Propelled by the perfect combination of xenon ions, hydrazine rocket propellant and adrenaline, Dawn is on the verge of its most ambitious exploits yet. Having flawlessly completed its latest assignment to study Ceres, the veteran explorer is now aiming for a new low. Earlier today Dawn ignited ion engine #2 to start maneuvering to its lowest altitude above the dwarf planet. Soon the spaceship will be skimming closer to the alien landscapes of rock, ice and salt than ever before, promising exciting new insights into the nature of a distant and mysterious world.
Almost once a day in its next orbit, Dawn will dive from 2,500 miles (4,000 kilometers) down to only 22 miles (35 kilometers), speeding above the ground at 1,050 mph (1,690 kph), and then shoot back up again. (Warning: Do not try this at home! Dawn is a trained professional.)
Before we (and Dawn) get to this new and final orbit, let's review the outstanding accomplishments this month. Dawn used its ion engine in April and May to descend to an orbit creatively known as extended mission orbit 6 (XMO6). (We showed the flight path last month and tracked the progress in mission status updates.) Ion thrusting concluded on schedule on May 14 when Dawn was in the targeted elliptical orbit, which ranged from 280 miles (450 kilometers) to 2,900 miles (4,700 kilometers).
Each of the 10 loops around Ceres took one and a half days, and Dawn successfully performed all of its planned observations. Every time Dawn flew northward over the sunlit hemisphere, the spacecraft used its cameras and other sensors to collect new data. During some orbits, as it flew southward over the hemisphere opposite the Sun, it turned to point its main antenna at faraway Earth and then radioed its findings to NASA's Deep Space Network. On other orbits, Dawn patiently continued looking down at Ceres. Of course, with the ground there hidden in the deep black of night on a moonless world, there was nothing to see, but by not turning, the spacecraft could conserve precious hydrazine for later in the mission. (Dawn used this strategy in most of the other phases at Ceres as well, starting with the third mapping orbit of the prime mission in 2015.) We will discuss more about hydrazine below.
As we saw in March's preview, Dawn's primary goal in XMO6 was to take advantage of it being summer in the southern hemisphere by making extensive observations in the far south. We also explained that XMO6 provided an opportunity for collecting new data (including higher resolution color pictures), providing new perspectives closer to the equator and in the northern hemisphere as well. Dawn spotted sites we have discussed before, including Ernutet Crater with deposits of organic materials, the smooth landscape around Ikapati Crater showing a history of flowing material, the volcano Ahuna Mons and other locations pictured above and below. Prior to three years ago, these places were all quite unknown (at least to Earthlings). In the intervening time, Dawn has studied many of them in exquisite detail, and at each one has discovered new questions to ask. XMO6 may provide new answers (and probably still newer questions.)
In addition to its normal photography and spectroscopy, the spacecraft took long exposure pictures to investigate areas that are in shadow throughout the Cerean year. We described before how water can be trapped in such locations, but when we last touched on this topic in December 2016 (along with a cool animation), we also mentioned that the seasons had precluded a good study in the southern hemisphere. XMO6 has helped rectify that, illustrating one benefit of being able to stay in orbit rather than catching whatever is to be seen during a fast flyby.
Dawn had one more assignment in XMO6. After the primary scientific observations were complete on the first, third, and tenth orbits, the spacecraft turned from pointing at the ground beneath it to the horizon. (The amount of hydrazine needed for a turn depends on the direction. In each case, mission controllers selected the most hydrazine-efficient direction.) As it turned, Dawn continued taking pictures. This showed terrain at new angles, contributing to the collection of stereo pictures taken in the third and fourth mapping orbits. But in this case, the scientific benefit, while real, was secondary. The primary objective was to get some cool new views of the limb of Ceres, including the one above. Loyal readers (and some others as well) may know that your correspondent finds such perspectives especially appealing, as described here (with other fine examples here, there and elsewhere). He decided the pure coolness of these XMO6 pictures would be reason enough to instruct Dawn to take them.
By the time Dawn completed XMO6, it had collected 1,800 new photos of Ceres in addition to a wealth of infrared spectra and visible spectra. As soon as its bounty was safely on Earth, the itinerant adventurer was ready for its next great challenge.
And now the blue lights are on again in mission control at JPL, as they were at the end of last month. The illumination is not designed to alter the circadian rhythm of the flight team but rather to provide a visual connection with the distant spacecraft as its ion engine emits a steady bluish glow. Dawn is now spiraling down, tightening its elliptical loops, getting lower and lower and lower. We described the previous descent last month, and you can see the current trajectory in the figure below.
Dawn will spend the rest of its operational life in the target orbit, XMO7, and most future Dawn Journals will be devoted to it. How long will that be? That's a good question (in contrast, perhaps, to all the absurd questions posed in previous Dawn Journals), but the answer is not easy.
We have discussed many times (here is a summary) that Dawn's lifetime is limited by its hydrazine, a conventional rocket propellant expelled from reaction control system thrusters to control its orientation in space. When that dwindling supply is exhausted, the robot will no longer be able to point its solar arrays at the Sun, its antenna at Earth, its sensors at Ceres or its ion engines in the direction needed to travel elsewhere. The mission will end, and the ship will become an inert celestial monument to the power of human ingenuity, creativity and curiosity, a lasting reminder orbiting one of the solar system worlds it unveiled that our passion for bold adventures and our noble aspirations to extend our reach into the universe can take us very, very far beyond the confines of our humble planetary home.
The rate at which Dawn consumes hydrazine depends very strongly on the nature of the orbit. The lower the height, the faster it uses hydrazine, because it must rotate more quickly to keep its sensors pointed at the ground. In addition, it has to fight harder to resist Ceres’ relentless gravitational tug on the very large solar arrays, creating an unwanted torque on the ship. In XMO7, Dawn will dip to less than one-tenth of its lowest altitude so far. The hydrazine is going to go fast. But that's okay. The hydrazine is there to be used in service of accomplishing the mission, and Dawn is going to use it very well indeed as it pursues fabulous new goals.
Dawn engineers have sophisticated mathematical models to predict just how quickly the hydrazine will be spent, and those models have done an excellent job throughout the mission. Nevertheless, as in all realistic and complex systems, there remains some degree of uncertainty. (As a courtesy to most readers, we will not delve into the recondite details.) We can predict only approximately how fast Dawn will expend hydrazine as it carries out its intricate assignments in the coming months. Glitches, which are inevitable on such a complex mission, can both consume hydrazine and compel the flight team to change the schedule and the plans, introducing further uncertainty.
As it turns out, there are two more aspects of this problem. Not only are we limited in our ability to predict how much hydrazine each activity will require but our measurement of how much hydrazine Dawn has remaining is imperfect too. We know that when it left Earth, riding atop a Delta rocket, the 12-gallon (45-liter) hydrazine tank was filled with 99.8 pounds (45.3 kilograms) of the propellant. In the subsequent 11.5 years, every time it has fired a thruster, the spacecraft has dutifully recorded the duration (in milliseconds) and reported that to mission control at JPL. It has also sent telemetry on the temperature and pressure in the hydrazine tank. With that information, engineers can calculate how much hydrazine is expended in each pulse of a thruster and, more to the point, how much is left in the tank. It is now down to about 1.8 gallons (7 liters). But no physical measurement is perfectly accurate. As only one example, the sensors that read the temperature and pressure have been subjected to violent shaking during the rocket's fiery ascent as well as almost a dozen years in space. Their readings now may be off a little bit one way or the other. The determination of how much hydrazine is still onboard thus has some uncertainty.
So, it is not possible to predict exactly how much hydrazine Dawn will need nor exactly how much it has. There is still another source of uncertainty. There is a complex network of tubing, valves and a filter between the tank and each of the 12 thrusters located around the spacecraft. Once the pressure in the lines is too low for a thruster to operate, the remaining hydrazine cannot be expelled. Of course, engineers can calculate how much of the hydrazine will be trapped in the system (known as the unusable hydrazine). That turns out to be 1.7 pints (0.8 liters), but, as with these other problems, they cannot know the answer with absolute precision, so it could be a little more or a little less.
Taken together, all these reasons prevent controllers from being able to pin down the day and time that Dawn will deplete the usable hydrazine. Experienced interplanetary explorers, like the Dawn flight team at JPL, are accustomed to dealing with such uncertainty.
The team will continue to guide Dawn in squeezing as much out of its time at Ceres as possible, acquiring new data until the spacecraft is unable to comply because it has expended the last puff of hydrazine. Right now, that is deemed most likely to be in September of this year (with a smaller chance it will be in August or maybe even October). Once Dawn has settled in to XMO7, and engineers have operational experience in the new orbit, they will update their estimate, and they will continue to refine it as the mission progresses.
And when the last of the hydrazine is used up, the spacecraft will actuate valves and try to fire thrusters to control its orientation, but hydrazine will no longer flow, so the torque it wants to exert will not be achieved. The spacecraft will be impotent, its attempts to point correctly futile. The struggle will be brief, as it will soon run out of electrical power, and the central computer will cease operating. We will address the details of its final moments in a future Dawn Journal.
For now, we needn't anticipate the end with despair. Dawn has already succeeded beyond our wildest expectations. The prime mission accomplished far more than planned at Vesta and at Ceres even though it confronted completely unanticipated and daunting obstacles, like the failures of two reaction wheels. The first extended mission (in XMO1 through XMO5) yielded many additional impressive bonuses as well as another reaction wheel failure. Now the second extension has provided further rewards in XMO6. And as we look ahead to XMO7, we can expect even more riches and, of course, more challenges (although no more reaction wheel failures).
