Hubble Space Telescope 2021 Part 2

• March 18, 2021: NASA's Hubble Space Telescope is giving astronomers a view of changes in Saturn's vast and turbulent atmosphere as the planet's northern hemisphere summer transitions to fall as shown in this series of images taken in 2018, 2019, and 2020 (left to right). ¹¹⁰⁾

- "These small year-to-year changes in Saturn's color bands are fascinating," said Amy Simon, planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "As Saturn moves towards fall in its northern hemisphere, we see the polar and equatorial regions changing, but we are also seeing that the atmosphere varies on much shorter timescales." Simon is lead author of a paper on these observations published March 11 in Planetary Science Journal.

- The Hubble data show that from 2018 to 2020 the equator got 5 to 10 percent brighter, and the winds changed slightly. In 2018, winds measured near the equator were about 1,000 miles per hour (roughly 1,600 kilometers per hour), higher than those measured by NASA's Cassini spacecraft during 2004-2009, when they were about 800 miles per hour (roughly 1,300 kilometers per hour). In 2019 and 2020 they decreased back to the Cassini speeds. Saturn's winds also vary with altitude, so the change in measured speeds could possibly mean the clouds in 2018 were around 37 miles (~ 60 km) deeper than those measured during the Cassini mission. Further observations are needed to tell which is happening.

- Saturn is the sixth planet from our Sun and orbits at a distance of about 886 million miles (1.4 billion kilometers) from the Sun. It takes around 29 Earth years to orbit the Sun, making each season on Saturn more than seven Earth years long. Earth is tilted with respect to the Sun, which alters the amount of sunlight each hemisphere receives as our planet moves in its orbit. This variation in solar energy is what drives our seasonal changes. Saturn is tilted also, so as the seasons change on that distant world, the change in sunlight could be causing some of the observed atmospheric changes.

- Like Jupiter, the solar system's largest planet, Saturn is a "gas giant" made mostly of hydrogen and helium, although there may be a rocky core deep inside. Enormous storms, some almost as large as Earth, occasionally erupt from deep within the atmosphere. Since many of the planets discovered around other stars are gas giants as well, astronomers are eager to learn more about how gas giant atmospheres work.

- Saturn is the second largest planet in the solar system, over 9 times wider than Earth, with more than 50 moons and a spectacular system of rings made primarily of water ice. Two of these moons, Titan and Enceladus, appear to have oceans beneath their icy crusts that might support life. Titan, Saturn's largest moon, is the only moon in our solar system with a thick atmosphere, including clouds that rain liquid methane and other hydrocarbons on to the surface, forming rivers, lakes, and seas. This mix of chemicals is thought to be similar to that on Earth billions of years ago when life first emerged. NASA's Dragonfly mission will fly over the surface of Titan, touching down in various locations to search for the primal building blocks of life.

- The Saturn observations are part of Hubble's Outer Planets Atmospheres Legacy (OPAL) program. "The OPAL program allows us to observe each of the outer planets with Hubble every year, enabling new discoveries and watching how each planet is changing over time," said Simon, principal investigator for OPAL.

- 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 conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

• March 18, 2021: Though our galaxy is an immense city of at least 200 billion stars, the details of how they formed remain largely cloaked in mystery. ¹¹¹⁾

- Scientists know that stars form from the collapse of huge hydrogen clouds that are squeezed under gravity to the point where nuclear fusion ignites. But only about 30 percent of the cloud's initial mass winds up as a newborn star. Where does the rest of the hydrogen go during such a terribly inefficient process?

- It has been assumed that a newly forming star blows off a lot of hot gas through lightsaber-shaped outflowing jets and hurricane-like winds launched from the encircling disk by powerful magnetic fields. These fireworks should squelch further growth of the central star. But a new, comprehensive Hubble survey shows that this most common explanation doesn't seem to work, leaving astronomers puzzled.

- Researchers used data previously collected from NASA's Hubble and Spitzer space telescopes and the European Space Agency's Herschel Space Telescope to analyze 304 developing stars, called protostars, in the Orion Complex, the nearest major star-forming region to Earth. (Spitzer and Herschel are no longer operational).

- In this largest-ever survey of nascent stars to date, researchers are finding that gas-clearing by a star's outflow may not be as important in determining its final mass as conventional theories suggest. The researchers' goal was to determine whether stellar outflows halt the infall of gas onto a star and stop it from growing.

- Instead, they found that the cavities in the surrounding gas cloud sculpted by a forming star's outflow did not grow regularly as they matured, as theories propose.

- "In one stellar formation model, if you start out with a small cavity, as the protostar rapidly becomes more evolved, its outflow creates an ever-larger cavity until the surrounding gas is eventually blown away, leaving an isolated star," explained lead researcher Nolan Habel of the University of Toledo in Ohio, USA.

- "Our observations indicate there is no progressive growth that we can find, so the cavities are not growing until they push out all of the mass in the cloud. So, there must be some other process going on that gets rid of the gas that doesn't end up in the star."

- The team's results will appear in an upcoming issue of The Astrophysical Journal. ¹¹²⁾

A Star is Born

- During a star's relatively brief birthing stage, lasting only about 500,000 years, the star quickly bulks up on mass. What gets messy is that, as the star grows, it launches a wind, as well as a pair of spinning, lawn-sprinkler-style jets shooting off in opposite directions. These outflows begin to eat away at the surrounding cloud, creating cavities in the gas.

- Popular theories predict that as the young star evolves and the outflows continue, the cavities grow wider until the entire gas cloud around the star is completely pushed away. With its gas tank empty, the star stops accreting mass – in other words, it stops growing.

- To look for cavity growth, the researchers first sorted the protostars by age by analyzing Herschel and Spitzer data of each star's light output. The protostars in the Hubble observations were also observed as part of the Herschel telescope's Herschel Orion Protostar Survey.

- Then the astronomers observed the cavities in near-infrared light with Hubble's Near-infrared Camera and Multi-object Spectrometer and Wide Field Camera 3. The observations were taken between 2008 and 2017. Although the stars themselves are shrouded in dust, they emit powerful radiation which strikes the cavity walls and scatters off dust grains, illuminating the gaps in the gaseous envelopes in infrared light.

- The Hubble images reveal the details of the cavities produced by protostars at various stages of evolution. Habel's team used the images to measure the structures' shapes and estimate the volumes of gas cleared out to form the cavities. From this analysis, they could estimate the amount of mass that had been cleared out by the stars' outbursts.

Figure 107: Though our galaxy is an immense city of at least 200 billion stars, the details of how they formed remain largely cloaked in mystery. Scientists know that stars form from the collapse of huge hydrogen clouds that are squeezed under gravity to the point where nuclear fusion ignites. But only about 30 percent of the cloud's initial mass winds up as a newborn star. Where does the rest of the hydrogen go during such a terribly inefficient process? (video credit: NASA Goddard Space Flight Center)

- "We find that at the end of the protostellar phase, where most of the gas has fallen from the surrounding cloud onto the star, a number of young stars still have fairly narrow cavities," said team member Tom Megeath of the University of Toledo. "So, this picture that is still commonly held of what determines the mass of a star and what halts the infall of gas is that this growing outflow cavity scoops up all of the gas. This has been pretty fundamental to our idea of how star formation proceeds, but it just doesn't seem to fit the data here."

- Future telescopes such as NASA's upcoming James Webb Space Telescope will probe deeper into a protostar's formation process. Webb spectroscopic observations will observe the inner regions of disks surrounding protostars in infrared light, looking for jets in the youngest sources. Webb also will help astronomers measure the accretion rate of material from the disk onto the star, and study how the inner disk is interacting with the outflow.

• March 12, 2021: NASA's Hubble Space Telescope resumed observations March 11 after a software error placed it in a protective safe mode several days earlier, but the incident is a reminder of the telescope's mortality. ¹¹³⁾

- NASA said Hubble resumed observations at 8 p.m. Eastern March 11, more than four and a half days after a software error caused the spacecraft to go into a safe mode, suspending normal operations of the nearly 31-year-old space telescope. ¹¹⁴⁾

- The software error was traced to what an agency statement called an "enhancement" recently uploaded to the spacecraft. That enhancement was intended to compensate for fluctuations from one of the telescope's gyroscopes, but a glitch in the software caused a broader problem with Hubble's main computer, triggering the safe mode early March 7.

- Controllers resolved the problem for now by disabling that software enhancement, and plan to correct the flaw and test the new software further before uploading it again.

- That safe mode, though, caused two other problems with Hubble. The telescope's aperture door, a cover on top of the telescope, is designed to automatically close when the spacecraft enters safe mode to prevent stray sunlight from entering, which could damage instruments and optics. During this safe mode, though, the door did not swing shut, a problem never before seen with Hubble.

- Engineers troubleshooting the problem found that the door did close once they switched to a backup motor. They have now set that motor as the primary one as they continue to study the problem with the other motor.

- One of Hubble's instruments, the Wide Field Camera 3, "experienced an unexpected error" during the recovery from safe mode. NASA did not elaborate on the error but said that observations using that instrument will remain on hold as engineers study the problem. The spacecraft's other instruments, including a camera and two spectrographs, are functioning.

