We explained in detail in the two August Dawn Journals that the spaceship would deplete its supply of hydrazine, which was essential for controlling its orientation as it orbited dwarf planet Ceres. We predicted that the last of the hydrazine would be spent between mid-September and mid-October (although we acknowledged that it could be earlier or later). Dawn, ever the overachiever, held on until the end of October, and the explorer was productive to the very end. This was the best way to end a mission. It was good to the last drop!

Dawn took us on a truly amazing solar system journey. Never content simply to do the same thing over and over, the flight team kept coming up with new kinds of activities and new kinds of observations from new orbital perspectives. With such a long and constantly active mission, it may have seemed like it would just keep going. For readers who did not keep up with our recent forecasts of the end, it might be worth recognizing Stein's Law: “If something cannot go on forever, it will stop.” (The eponymous economist who both conceived of and named this epigram was Herbert Stein.) 

Dawn demonstrates not only Stein's Law but also Davies’ Corollaries (proposed by Daniel Davies, a financial analyst):

1. Things that can’t go on forever, go on much longer than you think they will.

2. Corollary 1 applies even after taking into account Corollary 1.

When it stopped, Dawn was in its extended mission orbit 7 (XMO7). Every 27 hours, the intrepid ship dove from 2,500 miles (4,000 kilometers) to 22 miles (35 kilometers) above the ground, only three times higher than a typical passenger aircraft travels over Earth, and then soared back up again. It had a fantastic view as it streaked over the alien landscape at 1,050 mph (1,690 kph). (While the supply of hydrazine was very limited, it seemed there was no end to the adrenaline. Too bad Dawn's reaction control thrusters couldn't use that chemical instead.) The spacecraft expertly performed high-resolution measurements, providing scientists with a wealth of exquisite data and giving everyone incredibly detailed looks at the exotic sights. 

On Oct. 31, Dawn flew down to peridemeter (the low point of its elliptical orbit) shortly after 5:00 am PDT. As always, to keep its solar arrays pointed at the Sun and its sensors pointed at the ground, it had to fire its hydrazine thrusters extensively. Using the thrusters was routine after having operated for more than half of its time in space without the use of the reaction wheels that were intended for controlling its orientation, but which had failed. While the spacecraft didn't know the hydrazine was about to run out, mission controllers had known for quite some time.

As is typical for missions in deep space, Dawn operated most of the time out of radio contact. NASA's Deep Space Network (DSN) cannot serve all missions simultaneously, and often spacecraft have tasks to perform that are incompatible with radio communications. As it turned out, however, Dawn's final moments happened to be while the largest antenna at the Goldstone Deep Space Communications Complex in California was tracking it. The 230-foot (70-meter) antenna thus allowed the flight team to hear Dawn's swan song.

We have described before that with Dawn broadcasting through an auxiliary antenna when it flew close to the ground, scientists and engineers could learn about Ceres' interior. The spacecraft's radio signal was too weak to carry data to the DSN. Rather, it was as if Dawn were playing a single note with no variation. That may not make for an especially imaginative or melodious performance, but as different regions underground exerted their gravitational pulls and accelerated and decelerated the probe, the Doppler shift was music to the ears of planetary geologists. 

By observing changes in the strength and some other characteristics of the signal (and knowing the likely explanation), engineers were able to reconstruct some of the spacecraft's final actions. Around 20 minutes after it was at peridemeter, still quite low but with its momentum starting to carry it back up to high altitudes, the hydrazine thrusters became ineffective. Dawn recognized that it could no longer control its orientation (although it did not know the reason) and systematically proceeded through all the contingency procedures possible, such as swapping to backup equipment and even rebooting its main computer. It made valiant attempts and continued to operate with the professionalism of a dedicated, veteran space explorer, but without hydrazine, there was nothing it could do. The outcome was inevitable. Dawn was up against an unsolvable problem. 

Although the spacecraft's depletion of hydrazine and subsequent inability to communicate had been predicted for quite some time, your correspondent considered it worthwhile to verify the diagnosis. It was possible, albeit highly unlikely, that some other problem had befallen Dawn and that after the scheduled session with the Goldstone antenna, the sophisticated robot would solve it and try to reestablish radio contact. 

The plan then was for the flight team to look for Dawn at night. Hours after young trick-or-treaters everywhere had finished extorting sweets from their elders, when Earth had rotated so that another 230-foot (70-meter) antenna, the largest at the Madrid Deep Space Communications Complex, could point at Dawn's location in the sky, controllers listened again. Not even the faintest whisper was heard. The remote spacecraft was orbiting Ceres as silently as the cold vacuum of space itself. 

After more than 11 years of an incredibly exciting, fantastically fruitful, extremely difficult, deeply rewarding, super fun and just totally awesome interplanetary adventure, your correspondent declared the mission over shortly before 1:30 am PDT on Nov. 1. 

The mission had been a dream come true. Now the dream was over. 

Somehow, the stark reality of the end of the phantasmagorical mission in the middle of the night seemed to turn upside down the meaning of a proverb commonly (but vaguely) attributed to Africa: "However long the night, dawn will break."

This video captures some of the highlights of Dawn's interplanetary adventure, as well as some personal reflections on it.

With a rare excursion into first person, I wrote in my Aug. 22 Dawn Journal about how I felt with the mission coming to an end (and offered a fanciful additional perspective at the end of my Sept. 27 Dawn Journal). My feelings were unchanged when the end came. Nevertheless, in the actual event, I wrote down some of my thoughts, because Dawn was such a significant part of my life, and I am well aware of the fallibility of human memory. Memories, however vivid, are often more of a reconstruction than people like to believe.

But I quickly realized that it didn't matter how I was feeling! Here is an unedited excerpt of what I wrote after declaring the mission to be over: "These feelings are transitory, and I don't need to remember them anyway. It would be a mistake to consider how I feel now as somehow representing my overall experience or feelings about the mission. Indeed, this is very much the wrong time to try to put it into perspective. It would make a good story if I had some revelation or profound description of my feelings at this point, but there's no reason I should. It takes time to gain a good perspective. People construct and then gradually change their memories, all without any awareness. And I should not think that somehow now I will be imbued with the wisdom, insight, or other capability to put this into perspective. If I feel sad, elated, disappointed, relieved, proud, empty, gratified or any of myriad other feelings -- and, more to the point, a combination of myriad feelings -- I won't feel that way again. The end of Dawn is not what's important. All that preceded it is. And I cannot so easily grasp it all right now, so my feelings now are not as special or as meaningful as one might be tempted to think."

Finally, you can't appreciate the end of the mission if you don't appreciate the rest of the mission. So, feel free to reread the previous 310,000 words in Dawn Journals to gain the full appreciation.

There will be future opportunities to address some of the overall accomplishments of the mission and discoveries about Ceres. For now, we will devote more attention to this final phase. 

And there was no doubt about its finality. On Nov. 1-2, immediately after the official end of the mission, there was not enough time to reallocate previously scheduled DSN antenna time to other missions. So although confidence was high that Dawn would forever be silent, each of the three deep space communications complexes (Goldstone, then Canberra, and then Madrid) turned a sensitive ear to Ceres for one last time. No surprises occurred.

The final phase of Dawn's exploration began in June when it completed maneuvering to XMO7. We have explained that as the orbit precessed so that peridemeter gradually shifted from Ceres' day side to the night side, photography, infrared spectroscopy and visible spectroscopy became less valuable. The spacecraft had collected a tremendous number of such measurements earlier in the mission, so when it flew over illuminated terrain in XMO7 at higher altitudes than it had already been, new observations were not worthwhile. (Low altitude measurements of Ceres' nuclear radiation and gravity continued in darkness to the very end of the mission.)

Recognizing that the hydrazine would be long gone by the time peridemeter moved back to the day side, controllers took advantage of a nice opportunity at higher altitude for a last, fond look at Ceres on Sept. 1-2. As the dwarf planet pirouetted before the admiring eye of its permanent companion, Dawn recorded its final views of Ceres. One of them is shown above and another is below.

On Sept. 28 and 29, Dawn performed a calibration of the camera and the visible and infrared mapping spectrometer for one last time. They are precision scientific instruments, and the thorough analysis of their data depends on accurate knowledge of their sensitivity and other properties. The team has conducted calibrations throughout the mission so even slight changes could be detected and accounted for in interpreting the pictures and spectra and drawing conclusions about the nature of Vesta and Ceres. Dawn expended a little more of its remaining hydrazine to point the instruments at the stars Vega and Arcturus, which they had observed before. Indeed, the first time was less than three months after the journey began in 2007 (and Vega still holds special significance).

Even though Dawn took no more pictures nor infrared or visible spectra of Ceres after the beginning of September, it acquired a great many before that, far exceeding the team's expectations when planning this phase of the mission. In XMO7, the spacecraft sent more than 11,000 photographs of Ceres to Earth, almost all of them at very low altitude, revealing amazing new details. (This brought the total for Vesta plus Ceres to more than 100,000 pictures.) Also during XMO7, Dawn provided scientists with more than three million infrared spectra and almost 50,000 visible spectra. 

We have explained before that Ceres' nuclear glow is very faint, so the gamma ray and neutron detector (GRaND) requires a great deal of data to make its measurements, just as a camera needs a long exposure to record a dark scene. Despite its name, GRaND is meek and unprepossessing, but the instrument does do a wonderful job revealing the atomic composition of the material down to about a yard (meter) underground. GRaND does not need illumination, so it continued to operate even as Dawn glided over ground cloaked in the deep dark of night. 

In XMO7, GRaND acquired 140 hours of nuclear spectra from altitudes below Dawn's previous low altitude orbit, at 240 miles (385 kilometers) in 2015-2016. And it accumulated 50 hours of measurements of Ceres' radiation from within 60 miles (100 kilometers) of the ground. GRaND collected about four times as much data in XMO7 as scientists needed to meet their objectives. This will allow them to see Ceres' elemental abundances with much sharper resolution, like a close-up picture, than ever before. 

Like the other investigations, gravity measurements far surpassed what the team expected not only when planning XMO7 but even when the spacecraft was there.

With the smooth and productive operations in XMO7, these successes may all seem pretty simple. After all, it's only the cutting edge of rocket science, operating an ion propelled spaceship at incredibly low altitude around a dwarf planet well over a million times farther away than the International Space Station. But there were a few challenges to overcome. The team confronted and solved myriad problems to accomplish so much. 

Now, even if you don't have your own interplanetary spacecraft, you can explore Ceres and do so from the comfort of your home. Instead of going all the way to the main asteroid belt, bring that distant world to your computer with Cerestrek. You can also see all the sights on the first world Dawn unveiled with Vestatrek.

Dawn, however, will never again explore alien worlds. It will never again emit a bluish beam of xenon ions. It will never again communicate with beings on the faraway planet where its voyage began. It will never again perform any of the functions or tasks it executed so admirably on its remarkable journey. For decades, and quite possibly even for centuries, the ship that undertook a long, daring, difficult and successful deep-space expedition on behalf of humankind will remain silently in orbit around Ceres. It has become an inert celestial monument to the power of human ingenuity, creativity, and curiosity, a lasting reminder that our passion for bold adventures and our noble aspirations to know the cosmos can take us very, very far beyond the confines of our humble home.

Dawn is 1,800 miles (2,900 kilometers) from Ceres. It is also 3.53 AU (328 million miles, or 527 million kilometers) from Earth, or 1,320 times as far as the moon and 3.56 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, will never again make the round trip.

Dr. Marc D. Rayman
7:30 am PST November 11, 2018

TAGS: DAWN, CERES, DWARF PLANET, VESTA, ASTEROID, SPACE TRAVEL

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

After its short dawn ride to space on a Delta, Dawn began its long interplanetary expedition atop a cool blue beam from its ion engines. The spacecraft sailed past Mars in 2009 and in 2011 entered orbit around Vesta, the second largest body in the main asteroid belt. During 14 months there, it revealed Vesta to be more like the terrestrial planets, including the one on which Dawn was conceived and built (and where its controllers still reside), than like the much smaller chunks of rock we know as asteroids. In 2012, propelled by a zephyr of xenon ions, the ship set sail on the cosmic seas once again, its sights set on the largest uncharted world between the Sun and Pluto. Ceres took the traveler into a gentle but permanent gravitational hold in 2015. Thanks to curious and creative creatures on Earth, Ceres now has a moon named Dawn.

During its 11th year in deep space, the explorer undertook some of its most ambitious activities of the entire mission. Unlike missions limited to a brief glimpse of their targets during a flyby, Dawn has taken great advantage of being able to conduct comprehensive studies of Ceres (and Vesta). And thanks to the maneuverability afforded by its ion engines, the spacecraft has frequently changed its orbit to optimize its investigations. We described earlier this year how Dawn flew to extended mission orbit 6 (XMO6) for a new campaign of photography and other measurements. Following that, early in June the spaceship used its ion engine to descend to XMO7, in which it dives down to about 22 miles (35 kilometers) above the ground, only three times your altitude when you travel in a commercial jet. (Perhaps you ought to consider traveling by spaceship. Note that Dawn travels a little faster than a jet, although it does require some patience to reach the top speed. We will see more about this below.)

