Voyager grand tour mission of the solar systemCSIRO supporting NASA References
• May 18, 2022: While the spacecraft continues to return science data and otherwise operate as normal, the mission team is searching for the source of a system data issue. 1)
Figure 1: NASA’s Voyager 1 spacecraft, shown in this illustration, has been exploring our solar system since 1977, along with its twin, Voyager 2 (image credit: NASA/JPL-Caltech)
- The engineering team with NASA’s Voyager 1 spacecraft is trying to solve a mystery: The interstellar explorer is operating normally, receiving and executing commands from Earth, along with gathering and returning science data. But readouts from the probe’s attitude articulation and control system (AACS) don’t reflect what’s actually happening onboard.
- The AACS controls the 45-year-old spacecraft’s orientation. Among other tasks, it keeps Voyager 1’s high-gain antenna pointed precisely at Earth, enabling it to send data home. All signs suggest the AACS is still working, but the telemetry data it’s returning is invalid. For instance, the data may appear to be randomly generated, or does not reflect any possible state the AACS could be in.
- The issue hasn’t triggered any onboard fault protection systems, which are designed to put the spacecraft into “safe mode” – a state where only essential operations are carried out, giving engineers time to diagnose an issue. Voyager 1’s signal hasn’t weakened, either, which suggests the high-gain antenna remains in its prescribed orientation with Earth.
- The team will continue to monitor the signal closely as they continue to determine whether the invalid data is coming directly from the AACS or another system involved in producing and sending telemetry data. Until the nature of the issue is better understood, the team cannot anticipate whether this might affect how long the spacecraft can collect and transmit science data.
- Voyager 1 is currently 14.5 billion miles (23.3 billion km) from Earth, and it takes light 20 hours and 33 minutes to travel that difference. That means it takes roughly two days to send a message to Voyager 1 and get a response – a delay the mission team is well accustomed to.
- “A mystery like this is sort of par for the course at this stage of the Voyager mission,” said Suzanne Dodd, project manager for Voyager 1 and 2 at NASA’s Jet Propulsion Laboratory in Southern California. “The spacecraft are both almost 45 years old, which is far beyond what the mission planners anticipated. We’re also in interstellar space – a high-radiation environment that no spacecraft have flown in before. So there are some big challenges for the engineering team. But I think if there’s a way to solve this issue with the AACS, our team will find it.”
- It’s possible the team may not find the source of the anomaly and will instead adapt to it, Dodd said. If they do find the source, they may be able to solve the issue through software changes or potentially by using one of the spacecraft’s redundant hardware systems.
- It wouldn’t be the first time the Voyager team has relied on backup hardware: In 2017, Voyager 1’s primary thrusters showed signs of degradation, so engineers switched to another set of thrusters that had originally been used during the spacecraft’s planetary encounters. Those thrusters worked, despite having been unused for 37 years.
- Voyager 1’s twin, Voyager 2 (currently 12.1 billion miles, or 19.5 billion km, from Earth), continues to operate normally.
- Launched in 1977, both Voyagers have operated far longer than mission planners expected, and are the only spacecraft to collect data in interstellar space. The information they provide from this region has helped drive a deeper understanding of the heliosphere, the diffuse barrier the Sun creates around the planets in our solar system.
- Each spacecraft produces about 4 fewer watts of electrical power a year, limiting the number of systems the craft can run. The mission engineering team has switched off various subsystems and heaters in order to reserve power for science instruments and critical systems. No science instruments have been turned off yet as a result of the diminishing power, and the Voyager team is working to keep the two spacecraft operating and returning unique science beyond 2025.
- While the engineers continue to work at solving the mystery that Voyager 1 has presented them, the mission’s scientists will continue to make the most of the data coming down from the spacecraft’s unique vantage point.
• May 10, 2021: Until recently, every spacecraft in history had made all of its measurements inside our heliosphere, the magnetic bubble inflated by our Sun. But on August 25, 2012, NASA’s Voyager 1 changed that. As it crossed the heliosphere’s boundary, it became the first human-made object to enter – and measure – interstellar space. Now eight years into its interstellar journey, Voyager 1’s data is yielding new insights into what that frontier is like. 2)
- If our heliosphere is a ship sailing interstellar waters, Voyager 1 is a life raft just dropped from the deck, determined to survey the currents. For now, any rough waters it feels are mostly from our heliosphere’s wake. But farther out, it will sense the stirrings from sources deeper in the cosmos. Eventually, our heliosphere’s presence will fade from its measurements completely.
- “We have some ideas about how far Voyager will need to get to start seeing more pure interstellar waters, so to speak,” said Stella Ocker, a Ph.D. student at Cornell University in Ithaca, New York, and the newest member of the Voyager team. “But we're not entirely sure when we'll reach that point.”
Figure 2: Illustration of NASA's Voyager spacecraft showing the antennas used by the Plasma Wave Subsystem and other instruments (image credit: NASA/JPL-Caltech)
- Ocker’s new study, published on Monday in Nature Astronomy, reports what may be the first continuous measurement of the density of material in interstellar space. “This detection offers us a new way to measure the density of interstellar space and opens up a new pathway for us to explore the structure of the very nearby interstellar medium,” Ocker said. 3)
- When one pictures the stuff between the stars – astronomers call it the “interstellar medium,” a spread-out soup of particles and radiation – one might imagine a calm, silent, serene environment. That would be a mistake.
- “I have used the phrase ‘the quiescent interstellar medium’ – but you can find lots of places that are not particularly quiescent,” said Jim Cordes, space physicist at Cornell and co-author of the paper.
- Like the ocean, the interstellar medium is full of turbulent waves. The largest come from our galaxy’s rotation, as space smears against itself and sets forth undulations tens of light-years across. Smaller (though still gigantic) waves rush from supernova blasts, stretching billions of miles from crest to crest. The smallest ripples are usually from our own Sun, as solar eruptions send shockwaves through space that permeate our heliosphere’s lining.
- These crashing waves reveal clues about the density of the interstellar medium – a value that affects our understanding of the shape of our heliosphere, how stars form, and even our own location in the galaxy. As these waves reverberate through space, they vibrate the electrons around them, which ring out at characteristic frequencies depending on how crammed together they are. The higher the pitch of that ringing, the higher the electron density. Voyager 1’s Plasma Wave Subsystem – which includes two “bunny ear” antennas sticking out 30 feet (10 meters) behind the spacecraft – was designed to hear that ringing.
- In November 2012, three months after exiting the heliosphere, Voyager 1 heard interstellar sounds for the first time. Six months later, another “whistle” appeared – this time louder and even higher pitched. The interstellar medium appeared to be getting thicker, and quickly.
Figure 3: NASA's Voyager 1 spacecraft captured these sounds of interstellar space. Voyager 1's plasma wave instrument detected the vibrations of dense interstellar plasma, or ionized gas, from October to November 2012 and April to May 2013. The graphic shows the frequency of the waves, which indicate the density of the plasma. Colors indicate the intensity of the waves, or how "loud" they are. Red indicates the loudest waves and blue indicates the weakest. The soundtrack reproduces the amplitude and frequency of the plasma waves as "heard" by Voyager 1. The waves detected by the instrument antennas can be simply amplified and played through a speaker. These frequencies are within the range heard by human ears. Scientists noticed that each occurrence involved a rising tone. The dashed line indicates that the rising tones follow the same slope. This means a continuously increasing density (video credit: NASA/JPL-Caltech/University of Iowa)
- These momentary whistles continue at irregular intervals in Voyager’s data today. They’re an excellent way to study the interstellar medium’s density, but it does take some patience.
- “They've only been seen about once a year, so relying on these kind of fortuitous events meant that our map of the density of interstellar space was kind of sparse,” Ocker said.
- Ocker set out to find a running measure of interstellar medium density to fill in the gaps – one that doesn’t depend on the occasional shockwaves propagating out from the Sun. After filtering through Voyager 1’s data, looking for weak but consistent signals, she found a promising candidate. It started to pick up in mid-2017, right around the time of another whistle.
- “It’s virtually a single tone,” said Ocker. “And over time, we do see it change – but the way the frequency moves around tells us how the density is changing.”
Figure 4: Looking at signals only barely louder than noise, Ocker found a weak but nearly continuous signal — visible as a thin red line — connecting stronger plasma oscillation events in Voyager 1's Plasma Wave Subsystem data. This graph shows the filtered data with the weaker signals including the plasma wave emission (image credits: NASA's Voyager 1 Plasma Wave Subsystem/Stella Ocker)
Figure 5: This graph shows only the strong signals (blue background), image credits: NASA's Voyager 1 Plasma Wave Subsystem/Stella Ocker
- Ocker calls the new signal a plasma wave emission, and it, too, appeared to track the density of interstellar space. When the abrupt whistles appeared in the data, the tone of the emission rises and falls with them. The signal also resembles one observed in Earth’s upper atmosphere that’s known to track with the electron density there.
- “This is really exciting, because we are able to regularly sample the density over a very long stretch of space, the longest stretch of space that we have so far,” said Ocker. “This provides us with the most complete map of the density and the interstellar medium as seen by Voyager.”
- Based on the signal, electron density around Voyager 1 started rising in 2013 and reached its current levels about mid-2015, a roughly 40-fold increase in density. The spacecraft appears to be in a similar density range, with some fluctuations, through the entire dataset they analyzed which ended in early 2020.