- The safe mode, and related issues, is a reminder of Hubble's age. The spacecraft was launched in April 1990 and serviced by the space shuttle five times, most recently in May 2009. With the shuttle long since retired, astronomers know that, at some point, Hubble will suffer an unrecoverable failure that will end its historic mission.

- "Right now we're in the middle of what I think is a very good news story about Hubble," Jennifer Wiseman, an astronomer at NASA's Goddard Space Flight Center, said at a conference last year about the space telescope. She and others believe that the telescope can remain operational through much of this decade, based on trends in the performance of key components, such as its gyroscopes and batteries.

- Some have argued for a new servicing mission to Hubble using either a robotic or crewed spacecraft. John Grunsfeld, a former NASA astronaut who flew on three Hubble servicing missions and later served as the agency's associate administrator for science, presented a concept last year for a crewed servicing mission using an Orion spacecraft and a module equipped with a robotic arm and airlock. That spacecraft would dock with Hubble, with astronauts then performing repairs much as they did on past servicing missions.

- "We have the technology to go back to Hubble," he said in a presentation last June to the Space Transportation Association, noting that a commercial crew vehicle, like Crew Dragon, could be used in place of Orion. "We could keep Hubble going for another few decades."

- NASA, though, has shown no public interest in such a servicing mission, whose expense would run in the hundreds of millions to billions of dollars. "It's not currently on the books. Nobody is really talking about it a lot, at least publicly," Grunsfeld acknowledged.

- Without a servicing mission, Hubble could last for many years, or fail tomorrow, astronomers like Wiseman acknowledge. "We don't know how long Hubble's going to last," she said.

• March 11, 2021: For the first time, scientists using the NASA/ESA Hubble Space Telescope have found evidence of volcanic activity reforming the atmosphere on a rocky planet around a distant star. The planet, GJ 1132 b, has a similar density, size, and age to those of Earth. ^{115) 116)}

- The planet GJ 1132 b appears to have begun life as a gaseous world with a thick blanket of atmosphere. Starting out at several times the radius of Earth, this so-called "sub-Neptune" quickly lost its primordial hydrogen and helium atmosphere, which was stripped away by the intense radiation from its hot, young star. In a short period of time, it was reduced to a bare core about the size of Earth.

- To the surprise of astronomers, new observations from Hubble have uncovered a secondary atmosphere that has replaced the planet's first atmosphere. It is rich in hydrogen, hydrogen cyanide, methane and ammonia, and also has a hydrocarbon haze. Astronomers theorize that hydrogen from the original atmosphere was absorbed into the planet's molten magma mantle and is now being slowly released by volcanism to form a new atmosphere. This second atmosphere, which continues to leak away into space, is continually being replenished from the reservoir of hydrogen in the mantle's magma.

- "This second atmosphere comes from the surface and interior of the planet, and so it is a window onto the geology of another world," explained team member Paul Rimmer of the University of Cambridge, UK. "A lot more work needs to be done to properly look through it, but the discovery of this window is of great importance."

- "We first thought that these highly radiated planets would be pretty boring because we believed that they lost their atmospheres," said team member Raissa Estrela of the Jet Propulsion Laboratory at the California Institute of Technology in Pasadena, California, USA. But we looked at existing observations of this planet with Hubble and realized that there is an atmosphere there."

- "How many terrestrial planets don't begin as terrestrials? Some may start as sub-Neptunes, and they become terrestrials through a mechanism whereby light evaporates the primordial atmosphere. This process works early in a planet's life, when the star is hotter," said team leader Mark Swain of the Jet Propulsion Laboratory. "Then the star cools down and the planet's just sitting there. So you've got this mechanism that can cook off the atmosphere in the first 100 million years, and then things settle down. And if you can regenerate the atmosphere, maybe you can keep it."

- In some ways, GJ 1132 b has various parallels to Earth, but in some ways it is also very different. Both have similar densities, similar sizes, and similar ages, being about 4.5 billion years old. Both started with a hydrogen-dominated atmosphere, and both were hot before they cooled down. The team's work even suggests that GJ 1132 b and Earth have similar atmospheric pressure at the surface.

- However, the planets' formation histories are profoundly different. Earth is not believed to be the surviving core of a sub-Neptune. And Earth orbits at a comfortable distance from our yellow dwarf Sun. GJ 1132 b is so close to its host red dwarf star that it completes an orbit the star once every day and a half. This extremely close proximity keeps GJ 1132 b tidally locked, showing the same face to its star at all times — just as our moon keeps one hemisphere permanently facing Earth.

- "The question is, what is keeping the mantle hot enough to remain liquid and power volcanism?" asked Swain. "This system is special because it has the opportunity for quite a lot of tidal heating."

- The phenomenon of tidal heating occurs through friction, when energy from a planet's orbit and rotation is dispersed as heat inside the planet. GJ 1132 b is in an elliptical orbit, and the tidal forces acting on it are strongest when it is closest to or farthest from its host star. At least one other planet in the host star's system also exerts a gravitational pull on the planet. The consequences are that the planet is squeezed or stretched by this gravitational "pumping." That tidal heating keeps the mantle liquid for a long time. A nearby example in our own Solar System is the Jovian moon, Io, which has continuous volcanism as a result of a tidal tug-of-war between Jupiter and the neighboring Jovian moons.

- The team believes the crust of GJ 1132 b is extremely thin, perhaps only hundreds of feet thick. That's much too feeble to support anything resembling volcanic mountains. Its flat terrain may also be cracked like an eggshell by tidal flexing. Hydrogen and other gases could be released through such cracks.

- "This atmosphere, if it's thin — meaning if it has a surface pressure similar to Earth — probably means you can see right down to the ground at infrared wavelengths. That means that if astronomers use the James Webb Space Telescope to observe this planet, there's a possibility that they will see not the spectrum of the atmosphere, but rather the spectrum of the surface," explained Swain. "And if there are magma pools or volcanism going on, those areas will be hotter. That will generate more emission, and so they'll potentially be looking at the actual geological activity — which is exciting!"

• March 5, 2021: Hubble Space Telescope image of the week of the NGC2336 galaxy. Its spiral arms are glittered with young stars, visible in their bright blue light. In contrast, the redder central part of the galaxy is dominated by older stars. ^{117) 118)}

- NGC 2336 was discovered in 1876 by German astronomer Wilhelm Tempel, using a 28-centimeter telescope. This Hubble image is so much better than the view Tempel would have had — Hubble's main mirror is 2.4 meters across, nearly ten times the size of the telescope Tempel used. In 1987, NGC 2336 experienced a Type-Ia supernova, the only observed supernova in the galaxy since its discovery 111 years earlier.

• March 4, 2021: The red hypergiant VY Canis Majoris is enshrouded in huge clouds of dust. Stars come in an extraordinary range of sizes. One of the most colossal is VY Canis Majoris. If placed in the middle of our solar system it would engulf all the planets out to Saturn's orbit. This monster, appropriately called a red hypergiant, is as bright as 300,000 Suns. Yet it is so far away that, 200 years ago, it could be seen only as a faint star in the winter constellation of the Great Dog. Since then, it has faded and is no longer visible to the naked eye. Astronomers used Hubble to get a close-up look at the star and discovered the reason for the dimming. The star is expelling huge clouds of dust in the final stages of its life. Eventually, the bloated star may explode as a supernova, or may simply collapse and form a black hole. ¹¹⁹⁾

- Last year, astronomers were puzzled when Betelgeuse, the bright red supergiant star in the constellation Orion, dramatically faded, but then recovered. The dimming lasted for weeks. Now, astronomers have turned their sights toward a monster star in the adjoining constellation Canis Major, the Great Dog.

- The red hypergiant VY Canis Majoris—which is far larger, more massive, and more violent than Betelgeuse—experiences much longer, dimmer periods that last for years. New findings from NASA's Hubble Space Telescope suggest the same processes that occurred on Betelgeuse are happening in this hypergiant, but on a much grander scale.

- "VY Canis Majoris is behaving a lot like Betelgeuse on steroids," explained the study's leader, astrophysicist Roberta Humphreys of the University of Minnesota, Minneapolis.

- As with Betelgeuse, Hubble data suggest the answer for why this bigger star is dimming. For Betelgeuse, the dimming corresponded to a gaseous outflow that may have formed dust, which briefly obstructed some of Betelgeuse's light from our view, creating the dimming effect.

- "In VY Canis Majoris we see something similar, but on a much larger scale. Massive ejections of material which correspond to its very deep fading, which is probably due to dust that temporarily blocks light from the star," said Humphreys.

- The enormous red hypergiant is 300,000 times brighter than our Sun. If it replaced the Sun in our own solar system, the bloated monster would extend out for hundreds of millions of miles, between the orbits of Jupiter and Saturn.

- "This star is absolutely amazing. It's one of the largest stars that we know of—a very evolved, red supergiant. It has had multiple, giant eruptions," explained Humphreys.