The orbit was initially aligned so the low point, known as peridemeter, would be in the vicinity of Occator Crater on the dayside of Ceres, allowing the spacecraft to obtain stunning pictures and other data. We discussed in March and in June that the peridemeter gradually shifts south. As it did so, the focus of the close-up observations moved to Urvara Crater. In late August, to the delight of everyone interested in the exploration of space, Dawn was still operating productively, and by then, the peridemeter had moved to its greatest southerly latitude of 84° (corresponding to the orbital inclination, for those who understand orbits). Since then, it has progressed north on the other side of Ceres, opposite the Sun. (If this progression isn't clear, see the diagram in March and imagine continuing the trend of the orbital precession it illustrates.) Now the peridemeter has moved so far to the nightside that throughout Dawn's time over illuminated terrain, it is higher than it was in previous orbits. There is little new to see now from higher up. Therefore, Dawn no longer conducts visible or infrared observations, but it is continuing to measure nuclear radiation and the gravity field, both of which provide valuable insight into the nature of the dwarf planet.

On every Sept. 27, we reflect on this unique interplanetary adventure. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the 11th 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, ninth and tenth anniversaries.

In its 11 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,141 days (5.9 years), or 53 percent of the time (and 0.000000043 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 907 pounds (411 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 (261 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space. (Note that on the tenth anniversary, we actually gave a slightly higher xenon cost. Dawn has not refueled since then. For technical reasons we will not delve into, it is very difficult to compute the xenon consumption late in the mission. Engineers devoted extensive effort to refining their measurements of the xenon during the past year, resulting in a small change in their final calculation of how much xenon Dawn has used.) We saw in June that Dawn will never use its ion propulsion system again. We have the spacecraft right where we want it.

The thrusting since launch has achieved the equivalent of accelerating the probe by 25,700 mph (41,400 kph). 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 11 revolutions around the Sun, covering 69.1 AU (6.4 billion miles, or 10.3 billion kilometers). Orbiting farther from the Sun, and thus moving at a more leisurely pace, Dawn has traveled 46.4 AU (4.3 billion miles, or 6.9 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 11 years since Dawn began its voyage, Vesta has traveled only 44.9 AU (4.2 billion miles, or 6.7 billion kilometers), and the even more sedate Ceres has gone 41.8 AU (3.9 billion miles, or 6.3 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 11 years. You will see that as the strength of the Sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)

Comparing mileage with cars is highly misleading, but some readers can't help but try to make that comparison. The reason it is deceptive is that cars have to keep burning fuel to move as they overcome friction, but orbiting objects normally move without propulsion at all. Earth has completed its annual trip around the Sun (currently 584 million miles, or 940 million kilometers) for billions of years without requiring any propellant at all. Similarly, spacecraft coast most of the time. With ion propulsion, Dawn (and Deep Space 1 before it) were the exceptions, thrusting more often than coasting. But readers who require a comparison with their car (or their spaceship) can credit Dawn with 63 million miles per gallon (0.0000038 liters per 100 kilometers, or 3.8 liters per 100 million kilometers).

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.

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 seven 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 exotic world in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the 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.

Dawn has had a long and productive life. Indeed, many readers might agree that Dawn has accomplished much, much more in its life than, say, a Nathusius' pipistrelle does in its life, which lasts about the same length of time. (And while Nathusius' pipistrelle was honored as the first-ever "Bat Species of the Year," Dawn has been honored for its accomplishments too, although somewhat different ones.) For our chiroptophobic readers, Dawn's lifetime also is about the same as a paradoxical frog, a magnificent hummingbird (recently renamed Rivoli's hummingbird), and a Taipan beauty snake. They also tend not to achieve nearly as much in their lifetimes as Dawn has, although it's nice that all those names have some connection with Dawn's magnificent exploration of two worlds, paradoxically at similar distances from the Sun and yet dramatically different, and each beautiful in its way. 

Since the two August Dawn Journals, some people have expressed wishes that Dawn would live even longer. I would say it has already lasted longer! The hydrazine could have been depleted much earlier. Indeed, the mission could easily have ended years ago. Life is not easy in the forbidding depths of space, far from Earth. There are many reasons the mission could have concluded early, including the failures of the probe's reaction wheels. Dawn has flown more than half of its time in space without the use of those gyro-like devices, which had previously been considered indispensable for the mission. It is only through the near-heroic work of the flight team that, despite those failures and the many other challenges Dawn has faced, the prime mission concluded successfully in 2016. Dawn is now near the end of its second extension. One can even fantasize that Dawn did, in fact, fail early, succumbing to one of the risks during its unique and ambitious mission, dodging only 999 of 1,000 bullets, and that many of the fabulous pictures and other data from uncharted worlds were never acquired. And then somehow, we said, "If only..." with enough fervor, and we wished hard enough that the fatal problem had not occurred, and presto: we were granted a second chance! Then we could now be the beneficiaries. We might be living in that alternate universe, unaware that, in the original timeline, we were not so lucky. There is good reason not to believe that, but it may provide some perspective on our being fortunate that Dawn has lived so long and been so productive in its extraordinary extraterrestrial expedition. 

Dr. Marc D. Rayman
4:34 am PDT September 27, 2018

TAGS: DAWN, CERES, DWARF PLANET, VESTA, ASTEROID, SPACE TRAVEL, ION PROPULSION, ELECTRIC PROPULSION

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Dawn began its ambitious mission in 2007. (And on Aug. 17, 2018, it passed a milestone: three Vestan years of being in space.) But the mission is rapidly approaching its conclusion. In the previous Dawn Journal, we began an in-depth discussion of the end, and we continue it here.

We described how the spacecraft will lose the ability to control its orientation, perhaps as soon as September. It will struggle for a short time, but it will be impotent. Unable to point its electricity-generating solar panels at the Sun or its radio antenna to Earth, the seasoned explorer will go silent and will explore no more. Its expedition will be over.

We also took a short look at the long-term fate of the spacecraft. To ensure the integrity of possible future exploration that may focus on the chemistry related to life, planetary protection protocols dictate that Dawn not contact Ceres for at least 20 years. Despite being in an orbit that regularly dips so low, the spaceship will continue to revolve around its gravitational master for at least that long and, with very high confidence, for more than 50 years. The terrestrial materials that compose the probe will not contaminate the alien world before another Earth ship could arrive.

Like its human colleagues, Dawn started out on Earth, but now its permanent residence in the solar system, Ceres, is far, far away. Let's bring this cosmic landscape into perspective.

Imagine Earth reduced to the size of a soccer ball. On this scale, the International Space Station would orbit at an altitude of a bit more than one-quarter of an inch (7 millimeters). The moon would be a billiard ball almost 21 feet (6.4 meters) away. The Sun, the conductor of the solar system orchestra, would be 79 feet (24 meters) across at a distance of 1.6 miles (2.6 kilometers). More remote even than that, when Dawn ceases operating, it would be more than 5.5 miles (9.0 kilometers) from the soccer ball. The ship will stay locked in orbit around Ceres, the only dwarf planet in the inner solar system. The largest object between Mars and Jupiter, that distant orb would be five-eighths of an inch (1.6 centimeters) across, about the size of a grape. Of course, a grape has a higher water content than Ceres, but exploring this fascinating world of ice, rock and salt has been so much sweeter!

Now let's take a less terrestrial viewpoint and shift our reference to Ceres. Suppose it were the size of a soccer ball. In Dawn's final, elliptical orbit, which it entered in June, the spacecraft would travel only 37 inches (94 centimeters) away at its farthest point. Then it would go in to skim a mere one-third of an inch (8 millimeters) from the ball.

Dawn is one mission among many to explore the solar system, dating back almost 60 years and (we hope) continuing and even accelerating for much longer into the future. Learning about the cosmos is not a competition but rather a collective effort of humankind to advance our understanding. And to clarify one of the many popular mistaken notions about the solar system, let's take advantage of reducing Ceres to the size of a soccer ball to put some other bodies in perspective.

Because it is in the main asteroid belt, there is a common misconception that Ceres is just another asteroid, somehow like the ones visited by other spacecraft. It is not. The dwarf planet is distinctly unlike the small chunks of rock that are more typical asteroids. We have discussed various aspects of Ceres' complex geology, and much more remains to be gleaned from Dawn's data. Vesta too has a rich and complicated geology, and it is more akin to the terrestrial planets (including Earth) than to asteroids. But for now, let's focus simply on the size in order to make for an easy comparison. Of course, size is not a measure of interest or importance, but it will illustrate how dramatically different these objects are.

With a soccer-ball-sized Ceres, Vesta would be nearly five inches (more than 12 centimeters) in diameter. (This writer's comprehensive knowledge of sports inspires him to describe this as a ball nearly five inches, or more than 12 centimeters, in diameter.)

What about some of the asteroids being explored as Dawn's mission winds to an end? There are two wonderfully exciting missions with major events at asteroids (albeit ones much closer to Earth than the main asteroid belt) in the second half of 2018. Your correspondent, a lifelong space enthusiast, is as hopeful for success as anyone! Hayabusa2 is revealing Ryugu and OSIRIS-REx is on the verge of unveiling Bennu.

Ryugu and Bennu are more irregular in shape than Ceres and Vesta, but they would both be so small compared to the soccer ball that their specific shapes wouldn't matter. Ryugu would be less than a hundredth of an inch (a quarter of a millimeter) across. Bennu would be about half that size. They would be like two grains of sand compared to the soccer ball. In the first picture of the June Dawn Journal, we remarked on the detail visible in a feature photographed on one of Dawn's low streaks over the alien terrain. It is also visible in the first two pictures above. That one structure on Ceres is only a part of Cerealia Facula, which is the bright center of the much larger Occator Crater. Occator is a good-sized crater, but not even among the 10 largest on Ceres. Yet that one bright feature in the high-resolution photo is larger than either of these small asteroids. In many of Dawn's pictures that show the entire disk of the dwarf planet (like the rotation movie and the color picture here), Ryugu and Bennu would be less than a pixel, undetectably small, just as invisible specks of dust on a soccer ball.

The tremendous difference in size between Ceres (and Vesta) and small asteroids illustrates a widely unappreciated diversity in the solar system. Of course, that is part of the motivation for continuing to explore. There is a great deal yet to be learned!

Although Ryugu and Bennu aren't in the main asteroid belt, the belt contains many more Lilliputian asteroids closer in size to them than to the Brobdingnagian Ceres and Vesta. In fact, of the millions of objects in the main asteroid belt, Ceres by itself contains 35 percent of the total mass. Vesta has 10 percent of the total.

Readers with perfect memories may note that we used slightly smaller fractions in earlier Dawn Journals. Science advances! More recent estimates of the mass of the asteroid belt are slightly lower, so these percentages are now correspondingly higher. The difference is not significant, but the small increase only emphasizes how different Vesta and Ceres are from typical residents of the asteroid belt. It's also noteworthy -- or, at least, pretty cool -- that Dawn has single-handedly explored 45 percent of the mass between Mars and Jupiter.

Dawn will end its mission in the same orbit it is in now, looping around from a fraction of an inch (fraction of a centimeter) to a yard (a meter) from the soccer-ball-sized Ceres. In the previous Dawn Journal, we described what will happen onboard the spacecraft. We also saw that the most likely indication controllers will have that Dawn has run out of hydrazine will be its radio silence. They will take some carefully considered steps to verify that that is the correct conclusion.

But it is certain that emotions will be ahead of rationality. Even as team members are narrowing down the causes for the disappearance of the radio signal, many strong feelings about the end of the mission will arise. And they will be as varied as the people on the Dawn team, every one of whom has worked long and hard to make the mission so successful. Your correspondent can make reasonable guesses about their feelings but won't be so presumptuous as to do so.

As for my own feelings, well, I won't know until it happens, but I'm not too presumptuous to guess now. Long-time readers may recognize that your correspondent has avoided writing anything about himself (with a few rare exceptions), or even using first person, in his Dawn Journals. They are meant to be a record of a mission undertaken by humankind, for everyone who longs for knowledge and for adventures in the cosmos. But now I will devote a few words to my own perspective.

My love affair with the universe began when I was four, and my passion has burned brighter and brighter ever since. I knew when I was a starry-eyed nine-year-old that I wanted to get a Ph.D. in physics and work for NASA, although it was a few more years before I did. I had my own Galileo moment of discovery and awe when I first turned a telescope to the sky. Science and space exploration are part of me. They make me who I am. (My friend Mat Kaplan at The Planetary Society described me in the beginning of this video as "the ultimate space nerd." He's too kind!) Adding to my own understanding and contributing to humankind's knowledge are among my greatest rewards.

