On the other side of the solar system, invisible by virtue both of the blinding glare of the sun and by the vastness of the distance, Dawn is continuing its remarkable cosmic adventure.
Orbiting high above dwarf planet Ceres, the spacecraft is healthy and performing all of its assignments successfully even when confronted with what appears to be adversity.
In the last four Dawn Journals, we described the ambitious plans to maneuver the craft so it would cross the line from the sun to Ceres on April 29 and take pictures plus infrared and visible spectra from that special perspective. With Dawn between the sun and Ceres, the alignment is known as opposition, because from the spacecraft’s point of view, Ceres is opposite the sun.
As explained in March, those opposition measurements may provide clues to the nature of the material on the ground with much greater detail than the camera or other sensors could ever discern from orbit. The veteran explorer carried out its complex tasks admirably, and scientists are overjoyed with the quality of the data.
Dive into a sea of Oceans Melting Greenland data
"Get to work." The phrase stuck in my head.
I had just walked out of a two-and-a-half-hour debriefing with NASA’s Oceans Melting Greenland (OMG) Principal Investigator Josh Willis, but the whole meeting could be summed up in those three little words of his: Get to work.
It was as though he’d been ringing one of those big ol’ dinner gongs. Data! Hot off the press! Come and get your data! Calling all oceanographers, geologists, paleo-climate scientists: come and get a big ol’ helping of free data.
He made me hungry for data, too.
OMG has just returned from its second spring season. Every April for five years, just before the ice starts to melt, OMG flies a radar instrument over almost every glacier in Greenland that reaches the ocean and collects elevation measurements within a 6.2-mile (10-kilometer)-wide swath for each glacier individually so we can measure how quickly each one is thinning. That’s literally hundreds of glaciers.
“We have more than 70 of these swaths that cover a couple hundred glaciers to create new elevation maps that are high accuracy, high resolution and high quality,” Willis said.
OMG also has bathymetry data from sonar and gravimetry. And we have a year’s worth of Airborne Expendable Conductivity Temperature Depth Probes AXCTD data collected last September plus hundreds of vertical profiles of temperature and salinity taken from ship surveys. “We have temperature measurements in many glacial fjords that have never had a historical temperature profile before. And none of that data is being used to its fullest extent yet.” OMG will set the baseline so we know what the water temperatures are today, and as we look to the future, we can watch them warm. That’s huge.
I recounted all the times I’ve told someone that many parts of the ocean are still so unknown. I thought about all the times I’ve written about the OMG aircraft flying into remote, uncontrolled airspace, or researching the ocean water-ice interface around Greenland: So many of these places still nameless, still anonymous, still unidentified, still unknown. It’s mind blowing.
And somewhere in all this new data is information about the correlation between the ocean water and the ice as well as the answer to the question of how each glacier may or may not be affected by the waters offshore. “We know that warm water reaches a lot of glaciers. And there have been surveys in few places, but we’ve never had a comprehensive survey of the shelf water before,” Willis said.
OMG is mapping out the edges of glaciers and watching them change year on year on year. The mission measures glacial elevation in the last few kilometers before the glacier hits the water to see exactly how much the glacier shrank or retreated or both. In a few cases, the opposite might happen. Over a single year, a glacier might not have had as much calving or it might have slowed down, which would cause it to thicken and advance.
There are literally hundreds of glaciers to research and dozens of papers buried in that data. And anybody who wants to can sift through it and publish. “You could get a Ph.D. done really fast,” Willis added enticingly. Here are some recommendations for interesting scientific research:
- OMG’s temperature data could be used to write oceanography papers about where the warm water is on the shelf and to map out and catalogue which glaciers terminate in deep Atlantic water and which ones sit in shallow water. OMG has enough data to catalogue the depth of the faces of two-thirds of the glaciers around Greenland.
- Paleo-climatologists and geologists can use new clearly mapped-out OMG bathymetry data to study how ancient glaciers carved troughs in the sea floor. Looking at maps of the seafloor will help us understand the implications for Greenland’s ancient ice sheet. Some flat-bottomed troughs, for example, show evidence of where little ancient rivers must have carved their way through to erode the paleo-glaciers. And sea floor sediments could be analyzed to find out how far the ancient glaciers advanced.
- Overview papers that compare and contrast the east, west, north and south coasts of Greenland would be incredibly useful to have.
- Some elevation maps made from historical datasets as well as a few decades’ worth of temperature measurements already exist for some isolated regions across Greenland. Using these historical maps, it’s now possible to compare them with current measurements of temperature and elevation in these locations to observe the changes.
- OMG is also gathering oceanography data around Greenland. Since the Atlantic Ocean water is very warm and salty and the Arctic Ocean water is cold and fresh, the ratio of those two could be analyzed. Warm Atlantic Ocean water has been in the coastal area around Greenland forever, but how much Atlantic water makes it onto the shelf and reaches the glaciers? This is affected by the bathymetry and the winds, which affect the local currents. And according to Willis, “There’s really still a lot to learn.”
Already there are four downloadable datasets right here! So, come and get it, all you hungry Ph.D. oceanographers.
Get to work.
I can't wait to read your papers,
Dawn has accomplished an extraordinary orbital dance.
Dawn has accomplished an extraordinary orbital dance. It completed the cosmic choreography with the finesse and skill that have impressed fans since its debut in space nearly a decade ago. Dawn’s latest stellar performance with Ceres took two months and four acts. (Although Ceres played an essential role in the performance, it was much easier than Dawn’s. Ceres’ part was to exert a gravitational pull, which, thanks to all the mass within the dwarf planet, is pretty much inevitable.)
In February, we presented a detailed preview of the spacecraft’s extensive orbital maneuvering with its ion engine. Now, like so many of Dawn’s cool plans, that complex flight is more than an ambitious goal. It is real. (And the Dawn project will negotiate with any theme park that would like to turn that or any of our other deep-space feats into rides. Another good candidate is here.)
But there is more to do. The reason for such dramatic changes in the orbit is not to show off the flight team’s prowess in piloting an interplanetary spaceship. Rather, it is so Dawn’s new orbital path will cross the line from the sun to the gleaming center of Occator Crater on April 29. From the explorer’s point of view at that special position, Occator will be opposite the sun, which astronomers (and readers of the last three Dawn Journals) call opposition. Last month we explained the opposition surge, in which photographing the crater’s strikingly bright region, known as Cerealia Facula, may help scientists discover details of the reflective material covering the ground there, even at the microscopic level.
Dawn is multitasking. Even as it was executing its space acrobatics, and when it measures the opposition surge later this week, its most important duty is to continue monitoring cosmic rays. Scientists use the spacecraft’s recordings of the noise from this space radiation to improve the measurements it made at low altitude of radiation emitted by Ceres.
Now that Dawn is on course for opposition, let’s take a look at the observations that are planned. Measuring the opposition surge requires more than photographing Cerealia Facula right at opposition. The real information that scientists seek is how the brightness changes over a small range of angles very near opposition. They will compare what Dawn finds for Cerealia Facula with what they measure in carefully designed and conducted laboratory experiments.
To think about Dawn’s plan, let’s consider a clock. Ceres is at the center of the face with its north pole pointing toward the 12. As in this figure, the sun is far, far to the left, well outside the 9 and off the clock. This arrangement matches the alignment in this figure.
Now let’s put the spacecraft on the tip of the second hand, so it takes only one minute to orbit around Ceres. (In reality, it will take Dawn 59 days to complete one revolution in this new orbit, but we’ll speed things up here. We can also ignore for now that Dawn’s orbit is not circular. That would correspond, for example, to the length of the second hand changing as it goes around. This clock doesn’t have that feature.) If the clock were one foot (30 centimeters) across, Ceres would be a little more than a quarter of an inch (seven millimeters) in diameter, or smaller than a pea. Dawn is at a high altitude now, which is why Ceres is so small on the clock.