- Giant arcs of plasma surround the star at distances from it that are thousands of times farther away than the Earth is from the Sun. These arcs look like the solar prominences from our own Sun, only on a much grander scale. Also, they're not physically connected to the star, but rather, appear to have been thrown out and are moving away. Some of the other structures close to the star are still relatively compact, looking like little knots and nebulous features.

- In previous Hubble work, Humphreys and her team were able to determine when these large structures were ejected from the star. They found dates ranging over the past several hundred years, some as recently as the past 100 to 200 years.

- Now, in new work with Hubble, researchers resolved features much closer to the star that may be less than a century old. By using Hubble to determine the velocities and motions of the close-in knots of hot gas and other features, Humphreys and her team were able to date these eruptions more accurately. What they found was remarkable: many of these knots link to multiple episodes in the 19th and 20th centuries when VY Canis Majoris faded to one-sixth its usual brightness.

- Unlike Betelgeuse, VY Canis Majoris is now too faint to be seen by the naked eye. The star was once visible but has dimmed so much that it can now only be seen with telescopes.

- The hypergiant sheds 100 times as much mass as Betelgeuse. The mass in some of the knots is more than twice the mass of Jupiter. "It's amazing the star can do it," Humphreys said. "The origin of these high mass-loss episodes in both VY Canis Majoris and Betelgeuse is probably caused by large-scale surface activity, large convective cells like on the Sun. But on VY Canis Majoris, the cells may be as large as the whole Sun or larger."

- "This is probably more common in red supergiants than scientists thought and VY Canis Majoris is an extreme example," Humphreys continued. "It may even be the main mechanism that's driving the mass loss, which has always been a bit of a mystery for red supergiants."

- Though other red supergiants are comparably bright and eject a lot of dust, none of them is as complex as VY Canis Majoris. "So what's special about it? VY Canis Majoris may be in a unique evolutionary state that separates it from the other stars. It's probably this active over a very short period, maybe only a few thousand years. We're not going to see many of those around," said Humphreys.

- The star began life as a super-hot, brilliant, blue supergiant star perhaps as much as 35 to 40 times our Sun's mass. After a few million years, as the hydrogen fusion burning rate in its core changed, the star swelled up to a red supergiant. Humphreys suspects that the star may have briefly returned to a hotter state and then swelled back up to a red supergiant stage.

- "Maybe what makes VY Canis Majoris so special, so extreme, with this very complex ejecta, might be that it's a second-stage red supergiant," explained Humphreys. VY Canis Majoris may have already shed half of its mass. Rather than exploding as a supernova, it might simply collapse directly to a black hole.

- The team's findings appear in the February 4, 2021 edition of The Astronomical Journal. ¹²⁰⁾

- 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.

• February 26, 2021: This NASA/ESA Hubble Space Telescope Picture of the Week features NGC4826 — a spiral galaxy located 17 million light-years away in the constellation of Coma Berenices (Berenice's Hair). This galaxy is often referred to as the "Black Eye", or "Evil Eye", galaxy because of the dark band of dust that sweeps across one side of its bright nucleus. ^{121) 122)}

- NGC4826 is known by astronomers for its strange internal motion. The gas in the outer regions of this galaxy and the gas in its inner regions are rotating in opposite directions, which might be related to a recent merger. New stars are forming in the region where the counter rotating gases collide.

- This galaxy was first discovered in 1779 by the English astronomer Edward Pigott.

• February 25, 2021: For the first time, a wayward comet-like object has been spotted near the family of ancient asteroids. ¹²³⁾

- After traveling several billion miles toward the Sun, a wayward young comet-like object orbiting among the giant planets has found a temporary parking place along the way. The object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population.

- The unexpected visitor belongs to a class of icy bodies found in space between Jupiter and Neptune. Called Centaurs, they become active for the first time when heated as they approach the Sun, and dynamically transition into becoming more comet-like.

- Visible-light snapshots by NASA's Hubble Space Telescope reveal that the vagabond object shows signs of comet activity, such as a tail, outgassing in the form of jets, and an enshrouding coma of dust and gas. Earlier observations by NASA's Spitzer Space Telescope gave clues to the composition of the comet-like object and the gasses driving its activity.

- "Only Hubble could detect active comet-like features this far away at such high detail, and the images clearly show these features, such as a roughly 400,000-mile-long broad tail and high-resolution features near the nucleus due to a coma and jets," said lead Hubble researcher Bryce Bolin of Caltech in Pasadena, California.

- Describing the Centaur's capture as a rare event, Bolin added, "The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We're investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter."

- The team's paper appears in Feb. 11 issue of The Astronomical Journal. ¹²⁴⁾

- The research team's computer simulations show that the icy object, called P/2019 LD2 (LD2), probably swung close to Jupiter about two years ago. The planet then gravitationally punted the wayward visitor to the Trojan asteroid group's co-orbital location, leading Jupiter by about 437 million miles.

Bucket Brigade

- The nomadic object was discovered in early June 2019 by the University of Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes located on the extinct volcanoes, one on Mauna Kea and one on Haleakala. Japanese amateur astronomer Seiichi Yoshida tipped off the Hubble team to possible comet activity. The astronomers then scanned archival data from the Zwicky Transient Facility, a wide-field survey conducted at Palomar Observatory in California, and realized that the object was clearly active in images from April 2019.

- They followed up with observations from the Apache Point Observatory in New Mexico, which also hinted at the activity. The team observed the comet using Spitzer just days before the observatory's retirement in January 2020, and identified gas and dust around the comet nucleus. These observations convinced the team to use Hubble to take a closer look. Aided by Hubble's sharp vision, the researchers identified the tail, coma structure, the size of the dust particles, and their ejection velocity. These images helped them confirm that the features are due to relatively new comet-like activity.

- Although LD2's location is surprising, Bolin wonders whether this pit stop could be a common pull-off for some sunward-bound comets. "This could be part of the pathway from our solar system through the Jupiter Trojans to the inner solar system," he said.

- The unexpected guest probably will not stay among the asteroids for very long. Computer simulations show that it will have another close encounter with Jupiter in about another two years. The hefty planet will boot the comet from the system, and it will continue its journey to the inner solar system.

- "The cool thing is that you're actually catching Jupiter flinging this object around and changing its orbital behavior and bringing it into the inner system," said team member Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. "Jupiter controls what's going on with comets once they get into the inner system by altering their orbits."

- The icy interloper is most likely one of the latest members of the so-called "bucket brigade" of comets to get kicked out of its frigid home in the Kuiper Belt and into the giant planet region through interactions with another Kuiper Belt object. Located beyond Neptune's orbit, the Kuiper Belt is a haven of icy, leftover debris from our planets' construction 4.6 billion years ago, containing millions of objects, and occasionally these objects have near misses or collisions that drastically alter their orbits from the Kuiper Belt inward into the giant planet region.

- The bucket brigade of icy relics endure a bumpy ride during their journey sunward. They bounce gravitationally from one outer planet to the next in a game of celestial pinball before reaching the inner solar system, warming up as they come closer to the Sun. The researchers say the objects spend as much or even more time around the giant planets, which are gravitationally pulling on them – about 5 million years – than they do crossing into the inner system where we live.

- "Inner-system, ‘short-period' comets break up about once a century," Lisse explained. "So, in order to maintain the number of local comets we see today, we think the bucket brigade has to deliver a new short-period comet about once every 100 years."

An Early Bloomer

- Seeing outgassing activity on a comet 465 million miles away from the Sun (where the intensity of sunlight is 1/25th as strong as on Earth) surprised the researchers. "We were intrigued to see that the comet had just started to become active for the first time so far away from the Sun at distances where water ice is barely starting to sublimate," said Bolin.

- Water remains frozen on a comet until it reaches about 200 million miles from the Sun, where heat from sunlight converts water ice to gas that escapes from the nucleus in the form of jets. So the activity signals that the tail might not be made of water. In fact, observations by Spitzer indicated the presence of carbon monoxide and carbon dioxide gas, which could be driving the creation of the tail and jets seen on the Jupiter-orbiting comet. These volatiles do not need much sunlight to heat their frozen form and convert them to gas.

- Once the comet gets kicked out of Jupiter's orbit and continues its journey, it may meet up with the giant planet again. "Short-period comets like LD2 meet their fate by being thrown into the Sun and totally disintegrating, hitting a planet, or venturing too close to Jupiter once again and getting thrown out of the solar system, which is the usual fate," Lisse said. "Simulations show that in about 500,000 years, there's a 90% probability that this object will be ejected from the solar system and become an interstellar comet."

- 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. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, managed the Spitzer mission for NASA's Science Mission Directorate in Washington, D.C. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spitzer's entire science catalogue is available via the Spitzer data archive, housed at the Infrared Science Archive at IPAC. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado.

• February 19, 2021: Tantrums of a baby star - a Milky War star with circumstellar material outflow at a distance of 1400 light years.. Week in images. ¹²⁵⁾

- Before 1997 it was theorized by Schwartz and others that the objects could be a type of reflection nebula, or a type of shock wave formed from the gas emitted from a star interacting with the surrounding matter. The mystery was finally solved when a protostar, unseen in this image, was discovered at the centre of the long jets of matter. The outflows of matter, some 10 light-years across, were ejected from the newly born star and violently propelled outwards at speeds of over 150 kilometers per second. Upon reaching the surrounding gas, the collision created the bright shock waves seen here.