Passion and dedication are not the whole story. I recognize how incredibly lucky I am to be doing what I have loved for so long. I am lucky to have had access to the resources I have needed. I am lucky that I was able to do well in my formal education and in my own informal (but extensive) studies. I am lucky I could find the discipline and motivation within myself. For that matter, I am lucky to be able to communicate in terms that appeal to you, dear readers (or, at least, to some of you). My innate abilities and capabilities, and even many acquired ones, are, to a large extent, the product of factors out of my control, like my cognitive and psychological constitution.

That luck has paid off throughout my time at JPL. Working there has been a dream come true for me. It is so cool! I often have what amount to out-of-body experiences. When I am discussing a scientific or engineering point, or when I am explaining a conclusion or decision, sometimes a part of me pulls back and looks at the whole scene. Gosh! Listen to the cool things I get to say! Look at the cool things I get to do! Look at the cool things I know and understand! Imagine the cool spacecraft I'm working with and the cool world it is orbiting! I am still that starry-eyed kid, yet somehow, through luck and coincidence, I am doing the kind of things I love and once could only have dreamed of.

Dawn will continue to be exciting to the very end, performing new and valuable observations as it skims incredibly low over the dwarf planet on every orbital revolution. The spacecraft has almost always either been collecting new data or, thanks to the amazing ion propulsion, flying on a blue beam of xenon ions to somewhere else to gain a new perspective, see new sights and make more discoveries. Whether in orbit around Vesta or Ceres or traveling through the solar system between worlds, the mission was rarely anything like routine.

I love working on Dawn (although it was not my first space love). You won't be surprised that I think it is really cool. I could not be happier with its successes. I am not sad it is ending. I am thrilled beyond belief that it achieved so much!

I was very saddened in graduate school when my grandfather died. When I said something about it in my lab to a scientist from Shanghai I was working with, he asked how old my grandfather was. When I said he was 85, the wiser gentleman's smile lit up and he said, "Oh, you should be happy." And immediately I was! Of course I should be happy -- my grandfather had lived a long (and happy) life.

And so has Dawn. It has overcome problems not even imagined when we were designing and building it. It not only exceeded all of its original goals, but it has accomplished ambitious objectives not even conceived of until after it had experienced what could have been mission-ending failures. It has carried me, and uncounted others (including, I hope, you), on a truly amazing and exciting deep-space adventure with spectacular discoveries. Dawn is an extraordinary success by any measure.

It did not come easily. Dawn has consumed a tremendous amount of my life energy, many times at the expense of other desires and interests. (Perhaps ironically, it even comes at the expense of my many other deep interests in space exploration and in science, such as cosmology and particle physics, interests shared by my cats Quark and Lepton. Also, writing these Dawn Journals and doing my other outreach activities take up a very large fraction of what would otherwise be my personal time. As a result, I always write these in haste, and I'm never satisfied with them. That applies to this one as well. But I must rush ahead.) The challenges and the demands have been enormous, sometimes feeling insurmountable. That would not have been my preference, of course, yet it makes the endeavor's successful outcome that much more gratifying.

At the same time I have felt all the pressure, I have long been so overjoyed with the nature of the mission, I will miss it. There is nothing quite like controlling a spacecraft well over a thousand times farther than the Moon, farther even than the Sun. Silly, trite, perhaps even mawkish though it may seem, when spacecraft I have been responsible for have passed on the far side of the Sun, I have taken those opportunities to use that blinding signpost to experience some of the awe of the missions. I block the Sun with my hand and contemplate the significance, both to this particular big, starry-eyed kid and to humankind, of such an alignment. I -- we -- have a spacecraft on the far side of the Sun!

Every day I feel exhilarated knowing that, as my car's license plate frame proclaims, my other vehicle is in the main asteroid belt. It won't be the same when that vehicle is no longer operating.

But I will always have the memories, the thrills, the deep and powerful personal gratification. And I have good reason to believe they will persist, just as some prior space experiences still fill me with gratitude, pride, excitement and pure joy. (I also hope to have many more cool out-of-body experiences.)

And long after I'm gone and forgotten, Dawn’s successes will still be important. Its place in the annals of space exploration will be secure: a wealth of marvelous scientific discoveries, the first spacecraft to orbit an object in the asteroid belt, the first spacecraft to visit a dwarf planet (indeed, the first spacecraft to visit the first dwarf planet that was discovered), the first spacecraft to orbit a dwarf planet, the first spacecraft to orbit any two extraterrestrial destinations, and more.

For now, Dawn is continuing to operate beautifully (and you can read about it in subsequent Dawn Journals). The end of the mission, when it comes, will be bittersweet for me, a time to reflect and rejoice at how fantastically well it has gone, and a time to grieve that it is no more. I will have many powerful and conflicting feelings. Like Walt Whitman, I am large, I contain multitudes.

Thanks to Dawn, we now have Vesta and we now have Ceres. Soon, very soon, Dawn will be only a memory (save for those who visit Ceres and find it still in orbit) but the worlds it revealed will forever be a part of our intellectual universe, and the capabilities to explore the solar system that it advanced and devised will be applied to exciting new missions. And the experience of being intimately involved in this grand adventure will remain with me for as long as I am able to see the night sky and marvel at the mysteries of the universe that captivated me even as a starry-eyed child.

Dawn is 1,500 miles (2,400 kilometers) from Ceres. It is also 3.46 AU (322 million miles, or 518 million kilometers) from Earth, or 1,275 times as far as the Moon and 3.42 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
10:00 pm PDT August 22, 2018

TAGS: DAWN, CERES, VESTA, DWARF PLANET, ASTEROID BELT, ASTEROIDS, SPACECRAFT, SOLAR SYSTEM

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

When Dawn left Earth in 2007, it was outfitted with four reaction wheels, devices that were considered indispensable for controlling its orientation on its long expedition in deep space. Despite the failures of reaction wheels in 2010, 2012 and 2017, the team has accomplished an extremely successful mission, yielding riches at Vesta and at Ceres far beyond what had been anticipated when the interplanetary journey began. But now the rapidly dwindling supply of hydrazine propellant the robot uses in place of the reaction wheels is nearly exhausted.

With no friction to stabilize it, the large ship, with electricity-generating solar arrays stretching 65 feet (19.7 meters) wingtip-to-wingtip, holds its orientation in space by firing hydrazine propellant from the small jets of its reaction control system. The orientation should not be confused with the position. In the zero-gravity of spaceflight, they are quite independent. Unlike an aircraft, a spacecraft's position and the direction it travels are largely unrelated to its orientation. The probe's position is dictated by the principles of orbital motion, whether in orbit around the Sun, Vesta or (now) Ceres, and the ion propulsion system is used to change its trajectory. We are concerned here about orientation.

Dawn can hold its orientation quite stable, but it still lazily oscillates a little bit in pitch, roll and yaw. When the spacecraft points its main antenna to Earth, for example, spending many hours radioing its findings to the Deep Space Network (DSN) as it travels around Ceres, it rotates back and forth, but the angular motion is both tiny and slow. The ship turns about a thousand times slower than the hour hand on a clock. The clock hand continues its steady motion, going all the way around, rotating through a full circle in 12 hours. Dawn needs to keep its antenna pointed at Earth, however. If Dawn were at the center of the clock and Earth were at the 12, it wouldn't let the antenna point any farther away than the hour hand gets from the 12 in about a minute. The tiny angle is only about a tenth of the way from the 12 to the adjacent ticks (both on the left and on the right) that mark one second for the second hand. When Dawn's orientation approaches the maximum allowed angular deviation, the main computer instructs a jet to puff out a little hydrazine to reverse the motion.

When the spacecraft follows its elliptical orbit down to a low altitude, only three times higher than you are when you fly on a commercial jet, it needs to expel hydrazine to keep aiming its camera and spectrometers down as it rushes over the ground. If this isn't clear, try pointing your finger at an object and then circling around it. You are constantly changing the direction you're pointing. For Dawn to do that, especially in its elliptical orbit, requires hydrazine. (If you think Dawn could simply start rotating with hydrazine and then just point without using more, there are some subtleties here we will not describe. It really does require extensive hydrazine.)

Whether pointing at the landscape beneath it or at Earth, it might seem that Dawn could remain perfectly steady, but there are always tiny forces acting on it that would compromise its pointing. One is caused by the difference between Ceres' gravitational pull on the two ends of the solar arrays that occurs when the wings are not perfectly level. (We described this gravity gradient torque when Dawn was orbiting Vesta.) Also, sunlight reflecting in different ways from different components (some with polished, mirror-like surfaces, others with matte finishes) can exert a very small torque. Dawn uses hydrazine to counter these and other slight disturbances in its orientation.

As we have discussed extensively, very soon, the hydrazine will be depleted. Most likely between the middle of September and the middle of October (although possibly earlier or later), the computer will tell a reaction control jet to emit a small burst of hydrazine, as it has myriad times before in the mission, but the jet will not be able to do so. There won't be any usable hydrazine left. It will be like opening the end of a completely deflated balloon. No gas will escape. There will be no action, so there will be no reaction. Dawn's very slow angular motion will not be reversed but rather will continue, and the orientation will slowly move out of the tight bounds the ship normally maintains.

The computer will quickly recognize that the intended effect was not achieved. It will send more signals to the jet to fire, but the result will be no different. On a mission often operating out of radio contact with Earth and always very, very far away, help can never be immediate (after all, radio signals travel at the universal limit of the speed of light), so the robot is programmed to deal with problems on its own. There are several possibilities for what actions Dawn will take, depending on details we will not delve into, but a likely one is to try switching from the primary reaction control jets to the backup reaction control jets. Of course, that won't fix the problem, because the jets will not be at fault. In fact, with no hydrazine available, none of its attempts to correct the problem will succeed.

When Dawn experiences problems it can't resolve on its own, it invokes one of its safe modes, standard responses the craft uses when it encounters conditions its programming and logic cannot accommodate. (We have described the safe modes a number of times before, with perhaps the most exciting time being here.) In this case, the safe mode it will chose will go through many steps to reconfigure the spacecraft and prepare to wait for help from humans on a faraway planet (or anyone else who happens to lend assistance).

One of the first steps will be to temporarily power off the radio transmitter, one of the biggest consumers of electrical power on the ship. Until Dawn can make all of the necessary changes, including turning to point the solar panels at the Sun, it will not want to devote precious energy to unnecessary systems. Electrical power is vital. Without it, the spacecraft will be completely inoperative, just as your car, computer, smartphone or lights do nothing at all when they are deprived of power.

Dawn will try to do all its work using only the energy stored in its battery (which it keeps charged, using excess power from the solar arrays). It knows that later, once the arrays are in sunlight, it will have plenty of power, but in the meantime, it needs to be parsimonious. The computer, heaters, motors to rotate the solar arrays, and some other devices are essential to getting into safe mode. The radio is needed only after the spacecraft has completed other steps.

The spacecraft will not complete those other steps. One of them is to turn to point at the Sun, ensuring that the large solar arrays are fully illuminated. But without hydrazine, it will have no means to accomplish the necessary turn.

So, Dawn will not be able to achieve the planned orientation with the solar arrays generating electrical power. The computer will stubbornly refuse to turn on the radio, instead continuing to try to turn so the Sun will light up the arrays and infuse the robot with its electrical lifeblood.

Dawn will continue to try as long as it has power, whether flowing from partially lit solar arrays or from the battery. All the while, the spacecraft will continue to rotate at the same leisurely speed it did when it had hydrazine. But instead of gently oscillating back and forth, it will simply keep going in the same direction, like a clock's hour hand slowed down to measure months instead of hours.

Some of the time, the solar arrays will face away from the Sun and the battery will drain. Some of the time the solar arrays will point at (or near) the Sun just by luck. But Dawn doesn't rely on luck. Until it has a stable orientation with the arrays reliably on the Sun, the computer will insist that power not be devoted to the radio. First things first: first achieve a condition that can be safe for days, weeks, or even months, and then radio Earth for help. The programming did not anticipate being completely unable to control orientation.

Engineers have analyzed what will happen and observed many examples of it in the spacecraft simulator at JPL. Eventually, the computer may make some other attempts. But Dawn's struggle will be brief, lasting only hours before the battery is exhausted. The seasoned adventurer will sink into unconsciousness. At some later time, as its stately rotation brings the solar arrays back into the light, it may well begin to revive, but the cycle will repeat. The newly awakened Dawn will try to point at the Sun and hold that position, taking advantage of the power from the fortuitously illuminated solar arrays. But soon its continuing rotation will point the arrays into the dark of space again. It might seem that half the time the arrays would receive light and so it should be able to operate at half power, but it doesn't work that way. At Dawn's distance from the Sun, a little bit of that faint light on the solar arrays is not sufficient.

After an extraordinary extraterrestrial expedition, more than a decade of interplanetary travels, unveiling two of the last uncharted worlds in the inner solar system, performing unique and complex maneuvers, encountering and overcoming a host of unanticipated problems, Dawn will be on the losing end of a battle with the cold, hard reality of operation in deep space. Its mission will be over.

The spacecraft will be well over a million times farther from Earth than the International Space Station. How will we know when it has run out of hydrazine if its radio is off? (The reaction control system is expected to operate normally as long as there is usable hydrazine, so there will be no prior indication that its exhaustion is imminent.) 

