronomical Society: Letters. 20) 21)
- Planetary nebulae are the glowing shells of gas and dust formed from the outer layers of stars like our own Sun, which they throw off during the final stages of their evolution. In many cases, interaction with a nearby companion star plays an important role in the ejection of this material and the formation of the elaborate structures seen in the resulting planetary nebulae.
- The planetary nebula M3-1 is located in the constellation of Canis Major, at a distance of roughly 14,000 light years. M3-1 was a firm candidate to host a binary central star, as its structure with prominent jets and filaments is typical of these binary star interactions.
- Using the telescopes of the European Southern Observatory (ESO) in Chile, Jones's team looked at M3-1 over a period of several years. In the process they discovered and studied the binary stars in the center of the nebula.
- "We knew M3-1 had to host a binary star, so we set about acquiring the observations required to prove this and to relate the properties of the nebula with the evolution of the star or stars that formed it," says Brent Miszalski, researcher at the Southern African Large Telescope, and co-author of the study.
- The two stars are so close together that they cannot be resolved from the ground, so instead the presence of the second star is inferred from the variation of their observed combined brightness — most obviously by periodic eclipses of one star by the other which produce marked drops in the brightness.
- "When we began the observations, it was immediately clear that the system was a binary" explains Henri Boffin, researcher at the European Southern Observatory in Germany. "We saw that the apparently single star at the center of the nebula was rapidly changing in brightness, and we knew that this must be due to the presence of a companion star."
- The team discovered that the central star of the planetary nebula M3-1 has one of the shortest orbital period binary central stars known to date, at just over three hours. The ESO observations also show that the two stars — most likely a white dwarf with a low-mass main sequence companion — are almost touching.
- As a result, the pair are likely to undergo a so-called nova eruption, the result of the transfer of material from one star to the other. When this reaches a critical mass, a violent thermonuclear explosion takes place and the system temporarily increases in brightness by up to a million times.
- "After the various observing campaigns in Chile, we had enough data to begin to understand the properties of the two stars — their masses, temperatures and radii" says Paulina Sowicka, a PhD student at the Nicolas Copernicus Astronomical Center in Poland. "It was a real surprise that the two stars were so close together and so large that they were almost touching one another. A nova explosion could take place in just a few thousand years from now."
- Theory suggests that binary stars should be well separated after the formation of a planetary nebula. It should then take a long time before they begin to interact again and events such as novae become possible.
- In 2007, astronomers observed a different nova explosion, known as Nova Vul 2007, inside another planetary nebula. Jones comments: "The 2007 event was particularly difficult to explain. By the time the two stars are close enough for a nova, the material in the planetary nebula should have expanded and dissipated so much that it's no longer visible."
- The new event adds to the conundrum, adds Jones: "In the central stars of M3-1, we've found another candidate for a similar nova eruption in the relatively near future."
- The team now hope to carry out further study of the nebula and others like it, helping to shed light on the physical processes and origins of novae and supernovae, some of the most spectacular and violent phenomena in the Universe.
• 22 October 2018: NASA took great strides last week to press into service a Hubble Space Telescope backup gyroscope (gyro) that was incorrectly returning extremely high rotation rates. The backup gyro was turned on after the spacecraft entered safe mode due to a failed gyro on Friday, 5 October. The rotation rates produced by the backup gyro have since reduced and are now within an expected range. Additional tests will be performed to ensure Hubble can return to science operations with this gyro. 22)
- A gyro is a device that measures the speed at which the spacecraft is turning, and is needed to help Hubble turn and lock on to new targets.
- A wheel inside the gyro spins at a constant rate of 19,200 revolutions per minute. This wheel is mounted in a sealed cylinder, called a float, which is suspended in a thick fluid. Electricity is carried to the motor by thin wires, approximately the size of a human hair, that are immersed in the fluid. Electronics within the gyro detect very small movements of the axis of the wheel and communicate this information to Hubble’s central computer. These gyros have two modes — high and low. High mode is a coarse mode used to measure large rotation rates when the spacecraft turns across the sky from one target to the next. Low mode is a precision mode used to measure finer rotations when the spacecraft locks onto a target and needs to stay very still.
- In an attempt to correct the erroneously high rates produced by the backup gyro, the Hubble operations team executed a running restart of the gyro on 16 October. This procedure turned the gyro off for one second, and then restarted it before the wheel spun down. The intention was to clear any faults that may have occurred during startup on 6 October, after the gyro had been off for more than 7.5 years. However, the resulting data showed no improvement in the gyro’s performance.
- On 18 October, the Hubble operations team commanded a series of spacecraft maneuvers, or turns, in opposite directions to attempt to clear any blockage that may have caused the float to be off-center and produce the exceedingly high rates. During each maneuver, the gyro was switched from high mode to low mode to dislodge any blockage that may have accumulated around the float.
- Following the 18 October maneuvers, the team noticed a significant reduction in the high rates, allowing rates to be measured in low mode for brief periods of time. On 19 October, the operations team commanded Hubble to perform additional maneuvers and gyro mode switches, which appear to have cleared the issue. Gyro rates now look normal in both high and low mode.
- Hubble then executed additional maneuvers to make sure that the gyro remained stable within operational limits as the spacecraft moved. The team saw no problems and continued to observe the gyro through the weekend to ensure that it remained stable.
- The Hubble operations team plans to execute a series of tests to evaluate the performance of the gyro under conditions similar to those encountered during routine science observations, including moving to targets, locking on to a target, and performing precision pointing. After these engineering tests have been completed, Hubble is expected to soon return to normal science operations.
• 19 October 2018: Discovered in November 1834 by British astronomer John Herschel, NGC 1898 has been scrutinized numerous times by the NASA/ESA Hubble Space Telescope. Today we know that globular clusters are some of the oldest known objects in the universe and that they are relics of the first epochs of galaxy formation. While we already have a pretty good picture on the globular clusters of the Milky Way — still with many unanswered questions — our studies on globular clusters in nearby dwarf galaxies just started. The observations of NGC 1898 will help to determine whether their properties are similar to the ones found in the Milky Way, or if they have different features, due to being in a different cosmic environment. 23)
- The image of Figure 14 was taken by Hubble’s Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3).
• 18 October 2018: New observations by two Arizona State University astronomers using the Hubble Space Telescope have caught a red dwarf star in a violent outburst, or superflare. The blast of radiation was more powerful than any such outburst ever detected from the Sun, and would likely affect the habitability of any planets orbiting it. 24)
- Moreover, the astronomers say, such superflares appear more common in younger red dwarfs, which erupt 100 to 1000 times more powerfully than they will when they age.
- The superflare was detected as part of a Hubble Space Telescope observing program dubbed HAZMAT (HAbitable Zones and M dwarf Activity across Time). The program surveys red dwarfs (also known as M dwarfs) at three different ages—young, intermediate, and old—and observes them in ultraviolet light, where they show the most activity.
- "Red dwarf stars are the smallest, most common, and longest-lived stars in the galaxy," says Evgenya Shkolnik, an assistant professor in ASU's School of Earth and Space Exploration and the HAZMAT program's principal investigator. "In addition, we think that most red dwarf stars have systems of planets orbiting them."
- The Hubble telescope's orbit above Earth's atmosphere gives it clear, unhindered views at ultraviolet wavelengths. The flares are believed to be powered by intense magnetic fields that get tangled by the roiling motions of the stellar atmosphere. When the tangling gets too intense, the fields break and reconnect, unleashing tremendous amounts of energy.
- ASU postdoctoral researcher Parke Loyd is the first author on the paper (to be published in the Astrophysical Journal) that reports on the stellar outbursts. 25)
- He says, "When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, 'Whoa.'"
- Loyd notes, "Gathering data on young red dwarfs has been especially important because we suspected these stars would be quite unruly in their youth, which is the first hundred million years or so after they form." - He adds, "Most of the potentially-habitable planets in our galaxy have had to withstand intense flares like the ones we observed at some point in their life. That's a sobering thought."
Rough environment for planets
- About three-quarters of the stars in our Milky Way galaxy are red dwarfs. Most of the galaxy's "habitable-zone" planets—planets orbiting their stars at a distance where temperatures are moderate enough for liquid water to exist on their surface—orbit red dwarfs. In fact, the nearest star to our Sun, a red dwarf named Proxima Centauri, has an Earth-size planet in its habitable zone.
- However, red dwarfs—especially young red dwarfs—are active stars, producing flares that could blast out so much energy that it disrupts and possibly strips off the atmospheres of these fledgling planets.
- "The goal of the HAZMAT program is to understand the habitability of planets around low-mass stars," explains Shkolnik. "These low-mass stars are critically important in understanding planetary atmospheres." Ultraviolet radiation can modify the chemistry in a planet's atmosphere, or potentially remove that atmosphere.
- The observations reported in the Astrophysical Journal examined the flare frequency of 12 young (40 million year old) red dwarfs and represent just the first part of the HAZMAT program. These stars show that young low-mass stars flare much more frequently and more energetically than old stars and middle-age stars like our Sun—as evidenced by the superflare.
- "With the Sun, we have a hundred years of good observations," says Loyd. "And in that time, we've seen one, maybe two, flares that have an energy approaching that of the superflare."
- However, he says, "In a little less than a day's worth of Hubble observations of these young stars, we caught the superflare. This means that we're looking at superflares happening every day or even a few times a day."
- Could superflares of such frequency and intensity bathe young planets in so much ultraviolet radiation that they forever rule out any chance of habitability?
- According to Loyd, "Flares like we observed have the capacity to strip away the atmosphere from a planet. But that doesn't necessarily mean doom and gloom for life on the planet. It just might be different life than we imagine. Or there might be other processes that could replenish the atmosphere of the planet. It's certainly a harsh environment, but I would hesitate to call it a sterile environment."
- The next part of the HAZMAT study will be to study intermediate-age red dwarfs that are 650 million years old. Then the oldest red dwarfs will be analyzed and compared with the young and intermediate stars to understand the evolution of the high-energy-radiation environment for planets around these low-mass stars.
- Red dwarfs, which are estimated to burn as long as a trillion years, have a vast stretch of time available to eventually host evolving, habitable planets.
- "They just have many more opportunities for life to evolve, given their longevity," says Shkolnik. "I don't think we know for sure one way or another about whether planets orbiting red dwarfs are habitable just yet, but I think time will tell."
- She says, "It's great that we're living in a time when we have the technology to actually answer these kinds of questions, rather than just philosophize about them."
• 12 October 2018: This image from the NASA/ESA Hubble Space Telescope (Figure 17) reveals a spiral galaxy named Messier 95 (also known as M95 or NGC 3351). Located about 35 million light-years away in the constellation of Leo (The Lion), this swirling spiral was discovered by astronomer Pierre Méchain in 1781, and cataloged by French astronomer Charles Messier just four days later. Messier was primarily a comet hunter, and was often left frustrated by objects in the sky that resembled comets but turned out not to be. To help other astronomers avoid confusing these objects in the future, he created his famous catalog of Messier objects. 26)
- Most definitely not a comet, Messier 95 is actually a barred spiral galaxy. The galaxy has a bar cutting through its center, surrounded by an inner ring currently forming new stars. Also our own Milky Way is a barred spiral.
- As well as hosting this stellar nursery, Messier 95 is a known host of the dramatic and explosive final stages in the lives of massive stars: supernovae. In March 2016 a spectacular supernova named SN 2012aw was observed in the outer regions of one of Messier 95’s spiral arms. Once the light from the supernova had faded, astronomers were able to compare observations of the region before and after the explosion to find out which star had “disappeared” — the progenitor star. In this case, the star was an especially huge red supergiant up to 26 times more massive than the Sun.
• 08 October 2018: NASA is working to resume science operations of the Hubble Space Telescope after the spacecraft entered safe mode on Friday, 5 October , shortly after 6:00 p.m. EDT. Hubble’s instruments still are fully operational and are expected to produce excellent science for years to come. 27)
- Hubble entered safe mode after one of the three gyroscopes, actively being used to point and steady the telescope, failed. Safe mode puts the telescope into a stable configuration until ground control can correct the issue and return the mission to normal operation.
- Built with multiple redundancies, Hubble had six new gyros installed during Servicing Mission-4 in 2009. Hubble usually uses three gyros at a time for maximum efficiency, but can continue to make scientific observations with just one.
- The gyro that failed had been exhibiting end-of-life behavior for approximately a year, and its failure was not unexpected; two other gyros of the same type had already failed. The remaining three gyros available for use are technically enhanced and therefore expected to have significantly longer operational lives.
- Two of those enhanced gyros are currently running. Upon powering on the third enhanced gyro that had been held in reserve, analysis of spacecraft telemetry indicated that it was not performing at the level required for operations. As a result, Hubble remains in safe mode. Staff at NASA’s Goddard Space Flight Center and the Space Telescope Science Institute are currently performing analyses and tests to determine what options are available to recover the gyro to operational performance.
- Science operations with Hubble have been suspended while NASA investigates the anomaly. An Anomaly Review Board, including experts from the Hubble team and industry familiar with the design and performance of this type of gyro, is being formed to investigate this issue and develop the recovery plan. If the outcome of this investigation results in recovery of the malfunctioning gyro, Hubble will resume science operations in its standard three-gyro configuration.
- If the outcome indicates that the gyro is not usable, Hubble will resume science operations in an already defined “reduced-gyro” mode that uses only one gyro. While reduced-gyro mode offers less sky coverage at any particular time, there is relatively limited impact on the overall scientific capabilities.
• 03 October 2018: A pair of Columbia University astronomers using NASA's Hubble Space Telescope and the Kepler Space Telescope have assembled compelling evidence for the existence of a moon orbiting a gas-giant planet 8,000 light-years away. 28) 29)
- In a paper published 03 October in the journal Science Advances, Alex Teachey and David Kipping report that the detection of a candidate exomoon—that is, moons orbiting planets in other star systems—is unusual because of its large size, comparable to the diameter of Neptune. Such gargantuan moons do not exist in our own solar system, where nearly 200 natural satellites have been cataloged. 30)
- "This would be the first case of detecting a moon outside our solar system," said Kipping, an assistant professor of astronomy at Columbia. "If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets."