With this arrangement, opposition is when the second hand is on the 9 and Occator is pointed in that direction as well, so the sun, spacecraft and crater are all on the same line. All of the opposition surge measurements need to occur within about one second of the 9, and most of them have to be within a quarter of a second of that position. This precision has created quite a challenge to the flight team for navigating to and performing the observations.
Readers have long clamored for more information on clocks in the Dawn gift shops, which we have not addressed in more than three years. (Most, of course, clamor for refunds. For that, please take your clock in person to the refund center nearest you, which usually is near the largest black hole in your galaxy.) We hope the discussion this month has filled that horological void.
Dawn had this view in its third mapping orbit at an altitude of 915 miles (1,470 kilometers). It shows another example of material that flowed on the ground. A powerful impact occurred on the northwest rim of Datan Crater, creating the unnamed 12-mile (20-kilometer) near the top of the picture. The impact melted or even vaporized some material and unleashed a flow that extends south as much as 20 miles (32 kilometers). With a thickness of a few tens of yards (meters), it is not nearly as deep as the flow in the photo above. This scene is at 60°N, 247°E on this map. Dawn obtained more detailed photos of this region from a lower altitude, but this terrain covers such a large area that it’s easier to take it all in with this picture. (We presented an even broader view of this region here.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
The problem would be difficult enough if Ceres presented Occator to Dawn as a bright bullseye for the camera, but the dwarf planet is not that cooperative. Rather, like all planetary bodies, Ceres turns on its axis, so even if Dawn managed to hover on the line from the sun to Ceres, Occator would be visible only half the time. The rest of the time, the crater would be on the other side of Ceres, cloaked in the darkness of night (which would compromise a measurement of how much sunlight it reflects) and blocked from Dawn’s view by an opaque dwarf planet 584 miles (940 kilometers) in diameter.
Of course, Dawn can’t hover, and Occator is a moving target that’s not visible half the time. That introduces further complications. As Ceres’ rotation brings Occator from night into day (that is, it is sunrise -- dawn! -- at Occator), the crater will be on the limb from Dawn’s perspective. (Remember, Dawn is aligned with the sun.) The foreshortening would make a poor view for measuring the opposition surge. We need to have the crater closer to the center of the disc of Ceres, displaying its bright terrain for Dawn to see, not near the edge, where Cerealia Facula would appear compressed. (In November we saw a photo of Occator near the limb. When Dawn measures the opposition surge, it will be more than 13 times higher.)
Dawn’s orbit has been carefully designed so the spacecraft will cross the line from the sun to Occator when the crater is along the centerline of Ceres. That will give Dawn the best possible view. At that time, the sun will be as high as it can be that day from Occator’s perspective. Because the crater is at 20°N latitude, and Ceres’ axis is tilted only 4 degrees, the sun does not get directly overhead, but it reaches its highest point at noon.
If that is confusing, think about your own location on your planet. For most terrestrial readers, the sun never gets directly overhead (and for all, there are long stretches of the year in which it does not). But as the sun arcs across the sky from morning until evening, its highest point, closest to the zenith, is at noon. Now think about the same thing from the perspective of being far out in space, along the line from the sun to Earth, looking down on Earth as it rotates. That location will come over the limb at sunrise. (That sunrise is for someone still there on the ground. From your vantage point in space, the sun is behind you and Earth is in front of you.) Then the turning Earth will carry it to the other limb, where it will disappear over the horizon at sunset. The best view from space will be in the middle, at noon. If you have a globe, you can confirm this. Just remember that because of the tilt of Earth’s axis, the sun always stays between 23.5°N and 23.5°S. If it’s still confusing, don’t worry! You don’t need to understand this detail to follow the description of the observation plan, and you may rest assured that the Dawn team has a reasonably good grasp of the geometry.
Dawn observed this pair of overlapping craters near 50°N, 126°E from an altitude of 915 miles (1,470 kilometers) in its third mapping orbit. A broad landslide reaches as much as nine miles (15 kilometers) northeast from both craters. Flows with characteristics like this are found in many locations on Ceres, taking long paths on shallow slopes outside crater walls rather than inside. In general, they did not form at the time the associated craters did but are the result of subsequent processes. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Dawn’s orbital path is timed to make opposition occur as close as possible to 12:00:00 in the Occator Standard Time zone, and that happens to be 2:46:20 a.m. PDT on April 29. (We are glossing over many complications, but one fortunate simplification in the problem is that Cereans do not use daylight saving time. The Cerean day is only nine hours and four minutes long, but they’re so far from the sun that they don’t even bother trying to save daylight.)
Dawn will photograph Ceres extensively during the brief period around opposition. The spacecraft will be around 12,400 miles (20,000 kilometers) above Ceres, a view that would be equivalent to seeing a soccer ball 15 feet (4.7 meters) away. Occator Crater will be like a scar on the ball less than seven-eighths of an inch (2.2 centimeters) wide. The principal target, Cerealia Facula, would be a glowing pinhead, not even a tenth of an inch (about two millimeters) across, at the center of the crater.
Dawn took this photo of Ceres on March 28 from an altitude of 30,100 miles (48,400 kilometers) during its long coast to even greater heights. (The trajectory is described here.) Navigators used this and other pictures taken then to help pin down the spacecraft’s position in orbit in preparation for the third period of ion thrusting on April 4-12. (When we described the plan in February, the thrusting was scheduled for April 3-14. Dawn’s orbital trajectory following the two previous thrust segments was so good that not as much thrusting was needed.) Another navigation image taken after that maneuver is below. When Dawn photographs Occator Crater at opposition on April 29, they will be closer together, so Ceres will show up with 2.4 times more detail than here. More significant will be that the sun will be directly behind Dawn, so Ceres will appear as a fully illuminated disc (like a full moon rather than a half moon, or, to be more appropriate for this mission, like a full dwarf planet). This scene is centered at 33°S, 228°E, and most of what’s illuminated here is east of that location on this map. Near the top is Occator Crater, with its famously bright Cerealia Facula appearing as a bright spot. The crater is 57 miles (92 kilometers) across. Just below and to the right of center is the prominent Urvara Crater. At 106 miles (170 kilometers) in diameter, Urvara is the third largest crater on Ceres. We have seen Urvara in much finer detail several times before, most recently in October. To its right is Yalode, the second largest crater, 162 miles (260 kilometers) in diameter. We saw some intriguing details of its geology last month. The picture below includes the largest crater on Ceres. Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Dawn has spent a great deal of time scrutinizing Ceres from more than 50 times closer (see this table for a summary, including comparisons with a soccer ball for other altitudes). To accomplish this new goal, however, we don’t need high resolution. There are other technical considerations that require the greater altitude. We have already seen Cerealia Facula in as much detail as Dawn will ever reveal. But thanks to the team’s creativity, we have the possibility of learning about it on a far finer scale than had ever been considered.
As we have discussed before, scientists will study the handful of pixels in each image that contain Cerealia Facula to determine how the brightness changes as the viewing angle changes. Throughout its observations, Dawn will take pictures covering a range of exposures. After all, we don’t know how large or small the surge in brightness will be. The objective is to find out. The plan also includes taking pictures through the camera’s color filters to help determine whether the strength of the opposition surge depends on the wavelength of light. (Coherent backscatter may be more sensitive to the wavelength than shadow hiding.) In addition, the probe will collect visible and infrared spectra. (Dawn’s photos and spectra will capture a great many more locations on Ceres than Cerealia Facula. Indeed, well over half of the dwarf planet will be observed near opposition. The data for all these other locations will provide opportunities for still more valuable insights.)
