Dear Glutdawnous Readers,
The distant dwarf planet that Dawn is circling is full of mystery and yet growing ever more familiar. Ceres, which only last year was hardly more than a fuzzy blob against the stars, is now a richly detailed world, and our portrait grows more elaborate every day. Having greatly surpassed all of its original objectives, the reliable explorer is gathering still more data from its unique vantage point. Everyone who hungers for new knowledge about the cosmos or for bold adventures far from Earth can share in the sumptuous feast Dawn has been serving.
One of the major objectives of the mission was to photograph 80 percent of Ceres' vast landscape with a resolution of 660 feet (200 meters) per pixel. That would provide 150 times the clarity of the powerful Hubble Space Telescope. Dawn has now photographed 99.8 percent with a resolution of 120 feet (35 meters) per pixel.
This example of Dawn's extraordinary productivity may appear to be the limit of what it could achieve. After all, the spaceship is orbiting at an altitude of only 240 miles (385 kilometers), closer to the ground than the International Space Station is to Earth, and it will never go lower for more pictures. But it is already doing more.
Since April 11, instead of photographing the scenery directly beneath it, Dawn has been aiming its camera to the left and forward as it orbits and Ceres rotates. By May 25, it will have mapped most of the globe from that angle. Then it will start all over once more, looking instead to the right and forward from May 27 through July 10. The different perspectives on the terrain make stereo views, which scientists can combine to bring out the full three dimensionality of the alien world. Dawn already accomplished this in its third mapping orbit from four times its current altitude, but now that it is seeing the sights from so much lower, the new topographical map will be even more accurate.
Dawn is also earning extra credit on its assignment to measure the energy of gamma rays and neutrons. We have discussed before how the gamma ray and neutron detector (GRaND) can reveal the atomic composition down to about a yard (meter) underground, and last month we saw initial findings about the distribution of hydrogen. However, Ceres' nuclear glow is very faint. Scientists already have three times as much GRaND data from this low altitude as they had required, and both spectrometers in the instrument will continue to collect data. In effect, Dawn is achieving a longer exposure, making its nuclear picture of Ceres brighter and sharper.
In December we explained how using the radio signal to track the probe's movements allows scientists to chart the gravity field and thereby learn about the interior of Ceres, revealing regions of higher and lower density. Once again, Dawn performed even better than expected and achieved the mission's planned accuracy in the third mapping orbit. Because the strength of the dwarf planet's gravitational tug depends on the distance, even finer measurements of how it varies from location to location are possible in this final orbit. Thanks to the continued smooth operation of the mission, scientists now have a gravitational map fully twice as accurate as they had anticipated. With additional measurements, they may be able to squeeze out a little more detail, perhaps improving it by another 20 percent before reaching the method's limit.
Dawn has dramatically overachieved in acquiring spectra at both visible and infrared wavelengths. We have previously delved into how these measurements reveal the minerals on the ground and what some of the interesting discoveries are. Having already acquired more than seven times as many visible spectra and 21 times as many infrared spectra as originally called for, the spacecraft is adding to its riches with additional measurements. We saw in January that VIR has such a narrow view that it will never see all of Ceres from this close, so it is programmed to observe features that have caught scientists' interest based on the broad coverage from higher altitudes.
Dawn's remarkable success at Ceres was not a foregone conclusion. Of course, the flight team has confronted the familiar challenges people encounter every day in the normal routine of piloting an ion-propelled spaceship on a multibillion-mile (multibillion-kilometer) interplanetary journey to orbit and explore two uncharted worlds. But the mission was further complicated by the loss of two of the spacecraft's four reaction wheels, as we have recounted before. (In full disclosure, the devices aren’t actually lost. We know precisely where they are. But given that one stopped functioning in 2010 and the other in 2012, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Without three of these units to control its orientation in space, the robot has relied on its limited supply of hydrazine, which was not intended to serve this function. But the mission's careful stewardship of the precious propellant has continued to exceed even the optimistic predictions, allowing Dawn good prospects for carrying on its fruitful work. In an upcoming Dawn Journal, we will discuss how the last of the dwindling supply of hydrazine may be used for further discoveries.
In the meantime, Dawn is continuing its intensive campaign to reveal the dwarf planet's secrets, and as it does so, it is passing several milestones. The adventurer has now been held in Ceres' tender but firm gravitational embrace longer than it was in orbit around Vesta. (Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, and its mission would have been impossible without ion propulsion.) The spacecraft provided us with about 31,000 pictures of Vesta, and it has now acquired the same number of Ceres.
For an interplanetary traveler, terrestrial days have little meaning. They are merely a memory of how long a faraway planet takes to turn on its axis. Dawn left that planet long ago, and as one of Earth's ambassadors to the cosmos, it is an inhabitant of deep space. But for those who keep track of its progress yet are still tied to Earth, on May 3 the journey will be pi thousand days long. (And for our nerdier friends and selves, it will be shortly after 6:47 p.m. PDT.)
By any measure, Dawn has already accomplished an extraordinary mission, and there is more to look forward to as its ambitious expedition continues.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.73 AU (346 million miles, or 558 million kilometers) from Earth, or 1,455 times as far as the moon and 3.70 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and two minutes to make the round trip.
From 1967 through the early 1970s, a number of studies were conducted at JPL with the goal of reducing the size of computer memory and developing miniature storage media for spacecraft computers.
These early tests used Curie-point writing to communicate the bits (ones and zeroes) of computer data. In various tests, a hot wire stylus, an electron beam, or a ruby laser were used to heat tiny dots (around one micrometer in size) on thin ferromagnetic manganese bismuthide (MnBi) film. The material was heated to just above its Curie temperature (the point at which the material is demagnetized) then cooled within a magnetic field, controlling the direction of the magnetization for each dot. The recorded bits of information were observed with polarized light using the Faraday effect. The recorded information could be completely erased by saturating the film in an applied magnetic field, then the recording process could be repeated.
The newest Historical Photo of the Month http://beacon.jpl.nasa.gov/historical-photo-of-the-month shows Dr. George Lewicki and Dr. Dimiter Tchernev who worked on this task. It received NASA funding of $175,000 per year (about $1.2 million in 2016 dollars). The studies were documented in a series of published papers, articles in JPL Space Programs Summaries, and a press release. It was reported that one square inch of magnetic film could hold as much data as computer memory that (in 1967) took up ten cubic feet of space.