- In looking for exomoons, the researchers analyzed data from 284 Kepler-discovered planets that were in comparatively wide orbits, with periods greater than 30 days, around their host star. The observations measured the momentary dimming of starlight as a planet passed in front of its star, called a transit. The researchers found one instance, in Kepler 1625b, that had intriguing anomalies.
- "We saw little deviations and wobbles in the light curve that caught our attention," Kipping said.
- The Kepler results were enough for the team to get 40 hours of time with Hubble to intensively study the planet, obtaining data four times more precise than that of Kepler. The researchers monitored the planet before and during its 19-hour-long transit across the face of the star. After it ended, Hubble detected a second and much smaller decrease in the star's brightness 3.5 hours later, consistent with "a moon trailing the planet like a dog following its owner on a leash," Kipping said. "Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured."
- In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by measuring that the planet began its transit 1.25 hours earlier than predicted. This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location.
- "An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit," Kipping said.
- The researchers note that in principle this anomaly could be caused by the gravitational pull of a hypothetical second planet in the system, although Kepler found no evidence for additional planets around the star during its four-year mission.
- "A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," said lead author Teachey, NSF Graduate Fellow in astronomy at Columbia. "It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation."
- The moon is estimated to be only 1.5 percent the mass of its companion planet, which itself estimated to be several times the mass of Jupiter. This value is close to the mass-ratio between the Earth and its moon. But in the case of the Earth-Moon system and the Pluto-Charon system—the largest of the five known natural satellites of the dwarf planet Pluto—an early collision with a larger body is hypothesized to have blasted off material that later coalesced into a moon. Kepler 1625b and its satellite, however, are gaseous, not rocky, and, therefore, such a collision may not lead to the condensation of a satellite.
- Exomoons are difficult to find because they are smaller than their companion planet and so their transit signal is weak; they also shift position with each transit because the moon is orbiting the planet. In addition, the ideal candidate planets hosting moons are in large orbits, with long and infrequent transit times. In this search, the Neptune-sized moon would have been among the easiest to first detect because of its large size.
- The host planet and its moon lie within the solar mass star's (Kepler 1625) habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface. "Both bodies, however, are considered to be gaseous and therefore unsuitable for life as we know it," Kipping said.
- Future searches will target Jupiter-sized planets that are farther from their star than Earth is from the Sun. There are just a handful of these in the Kepler database. NASA's upcoming James Webb Space Telescope could really "clean-up" in the satellite search, Kipping said. "We can expect to see really tiny moons."
• 21 September 2018: In the northern constellation of Coma Berenices (Berenice's Hair) lies the impressive Coma Cluster — a structure of over a thousand galaxies bound together by gravity. Many of these galaxies are elliptical types, as is the brighter of the two galaxies dominating this image: NGC 4860 (center of Figure 20). However, the outskirts of the cluster also host younger spiral galaxies that proudly display their swirling arms. Again, this image shows a wonderful example of such a galaxy in the shape of the beautiful NGC 4858, which can be seen to the left of its bright neighbor and which stands out on account of its unusual, tangled, fiery appearance. 31)
- NGC 4858 is special. Rather than being a simple spiral, it is something called a “galaxy aggregate”, which is, just as the name suggests, a central galaxy surrounded by a handful of luminous knots of material that seem to stem from it, extending and tearing away and adding to or altering its overall structure. It is also experiencing an extremely high rate of star formation, possibly triggered by an earlier interaction with another galaxy. As we see it, NGC 4858 is forming stars so frantically that it will use up all of its gas long before it reaches the end of its life. The color of its bright knots indicates that they are formed of hydrogen, which glows in various shades of bright red as it is energized by the many young, hot stars lurking within.
• 14 September 2018: Gravity is so much a part of our daily lives that it is all too easy to forget its awesome power — but on a galactic scale, its power becomes both strikingly clear and visually stunning. 32)
Legend to Figure 21: Some distant galaxies even appear multiple times in this image. Since galaxies are wide objects, light from one side of the galaxy passes through the gravitational lens differently than light from the other side. When the galaxies’ light reaches Earth it can appear reflected, as seen with the galaxy on the lower left part of the lens, or distorted, as seen with the galaxy to the upper right. — These data were taken as part of a research project on star formation in the distant Universe, building on Hubble’s extensive legacy of deep-field images. Hubble observed 73 gravitationally-lensed galaxies for this project.
• 10 September 2018: Figure 22 is a composite image taken by Hubble on 6 June 2018 showing a fully-illuminated Saturn and its rings, along with six of its 62 known moons. The visible moons are (from left to right) Dione, Enceladus, Tethys, Janus, Epimetheus and Mimas (click here for an annotated version). Dione is the largest moon in the picture, with a diameter of 1123 km, compared to the smallest, oddly-shaped Epimetheus with a diameter around 116 km. 33)
- During Cassini’s mission, Enceladus was identified as one of the most intriguing moons, with the discovery of water vapor jets spewing from the surface implying the existence of a subsurface ocean. Icy moons with subsurface oceans could potentially offer the conditions to harbor life, and understanding their origins and properties are essential for furthering our knowledge of the Solar System. ESA's JUpiter ICy moons Explorer (Juice), due to launch in 2022, aims to continue this theme by studying Jupiter's ocean-bearing moons: Ganymede, Europa, and Callisto.
- The Hubble image of Figure 23 was taken shortly before Saturn's opposition on 27 June, when the Sun, Earth and Saturn were aligned so that the Sun fully illuminated Saturn as seen from Earth. Saturn's closest approach to Earth occurs around the same time as opposition, which makes it appear brighter and larger and allows the planet to be imaged in greater detail.
- In this image the planet’s rings are seen near their maximum tilt towards Earth. Towards the end of Cassini’s mission, the spacecraft made multiple dives through the gap between Saturn and its rings, gathering spectacular data in this previously unchartered territory.
- The image also shows a hexagonal atmospheric feature around the north pole, with the remnants of a storm, seen as a string of bright clouds. The hexagon-shaped cloud phenomenon is a stable and persistent feature first seen by the Voyager 1 space probe when it flew past Saturn 1981. In a study published just last week, scientists using Cassini data collected between 2013 and 2017, as the planet approached northern summer, identified a hexagonal vortex above the cloud structure, showing there is still much to learn about the dynamics of Saturn’s atmosphere.
- The Hubble observations making up this image were performed as part of the Outer Planet Atmospheres Legacy (OPAL) project, which uses Hubble to observe the outer planets to understand the dynamics and evolution of their complex atmospheres. This was the first time that Saturn was imaged as part of OPAL. This image was first published on 26 July.
• 30 August 2018: Astronomers using the NASA/ESA Hubble Space Telescope have taken a series of spectacular images featuring the fluttering auroras at the north pole of Saturn. The observations were taken in ultraviolet light and the resulting images provide astronomers with the most comprehensive picture so far of Saturn's northern aurora. 34)
- In 2017, over a period of seven months, the NASA/ESA Hubble Space Telescope took images of auroras above Saturn's north pole region using the Space Telescope Imaging Spectrograph. The observations were taken before and after the Saturnian northern summer solstice. These conditions provided the best achievable viewing of the northern auroral region for Hubble.
- On Earth, auroras are mainly created by particles originally emitted by the Sun in the form of solar wind. When this stream of electrically charged particles gets close to our planet, it interacts with the magnetic field, which acts as a gigantic shield. While it protects Earth's environment from solar wind particles, it can also trap a small fraction of them. Particles trapped within the magnetosphere — the region of space surrounding Earth in which charged particles are affected by its magnetic field — can be energized and then follow the magnetic field lines down to the magnetic poles. There, they interact with oxygen and nitrogen atoms in the upper layers of the atmosphere, creating the flickering, colorful lights visible in the polar regions here on Earth.
Note: The auroras here on Earth have different names depending on which pole they occur at. Aurora Borealis, or the northern lights, is the name given to auroras around the north pole and Aurora Australis, or the southern lights, is the name given for auroras around the south pole.
- However, these auroras are not unique to Earth. Other planets in our Solar System have been found to have similar auroras. Among them are the four gas giants Jupiter, Saturn, Uranus and Neptune. Because the atmosphere of each of the four outer planets in the Solar System is — unlike the Earth — dominated by hydrogen, Saturn's auroras can only be seen in ultraviolet wavelengths; a part of the electromagnetic spectrum which can only be studied from space.
- Hubble allowed researchers to monitor the behavior of the auroras at Saturn's north pole over an extended period of time. The Hubble observations were coordinated with the "Grand Finale" of the Cassini spacecraft, when the spacecraft simultaneously probed the auroral regions of Saturn. The Hubble data allowed astronomers to learn more about Saturn’s magnetosphere, which is the largest of any planet in the Solar System other than Jupiter.
Note: Cassini was a collaboration between NASA, ESA and the Italian Space Agency. It spent 13 years orbiting Saturn, gathering information and giving astronomers a great insight into the inner workings of Saturn. Cassini took more risks at the end of its mission, travelling through the gap between Saturn and its rings. No spacecraft had previously done this, and Cassini gathered spectacular images of Saturn as well as new data for scientists to work with. On 15 September 2017 Cassini was sent on a controlled crash into Saturn.
- The images show a rich variety of emissions with highly variable localized features. The variability of the auroras is influenced by both the solar wind and the rapid rotation of Saturn, which lasts only about 11 hours. On top of this, the northern aurora displays two distinct peaks in brightness — at dawn and just before midnight. The latter peak, unreported before, seems specific to the interaction of the solar wind with the magnetosphere at Saturn’s solstice.
- The main image presented here is a composite of observations made of Saturn in early 2018 in the optical and of the auroras on Saturn’s north pole region, made in 2017, demonstrating the size of the auroras along with the beautiful colors of Saturn.
- Hubble has studied Saturn's auroras in the past. In 2004, it studied the southern auroras shortly after the southern solstice (heic0504) and in 2009 it took advantage of a rare opportunity to record Saturn when its rings were edge-on (heic1003). This allowed Hubble to observe both poles and their auroras simultaneously.
• 24 August 2018: This dramatic image from the NASA/ESA Hubble Space Telescope shows the planetary nebula NGC 3918, a brilliant cloud of colorful gas in the constellation of Centaurus, around 4,900 light-years from Earth. 35)
- In the center of the cloud of gas, and completely dwarfed by the nebula, are the dying remnants of a red giant. During the final convulsive phase in the evolution of these stars, huge clouds of gas are ejected from the surface of the star before it emerges from its cocoon as a white dwarf. The intense ultraviolet radiation from the tiny remnant star then causes the surrounding gas to glow like a fluorescent sign. These extraordinary and colorful planetary nebulas are among the most dramatic sights in the night sky, and often have strange and irregular shapes, which are not yet fully explained.
- NGC 3918’s distinctive eye-like shape, with a bright inner shell of gas and a more diffuse outer shell that extends far from the nebula, looks as if it could be the result of two separate ejections of gas. But this is in fact not the case: studies of the object suggest that they were formed at the same time, but are being blown from the star at different speeds. The powerful jets of gas emerging from the ends of the large structure are estimated to be shooting away from the star at speeds of up to 350,000 km/hr.
- By the standards of astronomical phenomena, planetary nebulas like NGC 3918 are very short-lived, with a lifespan of just a few tens of thousands of years.
• 16 August 2018: Astronomers using the ultraviolet vision of NASA’s Hubble Space Telescope have captured one of the largest panoramic views of the fire and fury of star birth in the distant universe. The field features approximately 15,000 galaxies, about 12,000 of which are forming stars. Hubble’s ultraviolet vision opens a new window on the evolving universe, tracking the birth of stars over the last 11 billion years back to the cosmos’ busiest star-forming period, which happened about 3 billion years after the big bang. 36)
- Ultraviolet light has been the missing piece to the cosmic puzzle. Now, combined with infrared and visible-light data from Hubble and other space and ground-based telescopes, astronomers have assembled one of the most comprehensive portraits yet of the universe’s evolutionary history.
- The image straddles the gap between the very distant galaxies, which can only be viewed in infrared light, and closer galaxies, which can be seen across a broad spectrum. The light from distant star-forming regions in remote galaxies started out as ultraviolet. However, the expansion of the universe has shifted the light into infrared wavelengths. By comparing images of star formation in the distant and nearby universe, astronomers glean a better understanding of how nearby galaxies grew from small clumps of hot, young stars long ago.
- Because Earth’s atmosphere filters most ultraviolet light, Hubble can provide some of the most sensitive space-based ultraviolet observations possible.
- The program, called the Hubble Deep UV (HDUV) Legacy Survey, extends and builds on the previous Hubble multi-wavelength data in the CANDELS-Deep (Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey) fields within the central part of the GOODS (Great Observatories Origins Deep Survey) fields. This mosaic is 14 times the area of the Hubble Ultra Violet Ultra Deep Field released in 2014.
- The image of Figure 26 is a portion of the GOODS-North field, which is located in the northern constellation Ursa Major.
- The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
• 10 August 2018: This Picture of the Week shows the colorful globular cluster NGC 2108. The cluster is nestled within the Large Magellanic Cloud, in the constellation of the Swordfish (Dorado). It was discovered in 1835 by the astronomer, mathematician, chemist and inventor John Herschel, son of the famous William Herschel. 37)
- The most striking feature of this globular cluster is the gleaming ruby-red spot at the center left of the image (Figure 27). What looks like the cluster’s watchful eye is actually a carbon star. Carbon stars are almost always cool red giants, with atmospheres containing more carbon than oxygen — the opposite to our Sun. Carbon monoxide forms in the outer layer of the star through a combination of these elements, until there is no more oxygen available. Carbon atoms are then free to form a variety of other carbon compounds, such as C2, CH, CN, C3 and SiC2, which scatter blue light within the star, allowing red light to pass through undisturbed.
• 03 August 2018: Gravitational lenses — such as this galaxy cluster SDSS J1152+3313 — possess immense masses that wrap their surroundings and bend the light from faraway objects into rings, arcs, streaks, blurs, and other odd shapes. This lens, however, is not only wrapping the appearance of a distant galaxy — it is also amplifying its light, making it appear much brighter than it would be without the lens. Combined with the high image quality obtainable with Hubble, this gives valuable clues into how stars formed in the early Universe. 38)
- Star formation is a key process in astronomy. Everything that emits light is somehow connected to stars, so understanding how stars form is key to understanding countless objects lying across the cosmos. Astronomers can probe these early star-forming regions to learn about the sizes, luminosities, formation rates, and generations of different types of stars.