Dawn took this photo of Ceres on April 17 from an altitude of 27,800 miles (44,800 kilometers). Like the one above, this was taken to help navigate the spacecraft to opposition. Based on the navigation pictures and other data, the operations team developed a pair of trajectory correction maneuvers to fine tune the orbit. (This maneuvering was depicted in the figures in February as the fourth and final thrusting segment. The spacecraft executed the first with five hours of ion thrusting on April 22. It was scheduled to perform the second with a little less than 4.5 hours on April 23-24, but, as the last update to this Dawn Journal before it was posted, that did not occur. See the postscript.) This scene is centered at 52°S, 110°E, and the landscape in sunlight is to the east on this map. In the upper right is Kerwan, the largest Cerean crater at 174 miles (280 kilometers) in diameter. (We saw a close-up of part of this crater in October.) Kerwan is noticeably polygonal because the crater walls formed along preexisting underground fractures when the impactor struck. The largest crater in the grouping just below and right of center is Chaminuka Crater, which is 76 miles (122 kilometers) across. (Chaminuka was a spirit and prophet among the Shona people in what is now Zimbabwe. He could cause a barren tree to bear food and rain to come during a drought. Chaminuka also could turn into a child, a woman, an old man or even a ball. Despite these talents, there’s no evidence the prophet foretold anything about the geology of Ceres nor ever turned into a crater.) Full image and caption. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Although observing the opposition surge is a bonus in the extended mission, and not as high a priority as many of Dawn’s other scientific assignments, the operations team has taken extra measures to improve the likelihood of it working. Occasionally the camera experiences a glitch, perhaps from cosmic rays, that temporarily prevents the instrument from taking pictures. Therefore, for the opposition surge, the spacecraft will use both the primary camera and the backup camera. Even with well over 85,000 photos during Dawn’s exploration of Vesta and Ceres, the two cameras have been operated simultaneously only once. That was in February, and the purpose then was to verify that the cameras and all other systems (including spacecraft thermal control, data management and even extensive mission control software on distant Earth) would perform as engineers predicted. That test was successful and helped prepare for this upcoming observation.
The plan to measure the opposition surge on Ceres is complex and challenging, and the outcome is by no means assured. But that’s the nature of most efforts to uncover the universe’s secrets. After all, an expedition to orbit and explore two uncharted worlds that had appeared as little more than pinpoints of light among the stars for two centuries, the two largest bodies between Mars and Jupiter, is complex and challenging, and yet it has accomplished a great deal more than anticipated. The reward for such a bold undertaking is the thrill of new knowledge. But there are also rewards in engaging in the endeavor itself, as the spacecraft transports us far from the confines of our humble planetary residence. Such a journey fuels the fires of our passion for adventure far from home and our yearning for new sights and new perspectives on the cosmos.
Dawn is 17,800 miles (28,700 kilometers) from Ceres. It is also 3.64 AU (339 million miles, or 545 million kilometers) from Earth, or 1,505 times as far as the moon and 3.62 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and one minute to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT April 25, 2017
P.S. Just before this Dawn Journal was to be posted on April 24, when a scheduled telecommunications session began, the flight team discovered that the third of the spacecraft’s four reaction wheels had failed. We have written a great deal about these devices and the team’s extraordinary creativity in conducting an extremely successful mission without a full complement. The unit failed before the final, short period of ion thrusting, and the spacecraft correctly responded by entering one of its safe modes and assigning control of its orientation to the hydrazine thrusters. That meant it could not execute the brief maneuver, which would have changed the speed in orbit by 1.4 mph (2.3 kilometers per hour). The team quickly diagnosed the condition and returned the spacecraft to normal operation (still using hydrazine control) on April 25. They also determined that Dawn’s trajectory is close enough to the original plan that the opposition surge measurements can still be conducted. This experienced group of space explorers knows how to do it without the reaction wheels. (For most of the time since Dawn left Vesta in 2012, including the first year of Ceres operations, all four wheels were turned off. This will be no different.) See this mission status update for additional information. Next month’s Dawn Journal will include this new chapter in the reaction wheel story, the outcome of the attempt to observe the opposition surge and more.
NASA flies northward to monitor Greenland’s glaciers
I looked out the window of NASA’s modified G-III aircraft across the expanse. I knew what I would see. I knew it would look like white pillow-y ripples going on and on and on, way farther than anyone could see, like a vast field of white sand dunes stretching away into the distance.
The Oceans Melting Greenland (OMG) aircraft was flying across the entire top half of Greenland from the northwest coast to the northeast coast to make the day’s first science measurements. And the first science flight line was all the way across the Greenland Ice Sheet, across 620 miles (1,000 kilometers) of ice that’s up to 2 miles thick and hundreds of thousands of years old. And although I’d flown in Greenland a bunch of times before, I’d only ever flown over the coastal areas, where glaciers around the ice sheet’s edges carve their way through the Greenland terrain, to cut out deep, narrow fjords over centuries’ time.
Everything here is vast and expansive: the size, the views, the enormous quantity of ice.
Two days before, I’d trekked up to the ice sheet with a few members of the OMG team. We stood in the insanely cold, dry, biting air (Greenland is one of the least humid areas on planet Earth, with the cleanest, clearest air) and gazed into the incomprehensible distance. It was easy to use a snow boot to scrape the 2 inches or so of fine, dry, powdery snow away from the ice sheet to uncover the hard, greenish blue ice.
On the edge of the ice sheet, a slice of ancient ice layers was exposed like a glistening wall, and we’d walked past it on the way up to the top of the sheet. The ice wall was so vertical and so sheer, the snow that hid the other parts of the edge had fallen away, and we could see its smooth surface shining like a gem: striped blue and green. That ice is hundreds of thousands of years old, made from snow that fell year after year after year, eventually becoming compressed and preserved in this cold, dry desert environment.
Standing on top of the ice sheet, I imagined it under my feet, going down and down and down for a mile or more.
A mile—or more—of ice.
Everything here is vast and expansive: the size, the views, the enormous quantity of ice. Flying over them, the glaciers look like hundreds of broad frozen rivers, each one up to a few miles across, each one channeling its way from the interior of the landmass toward the sea over thousands of years. Each glacier carved out a fjord through the rock and out to sea in the same way a river erodes its channel, except it’s so much bigger, so much slower and the erosional power of the ice is so much more intense. From up here, the glacier’s impossibly slow creep seems frozen in both space and time. But the glaciers are moving. Stress fractures or crevasses, which are easy-to-observe evidence of glacier movement, form as the glaciers slope downhill toward the sea. And of course, we also have scientific measurements. Detailed satellite images show that the terminal edges of many glaciers such as Jakobshavn have receded by as much as 0.4 miles (600 meters) per year in recent times. Scientists also have time-lapse footage of seaward glacier flow.
But having evidence of glacier flow, and even glacier recession, is only part of the story. As a warmer atmosphere and a warmer ocean around the coastline continue to melt the massive amount of ice that covers Greenland, the ice ends up flowing into the ocean, which causes sea level rise worldwide.
As we flew over, the GLISTIN-A instrument received data from a 12-kilometer swath of whatever is below and off to the sides of it, in this case glaciers. Using these data, we can measure, with great precision, the height of each glacier we fly over. See, when the end of an individual glacier melts and calves into the ocean, the whole glacier speeds up and flows even faster downhill toward the ocean because there’s less friction against the sides and bottom to slow it down. The faster it moves, the more it stretches — like pulled taffy — and when a glacier is all stretched out, its elevation is lower. And because OMG will fly the same science lines along the same coastal glaciers every year for five years in a row, we’ll be able to find out how much elevation each glacier has lost, how fast it’s flowing into the ocean and how much ice has been lost.