- Ocker and her colleagues are currently trying to develop a physical model of how the plasma wave emission is produced that will be key to interpreting it. In the meantime, Voyager 1’s Plasma Wave Subsystem keeps sending back data farther and farther from home, where every new discovery has the potential to make us re-imagine our home in the cosmos.
• December 3, 2020: More than 40 years since they were launched, the Voyager spacecraft are still making discoveries. 4)
- In a new study, a team of physicists led by the University of Iowa, report the first detection of bursts of cosmic ray electrons accelerated by shock waves originating from major eruptions on the sun. The detection, made by instruments onboard both the Voyager 1 and Voyager 2 spacecraft, occurred as the Voyagers continue their journey outward through interstellar space, making them the first craft to record this unique phenomena in the realm between stars. 5)
- These newly detected electron bursts travel at nearly the speed of light, some 670 times faster than the shock waves that initially propelled them. The bursts were followed by plasma wave oscillations caused by lower-energy electrons arriving at the Voyagers’ instruments days later—and finally, in some cases, the shock wave itself as long as a month after that.
- The shock waves emanated from coronal mass ejections, which are expulsions of hot gas and energy that move outward from the sun at about 1 million mph. Even at that speed, it takes more than a year for the shock waves to reach the Voyager spacecraft, which have traveled further from the sun (more than 14 billion miles and counting) than any other human-made object.
- “What we see here, specifically, is a certain mechanism whereby when the shock wave first contacts the interstellar magnetic field lines passing through the spacecraft, it reflects and accelerates some of the cosmic ray electrons,” says Don Gurnett, professor emeritus in the Department of Physics and Astronomy and the study’s corresponding author. “We have identified through the cosmic ray instruments these are electrons that were reflected and accelerated by interstellar shocks propagating outward from energetic solar events at the sun. That is a new mechanism.”
- The discovery could help physicists better understand the dynamics of shock waves and cosmic radiation that come from flare stars (which can vary in brightness briefly due to violent activity on their surface) and exploding stars. The physics of such phenomena would be important to consider when sending astronauts on extended lunar or Martian excursions, for instance, during which they would be exposed to concentrations of cosmic rays far exceeding what can be experienced on Earth.
- The physicists believe these electrons in the interstellar medium are reflected off of a strengthened magnetic field at the edge of the shock wave and subsequently accelerated by the motion of the shock wave. The reflected electrons then spiral along interstellar magnetic field lines, gaining speed as the distance between them and the shock increases.
- In a 2014 paper in the journal Astrophysical Letters, physicists J. R. Jokipii and Jozsef Kota described theoretically how ions reflected from shock waves could be accelerated along interstellar magnetic field lines. The current study looks at bursts of electrons detected by the Voyager spacecraft that are thought to be accelerated by a similar process.
- “The idea that shock waves accelerate particles is not new,” Gurnett says. “It all has to do with how it works, the mechanism. And the fact we detected it in a new realm—the interstellar medium—which is much different than in the solar wind where similar processes have been observed. No one has seen it with an interstellar shock wave in a whole new pristine medium.”
• November 2, 2020: The only radio antenna that can command the 43-year-old spacecraft has been offline since March as it gets new hardware, but work is on track to wrap up in February. 6)
- On Oct. 29, 2020, mission operators sent a series of commands to NASA's Voyager 2 spacecraft for the first time since mid-March. The spacecraft has been flying solo while the 70-meter-wide (230-foot-wide) radio antenna used to talk to it has been offline for repairs and upgrades. Voyager 2 returned a signal confirming it had received the "call" and executed the commands without issue.
Figure 6: Crews conduct critical upgrades and repairs to the 70-meter-wide (230-foot-wide) radio antenna Deep Space Station 43 in Canberra, Australia. In this image, one of the antenna's white feed cones (which house portions of the antenna receivers) is being moved by a crane (image credit: CSIRO)
- The call to Voyager 2 was a test of new hardware recently installed on Deep Space Station 43, the only dish in the world that can send commands to Voyager 2. Located in Canberra, Australia, it is part of NASA's DSN (Deep Space Network), a collection of radio antennas around the world used primarily to communicate with spacecraft operating beyond the Moon. Since the dish went offline, mission operators have been able to receive health updates and science data from Voyager 2, but they haven't been able to send commands to the far-flung probe, which has traveled billions of miles from Earth since its 1977 launch.
- Among the upgrades to DSS43, as the dish is known, are two new radio transmitters. One of them, which is used to talk with Voyager 2, hasn't been replaced in over 47 years. Engineers have also upgraded heating and cooling equipment, power supply equipment, and other electronics needed to run the new transmitters.
- The successful call to Voyager 2 is just one indication that the dish will be back online in February 2021.
- "What makes this task unique is that we're doing work at all levels of the antenna, from the pedestal at ground level all the way up to the feedcones at the center of the dish that extend above the rim," said Brad Arnold, the DSN project manager at NASA's Jet Propulsion Lab in Southern California. "This test communication with Voyager 2 definitely tells us that things are on track with the work we're doing."
- The Deep Space Network consist of radio antenna facilities spaced equally around the globe in Canberra; Goldstone, California; and Madrid, Spain. The positioning of the three facilities ensures that almost any spacecraft with a line of sight to Earth can communicate with at least one of the facilities at any time.
- Voyager 2 is the rare exception. In order to make a close flyby of Neptune's moon Triton in 1989, the probe flew over the planet's north pole. That trajectory deflected it southward relative to the plane of the planets, and it has been heading in that direction ever since. Now more than 11.6 billion miles (18.8 billion kilometers) from Earth, the spacecraft is so far south that it doesn't have a line of sight with radio antennas in the Northern Hemisphere.
- DSS43 is the only dish in the Southern Hemisphere that has a transmitter powerful enough and that broadcasts the right frequency to send commands to the distant spacecraft. Voyager 2's faster-moving twin, Voyager 1, took a different path past Saturn and can communicate via antennas at the two DSN facilities in the Northern Hemisphere. The antennas must uplink commands to both Voyagers in a radio frequency range called S-band, and the antennas downlink data from the spacecraft in a range called X-band.
- While mission operators haven't been able to command Voyager 2 since DSS43 went offline, the three 34-meter-wide (111-foot-wide) radio antennas at the Canberra facility can be used together to capture the signals that Voyager 2 sends to Earth. The probe is sending back science data from interstellar space, or the region outside our Sun's heliosphere - the protective bubble of particles and magnetic fields created by the Sun that surrounds the planets and the Kuiper Belt (the collection of small, icy bodies beyond Neptune's orbit).
- DSS43 began operating in 1972 (five years before the launch of Voyager 2 and Voyager 1) and was only 64 meters (210 feet) wide at the time. It was expanded to 70 meters (230 feet) in 1987 and has received a variety of upgrades and repairs since then. But the engineers overseeing the current work say this is one of the most significant makeovers the dish has received and the longest it's been offline in over 30 years.
- "The DSS43 antenna is a highly specialized system; there are only two other similar antennas in the world, so having the antenna down for one year is not an ideal situation for Voyager or for many other NASA missions," said Philip Baldwin, operations manager for NASA's Space Communications and Navigation (SCaN) Program. "The agency made the decision to conduct these upgrades to ensure that the antenna can continue to be used for current and future missions. For an antenna that is almost 50 years old, it's better to be proactive than reactive with critical maintenance."
- The repairs will benefit other missions, including the Mars Perseverance rover, which will land on the Red Planet Feb. 18, 2021. The network will also play a critical role in Moon to Mars exploration efforts, ensuring communication and navigation support for both the precursor Moon and Mars missions and the crewed Artemis missions.
- The Deep Space Network is managed by JPL for the SCaN Program, located at NASA Headquarters within the Human Exploration and Operations Mission Directorate. The Canberra station is managed on NASA's behalf by Australia's national science agency, the Commonwealth Scientific and Industrial Research Organisation.
- The Voyager spacecraft were built by JPL, which continues to operate both. JPL is a division of Caltech in Pasadena. The Voyager missions are a part of the NASA Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate in Washington.
• March 25, 2020: Eight and a half years into its grand tour of the solar system, NASA's Voyager 2 spacecraft was ready for another encounter. It was January 24, 1986, and soon it would meet the mysterious seventh planet, icy-cold Uranus. 7)
- Over the next few hours, Voyager 2 flew within 50,600 miles (81,433 km) of Uranus' cloud tops, collecting data that revealed two new rings, 11 new moons and temperatures below minus 353º Fahrenheit (minus 214º Celsius). The dataset is still the only up-close measurements we have ever made of the planet.
- Three decades later, scientists reinspecting that data found one more secret.
- Unbeknownst to the entire space physics community, 34 years ago Voyager 2 flew through a plasmoid, a giant magnetic bubble that may have been whisking Uranus' atmosphere out to space. The finding, reported in Geophysical Research Letters, raises new questions about the planet's one-of-a-kind magnetic environment. 8)
A Wobbly Magnetic Oddball
- Planetary atmospheres all over the solar system are leaking into space. Hydrogen springs from Venus to join the solar wind, the continuous stream of particles escaping the Sun. Jupiter and Saturn eject globs of their electrically-charged air. Even Earth's atmosphere leaks. (Don't worry, it will stick around for another billion years or so.)
- The effects are tiny on human timescales, but given long enough, atmospheric escape can fundamentally alter a planet's fate. For a case in point, look at Mars.