For more detailed information about the history of JPL, contact the Library and Archives Reference Desk at (818) 354-4200 or email@example.com. If you have questions about the Historical Photo of the Month, please contact archivist Julie Cooper at Julie.A.Cooper@jpl.nasa.gov.
Cimate change news is intense. Ice caps are melting, the fire season lasts all year long; we have epic storms plus record-breaking floods, droughts and cyclones.
And this year will probably be the Hottest. Year. Ever.
When I interact with the public, I’m bombarded with questions such as “Are we all going to die?” and “How soon will humans go extinct?”
Happy Earth Day, everyone (wipes brow, rolls eyes).
Yet, when I wake up in the morning I'm excited to come to work. I'm energized. I’m amped, really amped. As in, kicking-butt-and-taking-names amped. Why? Because global warming is the greatest challenge of our lives, and challenge is what drives us. Challenge provides us with opportunity, challenge forces us to grow, challenge opens the way for amazing achievement. Challenge is exciting. Without challenge, without struggle, without discomfort, no one would ever advance.
So, when someone gets in my face and is super negative, I try to stay powerful, strong and confident. I tell myself that pressure is okay and I'm going to keep moving no matter what. Because I care about this planet so much that I choose to make a difference.
Yes, carbon dioxide levels are high and increasing rapidly. Yes, future generations will have some extraordinarily difficult challenges to deal with. But denial, avoidance and helplessness aren’t solutions. Can you imagine if we NASA peeps just sat there saying “Oh no, that’s too hard” when faced with huge obstacles? Are you kidding me? Come on! You think it’s easy to build science instruments on satellites and launch them into space? You think it’s easy to measure glaciers melting around the edges of Greenland, or the condition of coral reefs in the Pacific, or plankton blooms across the North Atlantic, or conduct eight field research campaigns in one year?
When the going gets tough—and it does, almost every day—we don’t just stop. We keep working. We know that no successful person got As on every test and that failure and struggle are part of accomplishment. We know that grit and determination will get you everywhere!
In this blog, I write about ocean pollution, sea level rise, climate change and decreasing biodiversity not to scare you, but to empower you, so we can make a difference—you and I, together. Someone reading this blog entry might be the creator of a new breakthrough technology, and then there will be a whole new reality.
So, when you think about the challenge of climate change this Earth Day, consider the possibility of welcoming that challenge. Our shared story could be a story about not giving up, about looking forward to growth, about saying, “Game on.”
Find out more about NASA earth expeditions here.
Join NASA for a #24Seven celebration of Earth Day.
Thank you for caring enough to make a difference and for being powerful in the world.
In between 1982 and 1997, JPL had no active missions on the surface of Mars. July 1986 was the 10th anniversary of the Viking mission, and an artist was hired to help show the possibilities of future Mars exploration. This artist’s rendering depicts a fleet of landers with astronauts aboard. The one on its side enabled cargo bay doors to open so a vehicle could be driven out onto the surface of the planet and other cargo unloaded.
Hey, readers: Our team reads your comments. We share them at our meetings. Sometimes they make us laugh, or sigh, or even scratch our heads.
We see that you see us. Yay for connecting!
And this is how I know you’ve noticed NASA’s latest airborne campaign, where NASA scientists fly a bunch of NASA instruments on a NASA airplane to study more details about Earth. Cool, right?
Lately, we’ve been flying around the edge of Greenland collecting radar data about how much its glaciers are melting into the sea. And the most common comment we get goes something like “Wheee! Let’s go. Take me with you.” When I told a friend about the possibility of joining the team in the field, she exclaimed, “All expenses paid?”
HAHAHA … no. As if a NASA expedition to Greenland is like a resort vacation instead of a giant heaping pile of hard work.
“When I looked down at the rivers of ice running into the ocean, it was shocking to think about the effects of rising sea levels as far away as California or Antarctica,” said Principle Investigator Josh Willis, two days after returning from his first trip to observe this pristine part of our planet as it melts into the sea and goes bye-bye. “Yet, I had a blast.” Because even though we all probably have many complex emotions about climate change, ice mass loss and sea level rise, we can still simultaneously feel super duper stoked about the chance to fly over the glaciers of Greenland in a freaking NASA plane. “The mountains, the ice, the water and the ice in the water are incredibly striking even though it’s lonely to see it disappearing at the hands of human activity,” he told me.
Yes, emotions are weird, and yes, there’s an awkward contrast or odd juxtaposition between feeling both thrill and grief at the same time.
But that’s life, I guess.
So just in case you’re still envisioning a champagne-swilling, caviar-scoffing, gangsta, hip-hop music video scene, here are a few things that might surprise you about the kind of major effort it takes to get on board NASA’s G-III plane and join the Oceans Melting Greenland field campaign:
Kick booty in a fire-resistant flight suit
So, you think you’d kick some booty in one of these flight suits? Oh, yeah. Totally. Well, so do we. Would you kill to have one? But the real reason the pilots think they’re so fab is because they’re fire-resistant. They. Resist. Fire! With racks and racks of science equipment wired with electrical cables, the crew has to be extra careful about fire on the plane. So wearing one of these flight suits is required.
A load of gas and no mistakes
A trip to Greenland sounds all romantic ‘n’ stuff, but operating a science instrument aboard a flying science lab on a six-hour flight every day is hard work. Just check out these flight paths. According to Project Manager Steve Dinardo, “You get a full load of gas and no mistakes.” Notice the flight path zigzags across the complicated coastline of the entire island. That’s because global warming of Earth’s atmosphere is melting the top of the ice sheet. But, aha! The ocean water around Greenland is even warmer than the air. That warm water is busy melting the glaciers from around their edges, hence the name, Oceans Melting Greenland, which will find out exactly how much of this melting is going on.
Instruments, instruments and more instruments. And did I mention some serious training?
The NASA modified G-III aircraft is … modified. (Did you notice the word “modified”?) What modified means is the plane has holes in it so experimental science instruments can stick out. And more scientific instruments are attached in, under and onto the plane in all sorts of configurations. To get to fly on this baby, you’d better have some training. Yep, some serious training: Safety training, first aid training, survival training. You get the idea.
Keep warm, in style
I can work the runway like a glamazon in this red coat, but it’s rated for survival in 50 degrees below zero. I said survival. In case of emergency. Does this sound like your all-inclusive vacation package now? With a survival coat? And there’s a survival vest too, with a beacon attached, and food rations, a pocketknife tool set, fishing gear, first aid supplies, a radio and a laser pointer for playing with cats—oops, I mean for signaling emergency and attracting rescue. The thing weighs about 20 pounds. Everyone on the plane has one of these puppies, and you’d better believe they know how to use it. If there’s a problem, the team would have to survive three to five days out in the wilderness until they're rescued. I don’t mean to scare you, but at NASA, when we say we care about safety, we’re not messing around.