They sure appear stable, still, enduring. But they’re not. They’re melting.
They sure appear stable, still, enduring. But they’re not. They’re melting.
And northern Greenland, along with the rest of the higher latitudes in the Northern Hemisphere, is experiencing some of the most intense impacts of global climate change right now, today.
Thank you for reading.
Now in its third year of orbiting a distant dwarf planet, a spacecraft from Earth is as active as ever. Like a master artist, Dawn is working hard to add fine details to its stunning portrait of Ceres.
In this phase of its extended mission, the spacecraft’s top priority is to record space radiation (known as cosmic rays) in order to refine its earlier measurements of the atomic species down to about a yard (meter) underground. The data Dawn has been collecting are excellent.
As we explained in January, the ambitious mission has added a complex bonus to its plans. The team is piloting the ship through an intricate set of space maneuvers to dramatically shift its orbit around Ceres. They are now about halfway through, and it has been smooth sailing. Dawn is on course and on schedule. (If you happen to be one of the few readers for whom it isn’t second nature to plan how to change a spacecraft’s orbit around a dwarf planet by 90 degrees and then fly it under control of ion engine, last month’s Dawn Journal presents a few of the details that may not be obvious. And you can follow the adventurer’s orbital progress with the regular mission status updates.)
If all goes well, on April 29 the new orbit will take Dawn exactly between the sun and the famous bright region at the center of Occator Crater. Named Cerealia Facula, the area is composed largely of salts. (Based on infrared spectra, the strongest candidate for the primary constituent is sodium carbonate). The probe will be at an altitude of about 12,400 miles (20,000 kilometers), or more than 50 times higher than it was in 2016 when it captured its sharpest photos of Occator (as well as the rest of Ceres’ 1.1 million square miles, or 2.8 million square kilometers). But the objective of reaching a position at which the sun and Ceres are in opposite directions, a special alignment known as opposition, is not to take pictures that display more details to our eyes. In fact, however, the pictures will contain intriguing new details that are not readily discerned by visual inspection. Dawn will take pictures as it gets closer and closer to opposition, covering a range of angles. In each image, scientists will scrutinize the handful of pixels on Cerealia Facula to track how the brightness changes as Dawn’s vantage point changes.
We described the opposition surge, in which the reflected sunlight at opposition may be significantly brighter than it is in any other geometrical arrangement. A few degrees or even a fraction of a degree can make a large difference. But why is that? What is the underlying reason for the opposition surge? What can we learn by measuring it? And is the best cake better than the best candy?
Those are all interesting and important questions. We will address some of them here and leave the rest for your own thorough investigation.
There are at least three separate physical effects that may contribute to the opposition surge. One of them is known as shadow hiding. When the sun shines on the ground, tiny irregularities in the surface, even at the microscopic level, will cast shadows. When you look at the ground, those shadows collectively detract from its overall brightness, even if each individual shadow is too small for you to see. The total amount of light reflected off the ground and into your eyes (or your camera) is less than it would be if every point, no matter how small, were well lit. However, if you look along the same direction as the incoming light, then all the shadows will be hidden. They will all be on the opposite side of those tiny irregularities, out of reach of both the incident light and your sight. In that case, anything you can see will be illuminated, and the scene will be brighter. The figure below is intended to illustrate this phenomenon of shadow hiding (and excluding the caption, the picture is probably worth almost 480 words).
The opposition surge was first described scientifically in 1887 by Hugo von Seeliger, an accomplished astronomer and highly esteemed teacher of astronomers. He analyzed data collected by Gustav Müller when Earth’s and Saturn’s orbits around the sun brought Saturn into opposition, and the brightness of the rings increased unexpectedly. Seeliger realized that shadow hiding among the myriad particles in the rings could explain Müller’s observations. The opposition surge is occasionally known as the Seeliger effect. (Although astronomers had been observing the rings for more than two centuries by then, a careful scientific analysis to show that the rings were not solid but rather composed of many small particles had only been completed about 30 years before Seeliger’s advance.)
Now astronomers recognize the opposition surge on many solar system bodies, including Earth’s moon and the moons of other planets, as well as Mars and asteroids. In fact, it also occurs on many materials on Earth, including vegetation. Scientists exploit the phenomenon to determine the character of materials at a distance when they can make careful measurements at opposition.
For many solar system objects, however, it is difficult or impossible to position the observer along the line between the sun and the target. But thanks to the extraordinary maneuverability provided by Dawn’s ion engine, we may be able to perform the desired measurement in Occator Crater.
It was nearly a century after Seeliger’s description of shadow hiding before scientists realized that there is another contributor to the opposition surge, which we mention only briefly here. It depends on the principle of constructive interference, which applies more in physics than in politics. Waves (in this case, light waves) that have their crests at the same places can add up to be especially strong (which makes the light bright). (Destructive interference, which may be more evident outside of the physics realm, occurs when troughs of one wave cancel crests of another.) We will not delve into why constructive interference tends to occur at opposition, but anyone with a thorough understanding of classical electromagnetic theory can work it out, as physicists did in the 1960s to 1980s. (More properly, it should be formulated not classically but quantum mechanically, but we recognize that some readers will prefer the former methodology because it is, as one physicist described it in 1968, "much simpler and more satisfying to the physical intuition." So, why make it hard?) For convenient use to ruin parties, the most common term for constructive interference in the opposition surge is coherent backscatter, but it sometimes goes by the other comparably self-explanatory terms weak photon localization and time reversal symmetry. Regardless of the name, as the light waves interact with the material they are illuminating at opposition, constructive interference can produce a surge in brightness.
The intensity of the opposition surge depends on the details of the material reflecting the light. Even the relative contributions of shadow hiding and coherent backscatter depend on the properties of the materials. (While both cause the reflected light to grow stronger as the angle to opposition shrinks, coherent backscatter tends to dominate at the very smallest angles.)
Especially sensitive laboratory measurements show that sometimes shadow hiding and coherent backscatter together are not sufficient to explain the result, so there must be even more to the opposition surge. The unique capability of science to explain the natural world, shown over and over and over again during the last half millennium, provides confidence that a detailed theoretical understanding eventually will be attained.
Part of science’s success derives from its combination of experiment and theory. For now, however, the opposition surge is more in the domain of the former than the latter. In other words, translating any opposition surge observation into a useful description of the properties of the reflecting material requires controlled laboratory measurements of well characterized materials. They provide the basis for interpreting the observation.
If Dawn accomplishes the tricky measurements (which we will describe next month), scientists will compare the Cerealia Facula opposition surge with lab measurements of the opposition surge. As always in good science, to establish the details of the experiments, they will start by integrating the knowledge already available, including the tremendous trove of data Dawn has already collected -- spectra of neutrons, gamma rays, visible light and infrared light plus extensive color and stereo photography and gravity measurements. In the context of their understanding of physics, chemistry and geology throughout the solar system, scientists will determine not only the mixtures of chemicals to test but also the properties such as grain sizes and how densely packed the particles are. They will perform experiments then on many combinations of credible facular composition and properties. Comparing those results with Dawn’s findings, they will be able to elucidate more about what really is on the ground in that mesmerizing crater. For example, if they determine the salt crystals are small, that may mean that salty water had been on the ground and sublimated quickly in the vacuum of space. But if the salt came out of solution more slowly underground and was later pushed to the surface by other geological processes, the crystals would be larger.