Even as it goes about trying to fix or recover from problems, the computer issues some brief status reports. (They often are more informative than the dialog boxes that pop up on your computer, and Dawn never asks you to click on something to proceed.) If the loss of hydrazine happens to occur while Dawn is communicating with Earth, one of those concise reports may be received before the computer turns off the transmitter. The short message would be like a farewell tweet that Dawn is ending its mission.

Most of the time, however, the probe does not point its main antenna at Earth. When it zips down to low altitude, it aims its sensors at the ground, so the antenna is pointed in an arbitrary direction. Dawn transmits a very broad radio signal through one of its auxiliary antennas so scientists and engineers can follow its motion very precisely. (We have explained before that this allows them to determine the interior structure of the dwarf planet.) That radio connection is too weak for anything else, so Dawn won't be able to tweet its news. If the last of the hydrazine is spent when Dawn's orbital motion is being tracked, the radio signal will simply disappear.

In its elliptical orbit, Dawn spends far less time traveling fast at low altitude than it does traveling slowly at high altitude, much as the girl on a swing we encountered in April. And when it is high up, we generally do not have radio contact at all. So it is more likely that the hydrazine will be depleted when Dawn is out of touch than when the DSN is recording its radio transmissions, through either the main antenna or an auxiliary antenna. Then the next time one of the antennas of the DSN aims at Dawn's location in the sky, it will strain to hear the faint radio whisper of the faraway probe, but all will be silent.

Dawn controllers and the DSN will work together to be sure the inability to detect the spacecraft isn't some other problem, perhaps in mission control or in the tremendously complex DSN. Over the course of a few days, they will use more than one antenna and will take a few other measures. After all, there could be other reasons for a temporary loss of signal, and engineers will work through the possibilities. But given Dawn's resilience and sophistication, if it remains uncommunicative during that time, the conclusion will not be in doubt. Even without a tweet, it will be clear Dawn has run out of hydrazine and is at the end of its operational life.

After conducting a systematic investigation, when the Dawn project is confident of the situation, we will announce the result. In the next Dawn Journal, we will consider a more personal side of this story.

But what of Dawn's long-term fate? Remember, its orientation in space is largely independent of its orbital motion. The spacecraft's inability to point where it wants, to power its systems, and to communicate with its human handlers will have virtually no effect on where it goes.

Dawn doesn't need propulsion to stay in orbit around Ceres, just as the Moon doesn't need it to stay in orbit around Earth and Earth doesn't need it to stay in orbit around the Sun. And that's important. We do not want Dawn to come into contact any time soon with the dwarf planet it orbits.

Ceres is subject to planetary protection, a set of standards designed to ensure the integrity of possible future "biological exploration" of the alien world. That terminology does not mean there is biology on Ceres but rather that that exotic world is of interest in the field of astrobiology. Ceres was once covered with an ocean and today harbors a vast inventory of water (mostly as ice but perhaps with some liquid still present underground). It also has a supply of heat (retained even now, long after radioactive elements decayed and warmed the interior), organics and a rich variety of other chemicals. With all these ingredients, Ceres could experience some of the chemistry related to the development of life. Scientists do not want to contaminate that pristine environment with Dawn's terrestrial materials.

Not all solar system bodies need such protection. The Moon, Mercury and Venus, for example, have not been of interest for searches for life or for prebiotic chemistry. For that reason, spacecraft are allowed to land or crash on those worlds because there is no expectation of subsequent biological exploration. Also exempt from such rules are tiny asteroids, including two that are being explored this year, Ryugu and Bennu. They are entirely unlike giant Ceres. They are often mistakenly thought of as being similar because of the oversimplified notion that all are asteroids. We will provide an illustration of the dramatic difference in the next Dawn Journal.

The planetary protection rules for Ceres specify that Dawn not be allowed to contact it for at least 20 years. There is a common misconception that the time is needed to allow the spacecraft to be sterilized by the radiation, vacuum and temperature extremes of spaceflight. That's not the case. Many terrestrial microbes are impressively hardy, and there is good reason to believe that some that have taken an unplanned interplanetary cruise with Dawn would remain viable after much longer than 20 years.

The requirement for 20 years is intended to allow enough time for a follow-up mission, if deemed of sufficiently high priority given the many goals NASA has for exploring the solar system. Two decades should be long enough to mount a mission that builds on Dawn's many discoveries. We would not want such a hypothetical mission to be misled by finding microorganisms or nonbiological organic chemicals that were deposited by our spacecraft. As we'll see below, the deadline for another mission to get there before Dawn contaminates Ceres is likely to be significantly more relaxed even than that.

Earlier this year, when the team was figuring out how to fly to and operate in an orbit like the one Dawn is in now, much of their work was guided by this planetary protection requirement. We did not want to enter an orbit that would not meet the 20-year lifetime. We could not take the chance of going to an orbit with a shorter lifetime and plan for subsequent maneuvers to increase the duration. We were not sufficiently confident Dawn would have enough hydrazine to remain operable long enough to make its observations and still be able to change its orbit.

The team studied elliptical orbits with different minimum altitudes. Trajectory experts investigated the long-term behavior of each orbit as Ceres' irregular gravity field tugs on the spacecraft revolution after revolution, year after year. Like Earth, Ceres has some regions of higher density and some of lower density. As Dawn orbits over these different regions, they gradually distort the orbit. The analyses also accounted for the slight pressure of sunlight, which not only can rotate the spacecraft but also can push it in its orbit. An orbit with a minimum of 22 miles (35 kilometers) was the lowest that the team was confident would comply with planetary protection, and that's why Dawn is now in just such an orbit.

And after 20 years? Calculations show that even over 50 years, the orbital perturbations are overwhelmingly likely to be too small to cause Dawn to crash. In fact, there is less than a one percent chance of the orbit being distorted enough that Dawn would hit Ceres. In other words, our analysis gives us more than 99 percent confidence that even in half a century, Dawn will still be revolving around Ceres, the largest object between Mars and Jupiter, the only dwarf planet in the inner solar system and the first dwarf planet discovered (129 years before Pluto).

Leaving the remarkable craft in orbit around the distant colossus will be a fitting and honorable conclusion to its historic journey of discovery at Vesta and Ceres. Dawn's scientific legacy is secure, having revealed myriad fascinating and exciting insights into two quite dissimilar and mysterious alien worlds. This interplanetary ambassador from Earth will be an inert celestial monument to the power of human ingenuity, creativity, and curiosity, a lasting reminder that our passion for bold adventures and our noble aspirations to know the cosmos can take us very, very far beyond the confines of our humble home.

Dawn is 1,400 miles (2,300 kilometers) from Ceres. It is also 3.46 AU (321 million miles, or 517 million kilometers) from Earth, or 1,270 times as far as the Moon and 3.42 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
7:00 pm PDT August 21, 2018

TAGS: DAWN, CERES, DWARF PLANET, ASTEROID BELT, SPACECRAFT, ASTROBIOLOGY

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

When Dawn dives low, it takes spectacular pictures, and you can see some of them here and more in the image gallery. But that's not all it does. The spacecraft also collects a trove of data on the nuclear radiation emanating from Ceres (which can reveal some of the atomic elements that are present), the gravity field (which can reveal the distribution of mass underground) and the infrared and visible light (which can reveal the minerals on the ground). Dawn has made all these kinds of measurements before, not only during more than three years at Ceres, the largest object in the main asteroid belt, but also during its 2011-2012 studies of Vesta, the second largest. But prior to this month, Dawn had never been this close and so never had such breathtaking sights and never been able to gather such high-resolution information.

We described the nature of this orbit in the three previous Dawn Journals. It is known as extended mission orbit 7 (XMO7) because Dawn's computer program for generating really cool and dramatic names was offline when it was time to come up with the name. Ever resourceful, the team activated the backup software that generates accurate but uninspiring names.

That kind of resourcefulness has served Dawn very well. Despite critical hardware failures that could have been disastrous for the mission, the flight team has accomplished success after success. The difficulty of flying so low -- only three times your altitude when you travel in a commercial jet -- and actually collecting useful data there seemed unachievable as recently as late last year. And now Dawn is doing it regularly.

Before XMO7, the spacecraft's lowest orbit around Ceres was 240 miles (385 kilometers), about the same height as the International Space Station is above Earth. Dawn spent eight months in 2015-2016 at that altitude, providing an exquisite view of the dwarf planet. It subsequently flew higher to pursue other scientific objectives.

Now Dawn is observing Ceres from as low as about 22 miles (35 kilometers). That tremendous reduction in altitude, a factor of 11, is the largest of the entire mission. At no other time at Vesta or Ceres did Dawn move in that much closer from its previous best vantage point. For those of you who enjoy the numbers, the table here has the distances for each of Dawn's observations of Ceres before the comprehensive mapping began, and this table shows the altitudes of the four mapping orbits of the prime mission, the last being the lowest. In those tables, we compared Dawn's view of Ceres to a view of a soccer ball. The low point of XMO7 would be like looking at a soccer ball from only one-third of an inch (eight millimeters) away. This is truly in-your-face exploration.

And the jump in resolution is amazing. With the fantastic new details, it seems we are seeing a whole new Ceres. A picture is worth a thousand words, but these pictures also merit a few exclamation points!!!

Dawn completed ion thrusting to XMO7 on June 6 and began its new observations of Ceres on schedule on June 9. Everywhere the spacecraft looked, it had fascinating new views. But the team had one special site in mind, and you might too. (Maybe it's even the same site.)

One of the bonus objectives was to try to get photos of Cerealia Facula, the mesmerizingly bright center of Occator Crater. We have explained why targeting a specific location is so hard. One of the attractive features of XMO7 was that it allowed two specially targeted attempts, thus increasing the chances that at least one would work. The team worked very hard to devise methods to take full advantage of that, while always quite well aware that it might not work.

Before we proceed, let's recall some terminology and introduce a new term. The high point in Dawn's orbit, 2,500 miles (4,000 kilometers), is known as apodemeter, analogous to the more common term apogee, which applies for orbits around Earth. (Demeter is the Greek counterpart of the Roman goddess Ceres.) The low point, 22 miles (35 kilometers), is peridemeter. Each 27.2-hour orbital revolution has one apodemeter and one peridemeter.

In April we discussed that Dawn travels much faster near peridemeter than near apodemeter, just as a swing moves faster at its low point than at its high point. As a fun fact, which does not bear on any of the following discussion, Dawn spends less than two hours over the dayside of Ceres (including peridemeter) and more than 25 hours over the nightside (including apodemeter). That may be surprising, but if you contemplate the illustrations of the elliptical XMO7 below and in March and think about the constantly changing velocity, it may make sense. (Or you may decide that it doesn't matter, accept it and move on.)

Mission planners had windows in the schedule for using the ion propulsion system to adjust the orbit. They instructed Dawn to fire its ion engine for 2 hours and 7 minutes on June 20 as the ship sailed upward. Fifteen hours later, on June 21, after it had crested in its orbit and was descending, it performed a second burn for 1 hour and 11 minutes.

The purpose of this pair of maneuvers was to bring Dawn's flight path at peridemeter right over Cerealia Facula. But the experienced explorers in mission control recognized that even with all their careful planning and Dawn's faithful execution of its assignments, there was a good chance the probe would not fly directly above that unique site as it sped northward. Therefore, they had also worked out plans to quickly determine how far east or west it would be at peridemeter and radio a (nearly) last minute adjustment in the angle it would point its sensors. 

After the second segment of ion maneuvering, Dawn's orbit took it down to peridemeter again on June 21 for another intensive period of close-up observations. Even before it had time to finish radioing those findings to Earth the next day, the team began preparing for the next dive down. On June 22, they made their final calculations of the orbital path and predicted that Dawn would fly a little west of Cerealia Facula that night and a little east of it the next time around. That afternoon, they transmitted instructions to Dawn to aim its camera and spectrometers just a little to the right the first time and just a little to the left the second time. (Sophisticated and capable though Dawn is, the instructions controllers sent were a little more specific and quantitative than the descriptions here.)

The team would have considered their extensive efforts successful if the spacecraft had photographed part of Cerealia Facula once. (Dawn flies so close to the ground that it would be impossible to photograph all of Cerealia Facula on any one orbit; its camera's view is simply not wide enough.) As it turned out, Dawn managed to get pictures of Cerealia Facula on three consecutive orbits, each time seeing different parts, yielding far better coverage of this exotic landscape than we had even hoped for.

Flying to this incredibly low orbit, getting such a wealth of data and even managing to photograph a good portion of Cerealia Facula truly tested the very limits of the mission's capabilities. Dawn has surpassed all expectations, accomplishing feats not even considered when it was designed.