A daring and exciting interplanetary adventure, journeying through the solar system atop a bluish beam of xenon ions, soaring past Mars and flying well over one million times farther from Earth than the International Space Station, orbiting Vesta and Ceres, the two largest bodies in the main asteroid belt (together representing about 40 percent of the combined mass of the millions of objects between Mars and Jupiter), exploring these mysterious uncharted worlds, revealing dramatic alien landscapes, powered by the collective passions of everyone exhilarated by new knowledge and everyone who longs to know the cosmos, Dawn has already surpassed any reasonable expectation for what it might achieve. What more may come, we do not yet know. That's part of the thrill of exploration and discovery. But when the end does come, it will represent the culmination of a truly extraordinary extraterrestrial expedition.
Dawn is 1,800 miles (2,900 kilometers) from Ceres. It is also 2.73 AU (254 million miles, or 408 million kilometers) from Earth, or 1,010 times as far as the Moon and 2.69 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 45 minutes to make the round trip.
Dr. Marc Rayman
6:30 pm PDT May 31, 2018
For the first time in almost a year, the Dawn mission control room at JPL is aglow with blue.
The rope lights strung around the room bathe it in a gentle light reminiscent of the beam emitted by an ion engine on the faraway spacecraft as it maneuvers in orbit around Ceres. Dawn had not thrust since June, but it is now using ion engine #2 to fly to a new orbit around the dwarf planet. Thanks to its uniquely capable ion propulsion system, Dawn has accomplished far more powered flight than any other spacecraft, and more is ahead.
Dawn has spent most of the last year revolving around Ceres once every 30 days in extended mission orbit 5 (XMO5), a designation that illustrates the team's flair for the dramatic. (Your correspondent, as passionate as anyone about the exploration of the cosmos, can imagine only a few names more inspiring than that. Fortunately, one of them happens to be "XMO7." Read on!) As the probe followed that elliptical course, it reached down to a little less than 2,800 miles (4,400 kilometers) above the alien world and up to 24,300 miles (39,100 kilometers).
Dawn flew to high altitude late in 2016. Its work there is now complete, and defying expectations, the aged adventurer still has life left in it. As we saw in last month's overview of the two upcoming orbits, Dawn's next assignment is to go much, much lower.
XMO5 and the subsequent two orbits are elliptical, as shown in the illustrations last month and the new one below. Observing Ceres from a very low altitude is possible only in an elliptical orbit, not a circular one. Dawn was not designed to operate at low altitude, and its reaction wheels, which are so important for controlling its orientation, have failed, making the problem even more difficult. We have discussed this before and will address another aspect of it this month for the lowest orbit.
Although the elliptical orbits introduce many new technical challenges for the team, Dawn still takes a spiral route from each orbit to the next, just as it did earlier at Ceres and at Vesta when the orbits were circular. In essence, the ion engine smoothly shrinks the starting ellipse until the new ellipse is the size needed. These trajectories are very complicated to plan and to execute, but with the expert piloting of the experienced team, the maneuvering is going very well. (You can follow the progress with the mission status updates.)
Dawn began its descent on April 16. On May 15, with the blue lights turned off in mission control, the veteran explorer will begin its observations in XMO6. (As suggested last month, the targeted minimum and maximum altitudes for XMO6 are being updated slightly even as Dawn is on its way. In the next Dawn Journal, we will present the actual altitude range.) If all goes well, the control room will be lit up in blue again from May 31 to June 7, as the ship sails down to XMO7.
In XMO7, Dawn will swoop down to an incredibly low 22 miles (35 kilometers) above the exotic terrain of ice, rock and salt. The last time it was that close to a solar system body was when it rode a rocket from Cape Canaveral over the Atlantic Ocean more than a decade ago. (For readers unfamiliar with solar system geography, that was Earth.) The XMO7 ellipse will then take the spacecraft up to 2,500 miles (4,000 kilometers). Each revolution will last 27 hours and 13 minutes. In considerably less time than that (assuming you read at a typical speed), we will discuss why this orbital period is important.
Last month, we described some of Dawn's planned low-altitude measurements of nuclear radiation to reveal more about Ceres' composition. As a bonus objective, scientists would like to study the elements in one of their favorite places (and perhaps one of yours as well): Occator Crater, site of the highly reflective salt deposits, famous not only on Ceres but also on Earth and everywhere else that readers follow Dawn's discoveries. Studying this one crater and the area around it (together known as a geological unit) could reveal more about the complex geology there. But doing so is quite a challenge, as Dawn would need to pass over that region 20 times to allow the gamma ray and neutron detector (GRaND) to record enough of the faint nuclear radiation. This is the equivalent of taking a long exposure with a camera when photographing a very dim scene.
Attempting to repeatedly fly low over that geological unit presents daunting obstacles, as we will discuss. It may not work, but the team will try. That's part of what makes for a daring adventure! And accomplishing such a feat requires a special trick. Fortunately, the Dawn team has several at its disposal.
Recall that Dawn will loop around Ceres, going south to north at low altitude and back to the south again at high altitude. Meanwhile, Ceres will turn on its axis toward the east, completing one rotation in just over 9 hours, 4 minutes. (Note that Ceres turns quite a bit faster than Earth. A Cerean day is much closer in duration to a day on Jupiter, which is 9 hours, 56 minutes. All three turn east.) Therefore, the flight team will synchronize the orbit so that each time Dawn swoops down to low altitude, it does so at just the right time so that Ceres' rotation will place the Occator geological unit under the probe's flight path.
We mentioned above that Dawn's orbit will take 27 hours, 13 minutes. This period is chosen to be exactly three times Ceres' rotation period. Experts (now including you) describe this as a three-to-one resonant orbit, meaning that for every three times Ceres turns, Dawn turns around it once.
If this synchronization is clear, feel free to skip this paragraph. Perhaps get a snack until it's time for the next paragraph or, better yet, use this time to gaze at the mesmerizing beauty of the night sky and contemplate the magnificence of the cosmos. If the synchronization is not clear, find a globe of Earth. Now imagine a satellite circling it, flying from the south pole to the north pole over one hemisphere and back to the south pole over the opposite hemisphere. Suppose the first passage occurs over your location. If Earth didn't rotate, the second orbit would take it over the same place. (Of course, if Earth didn't rotate, you might run out of patience waiting for tomorrow.) Now rotate the globe a little bit while your imagined satellite goes through one revolution. If it flew over your location the first time, it will not the second time. And you can see that with Earth rotating at a constant speed, it requires a carefully chosen speed for the satellite to pass over the desired target on each revolution. The Dawn flight team will work very hard to help our distant explorer have the orbit needed to achieve the three-to-one resonance.
The accuracy necessary will be difficult to achieve, even for the Dawn flight team at JPL, where the best celestial navigators in the solar system get to work. The problems that must be overcome are manifold. One of them is that, lacking functioning reaction wheels, Dawn fires its small hydrazine-fueled thrusters to control its orientation in space. Whether to turn to keep its sensors trained on the ground, even with the constantly changing altitude and velocity in the elliptical orbit, or to point its main antenna at Earth, the reaction from a little burst of hydrazine not only rotates the spacecraft but also nudges it in its orbit. (We have described this several times in great detail before.) Each small push from the thrusters distorts the orbit a little bit, desynchronizing it from the three-to-one resonance.
Another difficulty is that, just like Earth, Mars, the Moon and other solar system residents (not to mention cookie dough ice cream), Ceres is not uniform inside. Its complex geology has produced some regions of higher density and some of lower density (although not with the same delectable composition as the ice cream). The total gravitational pull on the spacecraft depends on the dwarf planet's internal structure. We have described before how scientists take advantage of it to map the interior. But we have measured the gravity from 240 miles (385 kilometers) high. When Dawn swoops down much lower, our gravity map will not be accurate enough to predict all the subtle details of the mass distribution that may cause slightly larger or slightly smaller pulls at some locations. It will take quite a while to formulate the new gravity map. That new map may reveal more about what's underground, but until then, it will be harder to keep the orbit in sync.
On two occasions in mid-June Dawn will use its ion engine to tweak its orbit (in what we have described before as a trajectory correction maneuver) to help maintain the synchronization, but there will still be residual discrepancies.
We described and depicted last month how the low point of Dawn's orbit will gradually shift southward on each successive revolution. That means we will have only a limited number of opportunities to fly over Occator before the low point is too far south. Given the complexity of the operations, the planned measurements are not at all assured.
There are other aspects of this problem as well. While we will not delve into them here, engineers have been working hard on every one of them.
We have mentioned before that photography will be extremely challenging in XMO7, because of both the high speed so close to the ground and the difficulty pointing the camera accurately enough to capture a specific target. Let's take a more careful look at the nature of the orbit to understand more about the problem of trying to see any particular site.
You can think of the motion in an elliptical orbit as being somewhat like that of a swing. Imagine a girl named Dawn on a swing. Perhaps she is 10 and a half years old (like our spacecraft), usually (but not always) does what we instruct (like our spacecraft), feels energized by the light of the Sun (like our spacecraft), loves the idea of exploring uncharted worlds (like our spacecraft) and uses photomultiplier tubes coupled to a bismuth germanate crystal scintillator, lithiated glass and boron-loaded plastic to measure the spectra of nuclear radiation (okay, she is not like our spacecraft in every way).
When Dawn rides her swing, her speed is constantly changing. As she approaches the top of her arc, gravity slows her down and even brings her momentarily to a stop. She then begins to fall, accelerating as she gets lower. As soon as she passes the lowest point, her upward motion and the downward pull of gravity oppose each other, and once again she begins to slow. When her swing is pumped up (whether with her legs or by the push of her friend or her friendly ion engine), her arc will reach higher, and then she will speed through the low point even faster.
Of course, the swing does not trace out an ellipse, and the girl does not loop all the way around, but the fundamental principles of motion are the same, as methodically investigated by Galileo Galilei four centuries ago and explained by Isaac Newton in the second half of the 17th century. Dawn's elliptical orbit around Ceres will behave somewhat like the swing. At high altitude, far above the dwarf planet, the spacecraft will move at only about 120 mph (190 kph). Then, as gravity pulls it back down, the spacecraft will accelerate until it skims over the ground at 1,050 mph (1,690 kph) before starting to swing up again.