“It’s not a triumph of human achievement that we’re melting the ice sheet,” said Willis. “When you see how huge these glaciers are and this huge chunk of this ice sheet disappearing into the ocean, it’s almost incomprehensible even when you see it from 40,000 feet.”
Find out more about Oceans Melting Greenland here.
View and download an OMG poster/infographic here.
Thank you for your comments.
Oceans Melting Greenland is part of NASA Earth Expeditions, a six-month field research campaign to study regions of critical change around the world.
Dear Resplendawnt Readers,
One year after taking up its new residence in the solar system, Dawn is continuing to witness extraordinary sights on dwarf planet Ceres. The indefatigable explorer is carrying out its intensive campaign of exploration from a tight orbit, circling its gravitational master at an altitude of only 240 miles (385 kilometers).
Even as we marvel at intriguing pictures and other discoveries, scientists are still in the early stages of putting together the pieces of the big puzzle of how (and where) Ceres formed, what its subsequent history has been, what geological processes are still occurring on this alien world and what all that reveals about the solar system.
For many readers who have not visited Ceres on their own, Occator Crater is the most mysterious and captivating feature. (To resolve the mystery of how to pronounce it, listen to the animation below.) As Dawn peered ahead at its destination in the beginning of 2015, the interplanetary traveler observed what appeared to be a bright spot, a shining beacon guiding the way for a ship sailing on the celestial seas. With its mesmerizing glow, the uncharted world beckoned, and Dawn answered the cosmic invitation by venturing in for a closer look, entering into Ceres' gravitational embrace. The latest pictures are one thousand times sharper than those early views. What was not so long ago a single bright spot has now come into focus as a complex distribution of reflective material in a 57-mile (92-kilometer) crater.
Scientists are still working on refining their understanding of this striking region. As we described in December, it seems that following the powerful impact that excavated Occator Crater, underground briny water reached the surface. The detailed photographs show many fractures cutting across the bright areas, and perhaps they provided a conduit. Water, whether as liquid or ice, would not last long there in the cold vacuum, eventually subliming. When the water molecules disperse, either escaping from Ceres into space or falling back to settle elsewhere, the dissolved salts are left behind. This reflective residue covers the ground, making the spellbinding and beautiful display Dawn now reveals.
While the crater is estimated to be a geological youngster at 80 million years old, that is an extremely long time for the material to remain so reflective. Exposed for so long to cosmic radiation and pelting from the rain of debris from space, it should have darkened. Scientists don't know (yet) what physical process are responsible, but perhaps it was replenished long after the crater itself formed, with more water, carrying dissolved salts, finding its way to the surface. As their analyses of the photos and spectra continue, scientists will gain a clearer picture and be able to answer this and other questions.
These latest Occator pictures did not come easily. Orbiting so close to Ceres, the adventurer’s camera captures only a small scene at a time, and it is challenging to cover the entirety of the expansive terrain. (Perhaps it comes as a surprise to those who have not read at least a few of the 123 Dawn Journals that precede this one that operating a spacecraft closer to a faraway dwarf planet than the International Space Station is to Earth is not as easy as, say, thinking about it.) But the patience and persistence in photographing the exotic landscapes have paid off handsomely.
We now have high resolution pictures of essentially all of Ceres save the small area around the south pole cloaked in the deep dark of a long winter night. Seasons last longer on Ceres than on Earth, and Dawn may not operate there long enough for the sun to rise at the south pole. By the beginning of southern hemisphere spring in November 2016, Dawn's mission to explore the first dwarf planet discovered may have come to its end.
In addition to photographing Ceres, Dawn conducts many other scientific observations, as we described in December and January. Among the probe's objectives at Ceres is to provide information for scientists to understand how much water is there, where it is, what form it is in and what role it plays in the geology.
We saw that extensive measurements of the faint nuclear radiation can help identify the atomic constituents. While the analysis of the data is complicated, and much more needs to be done, a picture is beginning to emerge from Dawn's neutron spectrometer (part of the gamma ray and neutron detector, GRaND). These subatomic particles are emitted from the nuclei of atoms buried within about a yard (meter) of the surface. Some manage to penetrate the material above them and fly into space, and the helpful ones then meet their fate upon hitting GRaND in orbit above. (Most others, however, will continue to fly through interplanetary space, decaying into a trio of other subatomic particles in less than an hour.) Before it escapes from the ground, a neutron's energy (and, equivalently, its speed) is strongly affected by any encounters with the nuclei of hydrogen atoms (although other atomic interactions can change the energy too). Therefore, the neutron energies can indicate to scientists the abundance of hydrogen. Among the most common forms in which hydrogen is found is water (composed of two hydrogen atoms and one oxygen atom), which can occur as ice or tied up in hydrated minerals.
GRaND shows Ceres is rich in hydrogen. Moreover, it detects more neutrons in an important energy range near the equator than near the poles, likely indicating there is more hydrogen, and hence more (frozen) water, in the ground at the high latitudes. Although Ceres is farther from the sun than Earth, and you would not consider it balmy there, it still receives some warmth. Just as at Earth, the sun's heating is less effective closer to the poles than at low latitudes, so this distribution of ice in the ground may reflect the temperature differences. Where it is warmer, ice close to the surface would have sublimed more quickly, thus depleting the inventory compared to the cooler ground far to the north or south.
Dawn spends most of its time measuring neutrons (and gamma rays), so it is providing a great deal of new data. And as scientists conduct additional analyses, they will learn more about the ice and other materials beneath the surface.
Another spectrometer is providing more tantalizing clues about the composition of Ceres, which is seen to vary widely. As the dwarf planet is not simply a huge rock but is a geologically active world, it is no surprise that it is not homogeneous. We discussed in December that the infrared mapping spectrometer had shown that minerals known as phyllosilicates are common on Ceres. Further studies of the data show evidence for the presence of two types: ammoniated phyllosilicates (described in December) and magnesium phyllosilicates. Scientists also find evidence of compounds known as carbonates, minerals that contain carbon and oxygen. There is also a dark substance in the mix that has not been identified yet.