• 26 July 2018: In the summer of 2018 the planets Mars and Saturn are, one after the other, in opposition to Earth. During this event the planets are relatively close to Earth, allowing astronomers to observe them in greater detail. Hubble took advantage of this preferred configuration and imaged both planets to continue its long-standing observation of the outer planets in the Solar System. 39) 40)
- Since the NASA/ESA Hubble Space Telescope was launched, its goal has always been to study not only distant astronomical objects, but also the planets within our Solar System. Hubble’s high-resolution images of our planetary neighbors can only be surpassed by pictures taken from spacecraft that actually visit these bodies. However, Hubble has one advantage over space probes: it can look at these objects periodically and observe them over much longer periods than any passing probe could.
- In the last months the planets Mars and Saturn have each been in opposition to Earth — Saturn on 27 June and Mars on 27 July. An opposition occurs when the Sun, Earth and an outer planet are lined up, with Earth sitting in between the Sun and the outer planet. During an opposition, a planet is fully lit by the Sun as seen from Earth, and it also marks the time when the planet is closest to Earth, allowing astronomers to see features on the planet’s surface in greater detail.
Note: The dates of opposition and closest approach differ slightly. This difference is caused by the elliptical orbit of the planets and the fact that the orbits are not in exactly the same plane.
- A month before Saturn's opposition — on 6 June — Hubble was used to observe the ringed planet . At this time Saturn was approximately 1.4 billion kilometers from Earth. The taken images show Saturn’s magnificent ring system near its maximum tilt toward Earth, allowing a spectacular view of the rings and the gaps between them. Though all of the gas giants boast rings, Saturn’s are the largest and most spectacular, stretching out to eight times the radius of the planet.
Note: The observations of Saturn were made as part of the Outer Planet Atmospheres Legacy (OPAL) project. OPAL is helping astronomers understand the atmospheric dynamics and evolution of the gas giant planets in our Solar System. Jupiter, Uranus and Neptune have already been observed several times as part of this project, but this is the first time Saturn was observed as part of OPAL.
- Alongside a beautiful view of the ring system, Hubble's new image reveals a hexagonal pattern around the north pole — a stable and persistent wind feature discovered during the flyby of the Voyager 1 space probe in 1981. To the south of this feature a string of bright clouds is visible: remnants of a disintegrating storm.
- While observing the planet, Hubble also managed to capture images of six of Saturn's 62 currently known moons: Dione, Enceladus, Tethys, Janus, Epimetheus, and Mimas. Scientists hypothesize that a small, wayward moon like one of these disintegrated 200 million years ago to form Saturn’s ring system.
- Hubble shot the second portrait, of the planet Mars, on 18 July, just 13 days before Mars reached its closest approach to Earth. This year Mars will get as close as 57.6 million km from Earth. This makes it the closest approach since 2003, when the red planet made its way closer to us than at any other time in almost 60 000 years (opo0322).
- While previous images showed detailed surface features of the planet, this new image is dominated by a gigantic sandstorm enshrouding the entire planet. Still visible are the white polar caps, Terra Meridiani, the Schiaparelli Crater, and Hellas Basin — but all of these features are slightly blurred by the dust in the atmosphere.
- Comparing these new images of Mars and Saturn with older data gathered by Hubble, other telescopes and even space probes allows astronomers to study how cloud patterns and large-scale structures on other planets in our Solar System change over time.
• 13 July 2018: In November 2008, 14-year-old Caroline Moore from New York discovered a supernova in UGC 12682. This made her the youngest person at the time to have discovered a supernova. Follow-up observations by professional astronomers of the so-called SN 2008ha showed that it was peculiarly interesting in many different ways: its host galaxy UGC 12862 rarely produces supernovae. It is one of the faintest supernovae ever observed and after the explosion it expanded very slowly, suggesting that the explosion did not release copious amounts of energy as usually expected. 41)
- Astronomers have now classified SN 2008ha as a subclass of a Type Ia supernova, which is the explosion of a white dwarf that hungrily accretes matter from a companion star. SN 2008ha may have been the result of a partially failed supernova, explaining why the explosion failed to decimate the whole star.
• 3 July 2018: Like a July 4 fireworks display, a young, glittering collection of stars resembles an aerial burst. The cluster is surrounded by clouds of interstellar gas and dust - the raw material for new star formation. The nebula, located 20,000 light-years away in the constellation Carina, contains a central cluster of huge, hot stars, called NGC 3603. 42)
- Appearing colorful and serene, this environment is anything but. Ultraviolet radiation and violent stellar winds have blown out an enormous cavity in the gas and dust enveloping the cluster. Most of the stars in the cluster were born around the same time but differ in size, mass, temperature and color. The course of a star's life is determined by its mass, so a cluster of a given age will contain stars in various stages of their lives, giving an opportunity for detailed analyses of stellar life cycles. NGC 3603 also contains some of the most massive stars known. These huge stars live fast and die young, burning through their hydrogen fuel quickly and ultimately ending their lives in supernova explosions.
- Star clusters like NGC 3603 provide important clues to understanding the origin of massive star formation in the early, distant universe. Astronomers also use massive clusters to study distant starbursts that occur when galaxies collide, igniting a flurry of star formation. The proximity of NGC 3603 makes it an excellent lab for studying such distant and momentous events.
- This Hubble Space Telescope image was captured in August 2009 and December 2009 with the Wide Field Camera 3 in both visible and infrared light, which trace the glow of sulfur, hydrogen, and iron.
• 25 June 2018: For years the Hubble Space Telescope has captured crisp spectral images of exoplanets transiting their host stars. Because those images include light filtered through the exoplanets’ atmospheres, they contain clues about atmospheric composition. Absorption features in such spectra have produced evidence of water, carbon dioxide, methane, and even clouds in the atmospheres of extrasolar planets. 43)
- But Hubble’s workhorse detector for exoplanet atmosphere observations, the Wide Field Camera 3, collects light in only 13 wavelength bins. The James Webb Space Telescope, scheduled for a 2020 launch, will be able to resolve spectra into hundreds of bins. The abundance of data could yield far more detailed portraits of extrasolar atmospheres, but it also creates a challenge: how to decipher all that information.
- Enter Kevin Heng and his coworkers at the University of Bern in Switzerland. The researchers have now demonstrated that machine learning can be used to extract atmospheric properties from even the most complicated transmission spectra. Heng and his colleagues trained their machine on tens of thousands of model spectra that were calculated analytically for atmospheres of varying temperature, cloudiness, and composition. The machine learning algorithm plots the spectra in N-dimensional space, where N is the number of wavelength bins in each spectrum, and then identifies clusters in that multidimensional space. Model atmospheres belonging to the same cluster tend to share similar physical attributes, so when the trained machine is given a real-life spectrum to analyze, it plots the spectrum and assigns to it the physical attributes of the nearest cluster.
- Reassuringly, a test-run analysis of the gas-giant planet WASP-12b yielded results similar to those of more conventional techniques. The test was implemented in 13-dimensional space, to match Hubble’s 13 spectral bins, but future implementations using more detailed spectra could include thousands of dimensions. 44)
• 25 June 2018: As if this Hubble Space Telescope picture isn't cluttered enough with myriad galaxies, nearby asteroids photobomb the image, their trails sometimes mimicking background astronomical phenomena. 45)
- The stunningly beautiful galaxy cluster Abell 370 (Figure 32) contains an astounding assortment of several hundred galaxies tied together by the mutual pull of gravity. Located approximately four billion light years away in the constellation Cetus, the Sea Monster, this immense cluster is a rich mix of a variety of galaxy shapes.
- Entangled among the galaxies are thin, white trails that look like curved or S-shaped streaks. These are trails from asteroids that reside, on average, only about 260 million kilometers from Earth – right around the corner in astronomical terms. The trails appear in multiple Hubble exposures that have been combined into one image. Of the 22 total asteroid sightings for this field, five are unique objects. These asteroids are so faint that they were not previously identified.
- The asteroid trails look curved due to an observational effect called parallax. As Hubble orbits around Earth, an asteroid will appear to move along an arc with respect to the vastly more distant background stars and galaxies. The motion of Earth around the Sun, and the motion of the asteroids along their orbits, are other contributing factors to the apparent skewing of asteroid paths.
- All the asteroids were found manually, the majority by “blinking” consecutive exposures to capture apparent asteroid motion. Astronomers found a unique asteroid for every 10 to 20 hours of exposure time.
- These asteroid trails should not be confused with the mysterious-looking arcs of blue light that are actually distorted images of distant galaxies behind the cluster. Many of these far-flung galaxies are too faint for Hubble to see directly. Instead, in a dramatic example of “gravitational lensing,” the cluster functions as a natural telescope, warping space and affecting light traveling through the cluster toward Earth.
- The study was part of the Frontier Fields program and the image, assembled from several exposures taken in visible and infrared light, was first published on 6 November 2017.
- Every year on 30 June, the global “Asteroid Day” event takes place to raise awareness about asteroids and what can be done to protect Earth from possible impact. The day falls on the anniversary of the Tunguska event that took place on 30 June 1908, the most harmful known asteroid related event in recent history. This year, ESA is co-hosting a live webcast with the European Southern Observatory packed with expert interviews, news on some of the most recent asteroid science results, and the truth about the dinosaurs. Watch 30 June at 13:00 CEST via http://www.esa.int/Our_Activities/Space_Engineering_Technology/Asteroid_day
• 21 June 2018: An international team of astronomers using the NASA/ESA Hubble Space Telescope and the European Southern Observatory's VLT (Very Large Telescope) has made the most precise test of general relativity yet outside our Milky Way. The nearby galaxy ESO 325-G004 acts as a strong gravitational lens, distorting light from a distant galaxy behind it to create an Einstein ring around its center. By comparing the mass of ESO 325-G004 with the curvature of space around it, the astronomers found that gravity on these astronomical length-scales behaves as predicted by general relativity. This rules out some alternative theories of gravity. 46) 47)
- Using the NASA/ESA Hubble Space Telescope and European Southern Observatory's VLT, a team led by Thomas Collett (University of Portsmouth, UK), was able to perform the most precise test of general relativity outside the Milky Way to date.
- The theory of general relativity predicts that objects deform spacetime, causing any light that passes by to be deflected and resulting in a phenomenon known as gravitational lensing. This effect is only noticeable for very massive objects. A few hundred strong gravitational lenses are known, but most are too distant to precisely measure their mass. However, the elliptical galaxy ESO 325-G004 is amongst the closest lenses at just 450 million light-years from Earth.
- Using the MUSE (Multi Unit Spectroscopic Explorer) instrument on the VLT the team calculated the mass of ESO 325-G004 by measuring the movement of stars within it. Using Hubble the scientists were able to observe an Einstein ring resulting from light from a distant galaxy being distorted by the intervening ESO 325-G004. Studying the ring allowed the astronomers to measure how light, and therefore spacetime, is being distorted by the huge mass of ESO 325-G004.
- Collett comments: "We know the mass of the foreground galaxy from MUSE and we measured the amount of gravitational lensing we see from Hubble. We then compared these two ways to measure the strength of gravity – and the result was just what general relativity predicts, with an uncertainty of only nine percent. This is the most precise test of general relativity outside the Milky Way to date. And this using just one galaxy!"
- General relativity has been tested with exquisite accuracy on Solar System scales, and the motions of stars around the black hole at the center of the Milky Way are under detailed study, but previously there had been no precise tests on larger astronomical scales. Testing the long range properties of gravity is vital to validate our current cosmological model.
- These findings may have important implications for models of gravity alternative to general relativity. These alternative theories predict that the effects of gravity on the curvature of spacetime are "scale dependent". This means that gravity should behave differently across astronomical length-scales from the way it behaves on the smaller scales of the Solar System. Collett and his team found that this is unlikely to be true unless these differences only occur on length scales larger than 6000 light-years.
- "The Universe is an amazing place providing such lenses which we can use as our laboratories," adds team member Bob Nichol (University of Portsmouth). "It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was."
• 31 May 2018: Though it resembles a peaceful rose swirling in the darkness of the cosmos, NGC 3256 is actually the site of a violent clash. This distorted galaxy is the relic of a collision between two spiral galaxies, estimated to have occurred 500 million years ago. Today it is still reeling in the aftermath of this event. 48)
- Located about 100 million light-years away in the constellation of Vela (The Sails), NGC 3256 is approximately the same size as our Milky Way and belongs to the Hydra-Centaurus Supercluster. It still bears the marks of its tumultuous past in the extended luminous tails that sprawl out around the galaxy, thought to have formed 500 million years ago during the initial encounter between the two galaxies, which today form NGC 3256. These tails are studded with young blue stars, which were born in the frantic but fertile collision of gas and dust.
- When two galaxies merge, individual stars rarely collide because they are separated by such enormous distances, but the gas and dust of the galaxies do interact – with spectacular results. The brightness blooming in the center of NGC 3256 gives away its status as a powerful starburst galaxy, host to vast amounts of infant stars born into groups and clusters. These stars shine most brightly in the far infrared, making NGC 3256 exceedingly luminous in this wavelength domain. Because of this radiation, it is classified as a Luminous Infrared Galaxy.
- NGC 3256 has been the subject of much study due to its luminosity, its proximity, and its orientation: astronomers observe its face-on orientation, that shows the disc in all its splendor. NGC 3256 provides an ideal target to investigate starbursts that have been triggered by galaxy mergers. It holds particular promise to further our understanding of the properties of young star clusters in tidal tails.
- As well as being lit up by over 1000 bright star clusters, the central region of NGC 3256 is also home to crisscrossing threads of dark dust and a large disc of molecular gas spinning around two distinct nuclei – the relics of the two original galaxies. One nucleus is largely obscured, only unveiled in infrared, radio and X-ray wavelengths.
- These two initial galaxies were gas-rich and had similar masses, as they seem to be exerting roughly equal influence on each other. Their spiral disks are no longer distinct, and in a few hundred million years' time, their nuclei will also merge and the two galaxies will likely become united as a large elliptical galaxy.