• February 12, 2021: This week's NASA/ESA Hubble Space Telescope Picture of the Week features an impressive portrait of M1-63, a beautifully captured example of a bipolar planetary nebula located in the constellation of Scutum (the Shield). A nebula like this one is formed when the star at its center sheds huge quantities of material from its outer layers, leaving behind a spectacular cloud of gas and dust. ¹²⁶⁾

- It is believed that a binary system of stars at the center of the bipolar nebula is capable of creating hourglass or butterfly-like shapes like the one in this image. This is because the material from the shedding star is funneled towards its poles, with the help of the companion, creating the distinctive double-lobed structure seen in nebulae such as M1-63.

• February 11, 2021: Scientists were expecting to find an intermediate-mass black hole at the heart of the globular cluster NGC 6397, but instead they found evidence of a concentration of smaller black holes lurking there. New data from the NASA/ESA Hubble Space Telescope have led to the first measurement of the extent of a collection of black holes in a core-collapsed globular cluster. ^{127) 128)}

- Globular clusters are extremely dense stellar systems, in which stars are packed closely together. They are also typically very old — the globular cluster that is the focus of this study, NGC 6397, is almost as old as the Universe itself. It resides 7800 light-years away, making it one of the closest globular clusters to Earth. Because of its very dense nucleus, it is known as a core-collapsed cluster.

- When Eduardo Vitral and Gary A. Mamon of the Institut d'Astrophysique de Paris set out to study the core of NGC 6397, they expected to find evidence for an "intermediate-mass" black hole (IMBH). These are smaller than the supermassive black holes that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of massive stars. IMBH are the long-sought "missing link" in black hole evolution and their mere existence is hotly debated, although a few candidates have been found (see Notes [1]).

- To look for the IMBH, Vitral and Mamon analyzed the positions and velocities of the cluster's stars. They did this using previous estimates of the stars' proper motions [2] from Hubble images of the cluster spanning several years [3], in addition to proper motions provided by ESA's Gaia space observatory, which precisely measures the positions, distances and motions of stars. Knowing the distance to the cluster allowed the astronomers to translate the proper motions of these stars into velocities.

- "Our analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated," explained Mamon.

- "We found very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster," added Vitral.

- This invisible component could only be made up of the remnants (white dwarfs, neutron stars, and black holes) of massive stars whose inner regions collapsed under their own gravity once their nuclear fuel was exhausted. The stars progressively sank to the cluster's centre after gravitational interactions with nearby less massive stars, leading to the small extent of the invisible mass concentration. Using the theory of stellar evolution, the scientists concluded that the bulk of the unseen concentration is made of stellar-mass black holes, rather than white dwarfs or neutron stars that are too faint to observe.

- Two recent studies had also proposed that stellar remnants and in particular, stellar-mass black holes, could populate the inner regions of globular clusters.

- "Our study is the first finding to provide both the mass and the extent of what appears to be a collection of mostly black holes in a core-collapsed globular cluster," said Vitral.

- "Our analysis would not have been possible without having both the Hubble data to constrain the inner regions of the cluster and the Gaia data to constrain the orbital shapes of the outer stars, which in turn indirectly constrain the velocities of foreground and background stars in the inner regions," added Mamon, attesting to an exemplary international collaboration.

- The astronomers also note that this discovery raises the question of whether mergers of these tightly packed black holes in core-collapsed globular clusters may be an important source of gravitational waves recently detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment.

Notes:

[1] In 2020, new data from the NASA/ESA Hubble Space Telescope provided the strongest evidence to date for a mid-sized black hole. Read the full press release on this result here.

[2] Proper motion describes how fast objects move in the sky.

[3] The Hubble data for this study were provided by A. Bellini, who measured the proper motions for over 1.3 million stars in 22 globular clusters, including NGC 6397.

• February 5, 2021: Picture of the Week: The Modest Galaxy (Astronomy). ¹²⁹⁾

- Many young blue stars are sprinkled throughout the circular patterns of UGC 3885's arms, contrasted and complemented by dark lanes of dust also following the spiral structure. A glancing look at UGC 3885 (Uppsala General Catalogue) of Galaxies may only leave you with an impression of the galaxy, but spare a moment longer and the intricacies of the galaxy begin to emerge. Located in the constellation of Lynx, a spiral galaxy some 180 million light-years away. UGC 3885 is a cosmic beauty to behold.

• January 29, 2021: The lives of planetary nebulae are often chaotic, from the death of their parent star to the scattering of its contents far out into space. Captured here by the NASA/ESA Hubble Space Telescope, ESO 455-10 is one such planetary nebula, located in the constellation of Scorpius (The Scorpion). ¹³⁰⁾

- The oblate shells of ESO 455-10, previously held tightly together as layers of its central star, not only give this planetary nebula its unique appearance, but also offer information about the nebula. Seen in a field of stars, the distinct asymmetrical arc of material over the north side of the nebula is a clear sign of interactions between ESO 455-10 and the interstellar medium.

• January 22, 2021: Located in the constellation of Virgo (The Virgin), around 50 million light-years from Earth, NGC 4535 is truly a stunning sight to behold. Despite the incredible quality of this image, taken from the NASA/ESA Hubble Space Telescope, NGC 4535 has a hazy, somewhat ghostly, appearance when viewed from a smaller telescope. This led amateur astronomer Leland S. Copeland to nickname NGC 4535 the "Lost Galaxy" in the 1950s. ^{131) 132)}

- This galaxy was studied as part of the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) survey, which aims to clarify many of the links between cold gas clouds, star formation, and the overall shape and other properties of galaxies. On 11 January 2021 the first release of the PHANGS-HST Collection was made publicly available. ¹³³⁾

• January 15, 2021: Images of two iconic planetary nebulae taken by the Hubble Space Telescope are revealing new information about how they develop their dramatic features. Researchers from Rochester Institute of Technology (RIT) and Green Bank Observatory presented new findings about the Butterfly Nebula (NGC 6302) and the Jewel Bug Nebula (NGC 7027) at the 237th meeting of the American Astronomical Society on Friday, Jan. 15. ¹³⁴⁾

- Hubble's Wide Field Camera 3 observed the nebulae in 2019 and early 2020 using its full, panchromatic capabilities, and the astronomers involved in the project have been using emission line images from near-ultraviolet to near-infrared light to learn more about their properties. The studies were first-of-their-kind panchromatic imaging surveys designed to understand the formation process and test models of binary-star-driven planetary nebula shaping.

- "We're dissecting them," said Joel Kastner, a professor in RIT's Chester F. Carlson Center for Imaging Science and School of Physics and Astronomy. "We're able to see the effect of the dying central star in how it's shedding and shredding its ejected material. We're now seeing where material that the central star has tossed away is being dominated by ionized gas, where it's dominated by cooler dust, and even how the hot gas is being ionized, whether by the star's UV or by collisions caused by its present, fast winds."

- Kastner said analysis of the new HST images of the Butterfly Nebula is confirming that the nebula was ejected only about 2,000 years ago—an eyeblink by the standards of astronomy – and established that the S-shaped iron emission that helps give it the "wings" of gas is even younger. Surprisingly, they found that while astronomers previously believed they had located the nebula's central star, that previously-identified star is actually not associated with the nebula and is instead much closer to Earth than the Butterfly Nebula. Kastner said he hopes that future studies with the James Webb Space Telescope could help locate the real dying star at the heart of the nebula.

• January 15, 2021: First discovered in 1798 by German-English astronomer William Hershel, NGC 613 is a galaxy which lies in the southern constellation of Sculptor 67 million light-years away. ¹³⁵⁾

- Recent studies have shown that bars are more common in galaxies now than they were in the past, which gives us important clues about galaxy formation and evolution.

• January 14, 2021: Astronomers are winding back the clock on the expanding remains of a nearby, exploded star. By using NASA's Hubble Space Telescope, they retraced the speedy shrapnel from the blast to calculate a more accurate estimate of the location and time of the stellar detonation. ^{136) 137)}

- The victim is a star that exploded long ago in the Small Magellanic Cloud, a satellite galaxy to our Milky Way. The doomed star left behind an expanding, gaseous corpse, a supernova remnant named 1E 0102.2-7219, which NASA's Einstein Observatory first discovered in X-rays. Like detectives, researchers sifted through archival images taken by Hubble, analyzing visible-light observations made 10 years apart.

- The research team, led by John Banovetz and Danny Milisavljevic of Purdue University in West Lafayette, Indiana, measured the velocities of 45 tadpole-shaped, oxygen-rich clumps of ejecta flung by the supernova blast. Ionized oxygen is an excellent tracer because it glows brightest in visible light.