And in one place (so far) on Ceres, this spectrometer has directly observed water, not below the surface but on the ground. The infrared signature shows up in a small crater named Oxo. (For the pronunciation, listen to the animation below.) As with the neutron spectra, it is too soon to know whether the water is in the form of ice or is chemically bound up in minerals.
At six miles (10 kilometers) in diameter, Oxo is small in comparison to the largest craters on Ceres, which are more than 25 times wider. (While geologists consider it a small crater, you might not agree if it formed in your backyard. Also note that when we showed Oxo Crater before, the diameter was slightly different. The crater's size has not changed since then, but as we receive sharper pictures, our measurements of feature sizes do change.) Dawn's first orbital destination, the fascinating protoplanet Vesta, is smaller than Ceres and yet has two craters far broader than the largest on Ceres. Based on studies of craters observed throughout the solar system, scientists have established methods of calculating the number and sizes of craters that could be formed on planetary surfaces. Those techniques show that Ceres is deficient in large craters. That is, more should have formed than appear in Dawn's pictures. Many other bodies (including Vesta and the moon) seem to preserve their craters for much longer, so this may be a clue about internal geological processes on Ceres that gradually erase the large craters.
Scientists are still in the initial stages of digesting and absorbing the tremendous wealth of data Dawn has been sending to Earth. The benefit of lingering in orbit (enabled by the remarkable ion propulsion system), rather than being limited to a brief glimpse during a fast flyby, is that the explorer can undertake much more thorough studies, and Dawn is continuing to make new measurements.
As recently as one year ago, controllers (and this writer) had great concern about the spacecraft's longevity given the loss of two reaction wheels, which are used for controlling the ship's orientation. And in 2014, when the flight team worked out the intricate instructions Dawn would follow in this fourth and final mapping orbit, they planned for three months of operation. That was deemed to be more than enough, because Dawn only needed half that time to accomplish the necessary measurements. Experienced spacecraft controllers recognize that there are myriad ways beautiful plans could go awry, so they planned for more time in order to ensure that the objectives would be met even if anomalies occurred. They also were keenly aware that the mission could very well conclude after three months of low altitude operations, with Dawn using up the last of its hydrazine. But their efforts since then to conserve hydrazine proved very effective. In addition, the two remaining wheels have been operating well since they were powered on in December, further reducing the consumption of the precious propellant.
As it turned out, operations have been virtually flawless in this orbit, and the first three months yielded a tremendous bounty, even including some new measurements that had not been part of the original plans. And because the entire mission at Ceres has gone so well, Dawn has not expended as much hydrazine as anticipated.
Dawn is now performing measurements that were not envisioned long in advance but rather developed only in the past two months, when it was apparent that the expedition could continue. And since March 19, Dawn has been following a new strategy to use even less hydrazine. Instead of pointing its sensors straight down at the scenery passing beneath it as the spacecraft orbits and Ceres rotates, the probe looks a little to the left. The angle is only five degrees (equal to the angle the minute hand of a clock moves in only 50 seconds, or less than the interval between adjacent minute tick marks), but that is enough to decrease the use of hydrazine and thus extend the spacecraft's lifetime. (We won't delve into the reason here. But for fellow nerds, it has to do with the alignment of the axes of the operable reaction wheels with the plane in which Dawn rotates to keep its instruments pointed at Ceres and its solar arrays pointed at the sun. The hydrazine saving depends on the wheels' ability to store angular momentum and applies only in hybrid control, not in pure hydrazine control. Have fun figuring out the details. We did!)
The angle is small enough now that the pictures will not look substantially different, but they will provide data that will help determine the topography. (Measurements of gravity and the neutron, gamma ray and infrared spectra are insensitive to this angle.) Dawn took pictures at a variety of angles during the third mapping orbit at Ceres (and in two of the mapping orbits at Vesta, HAMO1 and HAMO2) in order to get stereo views for topography. That worked exceedingly well, and photos from this lower altitude will allow an even finer determination of the three dimensional character of the landscape in selected regions. Beginning on April 11, Dawn will look at a new angle to gain still another perspective. That will actually increase the rate of hydrazine expenditure, but the savings now help make that more affordable. Besides, this is a mission of exploration and discovery, not a mission of hydrazine conservation. We save hydrazine when we can in order to spend it when we need it. Dawn's charge is to use the hydrazine to accomplish important scientific objectives and to pursue bold, exciting goals that lift our spirits and fuel our passion for knowledge and adventure. And that is exactly what it is has done and what it will continue to do.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.90 AU (362 million miles, or 583 million kilometers) from Earth, or 1,505 times as far as the moon and 3.90 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and five minutes to make the round trip.
We overlook Greenland ice loss at our own peril. It’s one of the largest contributors to accelerating sea level rise, and in the U.S. alone, nearly 5 million people live in 2.6 million homes at less than 4 feet above high tide. If you happen to be one of them, you should definitely pay attention to Greenland.
Yes, yes, Greenland is melting. You already knew that…probably. And the giant flux of fresh water pouring out of the second largest ice sheet on the planet isn’t slowing down anytime soon. Greenland’s ice melt is actually accelerating. In the last decade alone, NASA’s twin GRACE satellites measured it gushing 2 trillion tons of ice like a fire hose pouring fresh water into the North Atlantic.
But it’s easier to focus on politics, celebrity gossip, reality TV and cat videos than on Earth’s climate. It seems like everyone’s all “Greenland? Who cares. Whatever. Next.” And that upsets me.
Is it really that easy to pretend the effects of global warming don’t exist?
We overlook Greenland ice loss at our own peril. It’s one of the largest contributors to accelerating sea level rise, and in the U.S. alone, nearly 5 million people live in 2.6 million homes at less than 4 feet above high tide. If you happen to be one of them, you should definitely pay attention to Greenland.
Fortunately for all of us, NASA is paying attention to Greenland in a big way. We’re so concerned about the amount of ice loss that we’ve named a Greenland observing expedition Oceans Melting Greenland, or OMG for short, because that's the most appropriate response to the phenomenon.
This week, OMG heads up north on one of NASA’s G-III modified airplanes to continue a five-year mission that will look closely at how warming ocean water interacts with glaciers surrounding Greenland and melts them. The project began this past year by mapping undersea canyons via a ship equipped with an echo sounder. For this next part of the investigation, a radar instrument attached to the bottom of the G-III, called the Airborne Glacier and Land Ice Surface Topography Interferometer (GLISTIN-A), will be able to measure precisely how much the oceans are eating away at the edges of the ice on a glacier-by-glacier basis.