Dawn is much, much, much too far away for controllers to point its camera and other instruments as you might with a joystick or other controller in real time. Readers of the final paragraph of every Dawn Journal know that radio signals, traveling at the universal limit of the speed of light, usually take more than half an hour to complete the round trip. When Dawn is in XMO7 this summer, it will be about an hour. While the spacecraft is racing over the Cerean landscape, it can't wait for its radio signal to tell controllers what it sees and then, based on that, for a return radio signal to help it adjust the pointing of its camera. All the instructions from Earth have to be radioed in advance.
It is a very complicated process to go from measuring Dawn's orbit accurately to the probe actually aiming its camera and its spectrometers to collect new data, with many calculations and many steps in between, each of which has to be checked and double checked. The team has a special campaign planned for that purpose, and they will maneuver to XMO7 so that the best viewing will be in late June. But even when they work quickly for this dedicated attempt to get some bonus photographs of Occator, the entire process will take the better part of a week because of the spacecraft's orbital activities (e.g., while it observes Ceres, it cannot communicate with Earth), segments of its orbit where Ceres blocks its radio signal to Earth and so it is not possible to communicate, and the schedule for the large Deep Space Network antennas to shout so Dawn can then listen for what fades to become a long-distance radio whisper. Time needs to be allocated for computers and people to analyze data, to formulate and verify the new plans, to beam the instructions to Dawn and then Dawn finally to execute them. Meanwhile, even after the initial measurement of its orbit, while all this work is occurring on Earth, the ship will continue to be buffeted by the hydrazine winds and the gravitational currents, so its course will continue to change.
The consequence of all this is that by the time Dawn actually conducts its observations, its orbit will be different from what was measured days earlier. The carefully devised prediction that formed the basis of the plans could well be off one way or the other by four minutes or even more. (By the way, calculating now the credible magnitude of the error for this June campaign is a sophisticated science that, in itself, involves thousands and thousands of hours of computer calculations, performed on hundreds of computers working simultaneously. Epistemic knowledge does not come easily.)
From Dawn's perspective, descending and speeding north at 1,050 mph (1,690 kph) to the vicinity of Occator, faithfully pointing its sensors according to the plan worked out days before on a distant planet and stored in its computer, Ceres' rotation will carry the crater to the right at more than 190 mph (310 kph). Dawn's camera will take in a scene about 2.1 miles (3.4 kilometers) across, and at the spacecraft's high velocity, there won't be time to turn right and left to cover a broader swath. Even if the probe arrived at Occator's latitude a mere 20 seconds off schedule, a spot on the ground that was expected to be in the center of the camera would have moved entirely out of view and so would not even be glimpsed. If Dawn were four minutes too early or too late, the ground beneath the spacecraft (known as the ground track) would shift west or east by 13 miles (21 kilometers), and the terrain that's photographed could be entirely different from what was expected.
Occator Crater is 57 miles (92 kilometers) across, so all this work should allow GRaND, with its very wide field of view, to measure the composition in the geological unit that contains the crater. But the narrower view of the camera means we cannot be certain what features we will see. Fortunately, we already know that there is fascinating geology just about everywhere in and near Occator. Indeed, the dwarf planet is vast and varied, with a great many intriguing features. We are going to behold some amazing sights!
Before then, we will gain new perspectives from XMO6 in May. And as Dawn was getting closer to Ceres, together the pair were getting closer to the Sun until yesterday. Dawn isn't the only object in an elliptical orbit. Ceres, Earth, and all the other planets (whether dwarf or not) travel in elliptical orbits too, although they orbit the Sun. Ceres' orbit is more elliptical than Earth's but not as much as some of the other planets. The shape of Ceres' orbit is between that of Saturn's (which is more circular) and Mars' (which is more elliptical). (Of course, Ceres' orbit is larger than Mars' orbit -- it revolves farther from the Sun than the Red Planet does -- and smaller than Saturn's, but our focus here is on how much the orbit deviates from a perfect circle, regardless of the size.)
In its 4.6-year-long Cerean year, Ceres, with Dawn in tow, reached the minimum solar distance of just under 2.56 AU (238 million miles, or 383 million kilometers) on April 28. Dawn also was in residence at Ceres when they were at their maximum distance from the Sun in January 2016. Although the dwarf planet's orbit is not elliptical enough that the additional solar heating is expected to have much effect, the upcoming observations in XMO6 will provide scientists with the opportunity to look for any changes just in case. (The change Dawn detected at Juling Crater is more likely related to the seasonal change of the angle of the Sun rather than the distance to the Sun.)
The solar system constantly performs a complex and beautiful choreography, with everything in motion. Dawn will complete its current elegant spiral in another two weeks, and then it will be time for the next act, XMO6 and, after that, the finale, XMO7. A great many challenges are ahead but the allure of the rich rewards of new knowledge, new insight, and a new adventure is irresistible as Dawn delves further into the unknown.
Dawn is 1,400 miles (2,300 kilometers) from Ceres. It is also 2.34 AU (218 million miles, or 350 million kilometers) from Earth, or 900 times as far as the Moon and 2.32 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.
Dr. Marc Rayman
7:30 pm PDT April 29, 2018
A veteran explorer is leisurely orbiting the only dwarf planet in the inner solar system. Measuring space radiation high over Ceres, Dawn revolves once every 30 days in its gravitational master's firm grip. Dawn is well-known for its patience, and the pace of its activities has been decidedly relaxed in this orbit. That is about to change. There is now only one revolution to go before the spacecraft begins the final campaign of its long and rewarding deep-space adventure.
For eight months in 2015-2016, Dawn circled Ceres once every 5.4 hours at only 240 miles (385 kilometers). (The orbit has been variously designated as LAMO, then XMO1, and often as "the lowest orbit.") It then flew higher to pursue new objectives. The probe's orbit now takes it from slightly under 2,800 miles (4,400 kilometers) up to 24,300 miles (39,100 kilometers) and then back down again. (These values are a little different from what we presented in December, principally because the Sun's gravity gradually alters the orbit.) The orbit is known to people who call it extended mission orbit 5, or XMO5, as "extended mission orbit 5" or "XMO5" (following the nomenclature described here). XMO5 is illustrated in a figure below.
In contrast to the distant, serene probe, the operations team has been working quite intensively to prepare for a bold new phase of the mission. They have been assiduously working through all the tasks necessary to prepare for piloting this unique spaceship, late in its life and low on supplies, through maneuvers it was never designed for and to conduct observations never conceived of prior to late last year. Since the previous Dawn Journal, the team has generated more than 45,000 trajectories to study how to fly Dawn to two new orbits. Often there are more than 100 computers operating simultaneously to perform the necessary calculations. Many thousands more trajectories are yet to be computed and analyzed. If all goes well, by June, the probe will have followed an intricate flight plan that will allow it to glide a mere 22 miles (35 kilometers) above the alien landscapes almost every day in an orbit dramatically and poetically designated XMO7 (but occasionally summarized as "Whoa, that's low!").
Let's take a look at some of the plans the flight team is developing. As always, we will provide more details when Dawn is executing its complex assignments. In addition, as some parts of the plan are still being refined, there may be a few changes, and we will keep you updated on those as well. But plans are firm enough now that a preview is warranted.
On April 17, the spacecraft will fire up ion engine #2 and begin a downward spiral, gradually shrinking its elliptical orbit. Along the way to its final space destination, XMO7, the ship will moor at an intermediate orbit. On May 14, when it is in an orbit that ranges from about 235 miles (375 kilometers) to almost 3,000 miles (4,800 kilometers), it will shut down the engine. (This orbit is illustrated in the next two figures below.)
It is only coincidental that the lowest altitude of this intermediate orbit, XMO6, is so close to height of the lowest orbit so far. Indeed, the lowest point is not the most important point. The motivation for stopping in XMO6 is to collect infrared spectra and take pictures in the southern hemisphere in a range of about 900 miles (1,500 kilometers) to 1,600 miles (2,500 kilometers). It just so happens that when flying from XMO5 to XMO7, an orbit that provides that viewing opportunity dips down to the height of LAMO/XMO1 elsewhere in the orbit.
The XMO6 altitude in the south was chosen to be comparable to the altitude from which Dawn observed Ceres so extensively in its third and fifth mapping orbits (known as HAMO and XMO2, respectively). XMO6 will afford the probe views of the terrain with the illumination of southern summer that will make for the best comparison with what it has already observed farther north on the dwarf planet. Dawn photographed all of Ceres in full color in those earlier orbits, but it was not possible then to cover the vast surface with the infrared mapping spectrometer, which has a much smaller field of view than the camera. Therefore, scientists had focused their spectral mapping in the northern hemisphere, taking advantage of the lighting then. While some of the southern hemisphere was studied in infrared as well, the opportunity now to observe more of it will allow a more complete understanding of the distribution of minerals.
In XMO6 the spacecraft will fly over the south pole and then head north over the hemisphere of Ceres facing the Sun. It will go lower and lower as it does so. The lowest point in the orbit will occur between 50° and 60°N. Dawn already mapped that territory from LAMO/XMO1, but now it will take advantage of being low again to acquire some new color photography in the northern hemisphere.
As the spacecraft continues farther north, the altitude will increase again. It will sail higher as it travels over the night side before beginning its fall back down. It will take about 37 hours to complete one elliptical revolution.
Some readers may recall that for all of the mapping orbits at Vesta and Ceres, Dawn traveled south over the sunlit side and north over the hemisphere shrouded in the dark of night. (Readers who don't recall that are invited to trust that it's true.) Experts readily recognize that it is very, very difficult to reverse the orbital direction. Dawn did so, however, with the extensive maneuvering in February-April 2017 that allowed it to make the unique observation of opposition. Those who are interested can review the skilled piloting that reversed the direction.