- "Mars used to be a wet planet with a thick atmosphere," said Gina DiBraccio, space physicist at NASA's Goddard Space Flight Center and project scientist for the MAVEN (Mars Atmosphere and Volatile Evolution) mission. "It evolved over time" - 4 billion years of leakage to space - "to become the dry planet we see today."
- Atmospheric escape is driven by a planet's magnetic field, which can both help and hinder the process. Scientists believe magnetic fields can protect a planet, fending off the atmosphere-stripping blasts of the solar wind. But they can also create opportunities for escape, like the giant globs cut loose from Saturn and Jupiter when magnetic field lines become tangled. Either way, to understand how atmospheres change, scientists pay close attention to magnetism.
- That's one more reason Uranus is such a mystery. Voyager 2's 1986 flyby revealed just how magnetically weird the planet is.
"The structure, the way that it moves ... ," DiBraccio said, "Uranus is really on its own."
- Unlike any other planet in our solar system, Uranus spins almost perfectly on its side - like a pig on a spit roast - completing a barrel roll once every 17 hours. Its magnetic field axis points 60 degrees away from that spin axis, so as the planet spins, its magnetosphere - the space carved out by its magnetic field - wobbles like a poorly thrown football. Scientists still don't know how to model it.
- This oddity drew DiBraccio and her coauthor Dan Gershman, a fellow Goddard space physicist, to the project. Both were part of a team working out plans for a new mission to the "ice giants" Uranus and Neptune, and they were looking for mysteries to solve.
- Uranus' strange magnetic field, last measured more than 30 years ago, seemed like a good place to start.
- So they downloaded Voyager 2's magnetometer readings, which monitored the strength and direction of the magnetic fields near Uranus as the spacecraft flew by. With no idea what they'd find, they zoomed in closer than previous studies, plotting a new datapoint every 1.92 seconds. Smooth lines gave way to jagged spikes and dips. And that's when they saw it: a tiny zigzag with a big story.
- "Do you think that could be ... a plasmoid?" Gershman asked DiBraccio, catching sight of the squiggle.
- Little known at the time of Voyager 2's flyby, plasmoids have since become recognized as an important way planets lose mass. These giant bubbles of plasma, or electrified gas, pinch off from the end of a planet's magnetotail - the part of its magnetic field blown back by the Sun like a windsock. With enough time, escaping plasmoids can drain the ions from a planet's atmosphere, fundamentally changing its composition.
- They had been observed at Earth and other planets, but no one had detected plasmoids at Uranus - yet.
- DiBraccio ran the data through her processing pipeline, and the results came back clean. "I think it definitely is," she said.
The Bubble Escapes
- The plasmoid DiBraccio and Gershman found occupied a mere 60 seconds of Voyager 2's 45-hour-long flight by Uranus. It appeared as a quick up-down blip in the magnetometer data. "But if you plotted it in 3D, it would look like a cylinder," Gershman said.
Figure 7: Voyager 2 took this image as it approached the planet Uranus on 14 January 1986. The planet's hazy bluish color is due to the methane in its atmosphere, which absorbs red wavelengths of light (image credit: NASA/JPL-Caltech)
- Comparing their results to plasmoids observed at Jupiter, Saturn and Mercury, they estimated a cylindrical shape at least 127,000 miles (204,000 km) long, and up to roughly 250,000 miles (400,000 km) across. Like all planetary plasmoids, it was full of charged particles - mostly ionized hydrogen, the authors believe.?
- Readings from inside the plasmoid - as Voyager 2 flew through it - hinted at its origins. Whereas some plasmoids have a twisted internal magnetic field, DiBraccio and Gershman observed smooth, closed magnetic loops. Such loop-like plasmoids are typically formed as a spinning planet flings bits of its atmosphere to space. "Centrifugal forces take over, and the plasmoid pinches off," Gershman said. According to their estimates, plasmoids like that one could account for between 15% and 55% of atmospheric mass loss at Uranus, a greater proportion than either Jupiter or Saturn. It may well be the dominant way Uranus sheds its atmosphere to space.
- How has plasmoid escape changed Uranus over time? With only one set of observations, it's hard to say.
- "Imagine if one spacecraft just flew through this room and tried to characterize the entire Earth," DiBraccio said. "Obviously it's not going to show you anything about what the Sahara or Antarctica is like."
- But the findings help focus new questions about the planet. The remaining mystery is part of the draw. "It's why I love planetary science," DiBraccio said. "You're always going somewhere you don't really know."
- The twin Voyager spacecraft were built by and continue to be operated by NASA's Jet Propulsion Laboratory. JPL is a division of Caltech in Pasadena. The Voyager missions are a part of the NASA Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate in Washington.
• February 12, 2020: Thirty years ago on Feb. 14, 1990, NASA’s Voyager 1 spacecraft sent home a very special Valentine: A mosaic of 60 images that was intended as what the Voyager team called the first “Family Portrait” of our solar system. 9)
The spacecraft was out beyond Neptune when mission managers commanded it to look back for a final time and snap images of the worlds it was leaving behind on its journey into interstellar space.
It captured Neptune, Uranus, Saturn, Jupiter, Earth and Venus. A few key members didn’t make the shot: Mars was obscured by scattered sunlight bouncing around in the camera, Mercury was too close to the Sun and dwarf planet Pluto was too tiny, too far away and too dark to be detected. But the images gave humans an awe-inspiring and unprecedented view of their home world and its neighbors.
One of those images, the picture of Earth, would become known as the “Pale Blue Dot.” The unique view of Earth as a tiny speck in the cosmos inspired the title of scientist Carl Sagan's book, "Pale Blue Dot: A Vision of the Human Future in Space,"
But the image almost didn’t happen.
Here are 10 things you might not know about Voyager 1’s famous Pale Blue Dot photo.
1) Not in the Plan
Neither the “Family Portrait” nor the “Pale Blue Dot” photo was planned as part of the original Voyager mission. In fact, the Voyager team turned down several requests to take the images because of limited engineering resources and potential danger to the cameras from pointing them close to the Sun. It took eight years and six requests to get approval for the images.
2) A Unique Perspective
Voyager 1 remains the first and only spacecraft that has attempted to photograph our solar system. Only three spacecraft have been capable of making such an observation: Voyager 1, Voyager 2 and New Horizons. (Pioneer 10 and Pioneer 11 — the other two spacecraft headed into interstellar space — had similar vantage points, but technical challenges prevented them from getting such a shot.)
Figure 8: This data visualization uses actual spacecraft trajectory data to show the family portrait image from Voyager 1's perspective in February 1990 (image credit: NASA/JPL-Caltech)
3) A Mote of Dust
The Voyager imaging team wanted show Earth’s vulnerability — to illustrate how fragile and irreplaceable it is — and demonstrate what a small place it occupies in the universe. Earth in the image is only about a single a pixel, a pale blue dot.
4) A Happy Coincidence
The image contains scattered light that resembles beams of sunlight, making the tiny Earth appear even more dramatic. In fact, these sunbeams are camera artifacts that resulted from the necessity of pointing the camera within a few degrees of the Sun.
Voyager 1 was 40 astronomical units from the Sun at the time so Earth appeared very near our brilliant star from Voyager's vantage point. One astronomical unit is 93 million miles, or 150 million kilometers That one of the rays of light happened to intersect with Earth was a happy coincidence.
5) Carl Sagan's Dream Shot
The prominent planetary scientist Carl Sagan (1934-1996) — a member of the Voyager imaging team — had the original idea to use Voyager’s cameras to image Earth in 1981, following the mission's encounters with Saturn. Sagan later wrote in poetic detail about the image and its meaning in his book, "Pale Blue Dot: A Vision of the Human Future in Space." — "Look again at that dot." Sagan wrote. "That's here. That's home. That's us.”
Figure 9: The Pale Blue Dot is a photograph of Earth taken Feb. 14, 1990, by NASA’s Voyager 1 at a distance of 3.7 billion miles (6 billion kilometers) from the Sun. The image inspired the title of scientist Carl Sagan's book, "Pale Blue Dot: A Vision of the Human Future in Space," in which he wrote: "Look again at that dot. That's here. That's home. That's us." (image credit: NASA/JPL-Caltech)
6) Cold Cameras
Voyager 1 powered up its cameras for the images on Feb. 13 and it took three hours for them to warm up. The spacecraft’s onboard tape recorder saved all the images taken, for later playback to Earth.
7) Light Time
The images of Earth snapped by Voyager 1 captured light that had left our planet five hours and 36 minutes earlier. (This was, of course, reflected sunlight that had left the Sun eight minutes before that.)
Voyager 1 was so far from Earth it took several communications passes with NASA's Deep Space Network, over a couple of months, to transmit all the data. The last of the image data were finally downloaded on Earth on May 1, 1990.
9) Another Unique Perspective
Voyager 1 also took the first image of the entire Earth and Moon together near the start of its mission on Sept. 18, 1977. The images were taken 13 days after launch at a distance of about 7.3 million miles (11.66 million kilometers) from Earth.
10) Parting Shot
After taking the images for “The Family Portrait” at 05:22 GMT on Feb. 14, 1990, Voyager 1 powered down its cameras forever. As of early 2020 the spacecraft is still operating, but no longer has the capability to take images.