- To calculate an accurate explosion age, the astronomers picked the 22 fastest moving ejecta clumps, or knots. The researchers determined that these targets were the least likely to have been slowed down by passage through interstellar material. They then traced the knots' motion backward until the ejecta coalesced at one point, identifying the explosion site. Once that was known, they could calculate how long it took the speedy knots to travel from the explosion center to their current location.

- According to their estimate, light from the blast arrived at Earth 1,700 years ago, during the decline of the Roman Empire. However, the supernova would only have been visible to inhabitants of Earth's southern hemisphere. Unfortunately, there are no known records of this titanic event.

- The researchers' results differ from previous observations of the supernova's blast site and age. Earlier studies, for example, arrived at explosion ages of 2,000 and 1,000 years ago. However, Banovetz and Milisavljevic say their analysis is more robust.

- "A prior study compared images taken years apart with two different cameras on Hubble, the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys (ACS)," Milisavljevic said. "But our study compares data taken with the same camera, the ACS, making the comparison much more robust; the knots were much easier to track using the same instrument. It's a testament to the longevity of Hubble that we could do such a clean comparison of images taken 10 years apart."

- The astronomers also took advantage of the sharp ACS images in selecting which ejecta clumps to analyze. In prior studies, researchers averaged the speed of all of the gaseous debris to calculate an explosion age. However, the ACS data revealed regions where the ejecta slowed down because it was slamming into denser material shed by the star before it exploded as a supernova. Researchers didn't include those knots in the sample. They needed the ejecta that best reflected their original velocities from the explosion, using them to determine an accurate age estimate of the supernova blast.

- Hubble also clocked the speed of a suspected neutron star—the crushed core of the doomed star—that was ejected from the blast. Based on their estimates, the neutron star must be moving at more than 2 million miles per hour from the center of the explosion to have arrived at its current position. The suspected neutron star was identified in observations with the European Southern Observatory's VLT (Very Large Telescope) in Chile, in combination with data from NASA's Chandra X-ray Observatory.

- "That is pretty fast and at the extreme end of how fast we think a neutron star can be moving, even if it got a kick from the supernova explosion," Banovetz said. "More recent investigations call into question whether the object is actually the surviving neutron star of the supernova explosion. It is potentially just a compact clump of supernova ejecta that has been lit up, and our results generally support this conclusion."

- So the hunt may still be on for the neutron star. "Our study doesn't solve the mystery, but it gives an estimate of the velocity for the candidate neutron star," Banovetz said.

• January 8, 2021: The galaxy NGC 6946 is nothing short of spectacular. In the last century alone, NGC 6946 has experienced 10 observed supernovae, earning its nickname as the Fireworks Galaxy. In comparison, our Milky Way averages just one to two supernova events per century. This NASA/ESA Hubble Space Telescope image shows the stars, spiral arms, and various stellar environments of NGC 6946 in phenomenal detail. ¹³⁸⁾

- We are able to marvel at NGC 6946 as it is a face-on galaxy, which means that we see the galaxy "facing" us, rather than seeing it from the side (known as edge-on). The Fireworks Galaxy is further classified as an intermediate spiral galaxy and as a starburst galaxy. The former means the structure of NGC 6946 sits between a full spiral and a barred spiral galaxy, with only a slight bar in its center, and the latter means it has an exceptionally high rate of star formation.

• January 7, 2021: It is during rare merging events that galaxies undergo dramatic changes in their appearance and in their stellar content. These systems are excellent laboratories to trace the formation of star clusters under extreme physical conditions. ^{139) 140)}

- The Milky Way typically forms star clusters with masses that are 10 thousand times the mass of our Sun. This doesn't compare to the masses of the star clusters forming in colliding galaxies, which can reach millions of times the mass of our Sun.

- These dense stellar systems are also very luminous. Even after the collision, when the resulting galactic system begins to fade into a more quiescent phase, these very massive star clusters will shine throughout their host galaxy, as long-lasting witnesses of past merging events.

- By studying the six galaxy mergers shown here, the Hubble imaging Probe of Extreme Environments and Clusters (HiPEEC) survey has investigated how star clusters are affected during collisions by the rapid changes that drastically increase the rate at which new stars are formed in these galaxies. Hubble's capabilities have made it possible to resolve large star-forming "knots" into numerous compact young star clusters. Hubble's ultraviolet and near-infrared observations of these systems have been used to derive star cluster ages, masses, and extinctions and to analyze the star formation rate within these six merging galaxies. The HiPEEC study reveals that the star cluster populations undergo large and rapid variations in their properties, with the most massive clusters formed towards the end of the merger phase.

- Each of the merging systems shown here has been previously published by Hubble, as early as 2008 and as recently as October 2020. To celebrate it's 18th anniversary in 2008, the Hubble Space Telescope released a collection of 59 images of merging galaxies, which can be explored here.

1) "Our Milky Way May Be More Fluffy, Less Wiry," NASA Hubblesite, 16 December 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-061.html

2) "New Exhibit Displaying Hubble Science ‘Brings the Universe Home'," NASA Hubblesite, 14 December 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-067.html

3) "Four filter fusion," ESA Science & Exploration, 10 December 2021 Release ID: 2021-067 , URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_6_-_10_December_2021

4) Claire Andreoli, Ray Willard, "Mini-Jet Found Near Milky Way's Supermassive Black Hole," NASA Feature, 9 December 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-mini-jet-found-near-milky-ways-supermassive-black-hole

5) "Mini-Jet Found Near Milky Way's Supermassive Black Hole," NASA Hubblesite, 9 December 2021, Release ID: 2021-062, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-062.html

6) Gerald Cecil, Alexander Y. Wagner, Joss Bland-Hawthorn, Geoffrey V. Bicknell, and Dipanjan Mukherjee, "Tracing the Milky Way's Vestigial Nuclear Jet," The Astrophysical Journal, Volume 922, Number 2, Published: 6 December 2021, https://doi.org/10.3847/1538-4357/ac224f

7) "NASA Returns Hubble to Full Science Operations," NASA Feature, 7 December 2021, URL: https://www.nasa.gov/feature/goddard/2021/nasa-returns-hubble-to-full-science-operations

8) "One of a pair," ESA Science & Exploration, 03 December 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_29_November_-_3_December_2021

9) "NASA Takes Another Step Toward Full Hubble Ops: Spectrograph Returns," NASA Feature, 29 November 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-instruments-remain-in-safe-mode-nasa-team-investigating

10) Clair Andreoli, "Hubble's View of Planetary Nebula Reveals Complex Structure," NASA Feature, 29 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-s-view-of-planetary-nebula-reveals-complex-structure

11) Claire Andreoli, Andrea Gianopoulos, "Hubble Witnesses Shock Wave of Colliding Gases in Running Man Nebula," NASA Feature, 24 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-witnesses-shock-wave-of-colliding-gases-in-running-man-nebula

12) Claire Andreoli, Elizabeth Landau, "NASA Closer to Full Hubble Operations as Another Instrument Resumes Science," NASA Feature, 22 November 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-instruments-remain-in-safe-mode-nasa-team-investigating

13) Claire Andreoli, "Hubble Spots Swirls of Dust in the Flame Nebula," NASA Feature, 22 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-spots-swirls-of-dust-in-the-flame-nebula

14) Claire Andreoli, "Hubble Catches Celestial Prawn Drifting Through the Cosmic Deep," NASA Feature, 19 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-catches-celestial-prawn-drifting-through-the-cosmic-deep

15) "A spectacular side-on spiral," ESA Science & Exploration, 10 November 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_15_-_19_November_2021

16) "Hubble Takes a Grand Tour of the Solar System," ESA, heic2113 — Photo Release, 18 November 2021, URL: https://esahubble.org/news/heic2113/?lang

17) "Hubble's Grand Tour of the Outer Solar System," Hubblesite Release ID: 2021-047, 18 November 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-047.html

18) Claire Andreoli, "Hubble Spies Newly Forming Star Incubating in IC 2631," NASA Feature, 17 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-spies-newly-forming-star-incubates-in-ic-2631

19) Claire Andreoli, Cynthia M. O'Carroll, Elizabeth Landau, "NASA Extends Hubble Operations Contract, Provides Mission Update," NASA Feature, 16 November 2021, URL: https://www.nasa.gov/feature/nasa-extends-hubble-operations-contract-provides-mission-update

20) Claire Andreoli, "Nebula Churns Out Massive Stars in New Hubble Image," NASA Feature, 16 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/nebula-churns-out-massive-stars-in-new-hubble-image

21) Claire Andreoli, "Hubble Images Dark Nebula Cloaking Stars Within Dusty Depths," NASA Feature, 15 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-images-dark-nebula-cloaking-stars-within-dusty-depths

22) Claire Andreoli, "Reflected Starlight Bathes Forming Star in New Hubble Image," NASA Feature, 12 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/reflected-starlight-bathes-forming-star-in-new-hubble-image

23) "Cosmological curiosity," ESA Science & Exploration, 12 November 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_08_-_12_November_2021

24) Claire Andreoli, "Hubble Surveys A Snowman Sculpted from Gas and Dust," NASA Feature, 10 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-surveys-a-snowman-sculpted-from-gas-and-dust