The explorer will observe Ceres on 10 consecutive orbits in XMO6. To conserve precious hydrazine, Dawn will turn to point its main antenna to Earth and radio its findings after every other transit over the sunlit landscapes. In the other orbits, it will wait patiently, saving both data and hydrazine onboard for later.
On May 31, the spaceship will resume maneuvering. It will take about a week of ion thrusting to push down to the final orbit of the mission.
In XMO7 (shown in the two figures below), Dawn will range from as high as 2,500 miles (4,000 kilometers) to as low as about 22 miles (35 kilometers). (The minimum altitude will vary by a few miles, or kilometers, from revolution to revolution, for reasons we will explain in a future Dawn Journal.) It will take a little more than a day to complete one loop.
We have described before that photography will be very challenging, both because of the difficulty pointing the camera accurately enough to capture specific targets and the high speed so close to the ground. We will return to this problem in an upcoming Dawn Journal.
At the high point of XMO7, Dawn will move at only about 120 mph (190 kph). Then as gravity pulls it back down, the spacecraft will accelerate until it streaks northward at 1,050 mph (1,690 kph) above a relatively narrow strip of ground before starting to soar up again. Dawn was designed for mapping uncharted worlds, not making specialized observations under such conditions, and traveling so fast and so low means it cannot take pictures as sharp as you might expect. Nevertheless, even with a little bit of motion-induced blur at low altitude, any sights we photograph certainly will reveal finer details than we have seen before. This is going to be exciting!
The highest priority measurements will be the nuclear spectra, giving scientists the opportunity to take a sharper picture of the elemental composition of the faraway world, making a more accurate map of the concentration of atomic species that are important for Ceres' geology and chemistry. Dawn's gamma ray and neutron detector (GRaND) is not subject to the limitations of pointing accuracy and blur that can affect the photography. You can think of GRaND's gamma ray vision and its neutron vision as being broader but less acute than the camera's visible-light vision. Getting closer to the ground will help ensure the instrument sees a stronger nuclear signal than ever before and takes a clearer picture.
As the spacecraft races over the ground, GRaND will measure gamma rays and neutrons escaping into space from the atoms down to about a yard (meter) underground. It collected a large volume of such data from LAMO/XMO1, but being so much lower in XMO7 will allow scientists to identify and locate elements more accurately.
There are several GRaND (if not grand) objectives for XMO7. One is to see how the elemental composition differs at different latitudes. The instrument has already revealed that water is more plentiful near the surface at higher latitudes than near the equator, and now it may be able to refine this finding. One of the properties of XMO7 is that the low point will shift almost 2° of latitude south on each revolution. That is, each time Dawn swoops down to its lowest point, it will be south of the low point on the previous orbit. That will provide GRaND the opportunity to survey the concentration and distribution of underground ice at different latitudes. GRaND also may tell us more about other constituents, providing clues about the geological processes that shaped this exotic world.
Of course, as Dawn orbits Ceres, Ceres turns on its axis, pirouetting beneath her admiring companion. So each time Dawn zooms down for a close look, it will not only be farther south than the time before but it will also be at a different longitude. The next Dawn Journal will focus on this and what it means for GRaND and for photography.
Controlling Dawn's orientation in the zero-gravity of spaceflight is harder at low altitude, where Ceres' gravitational pull is stronger. Dawn will use hydrazine much more quickly in XMO7 than at any other part of the mission, and the last of the propellant will be expended before the end of this year.
Dawn just celebrated the third anniversary of arriving at its permanent residence in the solar system. In the natural perspective of its current home, Dawn arrived about two-thirds of a Cerean year ago, or nearly 3,000 Cerean days ago. The explorer has now completed 1,600 orbits. Although hydrazine is dwindling, and the adventure is nearing its end, there is still plenty to look forward to. Stay onboard as Dawn prepares to delve further into the unknown. It's going to be a great ride!
Dawn is 10,800 miles (17,400 kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 740 times as far as the Moon and 1.88 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.
Dr. Marc Rayman
9:15 am PDT March 20, 2018
Dawn has now logged 4 billion miles (6.4 billion kilometers) on its unique deep-space adventure. Sailing on a gentle breeze of xenon ions, the ambitious explorer journeyed for nearly four years to what had been only a small, fuzzy orb for over two centuries of terrestrial observations. Dawn spent more than a year there transforming it into a vast, complex protoplanet. Having sent its Vestan riches safely back to distant Earth, Dawn devoted another 2.5 years to reaching another blank canvas and there created another masterpiece of otherworldly beauty. Permanently in residence at dwarf planet Ceres, Dawn is now preparing to add some finishing touches.
The Dawn flight team at JPL did not even take notice as the odometer rolled over to 4,000,000,000. They have been focused on intensive investigations of how to maneuver the spaceship to lower altitudes than ever anticipated and operate there. For more than eight months in 2015-2016, Dawn circled 240 miles (385 kilometers) above the exotic Cerean landscape. From there, the team piloted the probe to higher orbits to undertake new studies, not anticipating that they might devise new methods to safely go much lower.
There are many challenges to overcome in flying closer to the dwarf planet, and although progress has been excellent, much more work lies ahead before maneuvering can begin. Indeed, even as some team members took time off in December, work never stopped. Many computers operated continuously, running sophisticated trajectory calculations. Engineers will assess the results when they return at the dawn of the new year and then set the computers to work on the next set of problems.
Meanwhile, Dawn waits patiently, safe and healthy in an orbit that ranges from a little more than 3,000 miles (4,800 kilometers) to nearly 24,000 miles (39,000 kilometers). It takes 30 days to complete one revolution. The spacecraft will continue operating in this elliptical orbit at least until April, the earliest opportunity to start its descent.
Having lost the use of the reaction wheels that controlled its orientation, Dawn now relies on hydrazine propellant fired from the small jets of its reaction control system. But after years of interplanetary travels and extensive maneuvering to observe Ceres, the remaining supply is very low. There simply is not enough left for a circular orbit lower than the one the spacecraft has already operated in. Dawn has plenty of xenon propellant to perform all the thrusting with its ion engine to change its orbit, but the available hydrazine is insufficient to perform all the necessary turns and to maintain a stable orientation for pointing its ion engine, solar arrays, antenna and sensors.
To fly low with a paucity of hydrazine, controllers are devising plans for an elliptical orbit. In the previous Dawn Journal, we saw that they might try to steer Dawn down to less than 125 miles (200 kilometers). While more work remains (including all those calculations that are occupying a cluster of computers), the progress has been encouraging. They are now analyzing orbits in which Dawn might even dive below 30 miles (50 kilometers) and then glide up to about 2,500 miles (4,000 kilometers) almost once a day. With many analyses still to perform and plans to refine, engineers anticipate that Dawn has enough hydrazine to maneuver to and operate in such an orbit for two months, and perhaps even a little longer.
If Dawn does go so low, it will be an exciting ride. How cool to skim so close to an alien world! But controllers must be careful that the spaceship doesn't dip too low. We have described before that Dawn complies with a set of protocols called planetary protection (not entirely unrelated to the Prime Directive). The team must ensure that the final orbit is stable enough that Dawn will not contaminate the astrobiologically interesting Ceres even for decades after the mission concludes.
The primary reason to plunge down so close to the mysterious landscapes of rock, ice and salt -- apart from pure awesomeness -- is to sense the nuclear radiation emanating from Ceres with greater clarity than ever before. With its gamma ray and neutron detector (GRaND), Dawn's measurements of this radiation provide insight into the atomic constituents down to about a yard (meter) underground. We have discussed this before in detail, including how the measurements work and why after operating so close to Ceres, Dawn flew to a higher orbit to improve its data.
The radiation is so faint, however, that some elements can only be detected from much closer range than Dawn has been. This is akin to looking at a very dim object or taking a picture of it. From far away, where little light reaches your eyes or your camera, colors are difficult to discern, so the view may be nearly black and white. But if you could move in close enough to capture much more light, you could see more colors. If Dawn can move in much closer to capture more of Ceres' nuclear glow, it may be able to see more of the elements of the periodic table -- in effect, taking a more colorful picture.
We see most objects by reflected light that originates either on the sun or artificial light sources. The nuclear radiation Dawn sees from Ceres is principally caused by cosmic rays. Cosmic rays are a form of radiation that fills space and originates far outside our solar system, mostly from supernovas elsewhere in the Milky Way Galaxy. The brighter the cosmic rays, the brighter Ceres will seem to be. The atoms on and underground don't reflect cosmic rays that strike them. Rather, the cosmic rays cause them to emit neutrons and gamma rays that escape back into space and carry with them the identities of the atoms. So, we can think of this as cosmic rays illuminating a scene, and Dawn will make nuclear photographs, revealing more details of Ceres' composition.
In addition to the advantage of going very low, it turns out that there is a special benefit to performing these measurements in 2018. The sun's magnetic field, which reaches out far beyond the planets, weakens cosmic rays entering our solar system, partially dimming the illumination. But our star's magnetism waxes and wanes in a cycle of 11 years. The sun now is entering the part of this regular cycle in which the magnetic field is weak. And it just so happens that this is an unusually weak solar cycle, so the sun's ability to hold cosmic rays at bay is less than at any time in the history of space exploration. Cosmic rays will be copious in the solar system. This won't matter much for people on or near Earth, because our planet's magnetic field (which extends well above where astronauts, cosmonauts and taikonauts work) resists most of the cosmic rays, and the thick blanket of atmosphere stops the rest. Ceres, like most residents of the solar system, does not have such protections. Thanks to the combination of the forecast of uniquely bright cosmic rays and the latest technology, 2018 will the best year so far in the history of solar system exploration to measure gamma rays or neutrons. Flying so close to the ground, Dawn should get superb readings.