- NGC 3256 was previously imaged through fewer filters by the NASA/ESA Hubble Space Telescope as part of a large collection of 59 images of merging galaxies, released for Hubble's 18th anniversary on 24 April 2008.
• 28 May 2018: This NASA/ESA Hubble Space Telescope image shows a cluster of hundreds of galaxies located about 7.5 billion light-years from Earth (Figure 35). The brightest galaxy within this cluster named SDSS J1156+1911 and known as the Brightest Cluster Galaxy (BCG), is visible in the lower middle of the frame. It was discovered by the Sloan Giant Arcs Survey which studied data maps covering huge parts of the sky from the Sloan Digital Sky Survey. The survey found more than 70 galaxies that look to be significantly affected by a cosmic phenomenon known as gravitational lensing. 49)
- Gravitational lensing is one of the predictions of Albert Einstein's General Theory of Relativity. The mass contained within a galaxy is so immense that it can actually warp and bend the very fabric of its surroundings (known as space-time), forcing light to travel along curved paths. As a result, the image of a more distant galaxy appears distorted and amplified to an observer, as the light from it has been bent around the intervening galaxy. This effect can be very useful in astronomy, allowing astronomers to see galaxies that are either obscured or too distant to be otherwise detected by our current instruments.
- Galaxy clusters are giant structures containing hundreds to thousands of galaxies, some with masses over one million billion times the mass of the Sun! SDSS J1156+1911 is only roughly 600 billion times the mass of the Sun, making it less massive than the average galaxy. However, it is massive enough to produce the fuzzy, greenish streak seen just below the brightest galaxy — the lensed image of a more distant galaxy.
Figure 35: This Hubble image shows a cluster of hundreds of galaxies located about 7.5 billion light-years from Earth (image credit: ESA/Hubble & NASA; Acknowledgment: Judy Schmidt (Geckzilla))
• 17 May 2018: Ultraviolet light is a major tracer of the youngest and hottest stars. These stars are short-lived and intensely bright. Astronomers have now finished a survey called LEGUS (Legacy ExtraGalactic UV Survey) that captured the details of 50 local galaxies within 60 million light-years of Earth in both visible and ultraviolet light. 50) 51)
- The LEGUS team carefully selected its targets from among 500 candidate galaxies compiled from ground-based surveys. They chose the galaxies based on their mass, star-formation rate, and their abundances of elements heavier than hydrogen and helium. Because of the proximity of the selected galaxies, Hubble was able to resolve them into their main components: stars and star clusters. With the LEGUS data, the team created a catalog with about 8000 young clusters and it also created a star catalog comprising about 39 million stars that are at least five times more massive than our Sun.
- The data, gathered with Hubble’s WFC3 (Wide Field Camera 3) and ACS (Advanced Camera for Surveys), provide detailed information on young, massive stars and star clusters, and how their environment affects their development. As such, the catalogue offers an extensive resource for understanding the complexities of star formation and galaxy evolution.
- One of the key questions the survey may help astronomers answer is the connection between star formation and the major structures, such as spiral arms, that make up a galaxy. These structured distributions are particularly visible in the youngest stellar populations.
- By resolving the fine details of the studied galaxies, while also studying the connection to larger galactic structures, the team aims to identify the physical mechanisms behind the observed distribution of stellar populations within galaxies.
- Figuring out the final link between gas and star formation is key to fully understanding galaxy evolution. Astronomers are studying this link by looking at the effects of the environment on star clusters, and how their survival is linked to their surroundings.
- LEGUS will not only allow astronomers to understand the local Universe. It will also help interpret views of distant galaxies, where the ultraviolet light from young stars is stretched to infrared wavelengths due to the expansion of space. The NASA/ESA/CSA James Webb Space Telescope and its ability to observe in the far infrared will complement the LEGUS views.
• 16 My 2018: Resembling a wizard’s staff set aglow, NGC 1032 cleaves the quiet darkness of space in two in this image from the NASA/ESA Hubble Space Telescope (Figure 38). 52)
- NGC 1032 is located about a hundred million light years away in the constellation Cetus (The Sea Monster). Although beautiful, this image perhaps does not do justice to the galaxy’s true aesthetic appeal: NGC 1032 is actually a spectacular spiral galaxy, but from Earth, the galaxy’s vast disc of gas, dust and stars is seen nearly edge-on.
• 02 May 2018: Astronomers using the NASA/ESA Hubble Space Telescope have detected helium in the atmosphere of the exoplanet WASP-107b. This is the first time this element has been detected in the atmosphere of a planet outside the Solar System. The discovery demonstrates the ability to use infrared spectra to study exoplanet extended atmospheres. 53)
- The international team of astronomers, led by Jessica Spake, a PhD student at the University of Exeter in the UK, used Hubble's Wide Field Camera 3 to discover helium in the atmosphere of the exoplanet WASP-107b. This is the first detection of its kind.
- Spake explains the importance of the discovery: "Helium is the second-most common element in the Universe after hydrogen. It is also one of the main constituents of the planets Jupiter and Saturn in our Solar System. However, up until now helium had not been detected on exoplanets - despite searches for it."
- The team made the detection by analyzing the infrared spectrum of the atmosphere of WASP-107b. Previous detections of extended exoplanet atmospheres have been made by studying the spectrum at ultraviolet and optical wavelengths; this detection therefore demonstrates that exoplanet atmospheres can also be studied at longer wavelengths.
Note: The measurement of an exoplanet's atmosphere is performed when the planet passes in front of its host star. A tiny portion of the star's light passes through the exoplanet's atmosphere, leaving detectable fingerprints in the spectrum of the star. The larger the amount of an element present in the atmosphere, the easier the detection becomes.
- "The strong signal from helium we measured demonstrates a new technique to study upper layers of exoplanet atmospheres in a wider range of planets," says Spake "Current methods, which use ultraviolet light, are limited to the closest exoplanets. We know there is helium in the Earth's upper atmosphere and this new technique may help us to detect atmospheres around Earth-sized exoplanets – which is very difficult with current technology."
- WASP-107b is one of the lowest density planets known: While the planet is about the same size as Jupiter, it has only 12% of Jupiter's mass. The exoplanet is about 200 light-years from Earth and takes less than six days to orbit its host star.
- The amount of helium detected in the atmosphere of WASP-107b is so large that its upper atmosphere must extend tens of thousands of kilometers out into space. This also makes it the first time that an extended atmosphere has been discovered at infrared wavelengths.
- Since its atmosphere is so extended, the planet is losing a significant amount of its atmospheric gases into space – between ~0.1-4% of its atmosphere's total mass every billion years.
Note: Stellar radiation has a significant effect on the rate at which a planet's atmosphere escapes. The star WASP-107 is highly active, supporting the atmospheric loss. As the atmosphere absorbs radiation it heats up, so the gas rapidly expands and escapes more quickly into space.
- As far back as the year 2000, it was predicted that helium would be one of the most readily-detectable gases on giant exoplanets, but until now, searches were unsuccessful.
- David Sing, co-author of the study also from the University of Exeter, concludes: "Our new method, along with future telescopes such as the NASA/ESA/CSA James Webb Space Telescope, will allow us to analyze atmospheres of exoplanets in far greater detail than ever before." 54)
• 19 April 2018: To celebrate its 28th anniversary in space the NASA/ESA Hubble Space Telescope took this amazing and colorful image of the Lagoon Nebula (Figure 40). The whole nebula, about 4000 light-years away, is an incredible 55 light-years wide and 20 light-years tall. This image shows only a small part of this turbulent star-formation region, about four light-years across. 55)
- This stunning nebula was first catalogued in 1654 by the Italian astronomer Giovanni Battista Hodierna, who sought to record nebulous objects in the night sky so they would not be mistaken for comets. Since Hodierna’s observations, the Lagoon Nebula has been photographed and analysed by many telescopes and astronomers all over the world.
• 10 April 2018: This NASA/ESA Hubble Space Telescope image (Figure 41) shows a massive galaxy cluster glowing brightly in the darkness. Despite its beauty, this cluster bears the distinctly unpoetic name of PLCK_G308.3-20.2. 56)
- Galaxy clusters can contain thousands of galaxies all held together by the glue of gravity. At on57)e point in time they were believed to be the largest structures in the Universe — until they were usurped in the 1980s by the discovery of superclusters, which typically contain dozens of galaxy clusters and groups and span hundreds of millions of light-years. However, clusters do have one thing to cling on to; superclusters are not held together by gravity, so galaxy clusters still retain the title of the biggest structures in the Universe bound by gravity.
- One of the most interesting features of galaxy clusters is the stuff that permeates the space between the constituent galaxies: the intracluster medium (ICM). High temperatures are created in these spaces by smaller structures forming within the cluster. This results in the ICM being made up of plasma — ordinary matter in a superheated state. Most luminous matter in the cluster resides in the ICM, which is very luminous X-rays. However, the majority of the mass in a galaxy cluster exists in the form of non-luminous dark matter. Unlike plasma, dark matter is not made from ordinary matter such as protons, neutrons and electrons. It is a hypothesized substance thought to make up 80 % of the Universe’s mass, yet it has never been directly observed.
• 02 April 2018: Astronomers using the NASA/ESA Hubble Space Telescope have found the most distant star ever discovered. The hot blue star existed only 4.4 billion years after the Big Bang. This discovery provides new insight into the formation and evolution of stars in the early Universe, the constituents of galaxy clusters and also on the nature of dark matter. 58)
- The international team, led by Patrick Kelly (University of Minnesota, USA), Jose Diego (Instituto de Física de Cantabria, Spain) and Steven Rodney (University of South Carolina, USA), discovered the distant star in the galaxy cluster MACS J1149-2223 in April 2016. The observations with Hubble were actually performed in order to detect and follow the latest appearance of the gravitationally lensed supernova explosion nicknamed "Refsdal" (heic1525), when an unexpected point source brightened in the same galaxy that hosted the supernova.
- "Like the Refsdal supernova explosion the light of this distant star got magnified, making it visible for Hubble," says Patrick Kelly. "This star is at least 100 times farther away than the next individual star we can study, except for supernova explosions."
- The observed light from the newly discovered star, called Lensed Star 1 (LS1) was emitted when the Universe was only about 30 percent of its current age – about 4.4 billion years after the Big Bang. The detection of the star through Hubble was only possible because the light from the star was magnified 2000 times.
- "The star became bright enough to be visible for Hubble thanks to a process called gravitational lensing," explains Jose Diego. The light from LS1 was magnified not only by the huge total mass of the galaxy cluster, but also by another compact object of about three times the mass of the Sun within the galaxy cluster itself; an effect known as gravitational microlensing.
Note: Gravitational lensing magnifies the light from fainter, background objects, allowing Hubble to see objects it would otherwise not be able to detect. The process was first predicted by Albert Einstein and is now used to find some of the most distant objects in the Universe. Usually the lensing object is a galaxy or a galaxy cluster, but in some cases it can also be a star or even a planet. When it involves these smaller objects the process is called microlensing.
- "The discovery of LS1 allows us to gather new insights into the constituents of the galaxy cluster. We know that the microlensing was caused by either a star, a neutron star, or a stellar-mass black hole," explains Steven Rodney. LS1 therefore allows astronomers to study neutron stars and black holes, which are otherwise invisible and they can estimate how many of these dark objects exist within this galaxy cluster.
- As galaxy clusters are among the largest and most massive structures in the Universe, learning about their constituents also increases our knowledge about the composition of the Universe overall. This includes additional information about the mysterious dark matter.
- "If dark matter is at least partially made up of comparatively low-mass black holes, as it was recently proposed, we should be able to see this in the light curve of LS1. Our observations do not favor the possibility that a high fraction of dark matter is made of these primordial black holes with about 30 times the mass of the Sun", highlights Kelly.
- After the discovery the researchers used Hubble again to measure a spectrum of LS1. Based on their analysis, the astronomers think that LS1 is a B-type supergiant star. These stars are extremely luminous and blue in color, with a surface temperature between 11,000 and 14,000 degrees Celsius; making them more than twice as hot as the Sun.
- But this was not the end of the story. Observations made in October 2016 suddenly showed a second image of the star. "We were actually surprised to not have seen this second image in earlier observations, as also the galaxy the star is located in can be seen twice," comments Diego. "We assume that the light from the second image has been deflected by another moving massive object for a long time – basically hiding the image from us. And only when the massive object moved out of the line of sight the second image of the star became visible." This second image and the blocking object add another piece of the puzzle to reveal the makeup of galaxy clusters.
- With more research and the arrival of new, more powerful telescopes like the NASA/ESA/CSA James Webb Space Telescope, the astronomers suggest that with microlensing, it will be possible to study the evolution of the earliest stars in the Universe in greater detail than ever expected.
•30 March 2018: The image of Figure 43, captured by the ACS (Advanced Camera for Surveys) on the NASA/ESA Hubble Space Telescope, shows the spiral galaxy NGC 5714, about 130 million light-years away in the constellation of Boötes (the Herdsman). NGC 5714 is classified as a Sc spiral galaxy, but its spiral arms — the dominating feature of spiral galaxies — are almost impossible to see, as NGC 1787 presents itself at an almost perfectly edge-on angle. 59) 60)
- Discovered by William Herschel in 1787, NGC 5714 was host to a fascinating and rare event in 2003. A faint supernova appeared about 8000 light-years below the central bulge of NGC 5714. Supernovae are the huge, violent explosions of dying stars, and the one that exploded in NGC 5714 — not visible in this much later image — was classified as a Type Ib/c supernova and named SN 2003dr. It was particularly interesting because its spectrum showed strong signatures of calcium.
- Calcium-rich supernovae are rare and hence of great interest to astronomers. Astronomers still struggle to explain these particular explosions as their existence presents a challenge to both observation and theory. In particular, their appearance outside of galaxies, their lower luminosity compared to other supernovae, and their rapid evolution are still open questions for researchers.