• November 4, 2019: One year ago, on Nov. 5, 2018, NASA's Voyager 2 became only the second spacecraft in history to leave the heliosphere - the protective bubble of particles and magnetic fields created by our Sun. At a distance of about 11 billion miles (18 billion kilometers) from Earth - well beyond the orbit of Pluto - Voyager 2 had entered interstellar space, or the region between stars. Today, five new research papers in the journal Nature Astronomy describe what scientists observed during and since Voyager 2's historic crossing. 10)
- Each paper details the findings from one of Voyager 2's five operating science instruments: a magnetic field sensor, two instruments to detect energetic particles in different energy ranges and two instruments for studying plasma (a gas composed of charged particles). Taken together, the findings help paint a picture of this cosmic shoreline, where the environment created by our Sun ends and the vast ocean of interstellar space begins.
- The Sun's heliosphere is like a ship sailing through interstellar space. Both the heliosphere and interstellar space are filled with plasma, a gas that has had some of its atoms stripped of their electrons. The plasma inside the heliosphere is hot and sparse, while the plasma in interstellar space is colder and denser. The space between stars also contains cosmic rays, or particles accelerated by exploding stars. Voyager 1 discovered that the heliosphere protects Earth and the other planets from more than 70% of that radiation.
- When Voyager 2 exited the heliosphere last year, scientists announced that its two energetic particle detectors noticed dramatic changes: The rate of heliospheric particles detected by the instruments plummeted, while the rate of cosmic rays (which typically have higher energies than the heliospheric particles) increased dramatically and remained high. The changes confirmed that the probe had entered a new region of space.
- Before Voyager 1 reached the edge of the heliosphere in 2012, scientists didn't know exactly how far this boundary was from the Sun. The two probes exited the heliosphere at different locations and also at different times in the constantly repeating, approximately 11-year solar cycle, over the course of which the Sun goes through a period of high and low activity. Scientists expected that the edge of the heliosphere, called the heliopause, can move as the Sun's activity changes, sort of like a lung expanding and contracting with breath. This was consistent with the fact that the two probes encountered the heliopause at different distances from the Sun.
- The new papers now confirm that Voyager 2 is not yet in undisturbed interstellar space: Like its twin, Voyager 1, Voyager 2 appears to be in a perturbed transitional region just beyond the heliosphere.
- "The Voyager probes are showing us how our Sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy," said Ed Stone, project scientist for Voyager and a professor of physics at Caltech. "Without this new data from Voyager 2, we wouldn't know if what we were seeing with Voyager 1 was characteristic of the entire heliosphere or specific just to the location and time when it crossed."
Pushing Through Plasma
- The two Voyager spacecraft have now confirmed that the plasma in local interstellar space is significantly denser than the plasma inside the heliosphere, as scientists expected. Voyager 2 has now also measured the temperature of the plasma in nearby interstellar space and confirmed it is colder than the plasma inside the heliosphere.
- In 2012, Voyager 1 observed a slightly higher-than-expected plasma density just outside the heliosphere, indicating that the plasma is being somewhat compressed. Voyager 2 observed that the plasma outside the heliosphere is slightly warmer than expected, which could also indicate it is being compressed. (The plasma outside is still colder than the plasma inside.) Voyager 2 also observed a slight increase in plasma density just before it exited the heliosphere, indicating that the plasma is compressed around the inside edge of the bubble. But scientists don't yet fully understand what is causing the compression on either side.
- If the heliosphere is like a ship sailing through interstellar space, it appears the hull is somewhat leaky. One of Voyager's particle instruments showed that a trickle of particles from inside the heliosphere is slipping through the boundary and into interstellar space. Voyager 1 exited close to the very "front" of the heliosphere, relative to the bubble's movement through space. Voyager 2, on the other hand, is located closer to the flank, and this region appears to be more porous than the region where Voyager 1 is located.
Magnetic Field Mystery
- An observation by Voyager 2's magnetic field instrument confirms a surprising result from Voyager 1: The magnetic field in the region just beyond the heliopause is parallel to the magnetic field inside the heliosphere. With Voyager 1, scientists had only one sample of these magnetic fields and couldn't say for sure whether the apparent alignment was characteristic of the entire exterior region or just a coincidence. Voyager 2's magnetometer observations confirm the Voyager 1 finding and indicate that the two fields align, according to Stone.
- The Voyager probes launched in 1978, and both flew by Jupiter and Saturn. Voyager 2 changed course at Saturn in order to fly by Uranus and Neptune, performing the only close flybys of those planets in history. The Voyager probes completed their Grand Tour of the planets and began their Interstellar Mission to reach the heliopause in 1989. Voyager 1, the faster of the two probes, is currently over 13.6 billion miles (22 billion kilometers) from the Sun, while Voyager 2 is 11.3 billion miles (18.2 billion kilometers) from the Sun. It takes light about 16.5 hours to travel from Voyager 2 to Earth. By comparison, light traveling from the Sun takes about eight minutes to reach Earth.
• October 8, 2019: Out at the boundary of our solar system, pressure runs high. This pressure, the force plasma, magnetic fields and particles like ions, cosmic rays and electrons exert on one another when they flow and collide, was recently measured by scientists in totality for the first time — and it was found to be greater than expected. 11)
- Using observations of galactic cosmic rays — a type of highly energetic particle — from NASA’s Voyager spacecraft scientists calculated the total pressure from particles in the outer region of the solar system, known as the heliosheath. At nearly 9 billion miles away, this region is hard to study. But the unique positioning of the Voyager spacecraft and the opportune timing of a solar event made measurements of the heliosheath possible. And the results are helping scientists understand how the Sun interacts with its surroundings.
- “In adding up the pieces known from previous studies, we found our new value is still larger than what’s been measured so far,” said Jamie Rankin, lead author on the new study and astronomer at Princeton University in New Jersey. “It says that there are some other parts to the pressure that aren’t being considered right now that could contribute.”
Figure 10: An illustration depicting the layers of the heliosphere (image credit: NASA/IBEX/Adler Planetarium)
- On Earth we have air pressure, created by air molecules drawn down by gravity. In space there’s also a pressure created by particles like ions and electrons. These particles, heated and accelerated by the Sun create a giant balloon known as the heliosphere extending millions of miles out past Pluto. The edge of this region, where the Sun’s influence is overcome by the pressures of particles from other stars and interstellar space, is where the Sun’s magnetic influence ends. (Its gravitational influence extends much farther, so the solar system itself extends farther, as well.)
- In order to measure the pressure in the heliosheath, the scientists used the Voyager spacecraft, which have been travelling steadily out of the solar system since 1977. At the time of the observations, Voyager 1 was already outside of the heliosphere in interstellar space, while Voyager 2 still remained in the heliosheath.
- “There was really unique timing for this event because we saw it right after Voyager 1 crossed into the local interstellar space,” Rankin said. “And while this is the first event that Voyager saw, there are more in the data that we can continue to look at to see how things in the heliosheath and interstellar space are changing over time.”
Figure 11: The Voyager spacecraft, one in the heliosheath and the other just beyond in interstellar space, took measurements as a solar even known as a global merged interaction region passed by each spacecraft four months apart. These measurements allowed scientists to calculate the total pressure in the heliosheath, as well as the speed of sound in the region (image credit: NASA's Goddard Space Flight Center/Mary Pat Hrybyk-Keith)
- The scientists used an event known as a global merged interaction region, which is caused by activity on the Sun. The Sun periodically flares up and releases enormous bursts of particles, like in coronal mass ejections. As a series of these events travel out into space, they can merge into a giant front, creating a wave of plasma pushed by magnetic fields.
- When one such wave reached the heliosheath in 2012, it was spotted by Voyager 2. The wave caused the number of galactic cosmic rays to temporarily decrease. Four months later, the scientists saw a similar decrease in observations from Voyager 1, just across the solar system’s boundary in interstellar space.
- Knowing the distance between the spacecraft allowed them to calculate the pressure in the heliosheath as well as the speed of sound. In the heliosheath sound travels at around 300 km/second — a thousand times faster than it moves through air.
- The scientists noted that the change in galactic cosmic rays wasn’t exactly identical at both spacecraft. At Voyager 2 inside the heliosheath, the number of cosmic rays decreased in all directions around the spacecraft. But at Voyager 1, outside the solar system, only the galactic cosmic rays that were traveling perpendicular to the magnetic field in the region decreased. This asymmetry suggests that something happens as the wave transmits across the solar system’s boundary.
- “Trying to understand why the change in the cosmic rays is different inside and outside of the heliosheath remains an open question,” Rankin said.
- Studying the pressure and sound speeds in this region at the boundary of the solar system can help scientists understand how the Sun influences interstellar space. This not only informs us about our own solar system, but also about the dynamics around other stars and planetary systems.
• July 8, 2019: With careful planning and dashes of creativity, engineers have been able to keep NASA's Voyager 1 and 2 spacecraft flying for nearly 42 years - longer than any other spacecraft in history. To ensure that these vintage robots continue to return the best science data possible from the frontiers of space, mission engineers are implementing a new plan to manage them. And that involves making difficult choices, particularly about instruments and thrusters. 12)
- One key issue is that both Voyagers, launched in 1977, have less and less power available over time to run their science instruments and the heaters that keep them warm in the coldness of deep space. Engineers have had to decide what parts get power and what parts have to be turned off on both spacecraft. But those decisions must be made sooner for Voyager 2 than Voyager 1 because Voyager 2 has one more science instrument collecting data - and drawing power - than its sibling.