In order to prepare for the long shot of attempting to capture Cerealia Facula, Dawn rotated to point its main antenna to Earth relatively often, sometimes after each peridemeter or after two or three. That allowed the flight team to work more closely with the spacecraft. Then it would turn again to bring its sensors to bear on Ceres shortly before soaring through the next peridemeter. But all that turning costs Dawn hydrazine, the resource that limits its operational life to only another few months. (We outlined this situation last month and will delve into it more fully next month.) Now Dawn will observe Ceres on five consecutive orbits, filling its memory with data, and then spend almost two full days, including one peridemeter, transmitting that valuable information back to Earth. While its antenna is trained on Earth, the spacecraft cannot simultaneously direct its sensors at Ceres. That actually yields especially good gravity measurements, which use the Doppler shift of the radio signal, because the signal is much stronger with the main antenna than with one of the auxiliary antennas. Pictures and spectra, however, cannot be acquired on that one peridemeter in every six during which Dawn sends its results to Earth. The flight team determined that the benefit of turning less often and thus reducing hydrazine consumption yields the best scientific return. (This savings was already accounted for when we described the end of the mission as likely being between August and October.)

We saw in March that the latitude at which Dawn reaches peridemeter shifts south with every revolution. That is, the low point of each orbit is about 2° south of the one before. As a result, each time the spacecraft flies over Occator Crater now, it is higher than the previous time. Occator is at 20°N. Now the peridemeter is close to the equator, and soon Dawn's best views of Ceres will be in the region of Urvara Crater.

Firing ion engine #2 on June 21 accomplished more than the orbital adjustment that allowed the ship to spot Cerealia Facula at three consecutive peridemeters. It also completed the planned use of the ion propulsion system for the entire mission. 

Dawn's ion engines have enabled this interplanetary spaceship to accomplish a journey unique in humankind's exploration of the solar system. After departing Earth with the help of a conventional rocket, Dawn used those engines to fly past Mars in 2009, to travel to Vesta and enter orbit in 2011, to maneuver extensively in orbit to optimize its observations there, to break out of orbit in 2012, to travel to Ceres and slip into orbit in 2015, and to perform even more maneuvering there than at Vesta. No other spacecraft has ever orbited two extraterrestrial destinations, and Dawn's mission to do so would have been impossible without ion propulsion.

We summarize the mission's ion thrusting on every Dawnniversary of launch, but since no further use is planned, we can give some final numbers here. Dawn thrust for a total of 2,141 days (5.9 years), or 55 percent of the time it has been in space (and 0.000000043 percent of the time since the Big Bang). The thrusting has achieved the equivalent of accelerating the probe by 25,700 mph (41,400 kilometers per hour). As we have often explained (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 Dawn Journal.) 

The engines have done their job admirably, and now we have no further use for them. As a reminder, they are not needed for Dawn to stay in orbit around Ceres, just as the Moon doesn't need propulsion to stay in orbit around Earth and Earth doesn't need propulsion to say in orbit around the Sun. Next month we will discuss what will happen to Dawn's orbit after the mission ends.

When the ion engine was programmed to stop thrusting on June 21, some Dawn team members gathered in mission control to mark the occasion. Dawn was busy and was not communicating with Earth at the time. Even if it had been, a radio signal confirming the end of thrust would have taken almost 25 minutes to reach our planet. But the team decided to neglect the limitation of the speed of light and mark the moment (1:15:03 pm PDT) that the blue glow on the distant ship's engine would extinguish for the last time. And at that same moment, the blue lights in mission control were turned off for the last time as well.

It's natural to feel some sadness or loss now that the engines will not fire again. After all, ion propulsion is cool, especially for those of us who first heard of it in science fiction. It is even cooler for those who appreciate its tremendous capability and how valuable that is for deep-space missions. We can feel wistful, of course, but the last use of the ion engines was a direct result of their great success. After a truly stupendous interplanetary mission, we have Dawn right where we want it: in an orbit optimized for getting the last, best data at the endlessly fascinating dwarf planet it has unveiled. We can be grateful the ion engines allowed Dawn to explore two of the last uncharted worlds in the inner solar system and that they captivated our imagination as the distant spacecraft traveled through the solar system on a blue-green beam of xenon ions. Not too long ago, ion propulsion was mostly in the domain of science fiction. NASA's Deep Space 1 put it firmly into the realm of science fact. Building on DS1, Dawn has rocketed far beyond, accomplishing a space trek that would have been impossible without ion propulsion. Its mission was to boldly go where -- well, you know. And it has! Dawn's engines will never emit their cool blue glow again, but their legacy will not fade.

Dawn is 100 miles (160 kilometers) from Ceres (and headed for peridemeter). It is also 3.06 AU (284 million miles, or 457 million kilometers) from Earth, or 1,125 times as far as the Moon and 3.01 times as far as the Sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 pm PDT June 30, 2018

TAGS: DAWN, CERES, VESTA, DWARF PLANET, JOURNAL

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Orbiting high above Ceres, the sophisticated Dawn spacecraft is hard at work unveiling the secrets of the exotic alien world that has been its home for almost two years.

Dawn’s primary objective in this sixth orbital phase at Ceres (known as extended mission orbit 3, XMO3 or "this sixth orbital phase at Ceres") is to record cosmic rays. Doing so will allow scientists to remove that "noise" from the nuclear radiation measurements performed during the eight months Dawn operated in a low, tight orbit around Ceres. The result will be a cleaner signal, revealing even more about the atomic constituents down to about a yard (meter) underground. As we will see below, in addition to this ongoing investigation, soon the adventurer will begin pursuing a new objective in its exploration of Ceres.

With its uniquely capable ion propulsion system, Dawn has flown to orbits with widely varying characteristics. In contrast to the previous five observation orbits (and all the observation orbits at Vesta), XMO3 is elliptical. Over the course of almost eight days, the spacecraft sails from a height of about 4,670 miles (7,520 kilometers) up to almost 5,810 miles (9,350 kilometers) and back down. Dutifully following principles discovered by Johannes Kepler at the beginning of the 17th century and explained by Isaac Newton at the end of that century, Dawn’s speed over this range of altitudes varies from 210 mph (330 kilometers per hour) when it is closest to Ceres to 170 mph (270 kilometers per hour) when it is farthest. Yesterday afternoon, the craft was at its highest for the current orbit. During the day today, the ship will descend from 5,790 miles (9,310 kilometers) to 5,550 miles (8,930 kilometers). As it does so, Ceres’ gravity will gradually accelerate it from 170 mph (273 kilometers per hour) to 177 mph (285 kilometers per hour). (Usually we round the orbital velocity to the nearest multiple of 10. In this case, however, to show the change during one day, the values presented are more precise.)

As we saw last month, the angle of XMO3 to the sun presents an opportunity to gain a new perspective on Ceres, with sunlight coming from a different angle. (We include the same figure here, because we will refer to it more below.) Last week, Dawn took advantage of that opportunity, seeing the alien landscapes in a new light as it took pictures for the first time since October.

Dawn takes more than a week to revolve around Ceres, but Ceres turns on its axis in just nine hours. Because Dawn moves through only a small segment of its orbit in one Cerean day, it is almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath it. During one such period on Jan. 27, Dawn’s high perch moved only from 11°N to 12°S latitude as Ceres presented her full range of longitudes to the explorer’s watchful eye. This made it very convenient to take pictures and visible spectra as the scenery helpfully paraded by. (The spacecraft was high enough to see much farther north and south than the latitudes immediately beneath it.) Dawn will make similar observations again twice in February.

As Dawn was expertly executing the elegant, complex spiral ascent from XMO2 to XMO3 in November, the flight team considered it to be the final choreography in the venerable probe’s multi-act grand interplanetary performance. By then, Dawn had already far exceeded all of its original objectives at Vesta and Ceres, and the last of the new scientific goals could be met in XMO3, the end of the encore. The primary consideration was to keep Dawn high enough to measure cosmic rays, meaning it needed to stay above about 4,500 miles (7,200 kilometers). There was no justification or motivation to go anywhere else. Well, that’s the way it was in November anyway. This is January. And now it’s (almost) time for a previously unanticipated new act, XMO4.

Always looking for ways to squeeze as much out of the mission as possible, the team has now devised a new and challenging investigation. It will consume the next five months (and much of the next five Dawn Journals). We begin this month with an overview, but follow along each month as we present the full story, including a detailed explanation of the underlying science, the observations themselves and the remarkable orbital maneuvering entirely unlike anything Dawn has done before. (You can also follow along with your correspondent’s uncharacteristically brief and more frequent mission status updates.)

From the XMO3 vantage point, with sunlight coming from the side, Ceres is gibbous and looks closer to a half moon than full. The new objective is to peer at Ceres when the sun is directly behind Dawn. This would be the same as looking at a full moon. (In the figure above, it would be like photographing Ceres from somewhere on the dashed line that points to the distant sun.)

While Dawn obtained pictures from near the line to the sun in its first Ceres orbit, there is a special importance to being even closer to that line. Let’s see why that alignment is valuable.

Most materials reflect light differently at different angles. You can investigate this yourself (and it’s probably easier to do at home than it is in orbit around a remote dwarf planet). To make it simpler, take some object that is relatively uniform (but with a matte finish, not a mirror-like finish) and vary the angles at which light hits it and from which you look at it. You may see that it appears dimmer or brighter as the angles change. It turns out that this effect may be used to help infer the nature of the reflecting material. (For the purposes of this exercise, if you can hold the angle of the object relative to your gaze fixed, and vary only the angle of the illumination, that’s best. But don’t worry about the details. Conducting this experiment represents only a small part of your final grade.)

Now when scientists carefully measure the reflected light under controlled conditions, they find that the intensity changes quite gradually over a wide range of angles. In other words, the apparent brightness of an object does not vary dramatically as the geometry changes. However, when the source of the illumination gets very close to being directly behind the observer, the reflection may become quite a bit stronger. (If you test this, of course, you have to ensure your shadow doesn’t interfere with the observation. Vampires don’t worry about this, and we’ll explain below why Dawn needn’t either.)

If you (or a helpful scientist friend of yours) measure how bright a partial moon is and then use that information to calculate how bright the full moon will be, you will wind up with an answer that’s too small. The full moon is significantly brighter than would be expected based on how lunar soil reflects light at other angles. (Of course, you will have to account for the fact that there is more illuminated area on a full moon, but this curious optical behavior is different. Here we are describing how the brightness of any given patch of ground changes.)

A full moon occurs when the moon and sun are in opposite directions from Earth’s perspective. That alignment is known as opposition. That is, an astronomical body (like the moon or a planet) is in opposition when the observer (you) is right in between it and the source of illumination (the sun), so all three are on a straight line. And because the brightness takes such a steep and unexpected jump there, this phenomenon is known as the opposition surge.

The observed magnitude of the opposition surge can reveal some of the nature of the illuminated object on much, much finer scales than are visible in photos. Knowing the degree to which the reflection strengthens at very small angles allows scientists to ascertain (or, at least, constrain) the texture of materials on planetary surfaces even at the microscopic level. If they are fortunate enough to have measurements of the reflectivity at different angles for a region on an airless solar system body (atmospheres complicate it too much), they compare them with laboratory measurements on candidate materials to determine which ones give the best match for the properties.

Dawn has already measured the light reflected over a wide range of angles, as is clear from the figure above showing the orbits. But the strongest discrimination among different textures relies on measuring the opposition surge. That is Dawn’s next objective, a bonus in the bonus extended mission.

You can see from the diagram that measuring the opposition surge will require a very large change in the plane of Dawn’s orbit. Shifting the plane of a spacecraft’s orbit can be energetically very, very expensive. (We will discuss this more next month.) Fortunately, the combination of the unique capabilities provided by the ion propulsion system and the ever-creative team makes it affordable.

Powered by an insatiable appetite for new knowledge, Dawn will begin ion thrusting on Feb. 23. After very complex maneuvers, it will be rewarded at the end of April with a view of a full Ceres from an altitude of around 12,400 miles (20,000 kilometers), about the height of GPS satellites above Earth. (That will be about 50 percent higher than the first science orbit, which is labeled as line 1 in the figure.) There are many daunting challenges in reaching XMO4 and measuring the opposition surge. Even though it is a recently added bonus, and the success of the extended mission does not depend on it, mission planners have already designed a backup opportunity in case the first attempt does not yield the desired data. The second window is late in June, allowing the spacecraft time to transmit its findings to Earth before the extended mission concludes at the end of that month.

For technical reasons, the measurements need to be made from a high altitude, and throughout the complex maneuvering to get there, Dawn will remain high enough to monitor cosmic rays. Ceres will appear to be around five times the width of the full moon we see from Earth. It will be about 500 pixels in diameter in Dawn’s camera, and more than 180,000 pixels will show light reflected from the ground. Of greatest scientific interest in the photographs will be just a handful of pixels that show the famous bright material in Occator Crater, known as Cerealia Facula and clearly visible in the picture above. Scientists will observe how those pixels surge in brightness over a narrow range of angles as Dawn’s XMO4 orbital motion takes it into opposition, exactly between Occator and the sun. Of course, the pictures also will provide information on how the widespread dark material covering most of the ground everywhere else on Ceres changes in brightness (or, if you prefer, in dimness). But the big reward here would be insight into the details of Cerealia Facula. Comparing the opposition surges with laboratory measurements may reveal characteristics that cannot be discerned any other way save direct sampling, which is far beyond Dawn’s capability (and authority). For example, scientists may be able to estimate the size of the salt crystals that make up the bright material, and that would help establish their geological history, including whether they formed underground or on the surface. We will discuss this more in March.