• 28 March 2018: An international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories have, for the first time, uncovered a galaxy in our cosmic neighborhood that is missing most – if not all – of its dark matter. This discovery of the galaxy NGC 1052-DF2 challenges currently-accepted theories of and galaxy formation and provides new insights into the nature of dark matter. The results are published in Nature. 61) 62)
- Astronomers using Hubble and several ground-based observatories have found a unique astronomical object: a galaxy that appears to contain almost no dark matter. Hubble helped to accurately confirm the distance of NGC 1052-DF2 to be 65 million light-years and determined its size and brightness. Based on these data the team discovered that NGC 1052-DF2 larger than the Milky Way, but contains about 250 times fewer stars, leading it to be classified as an ultra diffuse galaxy.
Note 1: The galaxy was identified with the Dragonfly Telephoto Array (DFA) and also observed by the Sloan Digital Sky Survey (SDSS). As well as the NASA/ESA Hubble Space Telescope, the Gemini Observatory and the Keck Observatory were used to study the object in more detail.
- "I spent an hour just staring at this image," lead researcher Pieter van Dokkum of Yale University says as he recalls first seeing the Hubble image of NGC 1052-DF2. "This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy."
- Further measurements of the dynamical properties of ten globular clusters orbiting the galaxy allowed the team to infer an independent value of the galaxies mass. This mass is comparable to the mass of the stars in the galaxy, leading to the conclusion that NGC 1052-DF2 contains at least 400 times less dark matter than astronomers predict for a galaxy of its mass, and possibly none at all. This discovery is unpredicted by current theories on the distribution of dark matter and its influence on galaxy formation.
Note 2: Since 1884 astronomers have invoked dark matter to explain why galaxies do not fly apart, given the speed at which the stars within galaxies move. From Kepler's Second Law it is expected that the rotation velocities of stars will decrease with distance from the center of a galaxy. This is not observed.
- "Dark matter is conventionally believed to be an integral part of all galaxies – the glue that holds them together and the underlying scaffolding upon which they are built," explains co-author Allison Merritt from Yale University and the Max Planck Institute for Astronomy, Germany. And van Dokkum adds: "This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work." - Merritt remarks: "There is no theory that predicts these types of galaxies – how you actually go about forming one of these things is completely unknown."
- Although counterintuitive, the existence of a galaxy without dark matter negates theories that try to explain the Universe without dark matter being a part of it. The discovery of NGC 1052-DF2 demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
Note3: The MOND theory – Modified Newtonian Dynamics – suggests that the phenomena usually attributed to dark matter can be explained by modifying the laws of gravity. The result of this would be that a signature usually attributed to dark matter should always be detected, and is an unavoidable consequence of the presence of ordinary matter.
- Meanwhile, the researchers already have some ideas about how to explain the missing dark matter in NGC 1052-DF2. Did a cataclysmic event such as the birth of a multitude of massive stars sweep out all the gas and dark matter? Or did the growth of the nearby massive elliptical galaxy NGC 1052 billions of years ago play a role in NGC 1052-DF2's dark matter deficiency?
- These ideas, however, still do not explain how this galaxy formed. To find an explanation, the team is already hunting for more dark-matter deficient galaxies as they analyze Hubble images of 23 ultra-diffuse galaxies – three of which appear to be similar to NGC 1052-DF2.
• April 24, 2017: Since its launch on 24 April 1990, Hubble has been nothing short of a revolution in astronomy. The first orbiting facility of its kind, for 27 years the telescope has been exploring the wonders of the cosmos. Astronomers and the public alike have witnessed what no other humans in history have before. In addition to revealing the beauty of the cosmos, Hubble has proved itself to be a treasure chest of scientific data that astronomers can access. 63) 64)
- NASA and ESA celebrate Hubble's birthday each year with a spectacular image. This year's anniversary image features a pair of spiral galaxies known as NGC 4302 – seen edge-on – and NGC 4298, both located 55 million light-years away in the northern constellation of Coma Berenices (Berenice's Hair). The pair, discovered by astronomer William Herschel in 1784, form part of the Virgo Cluster, a gravitationally bound collection of nearly 2000 individual galaxies. Such objects were first simply called "spiral nebulas," because it wasn't known how far away they were. In the early 20th century, Edwin Hubble discovered that galaxies are other island cities of stars far outside our Milky Way.
- At their closest points, the galaxies are separated from each other in projection by only around 7000 light-years. Given this very close arrangement, astronomers are intrigued by the galaxies' apparent lack of any significant gravitational interaction; only a faint bridge of neutral hydrogen gas – not visible in this image – appears to stretch between them. The long tidal tails and deformations in their structure that are typical of galaxies lying so close to each other are missing completely.
- The edge-on galaxy is called NGC 4302, and the tilted galaxy is NGC 4298. These galaxies look quite different because we see them angled at different positions on the sky. They are actually very similar in terms of their structure and contents.
- From our view on Earth, researchers report an inclination of 90 degrees for NGC 4302, which is exactly edge on. NGC 4298 is tilted 70 degrees.
- In NGC 4298, the telltale, pinwheel-like structure is visible, but it's not as prominent as in some other spiral galaxies. In the edge-on NGC 4302, dust in the disk is silhouetted against rich lanes of stars. Absorption by dust makes the galaxy appear darker and redder than its companion. A large blue patch appears to be a giant region of recent star formation.
- A typical spiral galaxy has arms of young stars that wind outward from its center. The bright arms are regions of intense star formation. Such galaxies have a central bulge and are surrounded by a faint halo of stars. Many spiral galaxies also have bars that extend from the central bulge to the arms.
- The edge-on NGC 4302 is about 87,000 light-years in diameter, which is about 60 percent the size of the Milky Way. It is about 110 billion solar masses, approximately one-tenth of the Milky Way's mass.
- The tilted NGC 4298 is about 45,000 light-years in diameter, about one third the size of the Milky Way. At 17 billion solar masses, it is less than 2 percent of the Milky Way galaxy's 1 trillion solar masses.
- The Hubble observations were taken between 2 - 22 January, 2017 with the WFC3 (Wide Field Camera 3) instrument in three visible light bands.
Hubble's 25th anniversary on orbit on April 24, 2015
From planets to planetary nebula, and from star formation to supernova explosions, the NASA/ESA Hubble Space Telescope has captured a wealth of astronomical objects in its 25-year career. The montage of Figure 47 presents 25 images that sample the space telescope’s rich contribution to our understanding of the Universe around us. 65) 66) 67)
The NASA/ESA Hubble was launched into orbit by the Space Shuttle on 24 April 1990 (12:33:51 UTC). It was the first space telescope of its kind, and has surpassed all expectations, providing a quarter of a century of discoveries, stunning images and outstanding science.
The anniversary image (Figure 49) is bursting with silver anniversary fireworks, showing off a giant young star cluster known as Westerlund 2, sparkling with the light of about 3000 stars. Hubble’s sharp vision resolves the dense concentration of stars in the central cluster, which measures only about 10 light-years across.
A new anniversary image of Hubble is released every year and shown in Figure 47.
At the center of the collage is Star cluster Westerlund 2, the image released on the occasion of Hubble’s 25th anniversary.
Top row (from left to right): Interacting galaxies , Abell 2218 , Comet ISON , Jupiter , Green filament in the Teacup galaxy , Star formation in 30 Doradus , Interacting galaxies Arp 273
Second row: Saturn , The Ring Nebula , “Mystic Mountain” in the Carina Nebula , Crab Nebula
Third row: The horsehead nebula , Carina Nebula , Planetary nebula NGC 6302 , Star formation in M17
Fourth row: Globular cluster NGC 121 , “Pillars of creation” , Ring galaxy AM 0644-741,
Fifth row: Colliding galaxies Arp 272, Star cluster NGC 602, Hubble Ultra Deep Field, Mars, Variable star RS Puppis , Orion Nebula
Figure 48: Horse of a Different Color: Hubble's Universe Unfiltered [video credit: Frank Summers, STScI (Space Telescope Science Institute)]
Legend to Figure 48: The Horsehead Nebula is a striking, dark gas cloud just below Orion's belt. It is a favorite of both professional and amateur astronomers. However, as a dark nebula, most of its true structure is hidden from visible light observations. To celebrate the 23rd anniversary of the Hubble Space Telescope, we revealed the considerable detail of that unseen nebular structure via an infrared portrait. The result is even more striking, and something one just doesn't see very often: a veritable astronomical horse of a different color. (Published on Sep 5, 2013).
"Hubble's Universe" is a recurring broadcast from HubbleSite, online home of the Hubble Space Telescope. Astrophysicist Frank Summers takes viewers on an in-depth tour of the latest Hubble discoveries. Find more episodes at: The Future of Space Astronomy: Hubble's Universe Unfiltered
• This glittering tapestry of young stars flaring into life in the star cluster Westerlund 2 has been released to celebrate the NASA/ESA Hubble Space Telescope’s 25th year in orbit and a quarter of a century of discoveries, stunning images and outstanding science. 68)
Legend to Figure 49: The sparkling centerpiece of Hubble’s anniversary fireworks is a giant cluster of about 3,000 stars called Westerlund 2, named for Swedish astronomer Bengt Westerlund who discovered the grouping in the 1960s. The cluster resides in a raucous stellar breeding ground known as Gum 29, located 20,000 light-years away from Earth in the constellation Carina.
The giant star cluster is only about two million years old, but contains some of the brightest, hottest and most massive stars ever discovered. Some of these are carving deep cavities in the surrounding material through their intense ultraviolet light and the high-speed charged particles contained in their stellar winds.
This image is a testament to Hubble’s observational power and demonstrates that, even with 25 years of operations under its belt, its story is by no means over. Hubble has set the stage for the James Webb Space Telescope – scheduled for launch in 2018 – but will not be immediately replaced by this next-generation observatory, instead working alongside it. Now, 25 years after launch, is the time to celebrate Hubble’s future potential as well as its remarkable history.
• November 2, 2015: Eerie, dramatic new pictures from NASA's Hubble Space Telescope show newborn stars emerging from "eggs" - not the barnyard variety - but rather dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). Hubble found the "EGGs," appropriately enough, in the Eagle nebula, a nearby star-forming region 6,500 light- years away in the constellation Serpens (Figure 50). 72)
- "For a long time astronomers have speculated about what processes control the sizes of stars - about why stars are the sizes that they are," said Jeff Hester of Arizona State University, Tempe, AZ. "Now in M16 we seem to be watching at least one such process at work right in front of our eyes."
- Striking pictures taken by Hester and co-investigators with Hubble's Wide Field and Planetary Camera 2 (WFPC2) resolve the EGGs at the tip of finger-like features protruding from monstrous columns of cold gas and dust in the Eagle nebula (also called M16 - 16th object in the Messier catalog). The columns - dubbed "elephant trunks" - protrude from the wall of a vast cloud of molecular hydrogen, like stalagmites rising above the floor of a cavern. Inside the gaseous towers, which are light-years long, the interstellar gas is dense enough to collapse under its own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings.
- Hubble gives a clear look at what happens as a torrent of ultraviolet light from nearby young, hot stars heats the gas along the surface of the pillars, "boiling it away" into interstellar space - a process called "photoevaporation. "The Hubble pictures show photoevaporating gas as ghostly streamers flowing away from the columns. But not all of the gas boils off at the same rate. The EGGs, which are denser than their surroundings, are left behind after the gas around them is gone.
- "It's a bit like a wind storm in the desert," said Hester. "As the wind blows away the lighter sand, heavier rocks buried in the sand are uncovered. But in M16, instead of rocks, the ultraviolet light is uncovering the denser egg-like globules of gas that surround stars that were forming inside the gigantic gas columns."
- Some EGGs appear as nothing but tiny bumps on the surface of the columns. Others have been uncovered more completely, and now resemble "fingers" of gas protruding from the larger cloud. (The fingers are gas that has been protected from photoevaporation by the shadows of the EGGs). Some EGGs have pinched off completely from the larger column from which they emerged, and now look like teardrops in space.
- By stringing together these pictures of EGGs caught at different stages of being uncovered, Hester and his colleagues from the Wide Field and Planetary Camera Investigation Definition Team are getting an unprecedented look at what stars and their surroundings look like before they are truly stars.
- "This is the first time that we have actually seen the process of forming stars being uncovered by photoevaporation," Hester emphasized. "In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed."
- "In a few cases we can see the stars in the EGGs directly in the WFPC2 images," says Hester. "As soon as the star in an EGG is exposed, the object looks something like an ice cream cone, with a newly uncovered star playing the role of the cherry on top."
- Ultimately, photoevaporation inhibits the further growth of the embyronic stars by dispersing the cloud of gas they were "feeding" from. "We believe that the stars in M16 were continuing to grow as more and more gas fell onto them, right up until the moment that they were cut off from that surrounding material by photoevaporation," said Hester.
Super Nova SN 1987A in the Large Magellanic Cloud
Thirty years ago, on 23 February 1987, the light from a stellar explosion marking the death of a massive star arrived at Earth to shine in Southern Hemisphere skies. Located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, SN 1987A was the closest observed supernova to Earth since the invention of the telescope. Studying it for the last 30 years has revolutionized our understanding of the explosive death of massive stars. 73)
- In operation since 1990, the NASA/ESA Hubble Space Telescope has observed the supernova remnant many times, as highlighted in this montage of Figure 51. The images show its evolution between 1994 and 2016, and highlight the main ring that blazes around the exploded star.
A new wide-field image(Figure 52) was also taken by Hubble in January 2017 to mark the 30 year anniversary. By observing the expanding remnant material over the years, Hubble has helped to show that the material within the ring was likely ejected 20,000 years before the actual explosion took place.
The initial burst of light from the supernova initially illuminated the rings. They slowly faded over the first decade after the explosion, until a fast-moving shell of gas ejected during the supernova slammed into the central ring, sending a powerful shockwave through the gas, heating it to searing temperatures and generating strong X-ray emission.
This caused clumps of denser gas within the ring to light up like a string of pearls, seen as the increasing number of bright spots, which are now fading again. As the shock wave continues to move through the shells ejected by the dying star in its final throes of life, who knows what new details will be revealed?
Since its launch in 1990 Hubble has observed the expanding dust cloud of SN 1987A several times and this way helped astronomers to create a better understanding of these cosmic explosions.
Supernova 1987A is located in the center of the image amidst a backdrop of stars. The bright ring around the central region of the exploded star is composed of material ejected by the star about 20,000 years before the actual explosion took place. The supernova is surrounded by gaseous clouds. The clouds' red color represents the glow of hydrogen gas.