In a future Dawn Journal we will discuss more of the specific objectives for the measurements and what they may reveal about Ceres, but now let's not forget about Dawn's other sensors. What about photography, infrared spectroscopy, visible spectroscopy, and gravity measurements?
We can look forward to some remarkable pictures. Some will be sharper than the best so far, but not by as much as you might expect. When it is in the low altitude segment of its orbit, Dawn will be moving faster than ever at Ceres. If you were in a plane traveling hundreds of miles (kilometers) per hour, it would not be hard to take a picture of the ground six miles (10 kilometers) beneath you. But if you were in a car driving at that speed or even faster, despite being closer to the ground, your pictures might not be better. (That wouldn't be the greatest of your worries, but the Dawn team is devoting a great deal of work to ensuring the ship's safety, as we'll discuss below.) The situation on Dawn isn't that severe, so the photography certainly will improve somewhat on what we already have.
Because the camera's field of view is so small and the hydrazine imposes such a stubborn limitation on Dawn's lifetime, we will see only a very small fraction of the dwarf planet's vast landscape with the improved clarity of low altitude.
In previous Dawn Journals (see, for example, this one), we have delved into details of how difficult it can be to predict the orbit with great accuracy. The dominant (but not exclusive) cause is that every time the hydrazine jets fire, whether to maintain a stable orientation or to turn (including to keep the sensors pointed at Ceres while Dawn swoops by in its elliptical orbit), they push the probe a little and so distort its orbit slightly. Predicting the subtleties of the changes in the spacecraft's orbit is a very complex problem. Although the outcome is not yet clear, the flight team is making progress in investigating methods to manage these orbital perturbations well enough to be able to have some control over where GRaND measures the atomic composition, because its gamma ray spectrometer and neutron spectrometer have broad views. They can tolerate the deviations in the orbit. But Dawn probably will not have the capability to capture any specific targets with its other spectrometers or cameras. Rather, controllers will take pictures of whatever terrain happens to be in view of the cameras. But on a world with as much fascinating diversity as Ceres, intriguing new details are likely to be discovered.
Along with studying the potential for improvements in pictures and spectra, the team is investigating refinements in Ceres' gravity field. They have already measured the gravity much more accurately than expected before Dawn arrived. Whether flying very close to some regions will allow them to improve their determination of the structure deep underground is the subject of ongoing work.
We will see in a Dawn Journal in a few months that the team will try to use certain properties of the orbit besides low altitude to provide attractive scientific opportunities. Nevertheless, it is clear that some goals simply will not be possible to achieve. To accomplish other objectives that are not feasible in that low ellipse, the team is analyzing the merits of pausing the ion-propelled spiral descent for a few weeks before reaching the final orbit. This could allow the spacecraft to view some regions of Ceres with the illumination of southern hemisphere summer, as we described in the previous Dawn Journal.
To ensure our distant ship remains ready to undertake extensive new observations, the infrared spectrometer, visible spectrometer, primary camera and backup camera each will be activated in January and run through their standard health checks and calibrations. For many of the observations in 2018, the two cameras will be used simultaneously to take as many pictures as possible, just as they were for special observations in 2017. Prior to this year, Dawn never used them concurrently.
With the help of a team of dedicated controllers, Dawn has shown itself to be a fantastically capable and resourceful explorer. Many new questions have to be answered and many challenges overcome for it to undertake another (and final) year in its bold expedition. But we can be hopeful that the creativity, ingenuity, and passion for knowledge and adventure that have propelled Dawn so very far already will soon allow it to add rich new details to what is already a celestial masterpiece.
Dawn is 17,200 miles (27,700 kilometers) from Ceres. It is also 1.77 AU (165 million miles, or 265 million kilometers) from Earth, or 705 times as far as the moon and 1.80 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 30 minutes to make the round trip.
Dr. Marc Rayman
4:30pm PST December 27, 2017
Dawn's long and productive expedition in deep space is about to enter a new phase.
Building on the successes of its primary mission and its first extended mission, NASA has approved the veteran explorer for a second extended mission. Dawn will undertake ambitious new investigations of dwarf planet Ceres, its permanent residence far from Earth.
It was not a foregone conclusion that Dawn would conduct further operations. In part, that's because it is only one of many exciting and important missions NASA has underway, and more are being designed and built. But the universe is a big place, as you may have noticed if you've ever gazed in awestruck reflection at the night sky (or had to search for a parking space in Los Angeles). It simply isn't possible to do everything we want. Entrusted with precious taxpayers' dollars, NASA has to make well-considered choices about what to do and what not to do.
In addition, as we have discussed in detail, Earth's ambassador to two giants in the main asteroid belt has had to contend with severe life-limiting problems. Dawn's reaction wheels have failed, and now it has consumed most of its original small supply of hydrazine that it uses in compensation. It has also expended most of the xenon propellant for its uniquely capable ion propulsion system. It was not clear that a truly productive future would be possible for this aged, damaged ship with some supplies that are so limited. Fortunately, Dawn has endless supplies of creativity, ingenuity, dedication and enthusiasm.
For several months, the flight team has been studying the feasibility of flying the spaceship closer to Ceres than had ever been seriously considered. Dawn spent more than eight months in 2015-2016 circling about 240 miles (385 kilometers) above the dwarf planet. It had spectacular views of mysterious landscapes and acquired a wealth of data far beyond what the team had anticipated. Then Dawn flew to a higher altitude during its first extended mission for new observations. Now engineers are making progress on ways to operate the spacecraft in an elliptical orbit that would allow it to swoop down to below 125 miles (200 kilometers) for a few minutes on each revolution. Their results so far are very encouraging. There are still many complex technical problems to solve, and months of additional work remain. Dawn can wait relatively patiently in its current orbit, where it expends hydrazine quite parsimoniously as it measures cosmic rays.
The promising potential for observing Ceres in elliptical orbits from closer than ever before makes a second extended mission there extremely attractive. NASA and the panel of scientists and engineers convened to provide an independent, objective assessment concluded that further exploration of Ceres would be the most valuable assignment for the spacecraft. It is noteworthy that Dawn is the only spacecraft ever to orbit two extraterrestrial destinations and even now, having significantly exceeded its original objectives, has the capability to leave Ceres and pay a brief visit to a third (although it does not have enough xenon left to orbit a third), but the prospects for new discoveries at Ceres are too great to pass up.
Ceres is not only the largest object between Mars and Jupiter but also certainly one of the most intriguing. In fact, motivated by what Dawn has found, there is now great interest in the possibility of sending a lander there someday. Anything more Dawn can do to learn about Ceres or to help pave the way for a subsequent mission will be of great importance.
Ceres is just too fascinating to abandon! Dawn has already revealed the dwarf planet to be an exotic world of ice, rock and salt, with organic materials and other chemical constituents, and now we can look forward to more discoveries. After all, the benefit of having the capability to orbit a distant destination, rather than being limited to a quick glimpse during a fleeting flyby, is that we can linger to scrutinize it and uncover even more of the secrets it holds. (Some readers may also draw inspiration from Ceres' ingredients to concoct recipes for treats to give out to Halloween visitors.)
In addition to the possibility of observing Ceres from unprecedentedly close, there are other benefits to keeping our sophisticated probe at work there. For now, let's consider two of them, both related to how long it takes Ceres to complete its stately orbit around the sun. One Cerean year is 4.6 terrestrial years.
The dwarf planet carries its robotic moon with it as it follows its elliptical path around the sun. In fact, all orbits, including Earth’s, are ellipses. Ceres’ orbit is more elliptical than Earth’s but not as much as some of the other planets. The shape of Ceres’ orbit is between that of Saturn (which is more circular) and Mars (which is more elliptical). (Of course, Ceres’ orbit is larger than Mars’ and smaller than Saturn’s, but here we are considering how much each orbit deviates from a perfect circle, regardless of the size.)
When Dawn arrived at Ceres in March 2015, they were 2.87 AU from the sun. That was well before the dwarf planet's orbit carried them to the maximum solar distance of 2.98 AU in January 2016. Now, with the second extended mission, the spacecraft will still be operating when Ceres reaches its minimum solar distance of 2.56 AU in April 2018. Dawn will keep a sharp eye out for any changes caused by being somewhat closer to the sun.
The extension also will give scientists the opportunity to examine Ceres with the different lighting caused by the change of seasons. Ceres' slower heliocentric orbit than Earth's means seasons last longer on that distant world. It was near the end of autumn in the southern hemisphere when Dawn took up residence at Ceres. Winter came to that hemisphere on July 24, 2015, when the sun reached its greatest northern latitude. The sun crossed the equator, bringing spring to the southern hemisphere, on Nov. 13, 2016, and summer begins on Dec. 22 of this year. Autumn, when the sun will leave the southern hemisphere, is more than one (terrestrial) year later. Most of Dawn's observations so far were made with the sun in the northern hemisphere. Now Dawn will have new opportunities to see the southern hemisphere with similar illumination.
In the coming months, as the team develops and refines its plans, we will describe how they will pilot the ship down to very low altitudes and what new measurements they will make. Before the new phase gets underway, however, you can explore Ceres (and other planets) yourself with Google maps (some functions don't work in some web browsers). Even though it does not use Dawn's sharpest photos, it should be more than adequate for most of your navigational needs. (It isn't quite adequate for Dawn's needs, but that's no cause for worry, because JPL navigators employ somewhat more sophisticated and accurate methods.)
What will Dawn find when it ventures closer to the ground than ever before? What will the new perspectives reveal about a strange world from the dawn of the solar system? What new challenges will the adventurer confront as it pushes further into uncharted territory? We don't know, but stay onboard as we find out together, for that is an essential element both of the tremendously successful process of science and the powerful thrill of exploration.
Dawn is 21,600 miles (34,700 kilometers) from Ceres. It is also 2.47 AU (229 million miles, or 369 million kilometers) from Earth, or 970 times as far as the moon and 2.49 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.