25) Claire Andreoli, "Hubble Captures the Shredded Remains of a Cosmic Explosion," NASA Feature, 9 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-captures-the-shredded-remains-of-a-cosmic-explosion

26) Claire Andreoli, Elizabeth Landau, "Hubble's Advanced Camera for Surveys Instrument Resumes Science, Investigation Continues," NASA Feature, 8 November 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-instruments-remain-in-safe-mode-nasa-team-investigating

27) "Cosmic fireworks," ESA Science & Exploration, 05 November 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_01_-_05_November_2021

28) "Hubble Images Colorful Planetary Nebula Ringed by Hazy Halo," NASA, 4 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-images-colorful-planetary-nebula-ringed-by-hazy-halo

29) Claire Andreoli, Elizabeth Landau, "NASA Takes Additional Steps to Investigate Hubble Instruments in Safe Mode," NASA Feature, 4 November 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-instruments-remain-in-safe-mode-nasa-team-investigating

30) "Hubble Spies Eye in the Sky," NASA, 3 November 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-spies-eye-in-the-sky

31) Claire Andreoli, "Hubble Remains in Safe Mode, NASA Team Investigating," NASA, 1 November 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-remains-in-safe-mode-nasa-team-investigating

32) Claire Andreoli, Ray Villard, "Hubble Celebrates Halloween with a Glowering, Dying Star," NASA Feature, 28 October 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-celebrates-halloween-with-a-glowering-dying-star

33) "Hubble Celebrates Halloween With A Glowering Carbon Star," ESA heic2112 — Photo Release, 28 October 2021, URL: https://esahubble.org/news/heic2112/?lang

34) "Hubble spies a pair of squabbling galaxies," ESA Science & Exploration, 22 October 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_18_-_22_October_2021

35) "Hubble Gives Unprecedented, Early View of a Doomed Star's Destruction," NASA, 21 October 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-gives-unprecedented-early-view-of-a-doomed-stars-destruction

36) Samaporn Tinyanont, et al., "Progenitor and Close-In Circumstellar Medium of Type II Supernova 2020fqv from High-Cadence Photometry and Ultra-Rapid UV Spectroscopy," MNRAS 000, 1–17 (2020), URL: https://stsci-opo.org/STScI-01FH8PXT7J03QWHH2JAZYN79DK.pdf

37) "Invisible galactic gale," ESA Science & Exploration, 15 October 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_11_-_15_October_2021

38) "Hubble Finds Evidence of Persistent Water Vapor in One Hemisphere of Europa," NASA Feature, 14 October 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-finds-evidence-of-persistent-water-vapor-in-one-hemisphere-of-europa

39) "Hubble Finds Evidence of Persistent Water Vapor Atmosphere on Europa," heic2111 — Science Release, 14 October 2021, URL: https://esahubble.org/news/heic2111/?lang

40) Lorenz Roth, "A Stable H₂O Atmosphere on Europa's Trailing Hemisphere From HST Images," Geophysical Research Letters, Volume48, Issue20, 28 October 2021, e2021GL094289, First published: 13 September 2021, https://doi.org/10.1029/2021GL094289, URL: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2021GL094289

41) "A dangerous dance," ESA Space Science, 8 October 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_4_-_8_October_2021

42) Donna Weaver, Lynn Jenner, "'Double' Galaxy Mystifies Hubble Astronomers," NASA Feature, 7 October 2021, URL: https://www.nasa.gov/feature/goddard/2021/double-galaxy-mystifies-hubble-astronomers

43) Richard E Griffiths, Mitchell Rudisel, Jenny Wagner, Timothy Hamilton, Po-Chieh Huang, Carolin Villforth, "Hamilton's Object – a clumpy galaxy straddling the gravitational caustic of a galaxy cluster: constraints on dark matter clumping," Monthly Notices of the Royal Astronomical Society, Volume 506, Issue 2, September 2021, Pages 1595–1608, Published: 17 May 2021, https://doi.org/10.1093/mnras/stab1375

44) "More than Meets the Eye," ESA Science & Exploration, 01 October 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_27_September_-_1_October_2021

45) "Hubble Shows Winds in Jupiter's Great Red Spot Are Speeding Up," ESA heic2110 — Science Release, 27 September 2021, URL: https://esahubble.org/news/heic2110/?lang

46) "Rings of Relativity," ESA Science & Exploration, 24 September 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_20_-_24_September_2021

47) "Hubble Snapshot of "Molten Ring" Galaxy Prompts New Research," NASA Feature, 23 September 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-snapshot-of-molten-ring-galaxy-prompts-new-research

48) "Hubble Finds Early, Massive Galaxies Running on Empty," NASA Hubblesite news release, 22 September 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-039.html

49) Katherine E. Whitaker, Christina C. Williams, Lamiya Mowla, Justin S. Spilker, Sune Toft, Desika Narayanan, Alexandra Pope, Georgios E. Magdis, Pieter G. van Dokkum, Mohammad Akhshik, Rachel Bezanson, Gabriel B. Brammer, Joel Leja, Allison Man, Erica J. Nelson, Johan Richard, Camilla Pacifici, Keren Sharon & Francesco Valentino, "Quenching of star formation from a lack of inflowing gas to galaxies," Nature, Volume 597, pp: 485-488, Published: 22 September 2021, https://doi.org/10.1038/s41586-021-03806-7

50) "A closer look at Hubble's 31st anniversary snapshot," ESA Science & Exploration, 17 September 2021, URL: https://www.esa.int/ESA_Multimedia/Images/2021/09/A_closer_look_at_Hubble_s_31st_anniversary_snapshot

51) "Rerun of Supernova Blast Is Expected to Appear in 2037," NASA Hubblesite, 13 September 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-030.html

52) Steven A. Rodney, Gabriel B. Brammer, Justin D. R. Pierel, Johan Richard, Sune Toft, Kyle F. O'Connor, Mohammad Akhshik & Katherine E. Whitaker, "A gravitationally lensed supernova with an observable two-decade time delay," Nature Astronomy Letter, Published: 13 September 2021, https://doi.org/10.1038/s41550-021-01450-9

53) "A glittering globular cluster," ESA Science & Exploration, 10 September 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_6_-_10_September_2021

54) Claire Andreoli, Ray Willard, Bethany Downer, "Hubble Discovers Hydrogen-Burning White Dwarfs Enjoying Slow Aging," NASA Feature, 6 September 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-discovers-hydrogen-burning-white-dwarfs-enjoying-slow-aging

55) "Cluster in the cloud," ESA Science & Exploration, 27 August 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_23_-_27_August_2021

56) "In the heart of the furnace," ESA Science & Exploration, 20 August 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_16_-_20_August_2021

57) "Comet ATLAS May Have Been a Blast from the Past," NASA Hubblesite, 19 August 2021 Release ID: 2021-031, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-031

58) Quanzhi Ye, David Jewitt, Man-To Hui, Qicheng Zhang, Jessica Agarwal, Michael S. P. Kelley, Yoonyoung Kim, Jing Li, Tim Lister, Max Mutchler, and Harold A. Weaver, "Disintegration of Long-period Comet C/2019 Y4 (ATLAS). I. Hubble Space Telescope Observations, The Astronomical Journal, Volume 162, No 2, Published: 21 July 2021, https://doi.org/10.3847/1538-3881/abfec3

59) Claire Andreoli, Rob Gutro, "Hubble Peers into a Dusty Stellar Nursery," NASA Feature, 13 August 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-peers-into-a-dusty-stellar-nursery

60) "Seeing quintuple," ESA Science & Exploration, 13 August 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_09_-_13_August_2021

61) "Portrait of a swirling galaxy," ESA Science & Exploration, 6 August 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_02_-_06_August_2021

62) "Squabbling galactic siblings," ESA Science & Exploration, 30 July 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_26_-_30_July_2021

63) "Hubble Finds First Evidence of Water Vapor at Jupiter's Moon Ganymede," ESA, heic2107 — Science Release, 26 July 2021, URL: https://esahubble.org/news/heic2107/?lang

64) "Hubble Finds First Evidence of Water Vapor at Jupiter's Moon Ganymede," NASA HubbleSite, 26 July 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-033

65) Lorenz Roth, Nickolay Ivchenko, G. Randall Gladstone, Joachim Saur, Denis Grodent, Bertrand Bonfond, Philippa M. Molyneux & Kurt D. Retherford, "A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations," Nature Astronomy, Published: 26 July 2021, https://doi.org/10.1038/s41550-021-01426-9

66) Claire Andreoli, Ray Villard, Bethany Downer, "Hubble Finds First Evidence of Water Vapor at Jupiter's Moon Ganymede," NASA Feature, 26 July 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-finds-first-evidence-of-water-vapor-at-jupiter-s-moon-ganymede

67) "Cosmic lens flare," ESA Science & Exploration, 23 July 2021, URL: https://www.esa.int/ESA_Multimedia/Images/2021/07/Cosmic_lens_flare