The colors of the foreground and background stars were added from observations taken by Hubble's WFPC2 ( Wide Field Planetary Camera 2).
Left: For comparison, a picture taken with the WFPC-1 camera in wide field mode, on November 27, 1993, just a few days prior to the STS-61 servicing mission. The effects of optical aberration in HST's 2.4-meter primary mirror blur starlight, smear out fine detail, and limit the telescope's ability to see faint structure. Both Hubble images are "raw;" they have not been subject to computer image reconstruction techniques commonly used in aberrated images made before the servicing mission.
Target Information of M100: The galaxy M100 (100th object in the Messier Catalog of non-stellar objects) is one of the brightest members of the Virgo Cluster of galaxies. The galaxy is in the spring constellation Coma Berenices and can be seen through a moderate-sized amateur telescope. M100 is spiral shaped, like our Milky Way, and tilted nearly face-on as seen from earth. The galaxy has two prominent arms of bright stars and several fainter arms. Though the galaxy is estimated to be tens of millions of light-years away, Hubble reveals the sort of detail only seen previously (with ground based telescopes) in neighboring galaxies that are ten times closer. Before HST, astronomers could only see such a level of detail in roughly a dozen galaxies in our Local Group. Now, with Hubble's improved vision, the portion of the universe which can be studied with such clarity has grown a thousand fold. Only the future will tell what revelations await as Hubble's spectacular vision is applied to a host of fascinating and important questions about the universe and our place in it.
Hubble Servicing Missions
HST (Hubble Space Telescope) was launched April 24, 1990 on Shuttle flight STS-31. However, after Hubble's deployment, scientist realized that the telescope's primary mirror had a flaw called “spherical aberration.” The outer edge of the mirror was ground too flat by a depth of 4 μm. This aberration resulted in images that were fuzzy because some of the light from the objects being studied was being scattered. After the amount of aberration was understood, scientists and engineers developed WFPC2 (Wide Field/Planetary Camera 2) and COSTAR (Corrective Optics Space Telescope Axial Replacement), which were installed in Hubble during the first space shuttle servicing mission in 1993. - Without periodic onboard servicing, HST would have been a disaster and would not have produced all of the great science it has. 74) 75) 76)
All Hubble Servicing Missions were conducted by berthing the Hubble Spacecraft inside the Shuttle Payload Bay. The Shuttle resources SRMS/OBSS (Shuttle Remote Manipulator Arm/Orbiter Boom Sensor System), SSRMS (Space Station Remote Manipulator System, Canadarm2 ), communication system, etc. were used to conduct the inspections and repairs of Hubble.
The servicing missions involved intensive coordination between NASA's Kennedy Space Center in Florida, Johnson Space Center in Houston, and Goddard Space Flight Center in Greenbelt, Maryland. Preparation activities included astronaut training at all three centers; simulations of telescope operations during the mission at the Space Telescope Operations Control Center (STOCC) at Goddard; testing and preparing instruments and hardware for flight at Goddard; and preparing the space shuttle for launch, flight and landing at Kennedy.
During the missions, operations took place primarily at Johnson and in Goddard's STOCC. Johnson’s Mission Control Center (MCC) monitored every aspect of the space shuttle and astronauts, including spacewalks, procedures and schedules, crew activities and health, and in-cabin and cargo bay systems and experiments. The STOCC ground crew handled telescope operations, sending commands to Hubble to place the instruments into “safe hold” (hibernation), close the aperture door (which covers the precious optical components), and perform maneuvers to position the telescope for grappling by the shuttle’s robotic arm, operated by astronauts to bring Hubble into the shuttle’s payload bay.
After each new system part or science instrument was installed, STOCC personnel performed tests to make sure each instrument and component had power and operated as it should. During the astronauts' sleep cycles, the STOCC team performed more detailed tests on each newly installed component to determine if additional service from astronauts would be required.
After all the servicing tasks had been performed via a three- to five-day series of spacewalks, STOCC controllers and Johnson Mission Control prepared the telescope for release. Often this also involved using the shuttle’s thrusters to climb into a higher orbit before releasing Hubble.
The astronaut crew then used the shuttle’s robotic arm to slowly raise Hubble from the payload bay and out into space, where controllers at STOCC opened Hubble's aperture door and made sure the telescope was functioning on its own. Returning Hubble to full science observations after a servicing mission usually took a few months.
SM-1 (Servicing Mission-1)
The primary goal of Servicing Mission 1 was to restore Hubble’s vision. Because Hubble’s primary mirror was incorrectly shaped, the telescope could not focus all the light from an object to a single sharp point. Instead, it saw a fuzzy halo around objects it observed. Astronauts on space shuttle Endeavour’s STS‐61 mission spent five days tuning it up. They installed two new devices - WFPC2 and COSTAR.
The first Hubble repair mission was launched Dec. 2, 1993 on STS-61 (Endeavour, landing on 13 Dec. 1993 at KSC). Installation of COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR deployed corrective optics in front of three of Hubble's first-generation instruments – the Faint Object Camera, the Goddard High Resolution Spectrometer, and the Faint Object Spectrograph. 77) 78) 79)
In addition, SM-1 included the installation and replacement of other components including: solar arrays, solar array drive electronics, magnetometers, two rate sensor units, two gyroscope electronic control units, etc.
After SM-1, Hubble became operational transmitting stupefying images of supernovas, gigantic explosions that marked the death of a star and revealed mysterious black holes in the center of virtually all galaxies. Thanks to these observations, delivered with 10 times the clarity of the most powerful telescopes on Earth, astronomers have been able to confirm that the universe is expanding at an accelerating rate and to calculate its age with greater precision as an estimated 13.7 billion years.
The shuttle flight of 1993 was one of most challenging and complex manned missions ever attempted. During a record five back-to-back space walks totaling 35 hours and 28 minutes, two teams of astronauts completed the first servicing of the Hubble Space Telescope (HST). In many instances, tasks were completed sooner than expected and a few contingencies that did arise were handled smoothly. 80)
WFPC2 significantly improved ultraviolet performance over WFPC1, the original instrument. In addition to having more advanced detectors and more stringent contamination control, it also incorporated built-in corrective optics.
• This comparison image of the core of galaxy M100 shows the dramatic improvement in the Hubble telescope's view of the universe of the universe after the first Hubble Servicing Mission in December 1993. The new image (right) was taken with the second generation Wide Field and Planetary Camera (WFPC2), which was installed during the STS-61 Hubble Servicing Mission. 81)
Legend to Figure 55: This picture beautifully demonstrates that the camera's corrective optics compensate fully for the optical aberration in Hubble's primary mirror. With the new camera, the Hubble explored the universe with unprecedented clarity and sensitivity, and fulfilled its most important scientific objectives for which the telescope was originally built. 82)
Right: The core of the grand design spiral galaxy M100, as imaged by Hubble Space Telescope's Wide Field Planetary Camera 2 in its high resolution channel. The WFPC-2 contains modified optics that correct for Hubble's previously blurry vision, allowing the telescope for the first time to cleanly resolve faint structure as small as 30 light-years across in a galaxy which is tens of millions of light years away. The image was taken on December 31, 1993.
Figure 56: Servicing Mission 1, December 1993: The primary goal of Servicing Mission 1 was to restore Hubble's vision. Because Hubble's primary mirror was incorrectly shaped, the telescope could not focus all the light from an object to a single sharp point. Instead, it saw a fuzzy halo around objects it observed. Astronauts on the space shuttle Endeavor (STS-61) spent five days tuning it up. They installed two new devices — the Wide Field and Planetary Camera 2 and the Corrective Optics Space Telescope Axial Replacement — to compensate for the primary mirror's incorrect shape. Astronauts also installed new solar arrays, to reduce the "jitter" caused by excessive flexing of the solar panels during the telescope's orbital transition from cold darkness into warm daylight, and new gyroscopes to help point and track the telescope, along with fuse plugs and electronic units (video credit: NASA, published on 7 January 2007)
• December 2, 2013. Although the SM-1 mission was a triumph, and it marked the beginning of the Hubble telescope's long and illustrious career, astronaut Jeffrey Hoffman recalled the SM-1 events at a symposium to mark the 20th anniversary of STS-61. "There were a lot of people who doubted that we could accomplish all the things we had set out to, but here we were at the end of the fifth of five [spacewalks], and we had accomplished 13 of the 12 tasks that had been assigned to us — so we were, justifiably, very happy." 83)
SM-2 (Servicing Mission-2)
The second Hubble service flight was on STS-82 (Feb. 11-21, 1997). 84) The installation of new instruments extended Hubble's wavelength range into the near infrared for imaging and spectroscopy, allowing to probe the most distant reaches of the universe. The replacement of the failed or degraded spacecraft components increased efficiency and performance. The newly installed instruments were: STIS (Space Telescope Imaging Spectrograph), and NICMOS (Near Infrared Camera and Multi-Object Spectrometer).
• NICMOS enabled Hubble to observe infrared wavelengths (0.8-2.5 µm), crucial for viewing very distant optical sources that have lost energy traveling across most of the visible universe and now radiate in the infrared band. NICMOS consists of three cameras. It is capable of both infrared imaging and spectroscopic observations of astronomical targets.
• STIS could take detailed pictures of celestial objects and hunt for black holes. Both instruments featured technology that wasn’t available when scientists designed and built the original Hubble instruments in the late 1970s. STIS's two-dimensional detectors have allowed the instrument to gather 30 times more spectral data and 500 times more spatial data than the previous spectrographs on Hubble. These were capable of only looking at one place at a time.
One of the greatest advantages to using STIS is in the study of supermassive black holes. STIS searches for massive black holes by studying the star and gas dynamics around galactic centers. It measures the distribution of matter in the universe by studying quasar absorption lines. It also uses its high sensitivity and spatial resolution to study star formation in distant galaxies and perform spectroscopic mapping of solar system objects.
The astronauts also installed a refurbished FGS (Fine Guidance Sensor), one of three essential instruments used to keep Hubble steady while viewing objects and to calculate celestial distances; a Solid State Recorder to replace one of Hubble’s data recorders; and a refurbished, spare Reaction Wheel Assembly, part of the Pointing Control Subsystem.
SM-3 (Servicing Mission-3)
Scheduled for June 2000, the third mission to the Hubble Space Telescope was originally planned to carry out preventive repairs. However, urgency to address the failure of Hubble’s third gyroscope led NASA managers to split SM3 into two parts (SM3A and SM3B), scheduling an early servicing mission (SM3A) for December 1999. 85)
The unexpected failure of the fourth of Hubble’s six gyroscopes on 13 November 1999, with SM3A already planned, caused NASA to place Hubble into safe mode. Unable to conduct science without at least three working gyros, Hubble went into a sort of protective hibernation until 19 December 1999, when a crew of astronauts aboard the Discovery Space Shuttle flew to its rescue and replaced all the gyroscopes.
Since the second Servicing Mission in February 1997, three of the gyroscopes had failed and caused some concern among NASA officials. Additionally, NASA deemed necessary the replacement of one of Hubble’s three Fine Guidance Sensors (FGS). Both devices are part of Hubble’s advanced pointing control system, and as such, they keep the telescope steady during observations.
SM-3A (Servicing Mission-3A)
The Hubble service flight on STS-103 took place Dec. 19-27, 1999. Objective: replacement of gyroscopes (after the third of Hubble's six gyroscopes failed), a fine guidance sensor and a S/C computer. Installation of six voltage/temperature kits for the S/C batteries. Installation of a new transmitter, solid-state recorder (12 Gbit), and thermal insulation blankets. 87) 88)
What was originally conceived as a mission of preventive maintenance turned more urgent on November 13, 1999, when the fourth of six gyros failed and Hubble temporarily closed its eyes on the universe. Unable to conduct science without three working gyros, Hubble entered a state of dormancy called safe mode. Essentially, Hubble "went to sleep" while it waited for help.
STS-103 restored the Hubble Space Telescope to working order and upgraded some of its systems, allowing the decade-old observatory to get ready to begin its second scheduled decade of astronomical observations (Ref. 86).
The first few days of the 8-day mission, the crew prepared for the rendezvous and capture of the Hubble Space Telescope and the three maintenance spacewalks to follow. After a 30-orbit chase Commander Brown and Kelly maneuvered the orbiter to a point directly beneath Hubble, then moved upward toward it. Mission Specialist Clervoy grappled Hubble using the orbiter's robotic arm and placed it on the Flight Support System in the rear of Discovery's cargo bay.
EVA No. 1: Mission Specialists Steven Smith and John Greenfield conducted the mission's first spacewalk. The two made numerous repairs, including replacing the telescope's three Rate Sensor Units — each containing two gyroscopes. They also installed six Voltage/Temperature Improvement Kits between Hubble's solar panels and its six 10-year-old batteries. The kits, the size of cell telephones, were designed to prevent any overheating or overcharging of those batteries. A few minor objectives were left undone, such as taking close-up photos of the Voltage/Temperature Improvement Kits. The 8-hour, 15-minute space walk was second to the longest space walk from Endeavour on STS-49 in May 1992. A few minor problems helped account for the length of the space walk. The astronauts had difficulty in removing one of the old RSUs, and opening valves and removing caps on the NICMOS. The tasks were eventually completed.
EVA No. 2: During the mission's second space walk, Mission Specialists Michael Foale and Claude Nicollier installed a new advanced computer — 20 times faster than Hubble's old one — and a new, 250 kg fine guidance sensor. This 8-hour, 10 minute space walk was the third longest in history. With all major activities accomplished, controllers reported that power was reaching both of the new pieces of equipment. "The brains of Hubble have been replaced," said Mission Specialist Grunsfeld. About 30 minutes later, Hubble began thinking with those new brains.
EVA No. 3: Smith and Grunsfeld again teamed up to make the mission's third and final space walk. Like the first two, it also lasted more than 8 hours, making it the fourth longest in history. The team installed a transmitter that sends scientific data from Hubble to the ground. It replaced one that failed in 1998. The astronauts used special tools developed for the task because transmitters, usually very reliable, were not designed to be replaced in orbit. Smith and Grunsfeld also installed a solid state digital recorder, replacing an older mechanical reel-to-reel recorder.