Most of the data on opposition surges on solar system objects use terrestrial observations, with astronomers waiting until Earth and the target happen to move into the necessary alignment with the sun. In those cases, the surge is averaged over the entire body, because the target is usually too far away to discern any details. Therefore, it is very difficult to learn about specific features when observing from near Earth. Few spacecraft have actively maneuvered to acquire such data because, as we alluded to above and will see next month, it is too difficult, especially at a massive body like Ceres. The recognition that Dawn might be able to complete this challenging measurement for a region of particular interest represents an important possibility for the mission to discover more about this intriguing dwarf planet’s geology.

Meeting the scientific goal will require a careful and quantitative analysis of the pixels, but the images of a fully illuminated Ceres will be visually appealing as well. Nevertheless, you are cautioned to avoid developing a mistaken notion about the view. (For that matter, you are cautioned to avoid developing mistaken notions about anything.) You might think (and some readers wondered about this in a different phase of the mission) that with Dawn being between the sun and Ceres (and not being a vampire), the spacecraft’s shadow might be visible in the pictures. It would look really cool if it were (although it also would interfere with the measurement of the opposition surge by introducing another factor into how the brightness changes). There will be no shadow. The spacecraft will simply be too high. Imagine you’re standing in Occator Crater, either wearing your spacesuit while engaged in a thrilling exploration of a mysterious and captivating extraterrestrial site or perhaps instead while you’re indoors enjoying some of the colony’s specially salted Cerean savory snacks, famous throughout the solar system. In any case, the distant sun you see would be a little more than one-third the size that it looks from Earth, comparable to a soccer ball at 213 feet (65 meters). Dawn would be 12,400 miles (20,000 kilometers) overhead. Although it’s one of the largest interplanetary spacecraft ever to take flight, with a wingspan of 65 feet (20 meters), it would be much too small for you to see at all without a telescope and would block an undetectably small amount of sunlight. It would appear smaller than a soccer ball seen from 135 miles (220 kilometers). Therefore, no shadow will be cast, the measurement will not be compromised by the spacecraft blocking some of the light reaching the ground and the pictures will not display any evidence of the photographer.

Even as the team was formulating plans for this ambitious new campaign, they successfully dealt with a glitch on the spacecraft this month. When a routine communications session with the Deep Space Network began on Jan. 17, controllers discovered that Dawn had previously entered its safe mode, a standard response the craft uses when it encounters conditions its programming and logic cannot accommodate. The main computer issues instructions to reconfigure systems, broadcasts a special radio signal through one of the antennas and then patiently awaits help from humans on a faraway planet (or anyone else who happens to lend assistance). The team soon determined what had occurred. Since it left Earth, Dawn has performed calculations five times per second about its location and speed in the solar system, whether in orbit around the sun, Vesta or Ceres. (Perhaps you do the same on your deep-space voyages.) However, it ran into difficulty in those calculations on Jan. 14 for the first time in more than nine years of interplanetary travel. To ensure the problematic calculations did not cause the ship to take any unsafe actions, it put itself into safe mode. Engineers have confirmed that the problem was in software, not hardware and not even a cosmic ray strike, which has occasionally triggered safe mode, most recently in September 2014.

Mission controllers guided the spacecraft out of safe mode within two days and finished returning all systems to their standard configurations shortly thereafter. Dawn was shipshape the subsequent week and resumed its scientific duties. When it activated safe mode, the computer correctly powered off the gamma ray and neutron detector, which had been measuring the cosmic rays, as we described above. The time that the instrument was off will be inconsequential, however, because there is more than enough time in the extended mission to acquire all the desired measurements.

The extended mission has already proven to be extremely productive, yielding a great deal of new data on this ancient world. But there is still more to look forward to as the veteran explorer prepares for a new and adventurous phase of its extraordinary extraterrestrial expedition.

Dawn is 5,650 miles (9,100 kilometers) from Ceres. It is also 2.87 AU (266 million miles, or 429 million kilometers) from Earth, or 1,135 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 48 minutes to make the round trip.

TAGS: CERES, DAWN, DWARF PLANET

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

The felicitous display is more than decorative. The illumination indicates that the interplanetary spacecraft is thrusting with one of its ion engines, which emit a lovely, soft bluish glow in the forbidding depths of space. Dawn is completing another elegant spiral around dwarf planet Ceres, maneuvering to its sixth science orbit.

Dawn’s ion propulsion system has allowed the probe to accomplish a mission unlike any other, orbiting two distant extraterrestrial destinations. Even more than that, Dawn has taken advantage of the exceptional efficiency of its ion engines to fly to orbits at different altitudes and orientations while at Vesta and at Ceres, gaining the best perspectives for its photography and other scientific investigations.

Dawn has thrust for a total of 5.7 years during its deep-space adventure. All that powered flight has imparted a change in the ship’s velocity of 25,000 mph (40,000 kilometers per hour). As we have seen, this is not the spacecraft’s actual speed, but it is a convenient measure of the effect of its propulsive work. Reaching Earth orbit requires only about 17,000 mph (less than 28,000 kilometers per hour). In fact, Dawn’s gentle ion engines have delivered almost 98 percent of the change in speed that its powerful Delta 7925H-9.5 rocket provided. With nine external rocket engines and a core consisting of a first stage, a second stage and a third stage, the Delta boosted Dawn by 25,640 mph (41,260 kilometers per hour) from Cape Canaveral out of Earth orbit and onto its interplanetary trajectory, after which the remarkable ion engines took over. No other spacecraft has accomplished such a large velocity change under its own power. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)

Early this year, we were highly confident Dawn would conclude its operational lifetime in its fourth orbit at Ceres (and remain there long after). But unexpectedly healthy and with an extension from NASA, Dawn is continuing its ambitious mission. After completing all of its tasks in its fifth scientific phase at Ceres, Dawn is pursuing new objectives by flying to another orbit for still more discoveries. Although we never anticipated adding a row to the table of Dawn’s orbits, last presented in December 2015, we now have an updated version.

As with the obscure Dawn code names for other orbits, this fifth orbit’s name requires some explanation. The extended mission is devoted to undertaking activities not envisioned in the prime mission. That began with two extra months in the fourth mapping orbit performing many new observations, but because it was then the extended mission, that orbit was designated extended mission orbit 1, or XMO1. (It should have been EMO1, of course, but the team’s spellchecker was offline on July 1, the day the extended mission started.) Therefore, the next orbit was XMO2. Dawn left XMO2 on Nov. 4, and we leave it to readers’ imaginations to devise a name for the orbit the spacecraft is now maneuvering to.

Surprisingly, Dawn is flying higher to enhance part of the scientific investigation that motivated going to the lowest orbit. We have explained before that Dawn’s objective in powering its way down to the fourth mapping orbit was to make the most accurate measurements possible of gravity and of nuclear radiation emitted by the dwarf planet.

For more than eight months, the explorer orbited closer to the alien world than the International Space Station is to Earth, and the gamma ray spectra and neutron spectra it acquired are outstanding, significantly exceeding all expectations. But ever-creative scientists have recognized that even with that tremendous wealth of data, Dawn can do still better. Let’s look at this more carefully and consider an example to resolve the paradox of how going higher can yield an improvement.

The gamma ray and neutron detector (GRaND) reveals some of Ceres’ atomic constituents down to about a yard (meter) underground. The principal limitation in analyzing these spectra is "noise." In fact, noise limits the achievable accuracy of many scientific measurements. It isn’t necessarily the kind of noise that you hear from loud machinery (nor from the mouth of your unhelpful parent, inattentive progeny or boring and verbose coworker), but all natural systems have something similar. Physical processes other than the ones of interest make unwanted contributions to the measurements. The part of a measurement scientists want is called the "signal." The part of a measurement scientists don’t want is called the "noise." The quality of a measurement may be characterized by comparing the strength of the signal to the strength of the noise. (This metric is called the "signal to noise ratio" by people who like to use jargon like "signal to noise ratio.")

We have discussed that cosmic rays, radiation that pervades space, strike atomic nuclei on Ceres, creating the signals that GRaND measures. Remaining at low altitude would have allowed Dawn to enhance its measurement of the Cerean nuclear signal. But scientists determined that an even better way to improve the spectra than to increase the signal is to decrease the noise. GRaND’s noise is a result of cosmic rays impinging directly on the instrument itself and on nearby parts of the spacecraft. With a more thorough measurement of the noise from cosmic rays, scientists will be able to mathematically remove that component of the low altitude measurements, leaving a clearer signal.

For an illustration of all this, suppose you want to hear the words of a song. The words are the signal and the instruments are the noise. (This is a scientific discussion, not a musical one.) It could be that the instruments are so loud and distracting that you can’t make the words out easily.

You might try turning up the volume, because that increases the signal, but it increases the noise as well. If the performance is live, you might even try to position yourself closer to the singer, perhaps making the signal stronger without increasing the noise too much. (Other alternatives are simply to Google the song or ask the singer for a copy of the lyrics, but those methods would ruin this example.)

If you’re doing this in the 21st century (or later), there’s another trick you can employ, taking advantage of computer processing. Suppose you had a recording of the singing with the instruments and then obtained separate recordings of the instruments. You could subtract the musical sounds that constitute the noise, removing the contributions from both guitars, the drums, the harp, both ukuleles, the kazoo and all the theremins. And when you eliminate the noise of the instruments, what remains is the signal of the words, making them much more intelligible.

To obtain a better measure of the noise, Dawn needs to go to higher altitude, where GRaND will no longer detect Ceres. It will make detailed measurements of cosmic ray noise, which scientists then will subtract from their measurements at low altitude, where GRaND observed Ceres signal plus cosmic ray noise. The powerful capability to raise its orbit so much affords Dawn the valuable opportunity to gain greater insight into the atomic composition. Of course, it’s not quite that simple, but essentially this method will help Dawn hear Ceres’ nuclear song more clearly.

To travel from one orbit to another, the sophisticated explorer has followed complex spiral routes. We have discussed the nature of these trajectories quite a bit, including how the operations team designs and flies them. But now they are using a slightly different method.

Those of you at Ceres who monitor the ship’s progress probably wouldn’t notice a difference in the type of trajectory. And the rest of you on Earth and elsewhere who keep track through our mission status updates also would not detect anything unusual in the ascent profile (to the extent that a spacecraft using ion propulsion to spiral around a dwarf planet is usual). But celestial navigators are now enjoying their use of a method they whimsically call local maximal energy spiral feedback control.

The details of the new technique are not as important for our discussion here as one of the consequences: Dawn’s next orbit will not be nearly as circular as any of its other orbits at Ceres (or at Vesta). Following the conclusion of this spiral ascent on Dec. 5, navigators will refine their computations of the orbit, and we will describe the details near the end of the month. We will see that as the spacecraft follows its elliptical loops around Ceres, each taking about a week, the altitude will vary smoothly, dipping below 4,700 miles (7,600 kilometers) and going above 5,700 miles (9,200 kilometers). Such a profile meets the mission’s needs, because as long as the craft stays higher than about 4,500 miles (7,200 kilometers), it can make the planned recordings of the cacophonous cosmic rays. We will present other plans for this next phase of the mission as well, including photography, in an upcoming Dawn Journal.

As Dawn continues its work at Ceres, the dwarf planet continues its stately 4.6-year-long orbit around the sun, carrying Earth’s robotic ambassador with it. Ceres follows an elliptical path around the sun (see, for example, this discussion, including the table). 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 describing how much each orbit deviates from a perfect circle.)

When Ceres tenderly took Dawn into its gravitational embrace in March 2015, they were 2.87 AU (267 million miles, or 429 million kilometers) from the sun. In January 2016, we mentioned that Ceres had reached its aphelion, or greatest distance from the sun, at 2.98 AU (277 million miles, or 445 million kilometers). Today at 2.85 AU (265 million miles, or 427 million kilometers), Ceres is closer to the sun than at any time since Dawn arrived, and the heliocentric distance will gradually decrease further throughout the extended mission. (If the number of numbers is overwhelming here, 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 relative distances.)

Another consequence of orbiting the sun is the progression of seasons. Right on schedule, as we boldly predicted in August 2015, Nov. 13 was the equinox on Ceres, marking the beginning of northern hemisphere autumn and southern hemisphere spring. Although it is celebrated on Ceres with less zeal than on Earth, it is fundamentally the same: the sun was directly over the equator that day, and now it is moving farther south. It takes Ceres so long to orbit the sun that this season will last until Dec. 22, 2017.

A celebration that might occur on Ceres (and which you, loyal Dawnophile, are welcome to attend) would honor Dawn itself. Although the spacecraft completed its ninth terrestrial year of spaceflight in September, on Dec. 12, it will have been two Cerean years since Dawn left Earth for its interplanetary journey. Be sure to attend in order to learn how a dawnniversary is commemorated in that part of the solar system.