Instrument integration (a fancy word for attaching instruments to planes and making sure they work and don’t come loose) went down at NASA’s Armstrong Flight Research Center, and Principal Invesigator Dr. Josh Willis, Project Manager Steve Dinardo, Co-Investigator Dr. Ian Fenty and I headed there to check it out.
Glaciers on the edge
As the technicians and engineers tweaked fistfuls of wires, we crawled in, under, through and around the aircraft. Then Dr. Ian Fenty (who helped design the flight plan) and I sat aboard our flying science lab and talked ice loss for a while. “We often find that a glacier that’s been retreating a lot might be in 1,000 feet of water,” he explained. “Whereas the glacier that’s not thinning very much is in water that’s only 100 or 200 feet deep.” That’s because the layers of ocean water around Greenland are in a very unique situation, where you have colder fresh glacier meltwater near the surface over salty ocean water that, due to climate change, has been warming. The water found at 600 feet and below is a relatively warm 4 degrees Celsius compared with the surface water, which is just near freezing at 0 degrees. This means that the “primary suspect” behind the acceleration of Greenland’s melting glaciers is the warming ocean waters that can get right up against the edge and interact with the glacier itself.
As the surface of lower elevation glaciers melts, the water percolates through the ice and forms giant subglacial channels, like a river system under the ice. If the ice running through these narrow rivers breaks off, the friction between the glacier and the substrate gets reduced a bit and literally stretches the ice so the glacier thins out. OMG’s GLISTIN-A radar is going to measure the height of the ice. “If we see a change in elevation from one year to the next, we can know how much ice is being lost and how much the movement of the glacier is speeding up.” Over the next five years OMG plans to go back to Greenland to look for more changes.
As I left the hangar and headed home, I thought about how Greenland is such a weird part of the world and how much I hope our society can put aside its troubles so we can work together to preserve it.
Find out more about Oceans Melting Greenland here.
Thank you for your comments.
Dear Indawnbitably Successful Readers,
A story of intense curiosity about the cosmos, passionate perseverance and bold ingenuity, a story more than two centuries in the making, has reached an extraordinary point. It begins with the discovery of dwarf planet Ceres in 1801 (129 years before its sibling Pluto; each was designated a planet for a time). Protoplanet Vesta was discovered in 1807. Following 200 years of telescopic observations, Dawn's daring mission was to explore these two uncharted worlds, the largest, most massive residents of the main asteroid belt between Mars and Jupiter. And now, as of February 2016, the spacecraft has accomplished all of the objectives that NASA defined for it in 2004, even before construction began (and before the very first Dawn Journal, nearly a decade ago).More than eight years after leaving its erstwhile planetary home behind for an ambitious deep space adventure, Dawn has now collected all of the data originally planned. Indeed, even prior to this third intercalary day of its expedition, the probe had already actually sent back a great deal more data for all investigations, significantly exceeding not only the original goals but also new ones added after the ship had set sail on the interplanetary seas. While scientists have a great deal of work still ahead to translate the bounty of data into knowledge, which is the greatest joy of science, the spacecraft can continue its work with the satisfaction that it has fulfilled its purpose and achieved an outstandingly successful mission.
Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, which would have been impossible without its advanced ion propulsion system. It is the only spacecraft ever to orbit an object in the main asteroid belt. It is also the only spacecraft ever to orbit massive bodies (apart from the sun and Earth) that had not been visited first by a flyby spacecraft to characterize the gravity and other properties. (By the way, Ceres is one of eight solar system bodies that operating spacecraft are orbiting now. The others are the sun, Venus, Earth, the moon, comet Churyumov-Gerasimenko, Mars and Saturn.)
Now in its fourth and final mapping orbit at Ceres, at an altitude of 240 miles (385 kilometers), Dawn is closer to the exotic terrain than the International Space Station is to Earth. The benefit of being in orbit is that the probe can linger rather than take only a brief look during a fast flyby. Even though Dawn has met its full list of objectives at Ceres, it continues to return new, valuable pictures and other measurements to provide even greater insight into this relict from the dawn of the solar system. For example, it is acquiring more nuclear spectra with its gamma ray and neutron detector, sharpening its picture of some atomic elements on Ceres. In addition, taking advantage of its unique vantage point, Dawn is collecting more infrared spectra of locations that are of special interest and soon will also take color photos and stereo photos (as it did in the third mapping orbit) of selected areas.
Dawn has completed more than 600 revolutions since taking up residence one year ago. The first few orbits took several weeks each, but as the spacecraft descended and Ceres' gravitational embrace grew more firm, its orbital velocity increased and the orbital period decreased. Now circling in less than five and a half hours, Dawn has made 370 orbits since reaching this altitude on Dec. 7.
The pace of observations here is higher than in the previous mapping orbits, where the orbital periods were longer. The spacecraft flies over the landscape faster now, and being closer to the ground, its instruments discern much more detail but capture a smaller area. Mission controllers have developed intricate plans for observing Ceres, but those plans depend on the spacecraft being at the right place at the right time. As we will see below, however, sometimes it may not be.
Suppose, for example, the intent is to observe a particular feature, perhaps the bright center of Occator crater, the lonely, towering mountain Ahuna Mons, the fractures in Dantu crater or artificial structures that definitively prove the existence of extraterrestrial intelligence, utterly transforming our understanding of the cosmos and shattering our naive perspectives on life in the universe. Trajectory analysis indicates when Dawn will fly over the designated location, and engineers will program it to take pictures or infrared spectra at that time. They will also include some margin, so they may program it to start 10 minutes before and end 10 minutes after. But they can't afford to put in too much margin. Data storage on the spacecraft is limited, so other geological features could not be observed. Also, transmitting data to Earth requires pointing the main antenna at that distant planet instead of pointing sensors at Ceres, so it would be unwise to collect much more than is necessary.
Even if devoting additional time (and data) to trying to observe the desired place were feasible, it wouldn't necessarily solve the problem. Dawn travels in a polar orbit, which is the only way to ensure that it passes over all latitudes. While Dawn soars from north to south over the sunlit hemisphere making its observations, the dwarf planet itself rotates on its axis, so the ground moves from east to west. If the spacecraft arrives at the planned orbital location a little early or a little late, the feature of interest may not even be beneath it but rather could be too far east or west, out of view of the instruments. In that case, increasing the duration of the observation period doesn't help.