Figure 12: This artist's concept depicts one of NASA's Voyager spacecraft, including the location of the CRS (Cosmic Ray Subsystem) instrument. Both Voyagers launched with operating CRS instruments (image credit: NASA/JPL-Caltech)
- After extensive discussions with the science team, mission managers recently turned off a heater for the cosmic ray subsystem instrument (CRS) on Voyager 2 as part of the new power management plan. The cosmic ray instrument played a crucial role last November in determining that Voyager 2 had exited the heliosphere, the protective bubble created by a constant outflow (or wind) of ionized particles from the Sun. Ever since, the two Voyagers have been sending back details of how our heliosphere interacts with the wind flowing in interstellar space, the space between stars.
- Not only are Voyager mission findings providing humanity with observations of truly uncharted territory, but they help us understand the very nature of energy and radiation in space - key information for protecting NASA's missions and astronauts even when closer to home.
- Mission team members can now preliminarily confirm that Voyager 2's cosmic ray instrument is still returning data, despite dropping to a chilly minus 74 degrees Fahrenheit (minus 59 degrees Celsius). This is lower than the temperatures at which CRS was tested more than 42 years ago (down to minus 49 degrees Fahrenheit, or minus 45 degrees Celsius). Another Voyager instrument also continued to function for years after it dropped below temperatures at which it was tested.
- ”It's incredible that Voyagers' instruments have proved so hardy," said Voyager Project Manager Suzanne Dodd, who is based at NASA's Jet Propulsion Laboratory in Pasadena, California. "We're proud they've withstood the test of time. The long lifetimes of the spacecraft mean we're dealing with scenarios we never thought we'd encounter. We will continue to explore every option we have in order to keep the Voyagers doing the best science possible."
- Voyager 2 continues to return data from five instruments as it travels through interstellar space. In addition to the cosmic ray instrument, which detects fast-moving particles that can originate from the Sun or from sources outside our solar system, the spacecraft is operating two instruments dedicated to studying plasma (a gas in which atoms have been ionized and electrons float freely) and a magnetometer (which measures magnetic fields) for understanding the sparse clouds of material in interstellar space.
- Taking data from a range of directions, the low-energy charged particle instrument is particularly useful for studying the probe's transition away from our heliosphere. Because CRS can look only in certain fixed directions, the Voyager science team decided to turn off CRS's heater first.
- Voyager 1, which crossed into interstellar space in August 2012, continues to collect data from its cosmic ray instrument as well, plus from one plasma instrument, the magnetometer and the low-energy charged particle instrument.
Why Turn Off Heaters?
- Launched separately in 1977, the two Voyagers are now over 11 billion miles (18 billion kilometers) from the Sun and far from its warmth. Engineers have to carefully control temperature on both spacecraft to keep them operating. For instance, if fuel lines powering the thrusters that keep the spacecraft oriented were to freeze, the Voyagers' antennae could stop pointing at Earth. That would prevent engineers from sending commands to the spacecraft or receiving scientific data. So the spacecraft were designed to heat themselves.
- But running heaters - and instruments - requires power, which is constantly diminishing on both Voyagers.
- Each of the probes is powered by three RTGs (Radioisotope Thermoelectric Generators), which produce heat via the natural decay of plutonium-238 radioisotopes and convert that heat into electrical power. Because the heat energy of the plutonium in the RTGs declines and their internal efficiency decreases over time, each spacecraft is producing about 4 fewer watts of electrical power each year. That means the generators produce about 40% less than what they did at launch nearly 42 years ago, limiting the number of systems that can run on the spacecraft.
- The mission's new power management plan explores multiple options for dealing with the diminishing power supply on both spacecraft, including shutting off additional instrument heaters over the next few years.
Revving Up Old Jet Packs
- Another challenge that engineers have faced is managing the degradation of some of the spacecraft thrusters, which fire in tiny pulses, or puffs, to subtly rotate the spacecraft. This became an issue in 2017, when mission controllers noticed that a set of thrusters on Voyager 1 needed to give off more puffs to keep the spacecraft's antenna pointed at Earth. To make sure the spacecraft could continue to maintain proper orientation, the team fired up another set of thrusters on Voyager 1 that hadn't been used in 37 years.
- Voyager 2's current thrusters have started to degrade, too. Mission managers have decided to make the same thruster switch on that probe this month. Voyager 2 last used these thrusters (known as trajectory correction maneuver thrusters) during its encounter with Neptune in 1989.
Many Miles to Go Before They Sleep
- The engineers' plan to manage power and aging parts should ensure that Voyager 1 and 2 can continue to collect data from interstellar space for several years to come. Data from the Voyagers continue to provide scientists with never-before-seen observations of our boundary with interstellar space, complementing NASA's IBEX (Interstellar Boundary Explorer), a mission that is remotely sensing that boundary. NASA is also preparing IMAP (Interstellar Mapping and Acceleration Probe), due to launch in 2024,to capitalize on the Voyagers' observations.
- "Both Voyager probes are exploring regions never before visited, so every day is a day of discovery," said Voyager Project Scientist Ed Stone, who is based at Caltech. "Voyager is going to keep surprising us with new insights about deep space."
- The Voyager spacecraft were built by JPL, which continues to operate both. JPL is a division of Caltech in Pasadena. The Voyager missions are a part of the NASA Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate in Washington.
• May 22, 2019: Former Jet Propulsion Laboratory Director Edward Stone - currently the David Morrisroe Professor of Physics at Caltech and the project scientist for NASA's Voyager mission for the past 47 years - has been awarded the prestigious Shaw Prize in Astronomy "for his leadership in the Voyager project, which has, over the past four decades, transformed our understanding of the four giant planets and the outer solar system, and has now begun to explore interstellar space," according to the award citation. The prize comes with a monetary award of $1.2 million. 13)
Figure 13: Ed Stone stands before a full-size model of Voyager at JPL (image credit: NASA/JPL-Caltech)
- "This is a tremendous honor," said Stone, "and a tribute to the teams who designed, developed, launched and operated Voyager on an inspiring journey of more than four decades."
- Since 1972, Stone has served as the project scientist for the Voyager mission, twin spacecraft designed to tour the solar system and its farthest reaches. The Voyager mission is managed by JPL in Pasadena, California, which Caltech manages for NASA.
- Voyager 2 launched in August 1977, and Voyager 1 soon followed, launching in September 1977. Some of the mission's many highlights include the first high-resolution images of the four giant planets of our solar system (Jupiter, Saturn, Uranus and Neptune); the discovery of volcanoes on Jupiter's moon Io; the first images of rings of Jupiter, Uranus, and Neptune; and the discovery of gaps and other complex structures in Saturn's rings.
- In 2012, Voyager 1 became the first human-made object to cross into interstellar space, beyond the protective bubble, or heliosphere, that surrounds our solar system. Voyager 2 achieved this milestone more recently, in 2018. Both missions carry Golden Records of Earth sounds, music, images and messages.
- Stone was born in Knoxville, Iowa, on January 23, 1936. He graduated from Iowa's Burlington Junior College in 1956 and earned his Ph.D. in physics from the University of Chicago in 1964. Since the Voyager spacecraft launched in 1977, Stone has led and coordinated 11 instrument teams on the project. He also served as the director of JPL from 1991 to 2001, overseeing many space-based missions, including Cassini, and a program of Mars exploration that included Mars Pathfinder and its Sojourner rover.
- Stone also played a key role in the development of the W. M. Keck Observatory in Hawaii. In the mid 1980s through the 1990s, he served as a vice chairman and chairman of the board of directors of the California Association for Research in Astronomy, which is responsible for building and operating Keck. He is also on the board of the W. M. Keck Foundation. He is currently playing a similar role in the development of the planned Thirty Meter Telescope, an international partnership that includes the U.S., Canada, China, Japan and India.
- Stone came to Caltech in 1964 as a research fellow, joining the faculty as an assistant professor in 1967. He became the Morrisroe professor in 1994 and, in 2004, became the vice provost for special projects at Caltech.
- He has served as a principal investigator on nine missions and as a co-investigator on five additional missions. He has more than 1,000 publications in professional journals and conference proceedings, and has mentored a large number of students, postdocs, and research scientists. Stone is the recipient of numerous awards, including the President's National Medal of Science (1991), the Magellanic Premium (1992), the Carl Sagan Memorial Award (1999), the Philip J. Klass Award for Lifetime Achievement (2007), the NASA Distinguished Public Service Medal (2013) and the Howard Hughes Memorial Award (2014). He is a member of the National Academy of Sciences.
- The Shaw Prize is awarded annually in three categories: Astronomy, Life Science and Medicine, and Mathematical Sciences. It is an international award managed and administered by The Shaw Prize Foundation based in Hong Kong. Mr. Shaw has also founded The Sir Run Run Shaw Charitable Trust and The Shaw Foundation Hong Kong, both dedicated to the promotion of education, scientific and technological research, medical and welfare services, and culture and the arts.
- The 2019 Shaw laureates will receive their awards in Hong Kong at the ceremonial prize-giving on Wednesday, Sept. 25, 2019.
• March 27, 2019: By all means, Voyager 1 and Voyager 2 shouldn’t even be here. Now in interstellar space, they are pushing the limits of spacecraft and exploration, journeying through the cosmic neighborhood, giving us our first direct look into the space beyond our star. 14)
But when they launched in 1977, Voyager 1 and Voyager 2 had a different mission: to explore the outer solar system and gather observations directly at the source, from outer planets we had only seen with remote studies. But now, four decades after launch, they’ve journeyed farther than any other spacecraft from Earth; into the cold, quiet world of interstellar space.