Although a year on Ceres lasts much longer than on Earth, 2016 is an unusually long year on our home planet. Not only was a leap day included, but a leap second will be added at the very end of the year to keep celestial navigators’ clocks in sync with nature. The Dawn team already has accounted for the extra second in the intricate plans formulated for the spacecraft. And at that second, on Dec. 31 at 23:59:60, we will be able to look back on 366 days and one second, an especially full and gratifying year in this remarkable deep-space expedition. But we needn’t wait. Even now, as mission control is bathed in a lovely glow, the members of the team as well as space enthusiasts everywhere are aglow with the thrill of new knowledge, the excitement of a daring, noble adventure and the anticipation of more to come.

Dawn is 3,150 miles (5,070 kilometers) from Ceres. It is also 2.08 AU (194 million miles, or 312 million kilometers) from Earth, or 770 times as far as the moon and 2.11 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 35 minutes to make the round trip.

TAGS: DAWN, CERES, ION ENGINE, ION PROPULSION, DWARF PLANET

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Ceres, which only last year was hardly more than a fuzzy blob against the stars, is now a richly detailed world, and our portrait grows more elaborate every day. Having greatly surpassed all of its original objectives, the reliable explorer is gathering still more data from its unique vantage point. Everyone who hungers for new knowledge about the cosmos or for bold adventures far from Earth can share in the sumptuous feast Dawn has been serving.

One of the major objectives of the mission was to photograph 80 percent of Ceres' vast landscape with a resolution of 660 feet (200 meters) per pixel. That would provide 150 times the clarity of the powerful Hubble Space Telescope. Dawn has now photographed 99.8 percent with a resolution of 120 feet (35 meters) per pixel.

This example of Dawn's extraordinary productivity may appear to be the limit of what it could achieve. After all, the spaceship is orbiting at an altitude of only 240 miles (385 kilometers), closer to the ground than the International Space Station is to Earth, and it will never go lower for more pictures. But it is already doing more.

Since April 11, instead of photographing the scenery directly beneath it, Dawn has been aiming its camera to the left and forward as it orbits and Ceres rotates. By May 25, it will have mapped most of the globe from that angle. Then it will start all over once more, looking instead to the right and forward from May 27 through July 10. The different perspectives on the terrain make stereo views, which scientists can combine to bring out the full three dimensionality of the alien world. Dawn already accomplished this in its third mapping orbit from four times its current altitude, but now that it is seeing the sights from so much lower, the new topographical map will be even more accurate.

Dawn is also earning extra credit on its assignment to measure the energy of gamma rays and neutrons. We have discussed before how the gamma ray and neutron detector (GRaND) can reveal the atomic composition down to about a yard (meter) underground, and last month we saw initial findings about the distribution of hydrogen. However, Ceres' nuclear glow is very faint. Scientists already have three times as much GRaND data from this low altitude as they had required, and both spectrometers in the instrument will continue to collect data. In effect, Dawn is achieving a longer exposure, making its nuclear picture of Ceres brighter and sharper.

In December we explained how using the radio signal to track the probe's movements allows scientists to chart the gravity field and thereby learn about the interior of Ceres, revealing regions of higher and lower density. Once again, Dawn performed even better than expected and achieved the mission's planned accuracy in the third mapping orbit. Because the strength of the dwarf planet's gravitational tug depends on the distance, even finer measurements of how it varies from location to location are possible in this final orbit. Thanks to the continued smooth operation of the mission, scientists now have a gravitational map fully twice as accurate as they had anticipated. With additional measurements, they may be able to squeeze out a little more detail, perhaps improving it by another 20 percent before reaching the method's limit.

Dawn has dramatically overachieved in acquiring spectra at both visible and infrared wavelengths. We have previously delved into how these measurements reveal the minerals on the ground and what some of the interesting discoveries are. Having already acquired more than seven times as many visible spectra and 21 times as many infrared spectra as originally called for, the spacecraft is adding to its riches with additional measurements. We saw in January that VIR has such a narrow view that it will never see all of Ceres from this close, so it is programmed to observe features that have caught scientists' interest based on the broad coverage from higher altitudes.

Dawn's remarkable success at Ceres was not a foregone conclusion. Of course, the flight team has confronted the familiar challenges people encounter every day in the normal routine of piloting an ion-propelled spaceship on a multibillion-mile (multibillion-kilometer) interplanetary journey to orbit and explore two uncharted worlds. But the mission was further complicated by the loss of two of the spacecraft's four reaction wheels, as we have recounted before. (In full disclosure, the devices aren’t actually lost. We know precisely where they are. But given that one stopped functioning in 2010 and the other in 2012, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Without three of these units to control its orientation in space, the robot has relied on its limited supply of hydrazine, which was not intended to serve this function. But the mission's careful stewardship of the precious propellant has continued to exceed even the optimistic predictions, allowing Dawn good prospects for carrying on its fruitful work. In an upcoming Dawn Journal, we will discuss how the last of the dwindling supply of hydrazine may be used for further discoveries.

In the meantime, Dawn is continuing its intensive campaign to reveal the dwarf planet's secrets, and as it does so, it is passing several milestones. The adventurer has now been held in Ceres' tender but firm gravitational embrace longer than it was in orbit around Vesta. (Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, and its mission would have been impossible without ion propulsion.) The spacecraft provided us with about 31,000 pictures of Vesta, and it has now acquired the same number of Ceres.

For an interplanetary traveler, terrestrial days have little meaning. They are merely a memory of how long a faraway planet takes to turn on its axis. Dawn left that planet long ago, and as one of Earth's ambassadors to the cosmos, it is an inhabitant of deep space. But for those who keep track of its progress yet are still tied to Earth, on May 3 the journey will be pi thousand days long. (And for our nerdier friends and selves, it will be shortly after 6:47 p.m. PDT.)

By any measure, Dawn has already accomplished an extraordinary mission, and there is more to look forward to as its ambitious expedition continues.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.73 AU (346 million miles, or 558 million kilometers) from Earth, or 1,455 times as far as the moon and 3.70 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and two minutes to make the round trip.

TAGS: CERES, DAWN, MISSION, SPACECRAFT, VESTA, DWARF PLANET

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Scientists are still working on refining their understanding of this striking region. As we described in December, it seems that following the powerful impact that excavated Occator Crater, underground briny water reached the surface. The detailed photographs show many fractures cutting across the bright areas, and perhaps they provided a conduit. Water, whether as liquid or ice, would not last long there in the cold vacuum, eventually subliming. When the water molecules disperse, either escaping from Ceres into space or falling back to settle elsewhere, the dissolved salts are left behind. This reflective residue covers the ground, making the spellbinding and beautiful display Dawn now reveals.

While the crater is estimated to be a geological youngster at 80 million years old, that is an extremely long time for the material to remain so reflective. Exposed for so long to cosmic radiation and pelting from the rain of debris from space, it should have darkened. Scientists don't know (yet) what physical process are responsible, but perhaps it was replenished long after the crater itself formed, with more water, carrying dissolved salts, finding its way to the surface. As their analyses of the photos and spectra continue, scientists will gain a clearer picture and be able to answer this and other questions.

These latest Occator pictures did not come easily. Orbiting so close to Ceres, the adventurer’s camera captures only a small scene at a time, and it is challenging to cover the entirety of the expansive terrain. (Perhaps it comes as a surprise to those who have not read at least a few of the 123 Dawn Journals that precede this one that operating a spacecraft closer to a faraway dwarf planet than the International Space Station is to Earth is not as easy as, say, thinking about it.) But the patience and persistence in photographing the exotic landscapes have paid off handsomely.

We now have high resolution pictures of essentially all of Ceres save the small area around the south pole cloaked in the deep dark of a long winter night. Seasons last longer on Ceres than on Earth, and Dawn may not operate there long enough for the sun to rise at the south pole. By the beginning of southern hemisphere spring in November 2016, Dawn's mission to explore the first dwarf planet discovered may have come to its end.

In addition to photographing Ceres, Dawn conducts many other scientific observations, as we described in December and January. Among the probe's objectives at Ceres is to provide information for scientists to understand how much water is there, where it is, what form it is in and what role it plays in the geology.

We saw that extensive measurements of the faint nuclear radiation can help identify the atomic constituents. While the analysis of the data is complicated, and much more needs to be done, a picture is beginning to emerge from Dawn's neutron spectrometer (part of the gamma ray and neutron detector, GRaND). These subatomic particles are emitted from the nuclei of atoms buried within about a yard (meter) of the surface. Some manage to penetrate the material above them and fly into space, and the helpful ones then meet their fate upon hitting GRaND in orbit above. (Most others, however, will continue to fly through interplanetary space, decaying into a trio of other subatomic particles in less than an hour.) Before it escapes from the ground, a neutron's energy (and, equivalently, its speed) is strongly affected by any encounters with the nuclei of hydrogen atoms (although other atomic interactions can change the energy too). Therefore, the neutron energies can indicate to scientists the abundance of hydrogen. Among the most common forms in which hydrogen is found is water (composed of two hydrogen atoms and one oxygen atom), which can occur as ice or tied up in hydrated minerals.

GRaND shows Ceres is rich in hydrogen. Moreover, it detects more neutrons in an important energy range near the equator than near the poles, likely indicating there is more hydrogen, and hence more (frozen) water, in the ground at the high latitudes. Although Ceres is farther from the sun than Earth, and you would not consider it balmy there, it still receives some warmth. Just as at Earth, the sun's heating is less effective closer to the poles than at low latitudes, so this distribution of ice in the ground may reflect the temperature differences. Where it is warmer, ice close to the surface would have sublimed more quickly, thus depleting the inventory compared to the cooler ground far to the north or south.

Dawn spends most of its time measuring neutrons (and gamma rays), so it is providing a great deal of new data. And as scientists conduct additional analyses, they will learn more about the ice and other materials beneath the surface.

Another spectrometer is providing more tantalizing clues about the composition of Ceres, which is seen to vary widely. As the dwarf planet is not simply a huge rock but is a geologically active world, it is no surprise that it is not homogeneous. We discussed in December that the infrared mapping spectrometer had shown that minerals known as phyllosilicates are common on Ceres. Further studies of the data show evidence for the presence of two types: ammoniated phyllosilicates (described in December) and magnesium phyllosilicates. Scientists also find evidence of compounds known as carbonates, minerals that contain carbon and oxygen. There is also a dark substance in the mix that has not been identified yet.

And in one place (so far) on Ceres, this spectrometer has directly observed water, not below the surface but on the ground. The infrared signature shows up in a small crater named Oxo. (For the pronunciation, listen to the animation below.) As with the neutron spectra, it is too soon to know whether the water is in the form of ice or is chemically bound up in minerals.

At six miles (10 kilometers) in diameter, Oxo is small in comparison to the largest craters on Ceres, which are more than 25 times wider. (While geologists consider it a small crater, you might not agree if it formed in your backyard. Also note that when we showed Oxo Crater before, the diameter was slightly different. The crater's size has not changed since then, but as we receive sharper pictures, our measurements of feature sizes do change.) Dawn's first orbital destination, the fascinating protoplanet Vesta, is smaller than Ceres and yet has two craters far broader than the largest on Ceres. Based on studies of craters observed throughout the solar system, scientists have established methods of calculating the number and sizes of craters that could be formed on planetary surfaces. Those techniques show that Ceres is deficient in large craters. That is, more should have formed than appear in Dawn's pictures. Many other bodies (including Vesta and the moon) seem to preserve their craters for much longer, so this may be a clue about internal geological processes on Ceres that gradually erase the large craters.

Scientists are still in the initial stages of digesting and absorbing the tremendous wealth of data Dawn has been sending to Earth. The benefit of lingering in orbit (enabled by the remarkable ion propulsion system), rather than being limited to a brief glimpse during a fast flyby, is that the explorer can undertake much more thorough studies, and Dawn is continuing to make new measurements.

As recently as one year ago, controllers (and this writer) had great concern about the spacecraft's longevity given the loss of two reaction wheels, which are used for controlling the ship's orientation. And in 2014, when the flight team worked out the intricate instructions Dawn would follow in this fourth and final mapping orbit, they planned for three months of operation. That was deemed to be more than enough, because Dawn only needed half that time to accomplish the necessary measurements. Experienced spacecraft controllers recognize that there are myriad ways beautiful plans could go awry, so they planned for more time in order to ensure that the objectives would be met even if anomalies occurred. They also were keenly aware that the mission could very well conclude after three months of low altitude operations, with Dawn using up the last of its hydrazine. But their efforts since then to conserve hydrazine proved very effective. In addition, the two remaining wheels have been operating well since they were powered on in December, further reducing the consumption of the precious propellant.

As it turned out, operations have been virtually flawless in this orbit, and the first three months yielded a tremendous bounty, even including some new measurements that had not been part of the original plans. And because the entire mission at Ceres has gone so well, Dawn has not expended as much hydrazine as anticipated.