Dr. Marc D. Rayman
2:30 p.m. PDT October 31, 2017
A decade after leaving its first home in the solar system, Dawn is healthy and successful at its current residence.
Even as the veteran explorer orbits high over dwarf planet Ceres and looks forward to continuing its mission, today it can reflect upon 10 exciting and productive years (or equivalently, with its present perspective, 2.17 exciting and productive Cerean years).
The ambitious adventurer embarked on an extraordinary extraterrestrial expedition on Sept. 27, 2007. With its advanced ion propulsion system, Dawn soared past Mars in 2009. The spacecraft took some of the Red Planet’s orbital energy around the sun to boost itself on its journey. (Nevertheless, this extra energy amounts to less than a quarter of what the ion engines have provided.) Ever a responsible citizen of the cosmos, Dawn fully adheres to the principle of the conservation of energy. So to compensate for speeding up, it slowed Mars down.
In 2011, the spacecraft arrived at Vesta, the second largest object in the main asteroid belt between Mars and Jupiter. Dawn gracefully entered into Vesta’s firm but gentle gravitational embrace. The probe maneuvered extensively in orbit, optimizing its views to get the best return possible from its photography and other observations. During 14 months in orbit, Dawn completed 1,298 revolutions around Vesta, taking nearly 31,000 pictures and collecting a wealth of other scientific measurements. From the perspective it had then, Dawn was in residence for nearly a third of a Vestan year (or almost 1,900 Vestan days). The explorer revealed a strange, ancient protoplanet, now recognized to be more closely related to the terrestrial planets (including the one Dawn left 10 years ago) than to the typical and smaller asteroids.
Unlike all other deep-space missions, Dawn had the capability to leave its first orbital destination and voyage to and enter orbit around another. After smoothly disengaging from Vesta, the interplanetary spaceship flew more than 900 million miles (1.5 billion kilometers) in 2.5 years to Ceres, the largest object in the asteroid belt. Indeed, prior to Dawn’s arrival, that dwarf planet was the largest body between the sun and dwarf planet Pluto that a spacecraft had not yet visited. And just as at Vesta, thanks to the maneuverability of ion propulsion, Dawn did not have to be content with a one-time flyby, gathering only as much data as possible during a brief encounter. By going into orbit around Ceres, the spacecraft could linger to scrutinize the exotic, alien world. And that is exactly what it has done.
Both Vesta and Ceres have held secrets since the dawn of the solar system, and both have beckoned since they were first spotted in telescopes at the dawn of the 19th century. For the next two centuries, they appeared as little more than faint smudges of light amidst myriad glittering stellar jewels, waiting for an inquisitive and admiring visitor from Earth. Finally, Dawn answered their cosmic invitations and eventually developed richly detailed, intimate portraits of each.
As the last stop on a unique interplanetary journey of discovery, Ceres has proven well worth the wait. Since arriving in March 2015 (more than half a Cerean year ago, or nearly 2,500 Cerean days ago), Dawn has completed 1,595 revolutions. It has beheld mysterious and fascinating landscapes and unveiled a complex world of rock, ice and salt, along with organic compounds and other intriguing constituents. The dwarf planet may have been covered by an ocean long ago, and there might even be liquid water underground now. The 57,000 pictures and numerous other measurements with the sophisticated sensors will keep scientists busy for many years (both terrestrial and Cerean).
By early 2016, during its ninth year in space, Dawn had accomplished so much that it exceeded all of the original objectives established for it by NASA before the ship set sail. Along the way, Dawn encountered and ultimately overcame many obstacles, including equipment failures that could well have sunk the mission. Against all odds and expectations, however, when its prime mission concluded in June 2016, the spacecraft was still healthy enough that NASA decided to extend the mission to learn still more about Ceres. Since then, Dawn has conducted many investigations that had never even been considered prior to last year. Now it has successfully achieved all of the extended mission objectives. And, once again defying predictions thanks to expert piloting by the flight team (and a small dose of good luck), Dawn still has some life left in it. Before the end of the year, NASA will formulate another new set of objectives that will take it to the end of its operational life.
Dawn has flown to many different orbital altitudes and orientations to examine Ceres. Now the probe is in an elliptical orbit, ranging from less than 3,200 miles (5,100 kilometers) up to 23,800 miles (38,300 kilometers). At these heights, it is measuring cosmic rays. Scientists mathematically remove the cosmic ray noise from Dawn’s 2015-2016 recordings of atomic elements from a low, tight orbit at only 240 miles (385 kilometers).
In its present orbit, Dawn can make these measurements to clarify Ceres’ nuclear signals while being very frugal with its precious hydrazine, which is so crucial because of the loss of three reaction wheels. (The small supply was not loaded onboard with the intention of compensating for failed reaction wheels.) When the hydrazine is expended, the mission will end. So this high elliptical orbit is a very good place to be while NASA and the Dawn project are determining how best to use the spacecraft in the future.
Meanwhile, this anniversary presents a convenient opportunity to look back on a remarkable spaceflight. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the tenth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to investigate Dawn’s ambitious journey in detail may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh, eighth and ninth anniversaries.
In its 10 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,109 days (5.8 years), or 58 percent of the time (and 0.000000042 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 908 pounds (412 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 69 of the 71 gallons (262 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.
The thrusting since then has achieved the equivalent of accelerating the probe by 25,400 mph (40,900 kilometers per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) It is remarkable that Dawn’s ion propulsion system has provided nearly the same change in speed as the entire Delta rocket.
Since launch, our readers who have remained on or near Earth have completed 10 revolutions around the sun, covering 62.8 AU (5.8 billion miles, or 9.4 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 42.4 AU (3.9 billion miles, or 6.3 billion kilometers). As it climbed away from the sun, up the solar system hill to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the 10 years since Dawn began its voyage, Vesta has traveled only 40.5 AU (3.8 billion miles, or 6.1 billion kilometers), and the even more sedate Ceres has gone 37.8 AU (3.5 billion miles, or 5.7 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the 10 years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)
Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with even more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. (If you prefer not to skip it, click here.) In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.
Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family (including Earth, Dawn, Vesta and Ceres) follow their individual paths around the sun, they sometimes move closer and sometimes move farther from it.
In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of Dawn’s journey has been changing the inclination of its orbit, an energetically expensive task.)
Now we can see how Dawn has done by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)
The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sept. 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.
from the Sun (AU)
from the Sun (AU)
|Dawn’s orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn’s orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn’s orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn’s orbit on Sept. 27, 2009||1.42||1.87||6.2°|
|Dawn’s orbit on Sept. 27, 2010||1.89||2.13||6.8°|
|Dawn’s orbit on Sept. 27, 2011||2.15||2.57||7.1°|
|Dawn’s orbit on Sept. 27, 2012||2.17||2.57||7.3°|
|Dawn’s orbit on Sept. 27, 2013||2.44||2.98||8.7°|
|Dawn’s orbit on Sept. 27, 2014||2.46||3.02||9.8°|
|Dawn’s orbit on Sept. 27, 2015||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2016||2.56||2.98||10.6°|
|Dawn’s orbit on Sept. 27, 2017||2.56||2.98||10.6°|
For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that six years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the stalwart adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.
Even after a decade of daring space travel, flying in deep space atop a blue-green pillar of xenon ions, exploring two of the last uncharted worlds in the inner solar system, overcoming the loss of three reaction wheels, working hard to stretch its shrinking supply of hydrazine, Dawn is ready for more. And so is everyone who yearns for new knowledge, everyone who is curious about the cosmos, and everyone who is exhilarated by bold adventures into the unknown. More is to come. Dawn -- and all those who find the lure of space irresistible -- can look forward to whatever lies ahead for this unique mission.
Dawn is 16,600 miles (26,700 kilometers) from Ceres. It is also 2.92 AU (271 million miles, or 437 million kilometers) from Earth, or 1,080 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 49 minutes to make the round trip.
Orbiting the only dwarf planet inside the orbit of Neptune, Dawn is healthy and continuing to carry out its assignments at Ceres with the masterful skill to be expected for such an experienced space explorer.
As Earth and Ceres took up positions on opposite sides of the sun for the first part of this month, the probe operated for almost two weeks without being able to count on assistance from its human handlers, even if it encountered a serious problem. The powerful interference of the sun could have prevented radio communications. But Dawn had no need. When the changing geometry allowed the radio silence to break, the ship confirmed that all was well.
Dawn’s primary responsibility in this phase of its mission continues to be monitoring cosmic rays. For eight months in 2015-2016, circling closer to Ceres than the International Space Station is to Earth, the probe measured nuclear radiation that contains the signatures of geologically important elements down to about a yard (meter) underground. Since December, when it reached a much greater altitude, it has been listening to the faint hiss of cosmic rays. Scientists will mathematically remove that from the earlier recordings of Ceres. This procedure will allow them to squeeze even more information out of the low-altitude census of atomic species.
Dawn had to fly far enough above Ceres that it could measure the cosmic rays alone, rather than the combination of Ceres radiation and cosmic radiation it detected at low altitude. The mission continued to go so well after they had sent the spacecraft to a high altitude, that the team devised more new objectives. To start, they had Dawn photograph some very nice scenes of a gibbous Ceres. Then they guided it through two months of intricate orbital maneuvers, allowing the spacecraft to fly across the line from the sun to Ceres, providing a view of the fully illuminated dwarf planet (like a full moon). In addition to yielding lovely new movies and color pictures, these opposition measurements may help scientists discover details of the material on the ground that would otherwise be impossible to descry from orbit.
That orbit extended so high that it took two months to complete one long elliptical loop around Ceres. The opposition observations worked extremely well, but it’s not a convenient orbit for most other investigations (except the cosmic ray measurements). Therefore, earlier this month, mission controllers instructed the spacecraft to use its ion engine to adjust the orbit again, this time reducing the period for one revolution to 30 days and improving the opportunities for future scientific measurements.