It is an impressive demonstration of the power of science that we can navigate an interplanetary spaceship to a particular location high above the mysterious, lustrous landscape of a distant alien world and gain insight into some details that would be too fine for you to see even if you were standing on the ground. Using the best of science, Dawn is teasing every secret it can from a relict from the dawn of the solar system. On behalf of everyone who appreciates the majesty of the cosmos, our dedicated, virtuoso artist is adding exquisite touches to what is already a masterpiece.
Dawn is 31,400 miles (50,500 kilometers) from Ceres. It is also 3.48 AU (324 million miles, or 521 million kilometers) from Earth, or 1,430 times as far as the moon and 3.48 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PDT March 30, 2017
In the 1970s and 80s, before advanced computer graphics, artist Ken Hodges was hired by JPL to create paintings that depicted many different missions – some in the planning stages and some only imagined.
Bruce Murray became JPL's Director in 1976, and he advocated new missions (Purple Pigeons) that would have enough pizzazz to attract public and scientific support. Hodges painted many of the Purple Pigeon images, including this scene of a Saturn orbiter with a lander going to the surface of Saturn's largest moon Titan. This artwork was done almost 30 years before Cassini's Huygens Probe reached the surface of Titan. Cassini was launched in 1997 and spent seven years traveling to Saturn. The probe was released in December 2004, and landed on Titan on January 14, 2005.
For more information about the history of JPL, contact the JPL Archives for assistance. [Archival and other sources: P-numbered photo albums and indexes, Cassini and Huygens web pages.]
Dear Pedawntic Readers,
A sophisticated spaceship in orbit around an alien world has been firing its advanced ion engine to execute complex and elegant orbital acrobatics.
On assignment from Earth at dwarf planet Ceres, Dawn is performing like the ace flier that it is.
The spacecraft’s activities are part of an ambitious bonus goal the team has recently devised for the extended mission. Dawn will maneuver to a location exactly on the line connecting Ceres and the sun and take pictures and spectra there. Measuring the opposition surge we explained last month will help scientists gain insight into the microscopic nature of the famous bright material in Occator Crater. Flying to that special position and acquiring the pictures and spectra will consume most of the rest of the extended mission, which concludes on June 30.
This month, we will look at the probe’s intricate maneuvers. Next month, we will delve more into the opposition surge itself, and in April we will describe Dawn’s detailed plans for photography and spectroscopy. In May we will discuss further maneuvers that could provide a backup opportunity for observing the opposition surge in June.
First, however, it is worth recalling that this is not Dawn’s primary responsibility, which is to continue to measure cosmic rays in order to improve scientists’ ability to establish the atomic species down to about a yard (meter) underground. Sensing the space radiation requires the spacecraft to stay more than 4,500 miles (7,200 kilometers) above the dwarf planet that is its gravitational master. The gamma ray and neutron detector will be operated continuously as Dawn changes its orbit and then performs the new observations. The ongoing high-priority radiation measurements will not be affected by the new plans.
The principal objective of the orbital maneuvers is to swivel Dawn’s orbit around Ceres. Imagine looking down on Ceres’ north pole, with the sun far to the left. (To help your imagination, you might refer to this figure from last month. As we will explain in May, Dawn’s orbital plane is slowly rotating clockwise, according to plan, and it is now even closer to vertical than depicted in January. That does not affect the following discussion.) From your perspective, looking edge-on at Dawn’s orbit, its elliptical path looks like a line, just as does a coin seen from the edge. In its current orbit (labeled 6 in that figure), Dawn moves from the bottom to the top over the north pole. When it is over the south pole, on the other side of the orbit, it flies from the top of the figure back to the bottom. The purpose of the current maneuvering is to make Dawn travel instead from the left to the right over the north pole (and from the right to the left over the south pole). This is equivalent to rotating the plane of the orbit around the axis that extends through Ceres’ poles and up to Dawn’s altitude. From the sun’s perspective, Dawn starts by revolving counterclockwise and the orbit is face-on. We want to turn it so it is edge-on to the sun.
That may not sound very difficult. After all, it amounts mostly to turning right at the north pole or left at the south pole. Spaceships in science fiction do that all the time (although sometimes they turn right at the south pole). However, it turns out to be extremely difficult in reality, not to mention lacking the cool sounds. When going over the south pole, from the top of the figure to the bottom, the spacecraft has momentum in that direction. To turn, it needs to cancel that out and then develop momentum to the left. That requires a great deal of work. It is energetically expensive. Fortunately, the ever-resourceful flight team has an affordable way.
As we discuss this more, we will present three diagrams of the trajectory. It may be challenging to follow Dawn’s three-dimensional motion on two-dimensional figures, especially if you are not accustomed to reading such depictions. Don’t worry! The team has it all under control, and it works. But consider that however complicated the figures seem, designing and flying the maneuvers is somewhat more complicated. Nevertheless, if you want to try, it might help to try to reproduce Dawn’s movements with your finger as you read the text and study the illustrations. (And if the figures are not helpful for understanding the trajectory, they may at least serve as fun optical illusions, as they did for one member of the test audience.)
Suppose you are driving from north to south and want to turn east at an intersection. You have to decrease your southward (forward) velocity somehow; otherwise, you will continue moving in that direction. You also have to increase your eastward (left) velocity, which initially is zero. That means putting on the brakes and then turning the wheel and reaccelerating, which takes work. (If you’re a stunt driver in the movies, it also may mean making smoke come out near the tires.) With your car, there are two major forces at work: the engine and the friction between the wheels and the road. For a spacecraft, the forces available are the propulsion system and the gravity of other bodies (like moons). Ceres’ only moon is Dawn itself, and there are no other helpful gravitational forces, so it’s all up to the probe’s ion engine.
Dawn was not built to perform these new maneuvers. The main tank and the xenon propellant loaded in it shortly before the spacecraft launched from Cape Canaveral did not account for such an addition to the interplanetary itinerary. The plan was to travel from Earth past Mars to Vesta, enter orbit and maneuver around the protoplanet, then break out of orbit and travel to Ceres, slip into orbit, and maneuver there. Dawn has now done all that with great distinction and already moved around more while orbiting Ceres than originally planned. Indeed, the mission has accomplished far, far more propulsive flight than any other, but now its xenon supply is very low. Navigators needed an efficient way to swivel the spacecraft’s orbit, and that meant finding an efficient way to change the direction of its orbital motion.
An orbit is the perfect balance between the inward tug of gravity and the fundamental tendency of free objects to travel in a straight line. Orbital velocity thus depends on the strength of the gravitational pull. At low altitude, orbiting objects travel faster than at higher altitude. (We have considered this topic in some detail, including with examples, several times before.) Dawn is flying to a very high altitude, where Ceres’ grip will not be as strong so the orbital velocity will naturally be much lower and therefore easier to change. Then it will turn left and swoop back down for the photo op. Any hotshot spaceship pilot would be proud to fly the same profile.
In December 2016, Dawn reached extended mission orbit 3 (XMO3), which ranged in altitude between 4,670 miles (7,520 kilometers) and 5,810 miles (9,350 kilometers). Now the spacecraft is climbing, and it will peak at more than 32,000 miles (52,000 kilometers) in early April when it will pivot the orbit almost 90 degrees. It will then glide down to about 12,400 miles (20,000 kilometers) for the targeted observations.