All of that is why, as we saw last month, it requires more pictures to fully map Ceres than you might expect. Many pictures may have to be taken in order to fill in gaps, and quite a few of the pictures overlap with others. Nevertheless, Dawn has done an excellent job. The spacecraft has photographed 99.6 percent of the dwarf planet from this low altitude. (If you aren't regularly visiting the image gallery, you are missing out on some truly out-of-this-world scenes.)
The flight team devises very detailed plans that tell the spacecraft what to do every second, including where to point and what data to collect with each sensor. When the observation plans are developed, they are checked and double-checked. Then they are translated into the appropriate software that the robotic ship will understand, and these instructions are checked and double-checked. That is integrated with all the other software that will be beamed to the spacecraft covering the same period of time, any conflicts are resolved and then the final version is checked and, well, you know.
This process is very involved, and it is usually well over a month between the formulation and the execution of the plan. During that time, Dawn's orbit can deviate slightly from the expert navigators' mathematical predictions, preventing the spacecraft from flying over the desired targets. There are several reasons the actual orbit may differ from the orbit used for developing the plan. (We have seen related examples of this, including as Dawn approached Mars, when it orbited Vesta and when it spiraled from one mapping orbit to another.) Let's briefly consider two.
One reason is that we do not have perfect knowledge of the variations in the strength of Ceres' gravitational pull from one location to another. We have discussed before that measuring these tiny irregularities in the gravity field provides insight into the distribution of mass within the dwarf planet that gives rise to them. The team has mapped the hills and valleys of the field quite well and even better than expected. Still, the remaining small uncertainty can lead to slight differences between what navigators calculate Dawn's motion will be and what its actual motion will be as it is buffeted by the gravitational currents.
A second source of discrepancy is that Dawn's own activities distort its orbit. Every time the reaction control system expels a tiny burst of hydrazine to control the spacecraft's orientation, keeping it pointed at its target, the force not only affects the orientation but also nudges the probe in its orbit, slowing it down or speeding it up very slightly. It's up to the spacecraft to decide exactly when to make these small adjustments, and it is not possible for controllers to predict their timing. (In a similar way, when you are driving, you occasionally move the steering wheel to keep going the direction you want, even if is straight ahead. It would be impossible to forecast each tiny movement, because they all depend on what has already happened plus the exact conditions at the moment.) The details of the reaction control system activity also depend on the use of the novel hybrid control scheme, which the joint Orbital/JPL team developed because of the failure of two of the spacecraft's four reaction wheels. The effect of each small firing of hydrazine is very small, but they can add up.
It took about a month in this mapping orbit to discover many of the subtleties of the gravity field and gain experience with how hybrid control affects the orbit. But even before descending to this altitude, the operations team understood the nature of these effects and was well prepared to deal with them.
They devised several strategies, all of which are being used to good effect. One of the ways to account for Dawn's actual orbit differing from its planned orbit is simply to change the orbit. Simply? Well, not really. It turns out to that to analyze the orbit and then maneuver to correct it in a timely way is a surprisingly complicated process, but, come to think of it, what isn't complicated when flying a spaceship around a distant, alien world? Nevertheless, every three weeks, the flight team makes a careful assessment of the orbit and determines whether a small refinement with the ion propulsion system is in order. For technical reasons, if maneuvers are needed, they will be executed in pairs, so mission planners have scheduled two windows (each 12 hours long and separated by eight days) about every 23 days.
Adjustments to resynchronize the actual orbit with the predicted orbit that formed the basis of the exploration plan are known as “orbit maintenance maneuvers.” Succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as if it were a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)
For both Vesta and Ceres, trajectory analyses long in advance determined that OMMs would not be needed in the higher orbits, so no windows were included in those schedules. There have been three OMM opportunities since arriving at the lowest altitude above Ceres, but only the first was needed. Dawn performed the pair on Dec. 31-Jan. 1 and on Jan. 8 with its famously efficient ion engine. The orbit was good enough the next two times that OMMs were deemed unnecessary. It is certain that some future OMMs will be required. Your faithful correspondent provides frequent (and uncharacteristically concise) reports on Dawn's day-to-day activities, including OMMs.
By the end of the Jan. 8 OMM, Dawn's ion propulsion system had accumulated 2,019 days of operation in space, more than 5.5 years. During that time, the effective change in speed was 24,600 mph (39,600 kilometers per hour). (We have discussed in detail that this is not Dawn's current speed but rather the amount by which the ion engines have changed it.) This is uniquely high for a spacecraft to accomplish with its own propulsion system and validates our description of ion propulsion as delivering acceleration with patience. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)
The effect of Dawn's gentle ion thrusting during its mission has been nearly the same as that of the entire Delta II 7925H-9.5 rocket, with its nine external rocket engines, first stage, second stage and third stage. To get started on its interplanetary adventure, Dawn's rocket boosted it from Cape Canaveral to out of Earth orbit with only four percent higher velocity than Dawn subsequently added on its own with its ion engines.
As Dawn and Earth follow their own independent orbits around the sun (Dawn's now tied permanently to its gravitational master, Ceres), next month they will reach their greatest separation of the entire mission. On March 4 (about one Earth year after Ceres took hold of Dawn), on opposite sides of the solar system, they will be 3.95278 AU (367.434 million miles, or 591.328 million kilometers) from each other. (For those of you with full schedules, note that the maximum separation will be 5:40 a.m. PST.) They won't be this far apart again until Feb. 6, 2025, long after Dawn has ceased operating (as discussed below). The figure below depicts the arrangement next month.
Dawn has faced many challenges in its unique voyage in the forbidding depths of space, but it has surmounted all of them. It has even overcome the dire threat posed by the loss of two reaction wheels (the second failure occurring in orbit around Vesta 3.5 years and 1.3 billion miles, or 2.0 billion kilometers, ago). With only two operable reaction wheels (and those no longer trustworthy), the ship's remaining lifetime is very limited.
A year ago, the team couldn't count on Dawn even having enough hydrazine to last beyond next month. But the creative methods of conserving that precious resource have proved to be quite efficacious, and the reliable explorer still has enough hydrazine to continue to return bonus data for a while longer. Now it seems highly likely that the spacecraft will keep functioning through the scheduled end of its primary mission on June 30, 2016.
NASA may choose to continue the mission even after that. Such decisions are difficult, as there is literally an entire universe full of interesting subjects to study, but resources are more limited. In any case, even if NASA extended the mission, and even if the two wheels operated without faltering, and even if the intensive campaign of investigating Ceres executed flawlessly, losing not an ounce (or even a gram) of hydrazine to the kinds of glitches that can occur in such a complex undertaking, the hydrazine would be exhausted early in 2017. Clearly an earlier termination remains quite possible.