Originally designed to measure the properties of the giant planets, the instruments on both spacecraft have spent the past few decades painting a picture of the propagation of solar events from our Sun. And the Voyagers' new mission focuses not only on effects on space from within our heliosphere — the giant bubble around the Sun filled up by the constant outflow of solar particles called the solar wind — but from outside of it. Though they once helped us look closer at the planets and their relationship to the Sun, they now give us clues about the nature of interstellar space as the spacecraft continue their journey.
The environment they explore is colder, subtler and more tenuous than ever before, and yet the Voyagers continue on, exploring and measuring the interstellar medium, a smorgasbord of gas, plasma and particles from stars and gas regions not originating from our system. Three of the spacecraft's 10 instruments are the major players that study how space inside the heliosphere differs from interstellar space. Looking at this data together allows scientist to piece together our best-yet picture of the edge of the heliosphere and the interstellar medium. Here are the stories they tell.
On the Sun Spot, we have been exploring the various instruments on Voyager 2 one at a time, and analyzing how scientists read the individual sets of data sent to Earth from the far-reaching spacecraft. But one instrument we have not yet talked about is Voyager 2’s Magnetometer, or MAG for short.
During the Voyagers' first planetary mission, the MAG was designed to investigate the magnetospheres of planets and their moons, determining the physical mechanics and processes of the interactions of those magnetic fields and the solar wind. After that mission ended, the Voyager spacecraft studied the magnetic field of the heliosphere and beyond, observing the magnetic reach of the Sun and the changes that occur within that reach during solar activity.
Getting the magnetic data as we travel further into space requires an interesting trick. Voyager spins itself around, in a calibration maneuver that allows Voyager to differentiate between the spacecraft's own magnetic field — that goes along for the ride as it spins — and the magnetic fields of the space it’s traveling through.
Figure 14: Illustration of NASA’s Voyager spacecraft, with the Magnetometer (MAG) instrument and its boom displayed (image credit: NASA’s Goddard Space Flight Center/Jet Propulsion Laboratory/Mary Pat Hrybyk-Keith)
The initial peek into the magnetic field beyond the Sun’s influence happened when Voyager 1 crossed the heliopause in 2012. Scientists saw that within the heliosphere, the strength of the magnetic field was quite variable, changing and jumping as Voyager 1 moved through the heliosphere. These changes are due to solar activity. But once Voyager 1 crossed into interstellar space, that variability was silenced. Although the strength of the field was similar to what it was inside the heliosphere, it no longer had the variability associated with the Sun’s outbursts.
Figure 15: Magnetometer (MAG) data taken from Voyager 1 during its transition into interstellar space in 2012 (image credit: NASA’s Goddard Space Flight Center/Jet Propulsion Laboratory)
This graph shows the magnitude, or the strength, of the magnetic field around the heliopause from January 2012 out to May 2014. Before encountering the heliopause, marked by the orange line, the magnetic strength fluctuates quite a bit. After a bumpy ride through the heliopause in 2012, the magnetic strength stops fluctuating and begins to stabilize in 2013, once the spacecraft is far enough out into the interstellar medium.
The Cosmic Ray Subsystem
Much like the MAG, the CRS (Cosmic Ray Subsystem) was originally designed to measure planetary systems. The CRS focused on the compositions of energetic particles in the magnetospheres of Jupiter, Saturn, Uranus and Neptune. Scientists used it to study the charged particles within the solar system and their distribution between the planets. Since it passed the planets, however, the CRS has been studying the heliosphere’s charged particles and — now — the particles in the interstellar medium.
The CRS measures the count rate, or how many particles detected per second. It does this by using two telescopes: the High Energy Telescope, which measures high energy particles (70MeV) identifiable as interstellar particles, and the Low Energy Telescope, which measures low-energy particles (5MeV) that originate from our Sun. You can think of these particles like a bowling ball hitting a bowling pin versus a bullet hitting the same pin — both will make a measurable impact on the detector, but they're moving at vastly different speeds. By measuring the amounts of the two kinds of particles, Voyager can provide a sense of the space environment it’s traveling through.
Figure 16: Illustration of NASA’s Voyager spacecraft, with the Cosmic Ray Subsystem (CRS) highlighted (image credit: NASA’s Goddard Space Flight Center/Jet Propulsion Laboratory/Mary Pat Hrybyk-Keith)
Figure 17: Scientists compared data from Voyager 1 with its 2012 crossing of the heliopause to watch for clue for when Voyager 2 would cross. In November 2018, the first clues came from the Cosmic Ray Subsystem! (image credit: NASA’s Jet Propulsion Laboratory/NASA Headquarters/Patrick Koehn)
These graphs show the count rate — how many particles per second are interacting with the CRS on average each day — of the galactic ray particles measured by the High Energy Telescope (top graph) and the heliospheric particles measured by the Low Energy Telescope (bottom graph). The line in red shows the data from Voyager 1, time shifted forward 6.32 years from 2012 to match up with the data from Voyager around November 2018, shown in blue.
CRS data from Voyager 2 on Nov. 5, 2018, showed the interstellar particle count rate of the High Energy Telescope increasing to count rates similar to what Voyager 1 saw then leveling out. Similarly, the Low Energy Telescope shows a severe decrease in heliospheric originating particles. This was a key indication that Voyager 2 had moved into interstellar space. Scientists can keep watching these counts to see if the composition of interstellar space particles changes along the journey.
The Plasma Instrument
The PLS (Plasma Science) instrument was made to measure plasma and ionized particles around the outer planets and to measure the solar wind’s influence on those planets. The PLS is made up of four Faraday cups, an instrument that measures the plasma as it passes through the cups and calculates the plasma’s speed, direction and density.
The plasma instrument on Voyager 1 was damaged during a fly-by of Saturn and had to be shut off long before Voyager 1 exited the heliosphere, making it unable to measure the interstellar medium’s plasma properties. With Voyager 2's crossing, scientists will get the first-ever plasma measurements of the interstellar medium.
Figure 18: Illustration of NASA’s Voyager spacecraft, with the PLS (Plasma Science) instrument displayed (image credit: NASA’s Goddard Space Flight Center/Jet Propulsion Laboratory/Mary Pat Hrybyk-Keith)
Scientists predicted that interstellar plasma measured by Voyager 2 would be higher in density but lower in temperature and speed than plasma inside the heliosphere. And in November 2018, the instrument saw just that for the first time. This suggests that the plasma in this region is getting colder and slower, and, like cars slowing down on a freeway, is beginning to pile up around the heliopause and into the interstellar medium.
And now, thanks to Voyager 2’s PLS, we have a never-before-seen perspective on our heliosphere: The plasma velocity from Earth to the heliopause.
Figure 19: With Voyager 2 crossing the heliopause, scientists now have a new view of solar wind plasma across the heliosphere (image credit: NASA's Jet Propulsion Laboratory/ Michigan Institute of Technology/John Richardson)
These three graphs tell an amazing story, summarizing a journey of 42 years in one plot. The top section of this graph shows the plasma velocity, how fast the plasma across the heliosphere is moving, against the distance out from Earth. The distance is in astronomical units; one astronomical unit is the average distance between the Sun and Earth, about 93 million miles (150 million km). For context, Saturn is 10 AU from Earth, while Pluto is about 40 AU away.
The heliopause crossing happened at 120 AU, when the velocity of plasma coming out from the Sun drops to zero (seen on the top graph), and the outward flow of the plasma is diverted — seen in the increase in the two bottom graphs, which show the upwards and downward speeds (the normal velocity, middle graph) and the sideways speed of the solar wind (the tangential velocity, bottom graph) of the solar wind plasma, respectively. This means as the solar wind begins to interact with the interstellar medium, it is pushed out and away, like a wave hitting the side of a cliff.
Looking at each instrument in isolation, however, does not tell the full story of what interstellar space at the heliopause looks like. Together, these instruments tell a story of the transition from the turbulent, active space within our Sun's influence to the relatively calm waters on the edge of interstellar space.
The MAG shows that the magnetic field strength decreases sharply in the interstellar medium. The CRS data shows an increase in interstellar cosmic rays, and a decrease in heliospheric particles. And finally, the PLS shows that there’s no longer any detectable solar wind.
Now that the Voyagers are outside of the heliosphere, their new perspective will provide new information about the formation and state of our Sun and how it interacts with interstellar space, along with insight into how other stars interact with the interstellar medium.
Voyager 1 and Voyager 2 are providing our first look at the space we would have to pass through if humanity ever were to travel beyond our home star — a glimpse of our neighborhood in space.
• December 10, 2018: For the second time in history, a human-made object has reached the space between the stars. NASA's Voyager 2 probe now has exited the heliosphere - the protective bubble of particles and magnetic fields created by the Sun. 15)
Figure 20: This illustration shows the position of NASA's Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause in November 2018 (image credit: NASA/JPL-Caltech)
Comparing data from different instruments aboard the trailblazing spacecraft, mission scientists determined the probe crossed the outer edge of the heliosphere on 5 November. This boundary, called the heliopause, is where the tenuous, hot solar wind meets the cold, dense interstellar medium. Its twin, Voyager 1, crossed this boundary in 2012, but Voyager 2 carries a working instrument that will provide first-of-its-kind observations of the nature of this gateway into interstellar space.