The maneuvering will be conducted in four stages. The first part of the ion powered ascent was Feb. 22-26, and the next will be March 8-12 when the orbital position is optimal. Although the spacecraft will stop thrusting then at an altitude of 8,000 miles (13,000 kilometers), it will have built up so much momentum that it will continue soaring upward for almost a month as Ceres’ gravitational attraction slows its down. (Dawn uses that pull as a means of putting on the brakes to reduce the forward momentum.) A third period of thrusting on April 3-14 at the apex of its arc will accomplish the turn. Dawn will then be in an orbit that will intersect the line between Occator Crater and the sun on April 29. (After turning, Dawn allows Ceres to do the work of accelerating it, as gravity brings the ship back down.)
This complex flight plan is different from all the prior powered flight, both at Vesta and at Ceres. Most of the orbit changes have been lovely spirals, and the ship rode the gravitational currents at Vesta to shift the orbital plane by a much smaller angle than it is working on now. Some of the graceful steps in this new choreography are especially delicate and require exquisite accuracy to reach just the right final trajectory. For the first time in almost two years, the spacecraft will need to take pictures of Ceres for the express purpose of helping navigators plot its progress. (In the intervening time, Dawn has taken more than 55,000 photos specifically to study the dwarf planet. Many of them also have been used for navigation.) Combining these "optical navigation" pictures with their other navigational techniques, the team will design a final, fourth stage of ion thrusting for April 22-24 to fine tune the orbit. We have described such trajectory correction maneuvers before. (It’s easier for you to chart the spacecraft’s progress than it is for the Dawn team. All you have to do is read the mission status reports.)
By the time it began ion thrusting last week, Dawn had successfully completed all of its assignments in XMO3. That included three photography sessions. In the last, the spacecraft used the primary and backup cameras simultaneously for the first time in the entire mission. In its extensive investigations of Vesta and Ceres, Dawn has taken more than 85,000 pictures, but all of them had been with only one camera powered on at a time, the other being held in reserve. In April we will discuss the reason for operating differently before leaving XMO3.
Dawn’s adventure has been long and its experiences manifold. In just a few days, the bold explorer will mark its second anniversary of arriving at Ceres. (That’s the second anniversary as reckoned by inhabitants of Earth. In contrast, for locals, the immigrant from distant Earth has been in residence for less than half a Cerean year, although more than 1,900 Cerean days.) In 2011-2012, the probe spent almost 14 months in orbit around the giant protoplanet Vesta, the second largest object in the main asteroid belt. The only craft ever to orbit two alien destinations, it is a denizen of deep space. In its nearly 9.5-year solar system journey, Dawn has traveled 3.7 billion miles (6.0 billion kilometers). For most of this time, the spaceship has been in orbit around the sun, just as its erstwhile home Earth is. Now it has been in orbit around remote worlds for a third of its total time in space. And for you numerologists, March 5 will mark Dawn’s being in orbit around its targets for pi years. (Happy pi-th anniversary.)
Readers on or near Earth who appreciate following such an extraordinary extraterrestrial expedition can take advantage of an opportunity this week to do a little celestial navigation of their own. On March 2, the moon will serve as a helpful signpost to locate the faraway ship on the interplanetary seas. From our terrestrial viewpoint, the moon will move very close to Dawn’s location in the sky. The specifics, of course, depend on your exact location. For many afternoon sky watchers in North America, the moon will come to within about a degree, or two lunar diameters, of Dawn. As viewed by some observers in South America, the moon will pass directly in front of Dawn. For most Earthlings, when the moon rises on the morning of March 2, it will be north and east of Dawn. During the day, the moon will gradually drift closer and, from many locations, pass the spacecraft and the dwarf planet it orbits. The angle separating them will be less than the width of your palm at arm’s length, providing a handy way to find our planet’s emissary. Although Dawn and Ceres will appear to be near the moon, they will not be close to it at all. The distant spacecraft will be more than 1,300 times farther away than the moon by then (and well over one million times farther than the International Space Station) and quite invisible. But your correspondent invites you to gaze in that direction as you raise a saluting hand to humankind’s insatiable appetite for knowledge, irresistible drive for exploration, passion for adventure, and longing to know the cosmos.
Dawn is 7,300 miles (11,800 kilometers) from Ceres. It is also 3.19 AU (296 million miles, or 477 million kilometers) from Earth, or 1,280 times as far as the moon and 3.22 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 53 minutes to make the round trip.
Dr. Marc D. Rayman
4:30 p.m. PST February 27, 2017
In early 1989, a series of thermal tests were conducted on the Microwave Limb Sounder (MLS) Instrument, which was part of the Upper Atmosphere Research Satellite (UARS).
The MLS System Thermal Vacuum (STV) test program was designed to evaluate its thermal integrity and functions in a simulated space environment. It included a 24-hour bakeout, six phases of thermal balance tests, and a thermal cycling test of the instrument in flight configuration, using a variety of heaters and lamps.
This photo shows the Ten-Foot Space Simulator located in Building 248, with a quartz lamp array approximately seven feet tall. This array faced the primary reflector during testing and helped to heat the chamber to 80°C (176°F). The vacuum chamber shroud was lowered over the test fixture, and the chamber walls and floor were maintained at -100°C to -179°C during testing.
For more information about the history of JPL, contact the JPL Archives for assistance.
“This year we’re gonna bring it!”
Oceans Melting Greenland (OMG) Principal Investigator Josh Willis told me excitedly. “It’s the beginning of year two of this five-year airborne mission, which means that by comparing data from the first and second years, we’ll be able to observe changes in Greenland’s glaciers and coastal ocean water for the first time.” Glaciers around Greenland’s jagged coastline have been melting into the ocean and causing increased sea level rise, so measuring the amount of ice mass loss will help us understand the impact of these changes, Willis said. “Will we see 5 feet of sea level rise this century … or more?”
See, Earth’s ocean, more than the atmosphere, is responsible for creating a stable climate. And as global warming has increased the temperature of the ocean waters surrounding Greenland, that warmer ocean water is melting the ice sheet from around its edges. “Hey! The ocean is eating away at the ice sheet!” Willis often cries when explaining the mission. And Team OMG is measuring how much of that warm water could be increasing due to climate change.
Decoding the environment
I understand how Willis and Project Manager Steve Dinardo get excited about measuring sea level rise. Greenland’s ice melt is accelerating, which explains why NASA is paying attention to it. Plus, after a successful first year, the team is fully aware of the stark beauty of Greenland’s rugged landscape and seascape and the rewards of bonding as a team. Dinardo told me he was “ecstatic about the incredible progress Team OMG has made in the last twenty-two months.”
As scientists, decoding the natural world is our way of caring about the environment. We care about Greenland’s icy coastline, so we go there. We go there and observe. We go there and measure. For there is something undeniable about the sheer beauty of this planet, and any time you get to experience it is a moment to feel exuberant and alive. Plus, flying around with a great team in a modified NASA G-III aircraft ain’t too shabby either.After a successful first year, the team is fully aware of the stark beauty of Greenland’s rugged landscape and seascape and the rewards of bonding as a team.
But wait. Before I continue, there’s something you probably noticed: Willis said he named this Greenland observing expedition Oceans Melting Greenland, or OMG for short, because, hey, OMG is the exact response you might have when you find out what’s going on up there.
Parts of Greenland’s coastline are so remote, so difficult to access by boat, that they’d remain uncharted, especially under areas that are seasonally covered with ice. Imagine the edge of an unimaginably complicated winding coastline, that unknown place where ice meets water meets seafloor. Big chunks of remnant sea ice clog up the water, and the glacier has retreated so recently that the coastline is changing as fast as, or even faster than, we can study it.