Regardless of when Dawn's end comes, it will not be a time for regret. The mission has realized its raison d'être and is reaping rewards even beyond those envisioned when it was conceived. It has taken us all on a marvelous interplanetary journey and allowed us to behold previously unseen sights of distant lands. The conclusion of the mission will be a time for gratitude that it was so successful. And until then, every new picture or other measurement adds to the richly detailed portrait of a faraway, exotic world. There is plenty more still to do before this remarkable story draws to a close.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.95 AU (367 million miles, or 591 million kilometers) from Earth, or 1,475 times as far as the moon and 3.99 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and six minutes to make the round trip.
In the mid-1970s, JPL evaluated several techniques for determining atmospheric water vapor effects on radiometric range. These experiments allowed the signals between spacecraft and the Deep Space Network antennas to be properly calibrated. One of the experiments was the Scanning Microwave Inversion Layer Experiment (SMILE). In May 1974, this test was conducted in El Monte, California, with a radiosonde suspended beneath a weather balloon. When the balloon reached 10,000 feet (about 3 km) it began measuring absolute pressure, ambient temperature, and relative humidity, then radioed the results to ground receivers.
A veteran interplanetary traveler is writing the closing chapter in its long and storied expedition. In its final orbit, where it will remain even beyond the end of its mission, at its lowest altitude, Dawn is circling dwarf planet Ceres, gathering an album of spellbinding pictures and other data to reveal the nature of this mysterious world of rock and ice.
Ceres turns on its axis in a little more than nine hours (one Cerean day). Meanwhile, its new permanent companion, a robotic emissary from Earth, revolves in a polar orbit, completing a loop in slightly under 5.5 hours. It flies from the north pole to the south over the side of Ceres facing the sun. Then when it heads north, the ground beneath it is cloaked in the deep dark of night on a world without a moon (save Dawn itself). As we discussed last month, Dawn's primary measurements do not depend on illumination. It can sense the nuclear radiation (specifically, gamma rays and neutrons) and the gravity field regardless of the lighting. This month, let's take a look at the other measurements our explorer is performing, most of which do depend on sunlight.
Of course the photographs do. Dawn had already mapped Ceres quite thoroughly from higher altitudes. The spacecraft acquired an extensive set of stereo and color pictures in its third mapping orbit. But now that Dawn is only about 240 miles (385 kilometers) high, its images are four times as sharp, revealing new details of the strange and beautiful landscapes.
Our spaceship is closer to Ceres than the International Space Station is to Earth. At that short range, it takes a long time to capture all of the vast territory, because each picture covers a relatively small area. Dawn’s camera sees a square about 23 miles (37 kilometers) on a side, less than one twentieth of one percent of the more than one million square miles (nearly 2.8 million square kilometers). In an ideal world (which is not the one Dawn is in or at), it would take just over two thousand photos from this altitude to see all the sights. However, as we will discuss in more detail next month, it is not possible to control the orbital motion and the pointing of the camera accurately enough to manage without more photos than that.
Most of the time, Dawn is programmed to turn at just the right rate to keep looking at the ground beneath it as it travels, synchronizing its rotation with its revolution around Ceres. It photographs the passing scenery, storing the pictures for later transmission to Earth. But some of the time, it cannot take pictures, because to send its bounty of data, it needs to point its main antenna at that distant planet, home not only to its controllers but also to many others (including you, loyal reader) who share in the thrill of a bold cosmic adventure. Dawn spends about three and a half days (nine Cerean days) with its camera and other sensors pointed at Ceres. Then it radios its findings home for a little more than one day (almost three Cerean days). During these communications sessions, even when it soars over lit terrain, it does not observe the sights below.
Mission planners have devised an intricate plan that should allow nearly complete coverage in about six weeks. To accomplish this, they guided Dawn to a carefully chosen orbit, and it has been doing an exceptionally good job there executing its complex activities.
Last month, we marveled at a stunning view that was not the typical perspective of peering straight down from orbit. Sometimes controllers now program Dawn to take a few more pictures after it stops aiming its instruments down, while it starts to turn to aim its antenna to Earth. This clever idea provides bonus views of whatever happens to be in the camera's sights as it slowly rotates from the point beneath the spacecraft off to the horizon. Who doesn't feel the attraction of the horizon and long to know what lies beyond?
Another of Dawn's scientific devices is two different sensors combined into one instrument. Like the camera, the visible and infrared mapping spectrometers (VIR) look at the sunlight reflected from the ground. (As we'll see below, however, VIR also can detect something more.) A spectrometer breaks up light into its constituent colors, just as a prism or a droplet of water does when revealing, quite literally, all the colors of the rainbow. Dawn's visible spectrometer would have a view very much like that. The infrared spectrometer, of course, looks at wavelengths of light our limited eyes cannot see, just as there are wavelengths of sound our limited ears cannot hear (consult with your dog for details).
A spectrometer does more than simply disperse the light into its components, however. It measures the intensity of that light at the different wavelengths. The materials on the surface leave their signature in the sunlight they reflect, making some wavelengths relatively brighter and some dimmer. That characteristic pattern is called a spectrum. By comparing these spectra with spectra measured in laboratories, scientists can infer the nature of the minerals on the ground. We described some of the intriguing conclusions last month.
VIR does still more. Rather than record the visible spectrum and the infrared spectrum from a single region, it takes spectra at 256 adjacent locations simultaneously. This would be like taking one column of 256 pixels in a picture and having a separate spectrum for each. By stitching columns together, you could construct the two dimensional picture but with the added dimension of an extensive spectrum at every location. (Because the extra information provides a sort of depth that flat pictures don't have, the result is sometimes called an “image cube.”) This capability to build up an image with spectra everywhere is what makes it a mapping spectrometer. VIR produces a remarkably rich view of its targets!
VIR's spectra contain much finer measurements of the colors and a wider range of wavelengths than the camera's images. In exchange, the camera has sharper vision and so can discern smaller geological features. In more technical terms, VIR achieves better spectral resolution and the camera achieves better spatial resolution. Fortunately, it is not a competition, because Dawn has both, and the instruments yield complementary measurements.
VIR generates a very large volume of data in each snapshot. As a result, Dawn can only capture and store relatively small areas of the dwarf planet with the mapping spectrometers, especially at this low altitude. Scientists have recognized from the first design of the mission that it would not be possible to cover all of Ceres (or Vesta) with VIR from the closer orbits. Nevertheless, Dawn has far exceeded expectations, returning a great many more spectra than anticipated. Still, as long as the spacecraft operates in this final mapping orbit, there will continue to be interesting targets to study with VIR.