In coming months, we will look ahead to new observations the team is just beginning to consider. It has not been assured that further activities would be possible. For half of the time since it embarked on its extraordinary extraterrestrial expedition, Dawn has managed to complete its work without the use of the full complement of equipment it was supposed to have at its disposal. Even with the failures of three reaction wheels, however, the mission has far exceeded its original objectives and well outlasted its expected lifetime. Nevertheless, the spacecraft’s lifetime certainly is limited, most likely by the dwindling supply of hydrazine, although possibly instead by one of the many risks that are part of the very nature of conducting complex operations in the unforgiving far reaches of space. For now, however, it appears that Dawn has enough life left in it to warrant pursuing even more new goals.
On July 16, as the sophisticated ship from distant Earth continues to carry out its mission, it will celebrate the 271st birthday of Giuseppe Piazzi, the first person to spot Ceres. It was a faint point of light amid the stars, one tiny jewel among too many to count. When the 54-year-old made his serendipitous discovery, which gave him an honored place in the history of science, he certainly could not have foreseen what Dawn has now seen. (And there's no reason he should have. He was an astronomer and mathematician, not a clairvoyant.)
In addition to revealing Ceres’ overall appearance, Dawn has acquired a wealth of pictures and other information that scientists are now actively studying. The mission has shown us mesmerizing bright regions and an extensive network of ground fractures in Occator Crater. The shapes and sizes of many craters provide intriguing clues about the strength and other properties of the interior, and the measurements of the gravity field yield still more insight into the inside. The towering cryovolcano Ahuna Mons rises up as a compelling monument to internal geological forces (which we will discuss below). Organic chemicals spotted in and near Ernutet Crater and elsewhere are of special interest for astrobiology. We see ice on the ground and have determined there is a tremendous amount underground (and there may be liquid underground as well). Piazzi discovered -- and Dawn uncovered -- a truly alien world, and its vastness and diversity are part of what make it so fascinating.
Among the minerals Dawn has found is a group known as carbonates, and they are abundant on Ceres. We see two types there. One, which is omnipresent, is known as dolomite and contains calcium and magnesium. It is mixed with another Cerean mineral, serpentine. A different type of carbonate is prominent in Occator Crater. The sodium carbonate there reflects so much sunlight that it seems almost to be luminous, like a giant spotlight casting its brilliance far out into space, perhaps to show off that it contains the highest concentration of any kind of carbonates known anywhere in the solar system except Earth. Occator’s specific kind, sodium carbonate, has been observed only on Earth and in the plumes of Saturn’s watery moon Enceladus. Interestingly, the carbonates and serpentine are formed by chemical reactions between rocks and water under high pressure. How could these minerals be both widespread and exposed?
One possibility is that they formed deep underground and were later pushed to the surface by internal geological processes. Just as on Earth, those internal forces are mostly powered by heat from the decay of radioactive elements. The heat is carried away by the motion of the material, just as heating water at the bottom of a pot causes it to rise and then make complex convection patterns. The strength of the forces depends on the rate at which the heat leaks from the deep interior to the ground. That is, heat is a form of energy, and a faster flow of heat energy (and thus of material) would provide a more powerful internal engine to drive minerals to the surface.
Heat flows from hot (far underground) to cold (the surface, which is exposed to space). It is at least 80 degrees Fahrenheit (50 degrees Celsius) colder near Ceres’ north and south poles than near the equator. That means the strength of the geological pressure pushing minerals to the surface should depend on the latitude, which would translate into different compositions at different latitudes. But that is not what Dawn sees. The minerals show up everywhere we look. Their prevalence is a fact that is inconsistent with a deep underground origin followed by a heat-driven movement to the surface. Science tells us we need to formulate a different explanation for why minerals produced in water under high pressure now can be found on the ground.
Scientists recognize a more likely explanation. The minerals may have formed in an ocean early in Ceres’ history, when radioactive elements were so abundant that it would have been warm enough to keep a large volume of water as a liquid. But as Ceres aged, it would have cooled (perhaps some readers have experienced this as well), because the supply of radioactive elements would have gradually been depleted as they decayed. Almost the entire ocean would have frozen, encasing Ceres in a shell of ice. But that wouldn’t be the end of the story.
Ice cannot last long on Ceres (except in special places). Cold though it is on that world, there is enough warmth from the distant sun that ice sublimates, turning from a solid into a gas as the water molecules escape into space. Even as that gradual phenomenon occurred at the microscopic level, ice was lost through a much more dramatic and abrupt process. It was blasted away by asteroids that slammed into it. The rain of rocks that fall onto Ceres over millions of years is a familiar hazard to anyone who has lived in the main asteroid belt for millions of years. In fact, scientists estimate that a frozen ocean three miles (five kilometers) thick could have been lost in only a few tens of millions of years, a blink in geological time. (And even if that ice shell had been much thicker, it would still have been lost on a geologically short timescale.)
Before it froze and dispersed, chemical reactions between the water and rocks would have produced a rich inventory of minerals. As Dawn peers down from its orbital perch, it sees their testimony to that long-lost ocean. And even now there may still be reservoirs of liquid within Ceres, as it is warm enough inside.
None of this could have been imagined by Piazzi on the night he first glimpsed Ceres from his observatory in Sicily. Because he wasn’t prescient, he also did not expect that what he discovered would be known at times as a planet, an asteroid, a dwarf planet and eventually as "home" by Dawn. Nor would he have anticipated the Tunisian-Sicilian War, the extraordinary intellectual achievements in the scientific discoveries of evolution, relativity and quantum mechanics, or the inventions of the safety pin, granola, integrated circuits and remotely controlled interplanetary spacecraft. If Piazzi thought seriously about the unique successes of science or about the nature of exploration, he did not leave much of a record.
For the perspective of someone who did, let’s go back to a time before Piazzi’s 1801 sighting of Ceres but after the dwarf planet’s formation nearly 4.6 billion years ago. Sometime between 1607 and 1620, the polymath and early champion of modern science
Francis Bacon wrote this in Cogitata et Visa (Thoughts and Conclusions):
It would disgrace us, now that the wide spaces of the material globe, the lands and seas, have been broached and explored, if the limits of the intellectual globe should be set by the narrow discoveries of the ancients. Nor are those two enterprises, the opening up of the earth and the opening up of the sciences, linked and yoked together in any trivial way. Distant voyages and travels have brought to light many things in nature, which may throw fresh light on human philosophy and science and correct by experience the opinions and conjectures of the ancients.
Bacon realized that archaic ideas had such a tight grip that they prevented the expansion of Europe’s intellectual horizons. The startling and exciting discoveries of the explorers who pushed the physical horizons during the century or so that preceded his writings broke that suffocating squeeze. New realizations about the reality of the natural world, and how dramatically it differed from the untested notions of old, inspired an ardor for intellectual exploration as daring and vigorous as what had been undertaken in traversing those distant lands and seas.
The reward has been discoveries by Piazzi and uncounted other scientists who have revealed the staggering richness of nature in all its forms, a universe of such majesty, such beauty, such complexity that it would seem to defy explanation. And yet science not only uncovers myriad mysteries but also lifts the veil, revealing inner workings and showing us why things are the way they are. The ultimate rewards of science are knowledge and understanding.
Dawn is both a beneficiary of and a contributor to the extraordinary successes of science since Bacon’s time. The mission’s "distant voyages and travels have brought to light many things in nature." And its exploration of alien lands and its journeys on interplanetary seas continue to "throw fresh light on human philosophy and science." The real beneficiaries are we ourselves. How fortunate we all are to behold what that light has illuminated!
Dawn is 20,000 miles (32,200 kilometers) from Ceres. It is also 3.67 AU (341 million miles, or 549 million kilometers) from Earth, or 1,400 times as far as the moon and 3.61 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.
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.
The flight team had worked out a plan to provide a backup opportunity to study Ceres at opposition on June 28. The results of the April 29 observations are so good, however, that the backup was deemed unnecessary and so has been canceled. In this phase of Dawn’s mission, the highest priority continues to be recording cosmic rays so scientists can improve their measurements of the atomic constituents down to about a yard (meter) underground.
Dawn’s latest success followed less than a week after what might have seemed to some people to be a very serious problem. Indeed, in other circumstances, it could have been devastating to the mission. Fortunately, the expert team piloting this spaceship was well prepared to steer clear of any dire scenarios.
On April 23, reaction wheel #1 failed. This was Dawn’s third incident of losing a reaction wheel. (In full disclosure, the units aren’t actually lost. We know precisely where they are. But given that they stopped functioning, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Reaction wheels are disks that spin to help control the orientation of the spacecraft, somewhat like gyroscopes. By electrically changing a wheel’s speed (as high as 75 revolutions per second), the spacecraft can turn or hold steady.
We have discussed Dawn’s reaction wheels many times, and reaction wheel enthusiasts are encouraged to review the detailed history by rereading the last 275,000 words posted. But because this is the last time we will ever need to discuss them, we will summarize the entire story to its conclusion here.
The wheels do not help propel Dawn through space. The ion propulsion system does that (and, by the way, does it amazingly well). The wheels are used to rotate the spacecraft around its three axes, which can be called pitch, roll and yaw; x, y and z; left-right, front-back and up-down; Kirk, Spock and McCoy; animal, vegetable and mineral; or many other names. Regardless of the designations, three wheels are needed because there are three dimensions of space. Always conservative, designers equipped Dawn with four wheels. On a nearly decade-long interplanetary odyssey to well over one million times farther from Earth than astronauts can travel, the probe was designed with enough spare hardware to tolerate the loss of almost any component, including a reaction wheel. (The spacecraft is also outfitted with a backup radio receiver, radio transmitter, central computer, ion engine, camera, heaters, valves and on and on.)