68) "A large tidal stream observed in the Sombrero galaxy," IAC (Instituto de Astrofísica de Canarias) Press Release, 21 July 2021, URL:https://www.iac.es/en/outreach/news/large-tidal-stream-observed-sombrero-galaxy

69) David Martínez-Delgado, Javier Román, Denis Erkal, Mischa Schirmer, Santi Roca-Fábrega, Seppo Laine, Giuseppe Donatiello, Manuel Jimenez, David Malin, Julio A Carballo-Bello, "A feather on the hat: Tracing the giant stellar stream around the Sombrero galaxy," MNRAS, stab1874, Published: 20 July 2021, https://doi.org/10.1093/mnras/stab1874

70) "Hubble Returns to Full Science Observations and Releases New Images," NASA Feature, 19 July 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-returns-to-full-science-observations-and-releases-new-images

71) Claire Andreoli, Alise Fisher, Elizabeth Landau, Rob Gutro, "NASA Returns Hubble Space Telescope to Science Operations," NASA Feature, 17 July 2021, URL: https://www.nasa.gov/feature/goddard/2021/operations-underway-to-restore-payload-computer-on-nasas-hubble-space-telescope

72) "NASA Successfully Switches to Backup Hardware on Hubble Space Telescope," NASA Feature, 16 July 2021, URL: https://www.nasa.gov/feature/goddard/2021/operations-underway-to-restore-payload-computer-on-nasas-hubble-space-telescope

73) "A star in a distant galaxy blew up in a powerful explosion, solving an astronomical mystery," Tel-Aviv University News Release, 14 July 2021, URL: https://www.eurekalert.org/pub_releases/2021-07/tu-asi071421.php

74) Daichi Hiramatsu, D. Andrew Howell, Schuyler D. Van Dyk, Jared A. Goldberg, Keiichi Maeda, Takashi J. Moriya, Nozomu Tominaga, Ken'ichi Nomoto, Griffin Hosseinzadeh, Iair Arcavi, Curtis McCully, Jamison Burke, K. Azalee Bostroem, Stefano Valenti, Yize Dong, Peter J. Brown, Jennifer E. Andrews, Christopher Bilinski, G. Grant Williams, Paul S. Smith, Nathan Smith, David J. Sand, Gagandeep S. Anand, Chengyuan Xu, Alexei V. Filippenko, Melina C. Bersten, Gastón Folatelli, Patrick L. Kelly, Toshihide Noguchi, Koichi Itagaki, "The electron-capture origin of supernova 2018zd," Nature Astronomy Letters, Published: 28 June 2021, https://doi.org/10.1038/s41550-021-01384-2, URL: https://is.gd/ADVWx3

75) "One of the greats," ESA Science & Exploration, 09 July 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_05_-_09_July_2021

76) "A scattering of stars," ESA Science & Exploration, 02 July 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_28_June_-_02_July_2021

77) Rob Gutro, "Operations Underway to Restore Payload Computer on NASA's Hubble Space Telescope," NASA Feature, 30 June 2021, URL: https://www.nasa.gov/feature/goddard/2021/operations-underway-to-restore-payload-computer-on-nasas-hubble-space-telescope

78) "Clash of the Titans," ESA Science & Exploration, 26 June 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_21_-_25_June_2021

79) Jeff Foust, "Computer problem takes Hubble offline," SpaceNews, 19 June 2021, URL: https://spacenews.com/computer-problem-takes-hubble-offline/

80) "A galactic powerhouse," ESA Science & Exploration, 18 June 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_14_-_18_June_2021

81) "Precise Distance to Galaxy Bolsters Missing Dark Matter Claim," NASA HubbleSite, 17 June 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-025.html

82) Claire Andreoli, Donna Weaver, Ray Villard, "Mystery of Galaxy's Missing Dark Matter Deepens," NASA Feature, 17 June 2021, URL: https://www.nasa.gov/feature/goddard/2021/mystery-of-the-galaxys-missing-dark-matter-deepens

83) Zili Shen, Shany Danieli, Pieter van Dokkum, Roberto Abraham, Jean P. Brodie, Charlie Conroy, Andrew E. Dolphin, Aaron J. Romanowsky, J. M. Diederik Kruijssen, and Dhruba Dutta Chowdhury, "A Tip of the Red Giant Branch Distance of 22.1 ± 1.2 Mpc to the Dark Matter Deficient Galaxy NGC 1052–DF2 from 40 Orbits of Hubble Space Telescope Imaging," The Astrophysical Journal Letters, Volume 914, Number 1, Published: 9 June 2021, https://doi.org/10.3847/2041-8213/ac0335

84) "Operations Underway to Restore Payload Computer on NASA's Hubble Space Telescope," NASA Feature, 16 June 2021, URL: https://www.nasa.gov/feature/goddard/2021/operations-underway-to-restore-payload-computer-on-nasas-hubble-space-telescope

85) "A Spiral Amongst Friends," ESA Science & Exploration, 11 June 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_07_-_11_June_2021

86) "Astronomers Probe Layer-Cake Structure of Brown Dwarf's Atmosphere," NASA HubbleSite, 09 June 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-028#section-id-2

87) "The eponymous NGC 691," ESA Science & Exploration, 4 June 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_31_May_-_04_June_2021

88) "Hubble Captures a Captivating Spiral," NASA Feature, 28 May 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-captures-a-captivating-spiral

89) "A distant spiral in Virgo," ESA Science & Exploration, 28 May 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_24_-_28_May_2021

90) "Lopsided galaxy 2276," NASA HubbleSite News, 27 May 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-029?news=true

91) "Hubble Inspects a Contorted Spiral Galaxy," heic2106 — Photo Release, 27 May 2021, URL: https://esahubble.org/news/heic2106/?lang

92) "A menagerie of galaxies," ESA Science & Exploration, 21 May 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_17_-_21_May_2021

93) "Hubble tracks down fast radio bursts to galaxies' spiral arms," NASA HubbleSite, 20 May 2021, Release ID: 2021-010, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-010.html#section-id-2

94) "Cosmic silver lining," ESA Science & Exploration, 14 May 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_10_-_14_May_2021

95) "Our giant universe," ESA Science & Exploration, 7 May 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_03_-_07_April_2021

96) "In the sky with diamonds," ESA Science & Exploration, 30 April 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_26_-_30_April_2021

97) Claire Andreoli, Claire Blome, Ray Villard, Lynn Jenner, "Hubble Watches How a Giant Planet Grows," NASA Feature, 29 April 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-watches-how-a-giant-planet-grows

98) Yifan Zhou, Brendan P. Bowler, Kevin R. Wagner, Glenn Schneider, Dániel Apai, Adam L. Kraus, Laird M. Close, Gregory J. Herczeg, and Min Fang, "Hubble Space Telescope UV and Hα Measurements of the Accretion Excess Emission from the Young Giant Planet PDS 70 b," The Astronomical Journal, Volume 161, Number 5, Published 29 April 2021, https://doi.org/10.3847/1538-3881/abeb7a

99) "Hubble Celebrates 31st Birthday with Giant Star on the Edge of Destruction," ESA/Hubble photo release heic2105, 23 April 2021, URL: https://esahubble.org/news/heic2105/?lang

100) "Galactic close-up," ESA science & Exploration, 23 April 2021, URL: https://esahubble.org/images/potw2116a/

101) "Light bends from the beyond," ESA Science & Exploration, 16 April 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_12_-_16_April_2021

102) "Spiral snapshot," ESA Science & Exploration, 9 April 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_05_-_09_April_2021

103) "Space Telescope Science Institute Celebrates Its 40th Anniversary," NASA HubbleSite, 08 April 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-11

104) "Hubble Spots Double Quasars in Merging Galaxies," NASA HubbleSite, 06 April 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-14

105) Yue Shen, Yu-Ching Chen, Hsiang-Chih Hwang, Xin Liu, Nadia Zakamska, Masamune Oguri, Jennifer I-Hsiu Li, Joseph Lazio & Peter Breiding , "A hidden population of high-redshift double quasars unveiled by astrometry," Nature Astronomy Letter, Published: 01 April 2021, https://doi.org/10.1038/s41550-021-01323-1

106) "Hubble Revisits the Veil Nebula," NASA, 2 April 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-revisits-the-veil-nebula

107) "NASA has selected 24 new Fellows for its prestigious NASA Hubble Fellowship Program (NHFP)," NASA HubbleSite, 31 March 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-16

108) "A peculiar sight," ESA Science & Exploration, 26 March 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_22_-_26_March_2021

109) "A flash of life," ESA Science & Exploration, 19 March 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_15_-_19_March_2021

110) "Hubble Sees Changing Seasons on Saturn," NASA Feature, 18 March 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/saturn-season-change

111) "Hubble Shows Torrential Outflows from Infant Stars May Not Stop Them from Growing," NASA Feature, 18 March 2021, URL: https://www.nasa.gov/feature/goddard/2021/hubble-shows-torrential-outflows-from-infant-stars-may-not-stop-them-from-growing