Based on the nearly 20 million spectra of Ceres that VIR acquired from higher altitudes, the team has determined that new infrared spectra will provide more insight into the dwarf planet's character than the visible spectra. Because of their composition, the minerals display more salient signatures in infrared wavelengths than visible. The excellent visible spectra from the first three mapping orbits are deemed more than sufficient. Therefore, to make the best use of our faithful probe and to dedicate the resources to what is most likely to yield new knowledge about Ceres, VIR is devoting its share of the mission data in this final orbit to its infrared mapping spectrometer. We have many more exciting discoveries to look forward to!
The infrared light Ceres reflects from the sun can tell scientists a great deal about the composition, but they can learn even more from analyzing VIR's measurements. The sun isn't the only source of infrared. Ceres itself is. Many people correctly associate infrared with heat, because warm objects emit infrared light, and the strength at different wavelengths depends on the temperature. That calls for measuring the spectrum! Distant from the sun though it is, Ceres is warmed slightly by the brilliant star, so it has a very faint infrared glow of its own. Scientists can distinguish in VIR's observations between the reflected infrared sunlight and the infrared light Ceres radiates. In essence, VIR can function as a remote thermometer.
Last month, in one of Dawn's best photos yet of Ceres, we considered planning a hike across a breathtaking landscape. In case we do, VIR has shown we should be prepared for chilly conditions. Observed temperatures (all rounded to the nearest multiple of five) during the day on the dwarf planet range from -135 degrees Fahrenheit (-95 degrees Celsius) to -30 degrees Fahrenheit (-35 degrees Celsius). (It is so cold in some locations and times, especially at night, that Ceres produces too little infrared light for VIR to measure. Temperatures below the coldest reported here actually don't register.) This finding provides compelling support for this writer's frequent claim that Ceres is really cool. In addition, knowing the temperatures will be very important for understanding geological processes on this icy, rocky world, just as we know the movement of terrestrial glaciers depends on temperature.
Your loyal correspondent can't -- or, at least, won't -- help but indulge his nerdiness with a brief tangent. The range of temperatures above represent the warmest on Ceres, given that VIR cannot measure lower values. It's amusing, if you have a similar weird sense of humor, that Ceres' average temperature apparently is not that far from what it would be for a black hole of the same mass. We won't delve into the physics here, but such a black hole would be -225 degrees Fahrenheit (-140 degrees Celsius). OK, enough hilarity. Back to Dawn and Ceres...
Ever creative, scientists are attempting another clever method to gain insight into the nature of this exotic orb. When Dawn is at just the right position in its orbit on the far side of Ceres, so that a straight line to Earth passes very close to the limb of Ceres itself, the spacecraft's radio signal will actually hit the dwarf planet. The radio waves interact with the materials on the surface, which can induce an exquisitely subtle distortion. After bouncing off the ground at a grazing angle, the radio signal continues on its way, heading toward Earth. The effect on the signal is much too small to affect the normal communications at all, but specialized equipment at NASA's Deep Space Network designed for this purpose might still be able to detect the tiny changes. The fantastically sensitive antennas measure the properties of the radio waves, and by studying the details, scientists may be able to learn more about the properties of the surface of the distant world. For example, this could help them distinguish between different types of materials (such as ice, rocks, sand, etc.) as well as reveal how rough or smooth the ground is at scales far, far smaller than the camera can discern. This is an extremely challenging measurement, and no small distortions have been detected so far, but always making the best possible use of the resources, scientists continue to look for them.
In addition to those bonus measurements, Dawn remains very productive in acquiring infrared spectra, photographs, gamma ray spectra and neutron spectra plus conducting measurements of the massive body's gravitational field, all of which contribute to unlocking the mysteries of the first dwarf planet ever discovered or explored. The venerable adventurer is in good condition and is operating flawlessly.
We have discussed extensively the failures of two of the four reaction wheels, devices Dawn used to depend on to control its orientation in space. Without three healthy reaction wheels, the probe has had to rely instead on hydrazine propellant expelled from the small jets of the reaction control system. (When Dawn uses its ion engine, that remarkable system does double duty, reducing the need for the hydrazine.)
For most of the time since escaping from Vesta's gravitational clutches in 2012, Dawn has kept the other two reaction wheels in reserve so any remaining lifetime from those devices could offset the high cost of hydrazine propellant to turn and point in this current tight orbit. Those two wheels have been on and functioning flawlessly since Dec. 14, 2015, and every day they operate, they keep the expenditure of the dwindling supply of hydrazine to half of what it would be without them. (Next month we will offer some estimates of how long Dawn might continue to operate.) But the ever-diligent team recognizes another wheel could falter at any moment, and they remain ready to continue the mission with pure hydrazine control after only a short recovery operation. If a third failure is at all like the two that have occurred already, the hapless wheel won't give an indication of a problem until it's too late. A reaction wheel failure evidently is entirely unpredictable. We'll know about it only after it occurs in the remote depths of space where Dawn resides at an alien world.
Earth and Ceres are so far from each other that their motions are essentially independent. The planet and the dwarf planet follow their own separate repetitive paths around the sun. And each carries its own retinue: Earth has thousands of artificial satellites and one prominent natural one, the moon. Ceres has one known satellite. It arrived there in March 2015, and its name is Dawn.
Coincidentally, both reached extremes earlier this month in their elliptical heliocentric orbits. Earth, in its annual journey around our star, was at perihelion, or the closest point to the sun, on Jan. 2, when it was 0.98 AU (91.4 million miles, or 147 million kilometers) away. Ceres, which takes 4.6 years (one Cerean year) for each loop, attained its aphelion, or greatest distance from the sun, on Jan. 6. On that day, it was 2.98 AU (277 million miles, or 445 million kilometers) from the gravitational master of the solar system.
Far, far from the planet where its deep-space voyage began, Dawn is now bound to Ceres, held in a firm but gentle gravitational embrace. The spacecraft continues to unveil new and fascinating secrets there for the benefit of all those who remain with Earth but who still look to the sky with wonder, who feel the lure of the unknown, who are thrilled by new knowledge, and who yearn to know the cosmos.
Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.87 AU (360 million miles, or 580 million kilometers) from Earth, or 1,440 times as far as the moon and 3.93 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and four minutes to make the round trip.