One reaction wheel failed in June 2010, about a year before Dawn arrived at its first destination, Vesta, the second largest body orbiting the sun between Mars and Jupiter. A second one failed in August 2012 as Dawn was escaping from Vesta, having far surpassed its objectives in exploring the protoplanet. (That second failure is so long ago, that now, for half of its time in space, Dawn has not had three operable wheels, despite the intent of its cautious designers.)
The flight team was able to overcome the loss of the two reaction wheels, even though that had never been planned for (nor even considered) when the spacecraft was being designed and built. It required not only a great deal of work but also exceptional ingenuity and diligence. That heroic effort paid off very handsomely in allowing the spacecraft to continue its ambitious deep-space expedition, trekking for 2.5 years from Vesta to Ceres and then conducting a comprehensive study of that dwarf planet, the first one humankind had ever seen. Dawn exceeded all of its goals and successfully concluded its prime mission in June 2016. And even with the malfunctions of two reaction wheels, the team kept the spacecraft so healthy and productive that it is now conducting an extended mission, gathering even more riches at Ceres.
There was no basis for predicting when another wheel would fail, but it was widely considered to be only a matter of time. Because the four wheels are of the same design, and some had failed on other spacecraft as well, confidence that the two remaining wheels would function for long was low. Indeed, your faithful correspondent, in his technical role on Dawn, occasionally referred to the "two failed wheels and two doomed wheels."
When the spacecraft reported on April 24 that another wheel had failed, no one on the team was very surprised. In fact, the biggest surprise was that the two doomed wheels had continued to operate as long as they did after the other two stopped.
The strategy for recovering from each of the two earlier failures and preparing for another was complex and multifaceted. Let’s recall just a few aspects.
Dawn carries a small supply of conventional rocket propellant called hydrazine, expelled from small jets of the reaction control system. (Yes, Dawn has a full set of backup jets.) The reaction wheels occasionally need a little bit of hydrazine help, and that is why the reaction control system is onboard. (For propulsion, it is far less efficient than the ion propulsion system, and Dawn has never used hydrazine for that purpose.) In principle, the reaction control system could do the job of the reaction wheels, but that would require a great deal more hydrazine than Dawn carried when it left Earth. Indeed, the reason for reaction wheels is that they control the orientation for much less mass. Well, to be more precise, they control the orientation when they work. When they fail, they don’t do as well. The flight team invested a tremendous effort in stretching the hydrazine so it could be used in place of the wheels, and that has proven to be extremely successful. In fact, Dawn arrived at Ceres ready to complete its mission here with zero wheels in case a third wheel was on the verge of failing.
The amount of hydrazine Dawn uses depends on its activities. Whenever it fires an ion engine, the engine controls two of the three axes, significantly reducing the consumption of hydrazine. In orbit around Vesta and Ceres, the probe often trains its sensors on the alien landscapes beneath it. The lower the orbital altitude, the faster the orbital velocity, so Dawn needs to turn faster to keep the ground in its sights. Also, the gravitational attraction of these massive worlds tends to tug on the unusually large solar arrays in a way that would turn the ship in an unwanted direction. (For more on this, see here.) That force is stronger at lower altitude, so Dawn needs to work harder to counter it. The consequence is that Dawn uses more hydrazine in orbit around Vesta and Ceres than when it is journeying between worlds, orbiting the sun and maneuvering with its ion engine. And it uses more hydrazine in lower orbits than in higher ones. Following the first reaction wheel problem, mission controllers decided to hold the wheels in reserve for the times that they would be most valuable in offsetting hydrazine use.
From August 2010 to May 2011, the spacecraft flew with the one failed wheel and the three healthy (but doomed) wheels all turned off. As it approached Vesta, controllers reactivated the three wheels, and they served well for almost all of Dawn’s work there. The second malfunction occurred in August 2012 as Dawn was ascending on its departure spiral, and the spacecraft correctly deactivated all of them and reverted to hydrazine control even before radioing the news to distant Earth. The wheels had been scheduled to be turned off again shortly after Dawn pulled free of Vesta, so the team decided to leave them off then and complete the escape without reaction wheels. They were not used again (except for four brief periods) until 1.2 billion miles (1.9 billion kilometers) later, in December 2015, when Dawn reached its lowest altitude orbit around Ceres.
At Ceres, of course, only two reaction wheels were operable, and Dawn was not designed to use fewer than three. But the day after the first reaction wheel problem occurred in 2010, engineers at JPL and Orbital ATK (back then, it was Orbital Sciences Corporation) began preparing for another failure. They started working on a method to control the orientation with two wheels plus hydrazine, a combination known as hybrid control. That would consume less hydrazine than using no wheels, although more than if three wheels were available. Following an unusually rapid development of such complex software for a probe in deep space, the team installed the new capability in Dawn’s central computer in April 2011, shortly before Vesta operations began. That software performed flawlessly from December 2015 until the third reaction wheel failed last month.
The team determined in 2010 that the benefits of operating the spacecraft with only one wheel would not justify the investment of effort required. So now that three have failed, the last operable wheel is turned off, and it will never be used again. But as we saw above, the team has a great deal of experience flying Dawn with no wheels at all. They had piloted the ship in that configuration through the solar system and around Ceres for a total of four years, so they were well prepared to continue.
With the third wheel failure, we can be grateful that each wheel provided as much benefit as it did. The wheels allowed Dawn to conduct extremely valuable work while using the hydrazine very sparingly. Now that we are finished with the wheels, the members of the flight team are not despondent, dear reader, and you shouldn’t be either. Dawn can continue to operate until the hydrazine is depleted or some unforeseen problem arises. But risks are the nature of venturing into the forbidding depths of space. For now, Dawn has life left in it. Next month we will describe the plans for using the remaining hydrazine.
Less than a week after the third reaction wheel failed, Dawn performed perfectly in collecting all of the planned pictures (using both the primary camera and the backup camera) as well as visible spectra and infrared spectra at opposition. Reaching that special position on the line from the sun to Ceres required two months of intricate maneuvers. By coincidence, another special alignment occurs very soon. This one is called conjunction.
Earth and Ceres follow independent orbits around the sun. Earth carries with it the moon and thousands of artificial satellites. The dwarf planet has one companion, a native of Earth, a temporary resident of Vesta and a resident of Ceres since March 2015.
Because Earth is closer to the sun than Ceres, it is bound by a stronger gravitational leash and so circles faster. Early next month, their separate orbital paths will bring them to opposite sides of the sun. From the terrestrial perspective (shared by some readers, perhaps even including you), the sun and Ceres will appear to be at the same location in the sky. This is conjunction.
Communicating with distant interplanetary spacecraft is not easy. (Surprise!) It is even more difficult near conjunction, when the radio signals between Earth and the spacecraft travel close to the sun on their way. The solar environment is fierce indeed, and the stormy plasma that surrounds the star interferes with the radio waves, like hot, turbulent air making light shimmer. Communications will be unreliable from May 31 to June 12. Even though some signals may get through, mission controllers can’t count on hearing from the spacecraft or contacting it. But they are confident the stalwart ship will manage on its own, executing the instructions transmitted to it beforehand and handling any problems until Earth and Ceres are better positioned for engineers to provide any help. Occasionally Deep Space Network antennas, pointing near the sun, will listen amid the roaring solar noise for Dawn’s faint whisper, but receiving any crackling messages will simply be a bonus. In essence, conjunction means radio silence.
Dawn’s proximity to the sun presents a convenient opportunity for terrestrial observers to locate Dawn in the sky. On June 5-6, it will be less than one solar diameter from the sun. Ceres does not orbit the sun in the same plane as Earth, so it does not always go directly behind the disk of the sun. The spacecraft and dwarf planet will be a little bit south of the sun.
If you hold three fingers (preferably your own) together at arm’s length and block the sun any time from June 1 to 10 (and you are encouraged to do so), you will also cover Dawn. From June 3 to June 8, you can cover the dazzling celestial signpost and Dawn at the same time with your thumb.Dawn is very big for an interplanetary spacecraft (or for an otherworldly dragonfly, for that matter), with a wingspan of nearly 65 feet (19.7 meters). However, it will be 346 million miles (557 million kilometers) away during conjunction, more than 3.7 times as far as the sun.
Those who lack the requisite superhuman (or even supertelescopic) vision to discern the fantastically remote spacecraft through the blinding light of the sun needn’t worry. We can overcome the limitation of our visual acuity with our passion for exploring the cosmos and our burning desire for bold adventures far from home. For this alignment is a fitting occasion to reflect once again upon missions deep into space.
There, in that direction, is Earth’s faraway emissary to alien worlds. You can point right to where it is. Dawn has traveled more than 3.8 billion miles (6.1 billion kilometers) on a remarkable odyssey. It is the product of creatures fortunate enough to be able to combine their powerful curiosity about the workings of the cosmos with their impressive abilities to wonder, investigate, and ultimately understand. While its builders remain in the vicinity of the planet upon which they evolved, their robotic ambassador now is passing on the far side of the extraordinarily distant sun.
This is the same sun that is more than 100 times the diameter of Earth and a third of a million times its mass. This is the same sun that has been the unchallenged master of our solar system for more than 4.5 billion years. This is the same sun that has shone down on Earth all that time and has been the ultimate source of much of the heat, light and other energy upon which residents of the planet have depended. This is the same sun that has so influenced human expression in art, literature, mythology and religion for uncounted millennia. This is the same sun that has motivated impressive scientific studies for centuries. This is the same sun that is our signpost in the Milky Way galaxy. Daring and noble missions like Dawn transport all of us well beyond it.
Dawn is 31,600 miles (50,800 kilometers) from Ceres. It is also 3.72 AU (346 million miles, or 557 million kilometers) from Earth, or 1,555 times as far as the moon and 3.68 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.
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.
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