Hubble was released from Discovery's cargo bay on Christmas Day.
The Hubble team has left the telescope far more fit and capable than ever before. The new, improved, and upgraded equipment included six fresh gyroscopes, six battery voltage/temperature improvement kits, a faster, more powerful, main computer, a next-generation solid state data recorder, a new transmitter, an enhanced fine guidance sensor, and new insulation.
SM-3B (Servicing Mission-3B)
A routine servicing mission to HST took place Mar. 1- 11, 2002 on STS-109 (Columbia). Installation of ACS (Advanced Camera for Surveys), built by Ball Aerospace for NASA and consisting of three cameras in the spectral range of 0.12-1.0 μm. The WFC (Wide Field Camera) uses a CCD area array of 16 Mpixel (4096 x 4096). The second is a HRC (High Resolution Camera) using a 1024 x 1024 CCD array and a high sensitivity in the UV. The third camera, the SBC (Solar-Blind Camera), is a far-ultraviolet, pulse-counting array that has a relatively high throughput at 121 nm. SA-3 (Solar Array-3) installation and PCU (Power Control Unit). Installation of a new experimental cryocooler for NICMOS (70 K cooling to revive its IR vision, and extend its life by several years). 89) 90) 91) 92)
Solar Array 3 (SA3) Installation: Four large flexible solar array (SA) panels (wings) provide power to the observatory. During SM1, the original arrays were replaced by SA2 and have powered Hubble for over 8 years. Radiation and debris take their toll on sensitive electronics, which will be replaced to ensure uninterrupted service for the remainder of the mission.
The new solar arrays (SA3) are rigid arrays, which do not roll up and therefore are more robust. Hubble gets a brand new look with its latest set of solar wings. Although one-third smaller than the first two pairs, the power increase was between 20 and 30 percent. They are less susceptible to extreme temperatures and their smaller-sized will reduce the effects of atmospheric drag on the spacecraft.
SM-4 (Servicing Mission-4)
The launch of SM-4 or flight STS-125 on Space Shuttle Atlantis, took place on May 11, 2009 (landing on May 24, 2009) with seven astronauts aboard (RMS capture, repair and upgrade of the 11,000 kg HST spacecraft at an orbital altitude of 560 km). Five spacewalks are required to refurbish Hubble with state-of-the-art science instruments designed to improve the telescope's discovery capabilities. The goal of the long overdue service mission is to extend the star-gazer's life by at least five years (the 2003 Columbia disaster that saw the shuttle disintegrate as it re-entered Earth's atmosphere, killing all seven crew members was the main reason for the long delay).
The priorities of the servicing mission are: 93)
• Three Rate Sensor Unit (gyroscope) removal and replacement (only two of the six gyros are currently in operation)
• WFC3 (Wide Field Camera 3). WFC3 replaces WFPC2 (Wide Field Planetary Camera 2). Use of 4 k x 2 k CCD e2v detector array providing full-frame imaging. — The WFPC2 was originally installed in the first Hubble servicing mission in 1993, and was nicknamed “the camera that saved Hubble” because its special optics were able to overcome the spherical aberration in the telescope’s main mirror.
- The WFC3 is configured as a two‐channel instrument. Its wide‐wavelength coverage with high efficiency is made possible by this dual‐channel design using two detector technologies. The incoming light beam from the Hubble telescope is directed into WFC3 using a pick‐off mirror, and is directed to either the Ultraviolet‐Visible (UVIS) channel or the Near‐Infrared (NIR) channel. The light‐sensing detectors in both channels are solid‐state devices. For the UVIS channel a large format CCD (Charge Coupled Device), similar to those found in digital cameras, is used. In the NIR detector the crystalline photosensitive surface is composed of mercury, cadmium and tellurium (HgCdTe). 94)
- The high sensitivity to light of the 16 megapixel UVIS CCD, combined with a wide field of view (160 x 160 arcsec), yields about a 35‐times improvement in discovery power versus Hubble’s current most sensitive ultraviolet imager, the ACS High Resolution Channel. The NIR channel’s HgCdTe detector is a highly advanced and larger (one megapixel) version of the 65,000 pixel detectors in the current near‐infrared instrument, NICMOS. The combination of field‐of‐view, sensitivity, and low detector noise results in a 15‐20 x enhancement in capability for WFC3 over NICMOS.
- An important design innovation for the WFC3 NIR channel results from tailoring its detector to reject infrared light (effectively “heat”) longer in wavelength than 1700 nm. In this way it becomes unnecessary to use a cryogen (e.g., liquid or solid nitrogen) to keep it cold. Instead the detector is chilled with an electrical device called a Thermo‐Electric Cooler (TEC). This greatly simplifies the design and will give WFC3 a longer operational life.
- Science instrument C&DH (Command & Data Handling) system swap out (replacement of a unit that failed in Sept. 2008)
- COS (Cosmic Origins Spectrograph) installation and replacement of COSTAR of SM-1. COS is a medium resolution spectrograph specifically designed to observe in the near and mid ultraviolet spectral range. COS was designed with one overriding objective in mind: to collect as many ultraviolet photons of light as possible and hence make possible the effective study of the huge, dark reservoir of gas that exists between the galaxies both near and far — t he so-called "cosmic web" of matter which represents the largest-scale structure in the universe.
- Battery module replacement installation (Bays 2 and 3). This is the first battery replacement in 19 years.
- Fine Guidance Sensor 2 removal and replacement (it is one of three sensors that help point and lock the telescope on targets)
- Repair of ACS (Advanced Camera for Surveys): ACS has been inoperable since January 2007, when its backup power supply system failed. Replacement of the entire electronics box, which will be powered by a separate low‐voltage power supply.
One piece of new technology is an ASIC, 96) 97) that enables an entire circuit board’s worth of electronics to be condensed into a very small package. It will be a part of the new CCD in the CEB (CCD Electronics Box) that will be installed to repair the failed ACS instrument. - The ASIC design is the same as the one already developed and tested for the JWST (James Webb Space Telescope) mission. However, the electronics packaging for Hubble is different because of the different operating conditions such as temperature and electronics environments.
- New Outer Blanket Layer installation (Bays 8, 5 & 7)
- Reboost of the HST spacecraft altitude.
- Reboost of the HST spacecraft altitude.
EVA1: The first spacewalk of the mission, performed by astronauts John Grunsfeld and Drew Feustel lasted a little over 7 1/2 hours. They successfully installed the new Wide Field Camera 3 science instrument and a new Science Instrument Command and Data Handling Unit. Both WFC-3 and the SI C&DH passed their “aliveness” tests, which essentially means the devices powered on correctly. The WFC-3 also passed its functional test, meaning the capabilities of the instrument itself were tested. The SI C&DH unit has also received an initial OK on its functional test, pending final review of data sent down to the ground. 98)
EVA2: The second EVA of the mission provided some challenges to astronauts Michael Good and Mike Massimino. However, they achieved all the objectives for this spacewalk, it just took them awhile — 7 hours and 56 minutes. They installed three Rate Sensor Units (RSUs), with a pair of gyros in each, and the first of two new battery module units.
EVA3: The third EVA of the mission went like clockwork as Grunsfeld and Feustel teamed up again. They removed the COSTAR (Corrective Optics Space Telescope Axial Replacement) and installed in its place the new COS (Cosmic Origins Spectrograph). They also completed an unprecedented repair of the Advanced Camera for Surveys replacing an electronic card and installed a new electronics box and cable.
To do the repairs on ACS, Grunsfeld removed 32 screws from an access panel to replace the camera’s four circuit boards and installed a new power supply. The two astronauts used specially designed tools to do a job that was never intended to be done on orbit. But they did it, and with efficiency.
Engineers at Goddard have already performed “aliveness” tests on both COS and ACS to verify they have electrical power. However while a functional test of the ACS indicated success in reviving the instrument’s heavily used wide-field channel, officials said early Sunday that it appears the repairs failed to resolve power problem with the camera’s stricken high-resolution channel and it appears “down for the count.”
On May 18, 2009, the astronauts successfully completed all the EVAs for the Hubble Servicing Mission, accomplishing all the mission goals. 99) 100)
Figure 71: During the Servicing Mission 4 (SM4), astronauts will make the final trip to the Hubble Telescope. Over the course of five spacewalks, they will install two new instruments, repair two inactive ones, and perform the component replacements that will keep the telescope functioning at least into 2014. The effort-intensive, rigorously researched, exhaustively tested mission also involves diverse groups of people on the ground throughout the country (video credit: NASA, published on 17 January 2009)
• 21 May 2019: Retired NASA astronaut John Grunsfeld hosts this six-part mini-series about the tools used on the Hubble Space Telescope servicing missions. Hubble was uniquely designed to be serviced in space so that components could be repaired and upgraded. Astronauts using custom-designed tools performed challenging spacewalks on five servicing missions from 1993 to 2009 to keep Hubble operating so that it could change our fundamental understanding of the universe. 101)
- Join John, EVA engineer Ed Rezac, and astronaut trainer Christy Hansen in this episode of Hubble Tool Time to learn about creating a Fastener Capture Plate to capture 111 screws in order to repair the Space Telescope Imaging Spectrograph on Servicing Mission 4 in 2009.
- In addition to enabling Hubble's scientific discoveries, the tools developed by teams at NASA's Goddard Space Flight Center and tested in collaboration with the Johnson Space Center furthered NASA's human exploration capabilities. These tools and the knowledge gleaned from the Hubble servicing missions are used today by astronauts on the International Space Station, and will be critical to NASA's future crewed missions to the Moon and Mars.
Figure 72: Hubble Tool Time Episode 6 - Servicing Mission 4 (video credit: NASA's Goddard Space Flight Center/Katrina Jackson)
Ground Segment of the HST (Hubble Space Telescope) Mission
The NASA/GSFC (Goddard Space Flight Center) in Greenbelt Maryland is home to the Hubble Space Telescope Operations Project, the government’s team of technical managers and scientists who oversee all aspects of the Hubble mission. Under its direction, an integrated group of civil servants and contractors at Goddard collectively known as the operations team is responsible for Hubble’s mission operations—those functions of the mission that operate together to assure the health, safety, and performance of the spacecraft. Examples include monitoring and adjusting the spacecraft’s subsystems (e.g., power, thermal, data management, pointing control, etc.), flight software development, sustaining engineering of the control center hardware and software, and systems administration of the network and ground system components. 102)
A separate contractor team at the Space Telescope Science Institute (STScI) in Baltimore is similarly responsible for science operations—the functions necessary to award telescope time, schedule observations, calibrate the received data, and archive the datasets. Working closely together, Goddard and the STScI operate Hubble 24 hours a day, 7 days a week, though most of the commanding to the telescope and receipt of its science data is accomplished by computers via automated operations.
The Space Telescope Operations Control Center (STOCC) is located at Goddard. It consists of a Mission Operations Room (MOR) and an Operations Support Room (OSR). The MOR is Hubble’s primary command and control room; both manual and automated operations are performed from this location.
Prior to the implementation of automated operations in May 2011, a team of console operators staffed the MOR around the clock. They executed the required procedures to acquire communications with the telescope, manage Hubble’s science and engineering data recorders, and load Hubble’s computers with command sequences. They also monitored telemetry from the spacecraft and reported any problems or concerns to the appropriate subsystem engineers.
These functions, which form Hubble’s daily routine, are now performed autonomously, enabling STOCC personnel to focus on special operations and various tests. Since the advent of automated operations, the STOCC is staffed only eight hours a day, five days a week. If an anomaly occurs on the spacecraft or within the ground system when the facility is unoccupied, a high-reliability text messaging system immediately alerts the appropriate members of the operations team.
In the Operations Support Room (OSR), STOCC personnel interface with a high-fidelity spacecraft simulator (or the spacecraft) to conduct a variety of tasks. These include testing any configuration changes planned for Hubble, analyzing engineering telemetry, testing flight or ground software updates, and running any contingency procedures or other special commanding in response to an observatory anomaly. The operations team continuously examines spacecraft subsystem performance, looking for trends that could signal component degradation and identifying ways to improve system performance and extend the mission’s lifetime.
Communications with Hubble are accomplished via a network interface from the STOCC to NASA’s White Sands Test Facility complex located in White Sands, New Mexico, near Las Cruces. The large antennas there transmit radio waves to NASA’s Tracking and Data Relay Satellite System (TDRSS), which forwards them to Hubble. Science observations and engineering data that the telescope stores on solid-state recorders are returned to Goddard using the reverse path. Once received and quality checked, the science data is forwarded to the Space Telescope Science Institute via dedicated high-speed network links where it is processed, archived, and distributed.
The Hubble mission and science operations teams strive to set the standard for NASA’s great observatories by continuously improving the telescope’s science productivity. The optical quality of the telescope, the excellent pointing performance of the spacecraft, the diverse capabilities of its instruments, and the skillful dedication of the mission’s staff keep Hubble at the forefront of observational astrophysics. The observatory is operating at peak performance thanks to the astronauts who successfully completed Servicing Mission 4 in May 2009. In that mission, space-shuttle-based astronauts installed two advanced-technology instruments, repaired two others, replaced a Fine Guidance Sensor, six gyros, six batteries, and the instrument computer, and outfitted certain equipment bays with protective thermal blankets. These upgrades position Hubble to continue its mission of discovery into the next decade.
The Vehicle Electrical System Test (VEST) facility provides Hubble operations and engineering personnel with a configurable, high-fidelity simulator of the Hubble observatory and its operating environment.
The VEST is used in three primary ways. First, it is employed to troubleshoot problems seen on the spacecraft by simulating the conditions under which the anomaly was observed and to test possible solutions. Second, it is used to verify updates to the software, commands, and engineering parameters before they are transmitted from the control center to the spacecraft. This process includes a thorough test of the ground procedures used for uplinking and installing these items. Finally, the VEST is used to verify operational procedures before they are executed by the flight operations team.
The VEST consists of several distinct elements. The largest is the integrated VEST structure itself. This is a mechanical and electrical engineering model of Hubble’s middle section (known as the Support Systems Module and the Optical Telescope Assembly Equipment Section) that contains the majority of the spacecraft’s hardware subsystems. Mounted within the structure are spaceflight-qualified modules developed as spares for servicing HST on orbit, and engineering duplicates (where no spares exist) of such items as computers, data recorders, interface boxes, power control units, etc.