The seawater around 400 meters deep is 3 to 4 degrees Celsius warmer than the water floating near the sea surface. And the sea floor bathymetry influences how much of that warm subsurface layer can reach far up into the fjords and melt the glaciers. So, to learn about the interface between where the bottom of the ice sheet reaches out over the seawater and down into the ocean, OMG began by mapping undersea canyons on the M/V Cape Race, a ship equipped with an echo sounder, which sailed right up the narrow fjords on the continental shelf surrounding Greenland to the places where the warmer Atlantic Ocean water meets the bottoms of the frozen, 0-degree glaciers. The crew had to snake in between floating icebergs and weave in and out of narrow fjords. The Cape Race used a multibeam echo sounder to map undersea canyons, where the warm seawater comes in contact with and melts the glaciers.
The next four years
In the spring of 2016, the Oceans Melting Greenland (OMG) team began surveying glacier elevation near the end of marine-terminating glaciers by precisely measuring the edges of the ice sheet on a glacier-by-glacier basis, using the Airborne Glacier and Land Ice Surface Topography Interferometer (GLISTIN-A), a radar instrument attached to the bottom of a modified NASA G-III aircraft. Data collected this spring and over the next four years can be compared with data collected in the spring of 2016 so we can determine how fast the glaciers are melting.
The investigation continued into last fall, with the team dropping more than 200 Aircraft eXpendable Conductivity Temperature Depth (AXCTD) probes that measured ocean temperature and salinity around Greenland, from the sea surface to the sea floor, through a hole in the bottom of the plane. “In most of these places,” Willis told me, “there’s been no temperature and salinity data collected. Ever.”As scientists, decoding the natural world is our way of caring about the environment.
The team will drop more ocean probes across the same locations to find out “how much ice melts when the water is this warm,” what the melt rate is, and how much that rate is increasing, because no one knows the melt rate yet.
Big picture project
“OMG is a big picture project,” Willis explained. ”We want to see what’s happening in the ocean on the large scale and what’s happening to the ice sheet on the largest scales.”
As part of Team OMG, I also flew on NASA’s G-III into uncontrolled airspace to places where no other aircraft had flown before, into narrow and steep ice-covered fjords, winding in and out, up and down, over and through to observe and measure, like scientists do. I saw the brilliant white ice carve its way through steep brown valleys into open ocean water. I saw the glorious expanse of white upon deep blue going on and on and on below us as we flew just 5,000 feet above the winding coastline. It was extraordinary.
And if you just thought “OMG,” Willis would be proud.
Thanks for reading.
A deep-space robotic emissary from Earth is continuing to carry out its extraordinary mission at a distant dwarf planet.
Orbiting high above Ceres, the sophisticated Dawn spacecraft is hard at work unveiling the secrets of the exotic alien world that has been its home for almost two years.
Dawn’s primary objective in this sixth orbital phase at Ceres (known as extended mission orbit 3, XMO3 or "this sixth orbital phase at Ceres") is to record cosmic rays. Doing so will allow scientists to remove that "noise" from the nuclear radiation measurements performed during the eight months Dawn operated in a low, tight orbit around Ceres. The result will be a cleaner signal, revealing even more about the atomic constituents down to about a yard (meter) underground. As we will see below, in addition to this ongoing investigation, soon the adventurer will begin pursuing a new objective in its exploration of Ceres.
With its uniquely capable ion propulsion system, Dawn has flown to orbits with widely varying characteristics. In contrast to the previous five observation orbits (and all the observation orbits at Vesta), XMO3 is elliptical. Over the course of almost eight days, the spacecraft sails from a height of about 4,670 miles (7,520 kilometers) up to almost 5,810 miles (9,350 kilometers) and back down. Dutifully following principles discovered by Johannes Kepler at the beginning of the 17th century and explained by Isaac Newton at the end of that century, Dawn’s speed over this range of altitudes varies from 210 mph (330 kilometers per hour) when it is closest to Ceres to 170 mph (270 kilometers per hour) when it is farthest. Yesterday afternoon, the craft was at its highest for the current orbit. During the day today, the ship will descend from 5,790 miles (9,310 kilometers) to 5,550 miles (8,930 kilometers). As it does so, Ceres’ gravity will gradually accelerate it from 170 mph (273 kilometers per hour) to 177 mph (285 kilometers per hour). (Usually we round the orbital velocity to the nearest multiple of 10. In this case, however, to show the change during one day, the values presented are more precise.)
As we saw last month, the angle of XMO3 to the sun presents an opportunity to gain a new perspective on Ceres, with sunlight coming from a different angle. (We include the same figure here, because we will refer to it more below.) Last week, Dawn took advantage of that opportunity, seeing the alien landscapes in a new light as it took pictures for the first time since October.
Dawn takes more than a week to revolve around Ceres, but Ceres turns on its axis in just nine hours. Because Dawn moves through only a small segment of its orbit in one Cerean day, it is almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath it. During one such period on Jan. 27, Dawn’s high perch moved only from 11°N to 12°S latitude as Ceres presented her full range of longitudes to the explorer’s watchful eye. This made it very convenient to take pictures and visible spectra as the scenery helpfully paraded by. (The spacecraft was high enough to see much farther north and south than the latitudes immediately beneath it.) Dawn will make similar observations again twice in February.
As Dawn was expertly executing the elegant, complex spiral ascent from XMO2 to XMO3 in November, the flight team considered it to be the final choreography in the venerable probe’s multi-act grand interplanetary performance. By then, Dawn had already far exceeded all of its original objectives at Vesta and Ceres, and the last of the new scientific goals could be met in XMO3, the end of the encore. The primary consideration was to keep Dawn high enough to measure cosmic rays, meaning it needed to stay above about 4,500 miles (7,200 kilometers). There was no justification or motivation to go anywhere else. Well, that’s the way it was in November anyway. This is January. And now it’s (almost) time for a previously unanticipated new act, XMO4.
Always looking for ways to squeeze as much out of the mission as possible, the team has now devised a new and challenging investigation. It will consume the next five months (and much of the next five Dawn Journals). We begin this month with an overview, but follow along each month as we present the full story, including a detailed explanation of the underlying science, the observations themselves and the remarkable orbital maneuvering entirely unlike anything Dawn has done before. (You can also follow along with your correspondent’s uncharacteristically brief and more frequent mission status updates.)
From the XMO3 vantage point, with sunlight coming from the side, Ceres is gibbous and looks closer to a half moon than full. The new objective is to peer at Ceres when the sun is directly behind Dawn. This would be the same as looking at a full moon. (In the figure above, it would be like photographing Ceres from somewhere on the dashed line that points to the distant sun.)
While Dawn obtained pictures from near the line to the sun in its first Ceres orbit, there is a special importance to being even closer to that line. Let’s see why that alignment is valuable.
Most materials reflect light differently at different angles. You can investigate this yourself (and it’s probably easier to do at home than it is in orbit around a remote dwarf planet). To make it simpler, take some object that is relatively uniform (but with a matte finish, not a mirror-like finish) and vary the angles at which light hits it and from which you look at it. You may see that it appears dimmer or brighter as the angles change. It turns out that this effect may be used to help infer the nature of the reflecting material. (For the purposes of this exercise, if you can hold the angle of the object relative to your gaze fixed, and vary only the angle of the illumination, that’s best. But don’t worry about the details. Conducting this experiment represents only a small part of your final grade.)
Now when scientists carefully measure the reflected light under controlled conditions, they find that the intensity changes quite gradually over a wide range of angles. In other words, the apparent brightness of an object does not vary dramatically as the geometry changes. However, when the source of the illumination gets very close to being directly behind the observer, the reflection may become quite a bit stronger. (If you test this, of course, you have to ensure your shadow doesn’t interfere with the observation. Vampires don’t worry about this, and we’ll explain below why Dawn needn’t either.)
