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

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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.

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.