A second element includes specially fabricated units designed to mimic the functionality of subsystems on Hubble that are too big or require emulation of the space environment. One such area is the spacecraft’s electrical power subsystem, which includes its two solar arrays, drive mechanisms (used to turn the arrays toward the sun) and six large batteries. Another area is the attitude guidance system that controls Hubble’s pointing and the observatory’s motion from target to target. In this area are units that simulate Hubble’s four 100-pound reaction wheels, six sensitive gyroscopes, sun sensors, and other miscellaneous components.
A third element simulates Hubble’s complex science instruments — both cameras and spectrographs. In this case, large computer racks developed for each instrument known as instrument benches simulate the electronics portion of an instrument and its optical parts like filter wheels, calibration lamps and channel-select mechanisms. Additional imbedded components generate realistic science data used to conduct end-to-end dataflow tests of the larger system.
Prior to each of the five servicing missions to Hubble, the VEST structure was a critical element for ensuring the fit of new spaceflight hardware into the spacecraft. Many of the items taken aloft by the astronauts to Hubble were first installed into the VEST structure to verify mechanical, electrical and software compatibility with the observatory. During that time, the VEST structure was located inside a large clean room whose environment was vital to maintaining the pristine condition of the hardware, as even a speck of dust could have potentially contaminated Hubble’s sensitive instruments and subsystems.
Some background of the HST (Hubble Stace Telescope) Program and STScI (Space Telescope Science Institute)
In 1979, NASA issued a request for proposals for a Space Telescope Science Institute (now also known as STScI), including a site where the institute would be located. After reviewing applications from prestigious institutions and universities, NASA selected the Association of Universities for Research in Astronomy (AURA) in 1981, which named the Johns Hopkins University’s Homewood campus in Baltimore, Maryland, as its base of operations. In the same year, Dr. Riccardo Giacconi was selected as STScI’s first director for his scientific leadership, technical knowledge, and his pioneering work in space astronomy and the creation of the field of X-ray astronomy. He initiated the hiring of STScI staff to support several critical science center functions: the definition of technical management and operations concepts, the development of a guide star selection system, and the creation of a science data reduction and analysis system. 103) 104)
NASA announced the Large Space Telescope’s official name in 1983: The Hubble Space Telescope (HST), honoring Dr. Edwin Hubble, who demonstrated in a 1929 publication that the universe is expanding based on observations showing that more distant galaxies are receding faster than nearby galaxies. His work also revealed that there were other galaxies beyond the Milky Way—revolutionizing our view of the cosmos and our place within it.
In January 1986, the loss of the entire crew of the space shuttle Challenger, which exploded after liftoff, led NASA to delay the launch of Hubble. However, STScI’s staff knew there was significant work yet to complete, which led to the 1989 release of the Guide Star Catalog and the software that supports it, revolutionizing how astronomers lock onto star positions to gather data from any ground- or space-based observatory, and ultimately automating Hubble’s observations.
In April 1990, the crew of the space shuttle Discovery successfully launched and deployed the Hubble Space Telescope. In May, its first image was released, showing a more clearly resolved star (HD96755 in the open cluster NGC 3532) compared to ground-based telescope observations. However, as institute staff collaborated with NASA and the scientific community to analyze additional data in the months that followed, they realized that Hubble’s primary mirror had an aberration; the images it returned were not perfectly focused.
Staff at the institute worked to implement software that allowed scientists to accurately revise Hubble’s data, providing improved images to the scientific community and the public. The issue was completely resolved during the first servicing mission in December 1993, which included the installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) and the Wide Field Planetary Camera 2 (WFPC 2), correcting Hubble’s optics and immediately clarifying the data. NASA astronauts completed four additional servicing missions through 2009, replacing and upgrading Hubble’s instruments with improved optics and increasing its resolution, power, and efficiency each time, allowing the telescope to become one of the world’s most productive scientific instruments.
Hubble’s success is due in part to STScI’s data archive, established at the outset of the mission in 1990. The archive was expanded to include data from other ultraviolet and optical space astronomy missions in 1997 and was renamed the Multi-mission Archive at Space Telescope (MAST). STScI provides secure storage and reliable retrieval services for observation data, creates user-friendly and scientifically useful search tools, and offers support services to the astronomy community. Today, MAST has a user community of more than 10,000 astronomers around the world. In addition to Hubble data, MAST houses the data of more than 20 missions in the ultraviolet, optical, and near-infrared wavelength range. In 2012, MAST was renamed in honor of Maryland’s U.S. Senator Barbara A. Mikulski, who was a steadfast supporter of NASA, the Hubble Space Telescope, and the Space Telescope Science Institute.
Even before Hubble was launched, STScI and NASA began organizing workshops to plan a “Next Generation Space Telescope” that would be significantly larger than Hubble and focused on infrared wavelengths of the electromagnetic spectrum. The idea gained momentum throughout the 1990s and was formally recommended as the top space-based priority by the National Academies of Sciences Decadal Survey in 2001. After construction on the new project began, it was renamed in honor of former NASA Administrator James E. Webb in 2002. The observatory is built with new technologies that will allow it to see deeper into the universe, answer fundamental questions about the origins of planets and stars in the Milky Way, and examine the nature of the first light in the universe.
In 2001 STScI was selected to oversee the science and mission operations of the James Webb Space Telescope, planned to launch in 2021. STScI staff are uniquely prepared to support Webb. Scientists and engineers have mined more than 30 years of experience preparing for and operating Hubble to create a framework for operations that will ensure the health of Webb and maximize its scientific productivity.
STScI will also play a key role in the science operations for NASA’s next flagship observatory following Webb, the Wide Field Infrared Survey Telescope (WFIRST), which is planned to launch in the mid-2020s. With a view 100 times wider than the Hubble Space Telescope at the same sensitivity and resolution, WFIRST will build wide-field maps of large regions of the sky in near-infrared light, and has the potential to answer vital questions in exoplanet and dark energy research. Looking forward, the staff at STScI will continue to follow the institute’s mission: to help humanity explore the universe with advanced space telescopes and ever-growing data archives.
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2) ”Faint starlight in Hubble images reveals distribution of dark matter [heic1820],”
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10) ”Astronomers Find Possible Elusive Star Behind Supernova,” NASA, 15 November 2018, URL: https://www.nasa.gov/feature/goddard/2018/astronomers-find-possible-elusive-star-behind-supernova
11) Schuyler D. Van Dyk, WeiKang Zheng, Thomas G. Brink, Alexei V. Filippenko, Dan Milisavljevic, Jennifer E. Andrews, Nathan Smith, Michele Cignoni, Ori D. Fox, Patrick L. Kelly, Angela Adamo, Sameen Yunus, Keto Zhang, and Sahana Kumar, ”SN 2017ein and the Possible First Identification of a Type Ic Supernova Progenitor,” The Astrophysical Journal, Vol. 860, No 2, URL of abstract: http://iopscience.iop.org/article/10.3847/1538-4357/aac32c/meta
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14) ”Polar lights on Uranus,” ESA, Space Science Image of the Week, 05 November 2018, URL: http://m.esa.int/spaceinimages/Images/2018/11/Polar_lights_on_Uranus
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17) ”The Ghost Nebula,” ESA, 26, October 2018, URL: http://m.esa.int/spaceinimages/Images/2018/10/The_Ghost_Nebula
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19) ”Hubble Captures the Ghost of Cassiopeia,” NASA, 25 October 2018, URL: https://www.nasa.gov/image-feature/goddard/2018/hubble-captures-the-ghost-of-cassiopeia
20) ”Ultra-close stars discovered inside a planetary nebula,” ScienceDaily, 23 October 2018, URL: https://www.sciencedaily.com/releases/2018/10/181023130545.htm
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24) ”Astronomers catch red dwarf star in a superflare outburst,” Arizona State University, 18 October 2018, URL: https://phys.org/news/2018-10-astronomers-red-dwarf-star-superflare.html
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35) Karl Hille, ”A Piercing Celestial Eye Stares Back at Hubble,” NASA, 24 August 2018, URL: https://www.nasa.gov/image-feature/goddard/2018/a-piercing-celestial-eye-stares-back-at-hubble
36) Ann Jenkins, Ray Villard, Pascal Oesch,Mireia Montes, ”Hubble Paints Picture of the Evolving Universe,” NASA, 16 August 2018, URL: https://www.nasa.gov
37) ”A globular cluster’s striking red eye,” ESA, 10 August 2018, URL: http://m.esa.int/spaceinimages/Images/2018/08/A_globular_cluster_s_striking_red_eye
38) ”Probing the distant past,” ESA, 03 August 2018, URL: http://m.esa.int/spaceinimages/Images/2018/07/Probing_the_distant_past
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42) ”Burst of Celestial Fireworks,” NASA, 3 July 2018, URL: https://www.nasa.gov
43) Ashley G. Smart, ”Machine learning solves an exoplanet atmosphere,” Physics Today, 25 June 2018, URL: https://tinyurl.com/ydh8o777
44) Pablo Márquez-Neila, Chloe Fischer, Raphael Sznitman, Kevin Heng, ”Supervised machine learning for analysing spectra of exoplanetary atmospheres,” Nature Astronomy, 25 July 2018, URL of abstract: https://www.nature.com/articles/s41550-018-0504-2
45) ”Dance of the asteroids:Nearby asteroids photobomb distant galaxies,” ESA Space Science Image of the Week, 25 June 2018, URL: http://m.esa.int/spaceinimages
46) ”Hubble proves Einstein correct on galactic scales (heic1812),” ESA, 21 June 2018, http://sci.esa.int/hubble/60441-hubble-proves-einstein-correct-on-galactic-scales-heic1812/
47) Thomas E. Collett, Lindsay J. Oldham, Russell J. Smith, Matthew W. Auger, Kyle B. Westfall, David Bacon, Robert C. Nichol, Karen L. Masters, Kazuya Koyama, Remco van den Bosch, ”A precise extragalactic test of General Relativity,” Science, Vol. 360, Issue 6395, pp. 1342-1346, 22 Jun 2018, DOI: 10.1126/science.aao2469
48) ”Cosmic collision lights up the darkness [heic1811],” ESA, 31 May 2018, URL: http://sci.esa.int/hubble/60372-cosmic-collision-lights-up-the-darkness/
49) ”Hubble spots a green cosmic arc,” NASA, 1 June 2018, URL: https://www.nasa.gov
50) ”Hubble shows the local Universe in ultraviolet,” Hubble Space Telescope, HEIC (Hubble European Space Agency Information Center) 1810, 17 May 2018, URL: http://www.spacetelescope.org/news/heic1810/?utm_source=feedburner&
51) ”Astronomers Release Most Complete Ultraviolet-Light Survey of Nearby Galaxies,” Hubblesite, 17 May 2018, URL: http://hubblesite.org/news_release/news/2018-27
52) ”A spiral disguised,” ESA, 16 May, 2018, URL: http://m.esa.int/spaceinimages/Images/2018/05/A_spiral_disguised
53) ”Hubble detects helium in the atmosphere of an exoplanet for the first time [heic1809],” ESA, 02 May 2018, URL: http://sci.esa.int/hubble
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55) ”Hubble's 28th birthday picture: The Lagoon Nebula,” Hubble Space Telescope, 19 April 2018, URL: https://www.spacetelescope.org/images/heic1808a/
56) ”A colossal cluster,” ESA, 10 April 2018, URL: http://m.esa.int/spaceinimages/Images/2018/04/A_colossal_cluster
57) ”Hubble Catches a Colossal Cluster,” NASA, 13 April, 2018, URL: https://www.nasa.gov
58) ”Hubble uses cosmic lens to discover most distant star ever observed [heic1807],” ESA, 02 April 2018, URL: http://sci.esa.int/hubble
59) ”The curious case of calcium-rich supernovae,” ESA, 30 March 2018, URL: http://m.esa.int/spaceinimages/Images/2018/03/The_curious_case_of_calcium-rich_supernovae
60) Steven K. Blau, ”A galaxy with surprisingly little dark matter,” Physics Today, 2 April 2018, URL: https://tinyurl.com/yargmbuf
61) ”Hubble finds first galaxy in the local Universe without dark matter [heic1806],” ESA, 28 March 2018, URL: http://sci.esa.int/hubble
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64) ”A New Angle on Two Spiral Galaxies for Hubble's 27th Birthday,” NASA, April 20, 2017, URL: https://www.nasa.gov/feature/goddard/2017
65) “Hubble 25,” ESA, April 23, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Hubble_25_without_title
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74) Edwin P. Hubble (Nov. 20, 1889-Sept. 28, 1953) was an American astronomer. He profoundly changed astronomers' understanding of the nature of the universe by demonstrating the existence of other galaxies besides the Milky Way. He also discovered that the degree of redshift observed in light coming from a galaxy increased in proportion to the distance of that galaxy from the Milky Way. This became known as Hubble's law, and would help establish that the universe is expanding.
75) Note: When originally planned in 1979, the Large Space Telescope program called for return to Earth, refurbishment, and re-launch every 5 years, with on-orbit servicing every 2.5 years. Hardware lifetime and reliability requirements were based on that 2.5 year interval between servicing missions. In 1985, contamination and structural loading concerns associated with return to Earth aboard the Shuttle eliminated the concept of ground return from the program. NASA decided that on-orbit servicing might be adequate to maintain HST for its 15 year design life.
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83) Denise Chow, ”Saving Hubble: Astronauts Recall 1st Space Telescope Repair Mission 20 Years Ago,” Space.com, 2 December 2013, URL: https://www.space.com
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97) Timothy J. Cole, “On-Orbit Repair of Satellites using Fastener Capture Plates to Eliminate Debris,” 2011 IEEE Aerospace Conference, Big Sky, MT, USA, March 5-12, 2011
98) Nancy Atkinson, “Hubble Servicing Mission 4 in Pictures, Part 1,” Universe Today, May 17, 2009, URL: http://www.universetoday.com/2009/05/17/hubble-servicing-mission-4-in-pictures-part-1/
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).