Dawn is now performing measurements that were not envisioned long in advance but rather developed only in the past two months, when it was apparent that the expedition could continue. And since March 19, Dawn has been following a new strategy to use even less hydrazine. Instead of pointing its sensors straight down at the scenery passing beneath it as the spacecraft orbits and Ceres rotates, the probe looks a little to the left. The angle is only five degrees (equal to the angle the minute hand of a clock moves in only 50 seconds, or less than the interval between adjacent minute tick marks), but that is enough to decrease the use of hydrazine and thus extend the spacecraft's lifetime. (We won't delve into the reason here. But for fellow nerds, it has to do with the alignment of the axes of the operable reaction wheels with the plane in which Dawn rotates to keep its instruments pointed at Ceres and its solar arrays pointed at the sun. The hydrazine saving depends on the wheels' ability to store angular momentum and applies only in hybrid control, not in pure hydrazine control. Have fun figuring out the details. We did!)

The angle is small enough now that the pictures will not look substantially different, but they will provide data that will help determine the topography. (Measurements of gravity and the neutron, gamma ray and infrared spectra are insensitive to this angle.) Dawn took pictures at a variety of angles during the third mapping orbit at Ceres (and in two of the mapping orbits at Vesta, HAMO1 and HAMO2) in order to get stereo views for topography. That worked exceedingly well, and photos from this lower altitude will allow an even finer determination of the three dimensional character of the landscape in selected regions. Beginning on April 11, Dawn will look at a new angle to gain still another perspective. That will actually increase the rate of hydrazine expenditure, but the savings now help make that more affordable. Besides, this is a mission of exploration and discovery, not a mission of hydrazine conservation. We save hydrazine when we can in order to spend it when we need it. Dawn's charge is to use the hydrazine to accomplish important scientific objectives and to pursue bold, exciting goals that lift our spirits and fuel our passion for knowledge and adventure. And that is exactly what it is has done and what it will continue to do.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.90 AU (362 million miles, or 583 million kilometers) from Earth, or 1,505 times as far as the moon and 3.90 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and five minutes to make the round trip.

Dr. Marc D. Rayman
4:00 p.m. PDT March 31, 2016

TAGS: CERES, DAWN, MISSION, SPACECRAFT, VESTA, DWARF PLANET

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

Dawn is the only spacecraft ever to orbit two extraterrestrial destinations, which would have been impossible without its advanced ion propulsion system. It is the only spacecraft ever to orbit an object in the main asteroid belt. It is also the only spacecraft ever to orbit massive bodies (apart from the sun and Earth) that had not been visited first by a flyby spacecraft to characterize the gravity and other properties. (By the way, Ceres is one of eight solar system bodies that operating spacecraft are orbiting now. The others are the sun, Venus, Earth, the moon, comet Churyumov-Gerasimenko, Mars and Saturn.)

Now in its fourth and final mapping orbit at Ceres, at an altitude of 240 miles (385 kilometers), Dawn is closer to the exotic terrain than the International Space Station is to Earth. The benefit of being in orbit is that the probe can linger rather than take only a brief look during a fast flyby. Even though Dawn has met its full list of objectives at Ceres, it continues to return new, valuable pictures and other measurements to provide even greater insight into this relict from the dawn of the solar system. For example, it is acquiring more nuclear spectra with its gamma ray and neutron detector, sharpening its picture of some atomic elements on Ceres. In addition, taking advantage of its unique vantage point, Dawn is collecting more infrared spectra of locations that are of special interest and soon will also take color photos and stereo photos (as it did in the third mapping orbit) of selected areas.

Dawn has completed more than 600 revolutions since taking up residence one year ago. The first few orbits took several weeks each, but as the spacecraft descended and Ceres' gravitational embrace grew more firm, its orbital velocity increased and the orbital period decreased. Now circling in less than five and a half hours, Dawn has made 370 orbits since reaching this altitude on Dec. 7.

The pace of observations here is higher than in the previous mapping orbits, where the orbital periods were longer. The spacecraft flies over the landscape faster now, and being closer to the ground, its instruments discern much more detail but capture a smaller area. Mission controllers have developed intricate plans for observing Ceres, but those plans depend on the spacecraft being at the right place at the right time. As we will see below, however, sometimes it may not be.

Suppose, for example, the intent is to observe a particular feature, perhaps the bright center of Occator crater, the lonely, towering mountain Ahuna Mons, the fractures in Dantu crater or artificial structures that definitively prove the existence of extraterrestrial intelligence, utterly transforming our understanding of the cosmos and shattering our naive perspectives on life in the universe. Trajectory analysis indicates when Dawn will fly over the designated location, and engineers will program it to take pictures or infrared spectra at that time. They will also include some margin, so they may program it to start 10 minutes before and end 10 minutes after. But they can't afford to put in too much margin. Data storage on the spacecraft is limited, so other geological features could not be observed. Also, transmitting data to Earth requires pointing the main antenna at that distant planet instead of pointing sensors at Ceres, so it would be unwise to collect much more than is necessary.

Even if devoting additional time (and data) to trying to observe the desired place were feasible, it wouldn't necessarily solve the problem. Dawn travels in a polar orbit, which is the only way to ensure that it passes over all latitudes. While Dawn soars from north to south over the sunlit hemisphere making its observations, the dwarf planet itself rotates on its axis, so the ground moves from east to west. If the spacecraft arrives at the planned orbital location a little early or a little late, the feature of interest may not even be beneath it but rather could be too far east or west, out of view of the instruments. In that case, increasing the duration of the observation period doesn't help.

All of that is why, as we saw last month, it requires more pictures to fully map Ceres than you might expect. Many pictures may have to be taken in order to fill in gaps, and quite a few of the pictures overlap with others. Nevertheless, Dawn has done an excellent job. The spacecraft has photographed 99.6 percent of the dwarf planet from this low altitude. (If you aren't regularly visiting the image gallery, you are missing out on some truly out-of-this-world scenes.)

The flight team devises very detailed plans that tell the spacecraft what to do every second, including where to point and what data to collect with each sensor. When the observation plans are developed, they are checked and double-checked. Then they are translated into the appropriate software that the robotic ship will understand, and these instructions are checked and double-checked. That is integrated with all the other software that will be beamed to the spacecraft covering the same period of time, any conflicts are resolved and then the final version is checked and, well, you know.

This process is very involved, and it is usually well over a month between the formulation and the execution of the plan. During that time, Dawn's orbit can deviate slightly from the expert navigators' mathematical predictions, preventing the spacecraft from flying over the desired targets. There are several reasons the actual orbit may differ from the orbit used for developing the plan. (We have seen related examples of this, including as Dawn approached Mars, when it orbited Vesta and when it spiraled from one mapping orbit to another.) Let's briefly consider two.

One reason is that we do not have perfect knowledge of the variations in the strength of Ceres' gravitational pull from one location to another. We have discussed before that measuring these tiny irregularities in the gravity field provides insight into the distribution of mass within the dwarf planet that gives rise to them. The team has mapped the hills and valleys of the field quite well and even better than expected. Still, the remaining small uncertainty can lead to slight differences between what navigators calculate Dawn's motion will be and what its actual motion will be as it is buffeted by the gravitational currents.

A second source of discrepancy is that Dawn's own activities distort its orbit. Every time the reaction control system expels a tiny burst of hydrazine to control the spacecraft's orientation, keeping it pointed at its target, the force not only affects the orientation but also nudges the probe in its orbit, slowing it down or speeding it up very slightly. It's up to the spacecraft to decide exactly when to make these small adjustments, and it is not possible for controllers to predict their timing. (In a similar way, when you are driving, you occasionally move the steering wheel to keep going the direction you want, even if is straight ahead. It would be impossible to forecast each tiny movement, because they all depend on what has already happened plus the exact conditions at the moment.) The details of the reaction control system activity also depend on the use of the novel hybrid control scheme, which the joint Orbital/JPL team developed because of the failure of two of the spacecraft's four reaction wheels. The effect of each small firing of hydrazine is very small, but they can add up.

It took about a month in this mapping orbit to discover many of the subtleties of the gravity field and gain experience with how hybrid control affects the orbit. But even before descending to this altitude, the operations team understood the nature of these effects and was well prepared to deal with them.

They devised several strategies, all of which are being used to good effect. One of the ways to account for Dawn's actual orbit differing from its planned orbit is simply to change the orbit. Simply? Well, not really. It turns out to that to analyze the orbit and then maneuver to correct it in a timely way is a surprisingly complicated process, but, come to think of it, what isn't complicated when flying a spaceship around a distant, alien world? Nevertheless, every three weeks, the flight team makes a careful assessment of the orbit and determines whether a small refinement with the ion propulsion system is in order. For technical reasons, if maneuvers are needed, they will be executed in pairs, so mission planners have scheduled two windows (each 12 hours long and separated by eight days) about every 23 days.

Adjustments to resynchronize the actual orbit with the predicted orbit that formed the basis of the exploration plan are known as “orbit maintenance maneuvers.” Succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as if it were a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)

For both Vesta and Ceres, trajectory analyses long in advance determined that OMMs would not be needed in the higher orbits, so no windows were included in those schedules. There have been three OMM opportunities since arriving at the lowest altitude above Ceres, but only the first was needed. Dawn performed the pair on Dec. 31-Jan. 1 and on Jan. 8 with its famously efficient ion engine. The orbit was good enough the next two times that OMMs were deemed unnecessary. It is certain that some future OMMs will be required. Your faithful correspondent provides frequent (and uncharacteristically concise) reports on Dawn's day-to-day activities, including OMMs.

By the end of the Jan. 8 OMM, Dawn's ion propulsion system had accumulated 2,019 days of operation in space, more than 5.5 years. During that time, the effective change in speed was 24,600 mph (39,600 kilometers per hour). (We have discussed in detail that this is not Dawn's current speed but rather the amount by which the ion engines have changed it.) This is uniquely high for a spacecraft to accomplish with its own propulsion system and validates our description of ion propulsion as delivering acceleration with patience. (The previous record holder, Deep Space 1, achieved 9,600 mph, or 15,000 kilometers per hour.)

The effect of Dawn's gentle ion thrusting during its mission has been nearly the same as that of the entire Delta II 7925H-9.5 rocket, with its nine external rocket engines, first stage, second stage and third stage. To get started on its interplanetary adventure, Dawn's rocket boosted it from Cape Canaveral to out of Earth orbit with only four percent higher velocity than Dawn subsequently added on its own with its ion engines.

As Dawn and Earth follow their own independent orbits around the sun (Dawn's now tied permanently to its gravitational master, Ceres), next month they will reach their greatest separation of the entire mission. On March 4 (about one Earth year after Ceres took hold of Dawn), on opposite sides of the solar system, they will be 3.95278 AU (367.434 million miles, or 591.328 million kilometers) from each other. (For those of you with full schedules, note that the maximum separation will be 5:40 a.m. PST.) They won't be this far apart again until Feb. 6, 2025, long after Dawn has ceased operating (as discussed below). The figure below depicts the arrangement next month.

Dawn has faced many challenges in its unique voyage in the forbidding depths of space, but it has surmounted all of them. It has even overcome the dire threat posed by the loss of two reaction wheels (the second failure occurring in orbit around Vesta 3.5 years and 1.3 billion miles, or 2.0 billion kilometers, ago). With only two operable reaction wheels (and those no longer trustworthy), the ship's remaining lifetime is very limited.

A year ago, the team couldn't count on Dawn even having enough hydrazine to last beyond next month. But the creative methods of conserving that precious resource have proved to be quite efficacious, and the reliable explorer still has enough hydrazine to continue to return bonus data for a while longer. Now it seems highly likely that the spacecraft will keep functioning through the scheduled end of its primary mission on June 30, 2016.

NASA may choose to continue the mission even after that. Such decisions are difficult, as there is literally an entire universe full of interesting subjects to study, but resources are more limited. In any case, even if NASA extended the mission, and even if the two wheels operated without faltering, and even if the intensive campaign of investigating Ceres executed flawlessly, losing not an ounce (or even a gram) of hydrazine to the kinds of glitches that can occur in such a complex undertaking, the hydrazine would be exhausted early in 2017. Clearly an earlier termination remains quite possible.

Regardless of when Dawn's end comes, it will not be a time for regret. The mission has realized its raison d'être and is reaping rewards even beyond those envisioned when it was conceived. It has taken us all on a marvelous interplanetary journey and allowed us to behold previously unseen sights of distant lands. The conclusion of the mission will be a time for gratitude that it was so successful. And until then, every new picture or other measurement adds to the richly detailed portrait of a faraway, exotic world. There is plenty more still to do before this remarkable story draws to a close.

Dawn is 240 miles (385 kilometers) from Ceres. It is also 3.95 AU (367 million miles, or 591 million kilometers) from Earth, or 1,475 times as far as the moon and 3.99 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take one hour and six minutes to make the round trip.

TAGS: DAWN, MISSION, SPACECRAFT, CERES, DWARF PLANET

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.