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

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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.

Even as the veteran explorer orbits high over dwarf planet Ceres and looks forward to continuing its mission, today it can reflect upon 10 exciting and productive years (or equivalently, with its present perspective, 2.17 exciting and productive Cerean years).

The ambitious adventurer embarked on an extraordinary extraterrestrial expedition on Sept. 27, 2007. With its advanced ion propulsion system, Dawn soared past Mars in 2009. The spacecraft took some of the Red Planet’s orbital energy around the sun to boost itself on its journey. (Nevertheless, this extra energy amounts to less than a quarter of what the ion engines have provided.) Ever a responsible citizen of the cosmos, Dawn fully adheres to the principle of the conservation of energy. So to compensate for speeding up, it slowed Mars down.

In 2011, the spacecraft arrived at Vesta, the second largest object in the main asteroid belt between Mars and Jupiter. Dawn gracefully entered into Vesta’s firm but gentle gravitational embrace. The probe maneuvered extensively in orbit, optimizing its views to get the best return possible from its photography and other observations. During 14 months in orbit, Dawn completed 1,298 revolutions around Vesta, taking nearly 31,000 pictures and collecting a wealth of other scientific measurements. From the perspective it had then, Dawn was in residence for nearly a third of a Vestan year (or almost 1,900 Vestan days). The explorer revealed a strange, ancient protoplanet, now recognized to be more closely related to the terrestrial planets (including the one Dawn left 10 years ago) than to the typical and smaller asteroids.

Unlike all other deep-space missions, Dawn had the capability to leave its first orbital destination and voyage to and enter orbit around another. After smoothly disengaging from Vesta, the interplanetary spaceship flew more than 900 million miles (1.5 billion kilometers) in 2.5 years to Ceres, the largest object in the asteroid belt. Indeed, prior to Dawn’s arrival, that dwarf planet was the largest body between the sun and dwarf planet Pluto that a spacecraft had not yet visited. And just as at Vesta, thanks to the maneuverability of ion propulsion, Dawn did not have to be content with a one-time flyby, gathering only as much data as possible during a brief encounter. By going into orbit around Ceres, the spacecraft could linger to scrutinize the exotic, alien world. And that is exactly what it has done.

Both Vesta and Ceres have held secrets since the dawn of the solar system, and both have beckoned since they were first spotted in telescopes at the dawn of the 19th century. For the next two centuries, they appeared as little more than faint smudges of light amidst myriad glittering stellar jewels, waiting for an inquisitive and admiring visitor from Earth. Finally, Dawn answered their cosmic invitations and eventually developed richly detailed, intimate portraits of each.

As the last stop on a unique interplanetary journey of discovery, Ceres has proven well worth the wait. Since arriving in March 2015 (more than half a Cerean year ago, or nearly 2,500 Cerean days ago), Dawn has completed 1,595 revolutions. It has beheld mysterious and fascinating landscapes and unveiled a complex world of rock, ice and salt, along with organic compounds and other intriguing constituents. The dwarf planet may have been covered by an ocean long ago, and there might even be liquid water underground now. The 57,000 pictures and numerous other measurements with the sophisticated sensors will keep scientists busy for many years (both terrestrial and Cerean).

By early 2016, during its ninth year in space, Dawn had accomplished so much that it exceeded all of the original objectives established for it by NASA before the ship set sail. Along the way, Dawn encountered and ultimately overcame many obstacles, including equipment failures that could well have sunk the mission. Against all odds and expectations, however, when its prime mission concluded in June 2016, the spacecraft was still healthy enough that NASA decided to extend the mission to learn still more about Ceres. Since then, Dawn has conducted many investigations that had never even been considered prior to last year. Now it has successfully achieved all of the extended mission objectives. And, once again defying predictions thanks to expert piloting by the flight team (and a small dose of good luck), Dawn still has some life left in it. Before the end of the year, NASA will formulate another new set of objectives that will take it to the end of its operational life.

Dawn has flown to many different orbital altitudes and orientations to examine Ceres. Now the probe is in an elliptical orbit, ranging from less than 3,200 miles (5,100 kilometers) up to 23,800 miles (38,300 kilometers). At these heights, it is measuring cosmic rays. Scientists mathematically remove the cosmic ray noise from Dawn’s 2015-2016 recordings of atomic elements from a low, tight orbit at only 240 miles (385 kilometers).

In its present orbit, Dawn can make these measurements to clarify Ceres’ nuclear signals while being very frugal with its precious hydrazine, which is so crucial because of the loss of three reaction wheels. (The small supply was not loaded onboard with the intention of compensating for failed reaction wheels.) When the hydrazine is expended, the mission will end. So this high elliptical orbit is a very good place to be while NASA and the Dawn project are determining how best to use the spacecraft in the future.

Meanwhile, this anniversary presents a convenient opportunity to look back on a remarkable spaceflight. For those who would like to track the probe’s progress in the same terms used on past anniversaries, we present here the tenth annual summary, reusing text from previous years with updates where appropriate. Readers who wish to investigate Dawn’s ambitious journey in detail may find it helpful to compare this material with the Dawn Journals from its first, second, third, fourth, fifth, sixth, seventh, eighth and ninth anniversaries.