Figure 21: NASA’s Voyager 2 enters interstellar space (video credit: NASA/JPL-Caltech)
Voyager 2 now is slightly more than 11 billion miles (18 billion km) from Earth. Mission operators still can communicate with Voyager 2 as it enters this new phase of its journey, but information – moving at the speed of light – takes about 16.5 hours to travel from the spacecraft to Earth. By comparison, light traveling from the Sun takes about eight minutes to reach Earth.
The most compelling evidence of Voyager 2’s exit from the heliosphere came from its onboard Plasma Science Experiment (PLS), an instrument that stopped working on Voyager 1 in 1980, long before that probe crossed the heliopause. Until recently, the space surrounding Voyager 2 was filled predominantly with plasma flowing out from our Sun. This outflow, called the solar wind, creates a bubble – the heliosphere – that envelopes the planets in our solar system. The PLS uses the electrical current of the plasma to detect the speed, density, temperature, pressure and flux of the solar wind. The PLS aboard Voyager 2 observed a steep decline in the speed of the solar wind particles on 5 November. Since that date, the plasma instrument has observed no solar wind flow in the environment around Voyager 2, which makes mission scientists confident the probe has left the heliosphere.
Figure 22: At the end of 2018, the cosmic ray subsystem aboard NASA’s Voyager 2 spacecraft provided evidence that Voyager 2 had left the heliosphere. There were steep drops in the rate of heliospheric particles that hit the instrument's radiation detector. At the same time, there were significant increases in the rate at which particles that originate outside our heliosphere (also known as galactic cosmic rays) hit the detector (image credit: NASA/JPL-Caltech/GSFC)
Legend to Figure 22: The graphs show data from Voyager 2's CRS, which averages the number of particle hits over a six-hour block of time. CRS detects both lower-energy particles that originate inside the heliosphere (greater than 0.5 MeV) and higher-energy particles that originate farther out in the galaxy (greater than 70 MeV).
In addition to the plasma data, Voyager’s science team members have seen evidence from three other onboard instruments – the cosmic ray subsystem, the low energy charged particle instrument and the magnetometer – that is consistent with the conclusion that Voyager 2 has crossed the heliopause. Voyager’s team members are eager to continue to study the data from these other onboard instruments to get a clearer picture of the environment through which Voyager 2 is traveling.
Figure 23: The set of graphs on the left illustrates the drop in electrical current detected in three directions by Voyager 2's plasma science experiment (PLS) to background levels. They are among the key pieces of data that Voyager scientists used to determine that Voyager 2 entered interstellar space, the space between stars, in November 2018. The disappearance in electrical current in the sunward-looking detectors indicates the spacecraft is no longer in the outward flow of solar wind plasma. It is instead in a new plasma environment — interstellar medium plasma. The image on the right shows the Faraday cups of the PLS. The three sunward pointed cups point in slightly different directions in order to measure the direction of the solar wind. The fourth cup (on the upper left) points perpendicular to the others (image credit: NASA/JPL-Caltech)
“There is still a lot to learn about the region of interstellar space immediately beyond the heliopause,” said Ed Stone, Voyager project scientist based at Caltech in Pasadena, California.
Together, the two Voyagers provide a detailed glimpse of how our heliosphere interacts with the constant interstellar wind flowing from beyond. Their observations complement data from NASA’s Interstellar Boundary Explorer (IBEX), a mission that is remotely sensing that boundary. NASA also is preparing an additional mission – the upcoming Interstellar Mapping and Acceleration Probe (IMAP), due to launch in 2024 – to capitalize on the Voyagers’ observations.
“Voyager has a very special place for us in our heliophysics fleet,” said Nicola Fox, director of the Heliophysics Division at NASA Headquarters. “Our studies start at the Sun and extend out to everything the solar wind touches. To have the Voyagers sending back information about the edge of the Sun’s influence gives us an unprecedented glimpse of truly uncharted territory.”
While the probes have left the heliosphere, Voyager 1 and Voyager 2 have not yet left the solar system, and won’t be leaving anytime soon. The boundary of the solar system is considered to be beyond the outer edge of the Oort Cloud, a collection of small objects that are still under the influence of the Sun’s gravity. The width of the Oort Cloud is not known precisely, but it is estimated to begin at about 1,000 astronomical units (AU) from the Sun and to extend to about 100,000 AU. One AU is the distance from the Sun to Earth. It will take about 300 years for Voyager 2 to reach the inner edge of the Oort Cloud and possibly 30,000 years to fly beyond it.
Figure 24: This artist's concept puts solar system distances in perspective. The scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the Sun to the Earth, which is about 150 million kilometers. Neptune, the most distant planet from the Sun, is about 30 AU (image credit: NASA/JPL-Caltech)
The Voyager probes are powered using heat from the decay of radioactive material, contained in a device called RTG (Radioisotope Thermal Generator). The power output of the RTGs diminishes by about four watts per year, which means that various parts of the Voyagers, including the cameras on both spacecraft, have been turned off over time to manage power.
“I think we’re all happy and relieved that the Voyager probes have both operated long enough to make it past this milestone,” said Suzanne Dodd, Voyager project manager at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “This is what we've all been waiting for. Now we’re looking forward to what we’ll be able to learn from having both probes outside the heliopause.”
Voyager 2 launched on 20 August 1977, 16 days before Voyager 1 (launch on 5 September 1977), and both have traveled well beyond their original destinations. The spacecraft were built to last five years and conduct close-up studies of Jupiter and Saturn. However, as the mission continued, additional flybys of the two outermost giant planets, Uranus and Neptune, proved possible. As the spacecraft flew across the solar system, remote-control reprogramming was used to endow the Voyagers with greater capabilities than they possessed when they left Earth. Their two-planet mission became a four-planet mission. Their five-year lifespans have stretched to 41 years, making Voyager 2 NASA’s longest running mission.
The Voyager story has impacted not only generations of current and future scientists and engineers, but also Earth's culture, including film, art and music. Each spacecraft carries a Golden Record of Earth sounds, pictures and messages. Since the spacecraft could last billions of years, these circular time capsules could one day be the only traces of human civilization.
Voyager’s mission controllers communicate with the probes using NASA’s Deep Space Network (DSN), a global system for communicating with interplanetary spacecraft. The DSN consists of three clusters of antennas in Goldstone, California; Madrid, Spain; and Canberra, Australia.
The Voyager Interstellar Mission is a part of NASA’s Heliophysics System Observatory, sponsored by the Heliophysics Division of NASA’s Science Mission Directorate in Washington. JPL built and operates the twin Voyager spacecraft. NASA’s DSN, managed by JPL, is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The network also supports selected Earth-orbiting missions. The CSIRO (Commonwealth Scientific and Industrial Research Organization), Australia’s national science agency, operates both the CDSCC (Canberra Deep Space Communication Complex), part of NASA's DSN, and the Parkes Observatory of CSIRO, which NASA has been using to downlink data from Voyager 2 since 8 November.
Australia's national science agency, CSIRO, is supporting NASA’s Voyager 2 spacecraft as it enters interstellar space.
• On 8 November 2018, CSIRO's Parkes radio telescope joined NASA's CDSCC (Canberra Deep Space Communication Complex), part of NASA's Deep Space Network, to receive unique and historic data from Voyager 2. This provides a clearer picture of the environment through which Voyager 2 is travelling. The Parkes telescope will continue to receive downlink data into early 2019. 16)
- NASA has engaged the Parkes telescope to support receiving this historic data from Voyager 2 while CDSCC is busy with communications for other deep space missions that are making their own important encounters during this period, such as New Horizons' flyby of the most distant object yet to be explored by a spacecraft, coming up on New Year's Day.
- Because of Voyager 2's location and distance from Earth, CDSCC and the Parkes telescope are the only facilities in the world that are capable of having contact with the spacecraft.
- Voyager 2 isn't able to record its data on board – it transmits it directly from the instruments back to Earth – making it essential to receive as much of this vital data as possible.
- CSIRO Chief Executive Dr Larry Marshall said CSIRO was here to solve the greatest challenges with science. "So we're proud to help NASA solve the scientific challenge of capturing this once in a lifetime opportunity as Voyager 2 ventures into interstellar space," Dr Marshall said.
- "Our team at Parkes has partnered with NASA on some of humanity's most momentous steps in space, including the landing of the Mars Rover Curiosity and, almost fifty years ago, the Apollo 11 Moon landing.
- CSIRO Director of Astronomy and Space Science Dr Douglas Bock explained how the additional support from Parkes would track Voyager 2. "The Canberra Deep Space Communication Complex, which CSIRO operates on behalf of NASA, has been providing command, telemetry and control for the twin Voyager spacecraft since their launch in 1977," Dr Bock said.
- "NASA has engaged our 64 m Parkes radio telescope to 'combine forces' with CDSCC's 70 m antenna, Deep Space Station 43 (DSS43), to capture as much scientifically valuable data as possible during this critical period.
- "The Parkes telescope will be tracking Voyager 2 for 11 hours a day while the spacecraft is observable from Parkes. CDSCC's DSS43 will also track Voyager 2 for a number of hours both before and after Parkes, expanding the available observation time. - This is a highlight of CSIRO's decades' worth of experience operating large, complex spacecraft tracking and radio astronomy infrastructure."