If you (or a helpful scientist friend of yours) measure how bright a partial moon is and then use that information to calculate how bright the full moon will be, you will wind up with an answer that’s too small. The full moon is significantly brighter than would be expected based on how lunar soil reflects light at other angles. (Of course, you will have to account for the fact that there is more illuminated area on a full moon, but this curious optical behavior is different. Here we are describing how the brightness of any given patch of ground changes.)
A full moon occurs when the moon and sun are in opposite directions from Earth’s perspective. That alignment is known as opposition. That is, an astronomical body (like the moon or a planet) is in opposition when the observer (you) is right in between it and the source of illumination (the sun), so all three are on a straight line. And because the brightness takes such a steep and unexpected jump there, this phenomenon is known as the opposition surge.
The observed magnitude of the opposition surge can reveal some of the nature of the illuminated object on much, much finer scales than are visible in photos. Knowing the degree to which the reflection strengthens at very small angles allows scientists to ascertain (or, at least, constrain) the texture of materials on planetary surfaces even at the microscopic level. If they are fortunate enough to have measurements of the reflectivity at different angles for a region on an airless solar system body (atmospheres complicate it too much), they compare them with laboratory measurements on candidate materials to determine which ones give the best match for the properties.
Dawn has already measured the light reflected over a wide range of angles, as is clear from the figure above showing the orbits. But the strongest discrimination among different textures relies on measuring the opposition surge. That is Dawn’s next objective, a bonus in the bonus extended mission.
You can see from the diagram that measuring the opposition surge will require a very large change in the plane of Dawn’s orbit. Shifting the plane of a spacecraft’s orbit can be energetically very, very expensive. (We will discuss this more next month.) Fortunately, the combination of the unique capabilities provided by the ion propulsion system and the ever-creative team makes it affordable.
Powered by an insatiable appetite for new knowledge, Dawn will begin ion thrusting on Feb. 23. After very complex maneuvers, it will be rewarded at the end of April with a view of a full Ceres from an altitude of around 12,400 miles (20,000 kilometers), about the height of GPS satellites above Earth. (That will be about 50 percent higher than the first science orbit, which is labeled as line 1 in the figure.) There are many daunting challenges in reaching XMO4 and measuring the opposition surge. Even though it is a recently added bonus, and the success of the extended mission does not depend on it, mission planners have already designed a backup opportunity in case the first attempt does not yield the desired data. The second window is late in June, allowing the spacecraft time to transmit its findings to Earth before the extended mission concludes at the end of that month.
For technical reasons, the measurements need to be made from a high altitude, and throughout the complex maneuvering to get there, Dawn will remain high enough to monitor cosmic rays. Ceres will appear to be around five times the width of the full moon we see from Earth. It will be about 500 pixels in diameter in Dawn’s camera, and more than 180,000 pixels will show light reflected from the ground. Of greatest scientific interest in the photographs will be just a handful of pixels that show the famous bright material in Occator Crater, known as Cerealia Facula and clearly visible in the picture above. Scientists will observe how those pixels surge in brightness over a narrow range of angles as Dawn’s XMO4 orbital motion takes it into opposition, exactly between Occator and the sun. Of course, the pictures also will provide information on how the widespread dark material covering most of the ground everywhere else on Ceres changes in brightness (or, if you prefer, in dimness). But the big reward here would be insight into the details of Cerealia Facula. Comparing the opposition surges with laboratory measurements may reveal characteristics that cannot be discerned any other way save direct sampling, which is far beyond Dawn’s capability (and authority). For example, scientists may be able to estimate the size of the salt crystals that make up the bright material, and that would help establish their geological history, including whether they formed underground or on the surface. We will discuss this more in March.
Most of the data on opposition surges on solar system objects use terrestrial observations, with astronomers waiting until Earth and the target happen to move into the necessary alignment with the sun. In those cases, the surge is averaged over the entire body, because the target is usually too far away to discern any details. Therefore, it is very difficult to learn about specific features when observing from near Earth. Few spacecraft have actively maneuvered to acquire such data because, as we alluded to above and will see next month, it is too difficult, especially at a massive body like Ceres. The recognition that Dawn might be able to complete this challenging measurement for a region of particular interest represents an important possibility for the mission to discover more about this intriguing dwarf planet’s geology.
Meeting the scientific goal will require a careful and quantitative analysis of the pixels, but the images of a fully illuminated Ceres will be visually appealing as well. Nevertheless, you are cautioned to avoid developing a mistaken notion about the view. (For that matter, you are cautioned to avoid developing mistaken notions about anything.) You might think (and some readers wondered about this in a different phase of the mission) that with Dawn being between the sun and Ceres (and not being a vampire), the spacecraft’s shadow might be visible in the pictures. It would look really cool if it were (although it also would interfere with the measurement of the opposition surge by introducing another factor into how the brightness changes). There will be no shadow. The spacecraft will simply be too high. Imagine you’re standing in Occator Crater, either wearing your spacesuit while engaged in a thrilling exploration of a mysterious and captivating extraterrestrial site or perhaps instead while you’re indoors enjoying some of the colony’s specially salted Cerean savory snacks, famous throughout the solar system. In any case, the distant sun you see would be a little more than one-third the size that it looks from Earth, comparable to a soccer ball at 213 feet (65 meters). Dawn would be 12,400 miles (20,000 kilometers) overhead. Although it’s one of the largest interplanetary spacecraft ever to take flight, with a wingspan of 65 feet (20 meters), it would be much too small for you to see at all without a telescope and would block an undetectably small amount of sunlight. It would appear smaller than a soccer ball seen from 135 miles (220 kilometers). Therefore, no shadow will be cast, the measurement will not be compromised by the spacecraft blocking some of the light reaching the ground and the pictures will not display any evidence of the photographer.
Even as the team was formulating plans for this ambitious new campaign, they successfully dealt with a glitch on the spacecraft this month. When a routine communications session with the Deep Space Network began on Jan. 17, controllers discovered that Dawn had previously entered its safe mode, a standard response the craft uses when it encounters conditions its programming and logic cannot accommodate. The main computer issues instructions to reconfigure systems, broadcasts a special radio signal through one of the antennas and then patiently awaits help from humans on a faraway planet (or anyone else who happens to lend assistance). The team soon determined what had occurred. Since it left Earth, Dawn has performed calculations five times per second about its location and speed in the solar system, whether in orbit around the sun, Vesta or Ceres. (Perhaps you do the same on your deep-space voyages.) However, it ran into difficulty in those calculations on Jan. 14 for the first time in more than nine years of interplanetary travel. To ensure the problematic calculations did not cause the ship to take any unsafe actions, it put itself into safe mode. Engineers have confirmed that the problem was in software, not hardware and not even a cosmic ray strike, which has occasionally triggered safe mode, most recently in September 2014.
Mission controllers guided the spacecraft out of safe mode within two days and finished returning all systems to their standard configurations shortly thereafter. Dawn was shipshape the subsequent week and resumed its scientific duties. When it activated safe mode, the computer correctly powered off the gamma ray and neutron detector, which had been measuring the cosmic rays, as we described above. The time that the instrument was off will be inconsequential, however, because there is more than enough time in the extended mission to acquire all the desired measurements.
The extended mission has already proven to be extremely productive, yielding a great deal of new data on this ancient world. But there is still more to look forward to as the veteran explorer prepares for a new and adventurous phase of its extraordinary extraterrestrial expedition.
Dawn is 5,650 miles (9,100 kilometers) from Ceres. It is also 2.87 AU (266 million miles, or 429 million kilometers) from Earth, or 1,135 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 48 minutes to make the round trip.
Dr. Marc D. Rayman
4:00 p.m. PST January 31, 2017