112) Nolan M. Habel, S. Thomas Megeath, Joseph Jon Booker, William J. Fischer, Marina Kounkel, Charles Poteet, Elise Furlan, Amelia Stutz, P. Manoj, John J. Tobin, Zsofia Nagy, Riwaj Pokhrel, Dan Watson, "An HST Survey of Protostellar Outflow Cavities: Does Feedback Clear Envelopes?," The Astrophysical Journal, Draft version February 16, 2021, URL: https://arxiv.org/pdf/2102.06717.pdf

113) Jeff Foust, "Aging Hubble returns to operations after software glitch," Space News, 12 March 2021, URL: https://spacenews.com/aging-hubble-returns-to-operations-after-software-glitch/

114) "NASA's Hubble Space Telescope Resumes Science Operations," NASA, 12 March 2021, URL: https://www.nasa.gov/feature/goddard/2021/nasa-s-hubble-space-telescope-set-to-resume-science-operations

115) "Hubble Sees New Atmosphere Forming on a Rocky Exoplanet," heic2104 — Science Release, 11 March 2021, URL: https://esahubble.org/news/heic2104/?lang

116) "Distant Planet May Be on Its Second Atmosphere, NASA's Hubble Finds," NASA/JPL, 11 March 2021, URL: https://www.jpl.nasa.gov/news/distant-planet-may-be-on-its-second-atmosphere-nasas-hubble-finds?utm_source=iContact&utm_medium=email&utm_campaign=nasajpl&utm_content=Exoplanet20210311-1

117) "Big, beautiful and blue," ESA Science & Exploration, 05 March 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_01_-_05_March_2021

118) Claire Andreoli, Lynn Jenner, "Hubble Beholds a Big, Beautiful Blue Galaxy," NASA, 5 March 2021, URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-beholds-a-big-beautiful-blue-galaxy

119) "Hubble Solves Mystery of Monster Star's Dimming," HubbleSite News, 04 March 2021, URL: https://hubblesite.org/contents/news-releases/2021/news-2021-09

120) Roberta M. Humphreys, Kris Davidson, A. M. S. Richards, L. M. Ziurys, Terry J. Jones, and Kazunori Ishibashi, "The Mass-loss History of the Red Hypergiant VY CMa*," The Astronomical Journal, Volume 161, Number 3, Published: 4 February 2021, https://doi.org/10.3847/1538-3881/abd316

121) Eye in the sky," ESA Science & Exploration, 26 February 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_22_-_26_February_2021

122) "Hubble Captures an Eye in the Sky – "Evil Eye" Galaxy With Strange Internal Motion," SciTech Daily, 21 February, 2021, URL: https://scitechdaily.com/hubble-captures-an-eye-in-the-sky-evil-eye-galaxy-with-strange-internal-motion/

123) "Comet Makes a Pit Stop Near Jupiter's Asteroids," NASA/JPL News, 25 February 2021, URL: https://www.jpl.nasa.gov/news/comet-makes-a-pit-stop-near-jupiters-asteroids?utm_source=iContact&utm_medium=email&utm_campaign=nasajpl&utm_content=hubble20210225-1

124) Bryce T. Bolin, Yanga R. Fernandez, Carey M. Lisse, Timothy R. Holt, Zhong-Yi Lin, Josiah N. Purdum, Kunal P. Deshmukh, James M. Bauer, Eric C. Bellm, Dennis Bodewits, Kevin B. Burdge, Sean J. Carey, Chris M. Copperwheat, George Helou, Anna Y. Q. Ho, Jonathan Horner, Jan van Roestel, Varun Bhalerao, Chan-Kao Chang, Christine Chen, Chen-Yen Hsu, Wing-Huen Ip, Mansi M. Kasliwal, Frank J. Masci, Chow-Choong Ngeow, Robert Quimby, Rick Burruss, Michael Coughlin, Richard Dekany, Alexandre Delacroix, Andrew Drake, Dmitry A. Duev, Matthew Graham, David Hale, Thomas Kupfer, Russ R. Laher, Ashish Mahabal, Przemyslaw J. Mróz, James D. Neill, Reed Riddle, Hector Rodriguez, Roger M. Smith, Maayane T. Soumagnac, Richard Walters, Lin Yan, and Jeffry Zolkower, "Initial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), Using Hubble, Spitzer, ZTF, Keck, Apache Point Observatory, and GROWTH Visible and Infrared Imaging and Spectroscopy," The Astronomical Journal, Volume 161, No 3, Published: 11 February 2021, https://doi.org/10.3847/1538-3881/abd94b

125) "Tantrums of a baby star," ESA Science & Exploration, 19 February 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_15_-_19_February_2021

126) A cosmic hourglass," ESA Science & Exploration, 12 February 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_08_-_12_February_2021

127) "Hubble Uncovers Concentration of Small Black Holes," heic2103 — Science Release, 11 February 2021, URL: https://esahubble.org/news/heic2103/?lang

128) Eduardo Vitral and Gary A. Mamon, "Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?," Astronomy & Astrophysics, Volume 646, Published: 11 February 2021, Article No: A63, https://doi.org/10.1051/0004-6361/202039650

129) "The modest galaxy," ESA Science & Exploration, 5 February 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_01_-_05_February_2021

130) "An interstellar distributor," ESA Science & Exploration, 29 January 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_25_-_29_January_2021

131) "Colors of the lost galaxy," ESA Science & Exploration, 22 January 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_18_-_22_January_2021

132) "Hubble Takes Portrait of the ‘Lost Galaxy'," NASA, 21 January 2021: URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-takes-portrait-of-the-lost-galaxy

133) Janice C. Lee,Bradley C. Whitmore,David A. Thilker, Sinan Deger, Kirsten L. Larson, Leonardo Ubeda,Gagandeep S. Anand, Mederic Boquien, Rupali Chandar,Daniel A. Dale,Eric Emsellem, Adam K. Leroy, Erik Rosolowsky, Eva Schinnerer, Judy Schmidt, Jordan Turner, Schuyler Van Dyk, Richard L. White, Ashley T. Barnes, Francesco Belfiore, Frank Bigiel, Guillermo A. Blanc, Yixian Cao, Melanie Chevance, Enrico Congiu, Oleg V. Egorov, Simon C. O. Glover, Kathryn Grasha, Brent Groves, Jonathan Henshaw, Annie Hughes, Ralf S. Klessen, Eric Koch, Kathryn Kreckel, J. M. Diederik Kruijssen, Daizhong Liu, Laura A. Lopez, Ness Mayker, Sharon E. Meidt, Eric J. Murphy, Hsi-An Pan, J éróme Pety, Miguel Querejeta, Alessandro Razza, Toshiki Saito, Patricia S ánchez-Bl ázquez, Francesco Santoro, Amy Sardone, Fabian Scheuermann, Andreas Schruba, Jiayi Sun, Antonio Usero, E. Watkins, and Thomas G. Williams, "The PHANGS-HST Survey:Physics at High Angular resolution in Nearby GalaxieS with the Hubble Space Telescope," Astrophysics of Galaxies, Draft version January 11, 2021, https://arxiv.org/pdf/2101.02855.pdf

134) "Astronomers dissect the anatomy of planetary nebulae using Hubble Space Telescope images, Researchers from RIT and Green Bank Observatory shed new light on nebula formation processes," RIT News, 15 January 2021, URL: https://www.rit.edu/news/astronomers-dissect-anatomy-planetary-nebulae-using-hubble-space-telescope-images

135) "Sculpted in sculptor," ESA Science & Exploration, 15 January 2021, URL: https://www.esa.int/About_Us/Week_in_images/Week_in_images_11_-_15_January_2021

136) Claire Andreoli, Donna Weaver, Ray Villard, John Banovetz, Danny Milisavljevic, "Researchers Rewind the Clock to Calculate Age and Site of Supernova Blast," NASA Feature, 14 January 2021, URL: https://www.nasa.gov/feature/goddard/2021/researchers-rewind-the-clock-to-calculate-age-and-site-of-supernova-blast

137) "Hubble Pinpoints Supernona Blast," ESA Hubble heic2102 — Photo Release, 15 January 2021, URL: https://esahubble.org/news/heic2102/?lang

138) Lynn Jenner, "Hubble Views a Dazzling ‘Fireworks Galaxy'," NASA Feature, 8 January 2021: URL: https://www.nasa.gov/image-feature/goddard/2021/hubble-views-a-dazzling-fireworks-galaxy/

139) "When Galaxies Collide: Hubble Showcases 6 Beautiful Galaxy Mergers," Hubble, heic2101 — Photo Release, 7 January 2021, URL: https://esahubble.org/news/heic2101/

140) A. Adamo, K. Hollyhead, M. Messa, J. E. Ryon, V. Bajaj, A. Runnholm, S. Aalto, D. Calzetti, J. S. Gallagher, M. J. Hayes, J. M. D. Kruijssen, S. König, S. S. Larsen, J. Melinder, E. Sabbi, L. J. Smith, G. Östlin, "Star cluster formation in the most extreme environments: insights from the HiPEEC survey," MNRAS, Volume 499, Issue 3, December 2020, Pages 3267–3294, https://doi.org/10.1093/mnras/staa2380
 

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 (eoportal@symbios.space).