Figure 25: Photo of CSIRO's Parkes 64 m radio telescope (image credit: CSIRO)
Legend to Figure 25: The Parkes radio telescope is located outside the town of Parkes in the central-west region of New South Wales, about 380 km from Sydney. It's one of three instruments that make up the Australia Telescope National Facility. Parkes is one of the largest single-dish telescopes in the southern hemisphere dedicated to astronomy. It started operating in 1961, but only its basic structure has remained unchanged. The surface, control system, focus cabin, receivers, computers and cabling have all been upgraded – some parts many times – to keep the telescope at the cutting edge of radio astronomy. The telescope is now 10,000 times more sensitive than when it was commissioned.
Figure 26: Photo of NASA's CDSCC (Canberra Deep Space Communication Complex), with the 70 m antenna in the foreground, located 35 km outside Australia's capital (image credit: NASA, CSIRO)
Legend to Figure 26: CDSCC is part of NASA’s Deep Space Network (DSN) which connects scientists around the world with their robotic spacecraft exploring the Solar System and beyond. Since 2010, CSIRO has partnered with NASA to manage the Canberra facility on their behalf. The DSN also has two other station complexs located near Madrid, Spain and Goldstone, California.The Canberra facility is dominated by a massive 70 m dish, the largest steerable parabolic antenna in the southern hemisphere. Along with three additional 34 m dishes across the site, they transmit and receive data from over 40 missions in deep space, including both of NASA’s Voyager spacecraft.
NASA's Deep Space Antenna Upgrades to Affect Voyager Communications
• February 12, 2021: In the nearly 44 years since NASA launched Voyager 2, the spacecraft has gone beyond the frontiers of human exploration by visiting Uranus, Neptune and, eventually, interstellar space. 17)
Figure 27: Repairs to the Deep Space Station 43 in Canberra, Australia, the only means of communication with the Voyager 2 spacecraft (image credit: NASA)
- Last March, the agency was compelled to shut down its only means of reaching 12 billion miles across the heavens to this robotic trailblazer. On Friday, Earth’s haunting silence will come to an end as NASA switches that communications channel back on, restoring humanity’s ability to say hello to its distant explorer.
- Because of the direction in which it is flying out of the solar system, Voyager 2 can only receive commands from Earth via one antenna in the entire world. It’s called DSS 43 and it is in Canberra, Australia. It is part of the Deep Space Network, or DSN, which along with stations in California and Spain, is how NASA and allied space agencies stay in touch with the armada of robotic spacecraft exploring everything from the sun’s corona to the regions of the Kuiper belt beyond the orbit of Pluto. (Voyager 2’s twin, Voyager 1, is able to communicate with the other two stations.)
- A round-trip communication with Voyager 2 takes about 35 hours — 17 hours and 35 minutes each way.
- DSS 43 is a 70-meter dish that has been operating since 1973. It was long overdue for upgrades, especially with new robotic missions headed to Mars this year and even more preparing to launch to study other worlds in the months and years to come. So last year, the dish was switched off and dismantled, even though the shutdown posed considerable risk to the geriatric Voyager 2 probe.
- Like everything in 2020, what would have been a normal antenna upgrade was anything but. Usually, the mission’s managers at NASA’s Jet Propulsion Laboratory in California would send about 30 experts to oversee the dish’s makeover. But restrictions imposed during the COVID-19 pandemic reduced the team to four.
- At the Canberra station, the crew working on the upgrade had to be separated into three smaller teams, said Glen Nagle, outreach manager at the Canberra Deep Space Communication Complex. “So there was always a backup team in case anybody got sick, and you could put that team in isolation, and the other team could come in and cover for them.” They also split the teams into morning and evening shifts to ensure social distancing.
- While Voyager 2 was able to call home on the Canberra site’s smaller dishes during the shutdown, none of them could send commands to the probe. If anything had gone wrong aboard the probe during the last year, NASA would have been powerless to fix it.
- Although NASA has been unable to send full commands to Voyager 2, it did send one test message to the spacecraft at the end of October when the antenna was mostly reassembled. A device on board called the command loss timer, something like a dead man’s switch, is used to help the spacecraft determine whether it’s lost contact with Earth and should protect itself by going into a form of electronic slumber. The October test reset the timer, and successfully told the spacecraft to continue operating.
- “I think there was probably a big sigh of relief there,” Mr. Nagle said. “And we were very pleased to be able to confirm that the spacecraft was still talking to us.”
- The work got high marks from NASA officials in the United States.
- “The DSN folks in Canberra did a remarkable job under the pandemic conditions just to upgrade DSS 43,” said Suzanne Dodd, the Voyager mission project manager and director of the Interplanetary Network Directorate at the Jet Propulsion Laboratory. “I’ve got 100 percent confidence in that antenna, that it will operate just fine for a few more decades. Long past when the Voyagers are done.”
- Both Voyager 1 and Voyager 2 hold the records for the farthest a spacecraft has ever traveled and for the longest operating mission. Voyager 2 has had a few hiccups over the years, but it is still feeling its way around in the dark, making discoveries about the boundaries that separate our solar system from the rest of the Milky Way galaxy.
- “I’ve seen scientists whose backgrounds are in astrophysics now looking at Voyager data and trying to match that up with data they have from ground-based telescopes or other space-based telescopes,” Ms. Dodd said. “That’s kind of exciting to go from a planetary mission to the heliophysics mission and now, practically into an astrophysics mission.”
- While Voyager 2 keeps chugging along, Ms. Dodd and her colleagues are preparing to switch off one of its scientific sensors, the Low Energy Charged Particle instrument. Doing so will ensure that the spacecraft’s limited power supply can keep its other systems, particularly its communications antenna, warm enough to function.
- While that will reduce the spacecraft’s scientific output, the main goal now is longevity.
- “The challenge is not in the new technology, or the great discoveries,” Ms. Dodd said. “The challenge is in keeping it operating as long as possible, and returning the science data as long as possible.”
- The team estimates that both spacecraft can operate for another four to eight years, and NASA last year granted the team three more years of flying time.
- “The spacecraft continues to plug along,” Ms. Dodd said. “It always surprises me.”
Figure 28: The “Sounds of Earth” record being mounted on the Voyager 2 spacecraft before its launch in 1977 (image credit: NASA, via Associated Press)
• Starting in early March 2020, NASA's Voyager 2 will quietly coast through interstellar space without receiving commands from Earth. That's because the Voyager's primary means of communication, the Deep Space Network's 70-meter-wide (230-feet-wide) radio antenna in Canberra, Australia, will be undergoing critical upgrades for about 11 months. During this time, the Voyager team will still be able to receive science data from Voyager 2 on its mission to explore the outermost edge of the Sun's domain and beyond. 18)
The Deep Space Network's Canberra facility in Australia is the only antenna that can send commands to the Voyager 2 spacecraft. The antenna enhancements will improve future spacecraft communications, but during the upgrades, Voyager 2 will not be able to receive new commands from Earth.
About the size of a 20-story office building, the dish has been in service for 48 years. Some parts of the 70-meter antenna, including the transmitters that send commands to various spacecraft, are 40 years old and increasingly unreliable. The Deep Space Network (DSN) upgrades are planned to start now that Voyager 2 has returned to normal operations, after accidentally overdrawing its power supply and automatically turning off its science instruments in January.
The network operates 24 hours a day, 365 days a year and is spread over three sites around the world, in California, Spain and Australia. This allows navigators to communicate with spacecraft at the Moon and beyond at all times during Earth's rotation. Voyager 2, which launched in 1977, is currently more than 11 billion miles (17 billion kilometers) from Earth. It is flying in a downward direction relative to Earth's orbital plane, where it can be seen only from the southern hemisphere and thus can communicate only with the Australian site.
Moreover, a special S-band transmitter is required to send commands to Voyager 2 - one both powerful enough to reach interstellar space and on a frequency that can communicate with Voyager's dated technology. The Canberra 70-meter antenna (called "DSS43") is the only such antenna in the southern hemisphere. As the equipment in the antenna ages, the risk of unplanned outages will increase, which adds more risk to the Voyager mission. The planned upgrades will not only reduce that risk, but will also add state-of-the art technology upgrades that will benefit future missions.
"Obviously, the 11 months of repairs puts more constraints on the other DSN sites," said Jeff Berner, Deep Space Network's chief engineer. "But the advantage is that when we come back, the Canberra antenna will be much more reliable."
The repairs will benefit far more than Voyager 2, including future missions like the Mars 2020 rover and Moon to Mars exploration efforts. The network will play a critical role in ensuring communication and navigation support for both the precursor Moon and Mars missions and the crewed Artemis missions. "The maintenance is needed to support the missions that NASA is developing and launching in the future, as well as supporting the missions that are operating right now," said Suzanne Dodd, Voyager project manager and JPL Director for the Interplanetary Network.
The three Canberra 34-meter (111-foot) antennas can be configured to listen to Voyager 2's signal; they just won't be able to transmit commands. In the meantime, said Dodd, the Voyager team will put the spacecraft into a quiescent state, which will still allow it to send back science data during the 11-month downtime.
"We put the spacecraft back into a state where it will be just fine, assuming that everything goes normally with it during the time that the antenna is down," said Dodd. "If things don't go normally - which is always a possibility, especially with an aging spacecraft - then the onboard fault protection that's there can handle the situation."
Berner says the work on the 70-meter antenna is like bringing an old car into the shop: There's never a good time to do it, but it will make the car much more dependable if you do.
The work on the Canberra DSN station is expected to be completed by January 2021. The DSN is managed by NASA's Jet Propulsion Laboratory for the agency's Human Exploration and Operations' Space Communication and Navigation program.
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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).
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