In its 10 years of interplanetary travels, the spacecraft has thrust with its ion engines for a total of 2,109 days (5.8 years), or 58 percent of the time (and 0.000000042 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 908 pounds (412 kilograms) of its supply of xenon propellant, which was 937 pounds (425 kilograms) on Sept. 27, 2007. The spacecraft has used 69 of the 71 gallons (262 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.

The thrusting since then has achieved the equivalent of accelerating the probe by 25,400 mph (40,900 kilometers per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) It is remarkable that Dawn’s ion propulsion system has provided nearly the same change in speed as the entire Delta rocket.

Since launch, our readers who have remained on or near Earth have completed 10 revolutions around the sun, covering 62.8 AU (5.8 billion miles, or 9.4 billion kilometers). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 42.4 AU (3.9 billion miles, or 6.3 billion kilometers). As it climbed away from the sun, up the solar system hill to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It had to go even slower to perform its graceful rendezvous with Ceres. In the 10 years since Dawn began its voyage, Vesta has traveled only 40.5 AU (3.8 billion miles, or 6.1 billion kilometers), and the even more sedate Ceres has gone 37.8 AU (3.5 billion miles, or 5.7 billion kilometers). (To develop a feeling for the relative speeds, you might reread this paragraph while paying attention to only one set of units, whether you choose AU, miles, or kilometers. Ignore the other two scales so you can focus on the differences in distance among Earth, Dawn, Vesta and Ceres over the 10 years. You will see that as the strength of the sun’s gravitational grip weakens at greater distance, the corresponding orbital speed decreases.)

Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with even more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. (If you prefer not to skip it, click here.) In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.

Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family (including Earth, Dawn, Vesta and Ceres) follow their individual paths around the sun, they sometimes move closer and sometimes move farther from it.

In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft may travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and part of the arduousness of Dawn’s journey has been changing the inclination of its orbit, an energetically expensive task.)

Now we can see how Dawn has done by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft had terminated ion thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on Sept. 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.

For readers who are not overwhelmed by the number of numbers, investing the effort to study the table may help to demonstrate how Dawn patiently transformed its orbit during the course of its mission. Note that six years ago, the spacecraft’s path around the sun was exactly the same as Vesta’s. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to slip into orbit around Vesta. While simply flying by it would have been far easier, matching orbits with Vesta required the exceptional capability of the ion propulsion system. Without that technology, NASA’s Discovery Program would not have been able to afford a mission to explore the massive protoplanet in such detail. Dawn has long since gone well beyond that. Having discovered so many of Vesta’s secrets, the adventurer left it behind. No other spacecraft has ever escaped from orbit around one distant solar system object to travel to and orbit still another extraterrestrial destination. From 2012 to 2015, the stalwart craft reshaped and tilted its orbit even more so that now it is identical to Ceres’. Once again, that was essential to accomplishing the intricate celestial choreography in which the behemoth reached out with its gravity and tenderly took hold of the spacecraft. They have been performing an elegant pas de deux ever since.

Even after a decade of daring space travel, flying in deep space atop a blue-green pillar of xenon ions, exploring two of the last uncharted worlds in the inner solar system, overcoming the loss of three reaction wheels, working hard to stretch its shrinking supply of hydrazine, Dawn is ready for more. And so is everyone who yearns for new knowledge, everyone who is curious about the cosmos, and everyone who is exhilarated by bold adventures into the unknown. More is to come. Dawn -- and all those who find the lure of space irresistible -- can look forward to whatever lies ahead for this unique mission.

Dawn is 16,600 miles (26,700 kilometers) from Ceres. It is also 2.92 AU (271 million miles, or 437 million kilometers) from Earth, or 1,080 times as far as the moon and 2.91 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 am PDT September 27, 2017

TAGS: DAWN, CERES, VESTA, ASTEROID BELT, ION 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.

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

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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 precious pictures and other data have just begun to arrive on Earth, and it is too soon to say anything about the latest findings, but stand by for stunning new discoveries. Actually, you could get pictures about as good as Dawn’s are now with a telescope 217 times the diameter of Hubble Space Telescope. An alternative is to build your own interplanetary spaceship, travel through the depths of space to the only dwarf planet in the inner solar system, and look out the window. Or go to the Ceres image gallery.

Dawn has already gained fabulous perspectives on this mysterious world from its first and second mapping orbits. Now at one third the altitude of the mapping campaign that completed in June, its view is three times as sharp. (Exploring the cosmos is so cool!) That also means each picture takes in a correspondingly smaller area, so more pictures are needed now to cover the entire vast and varied landscape. At this height, Dawn’s camera sees a square about 88 miles (140 kilometers) on a side, less than one percent of the more than one million square miles (nearly 2.8 million square kilometers). The orbital parameters were chosen carefully so that as Ceres rotates on its axis every nine hours (one Cerean day), Dawn will be able to photograph nearly all of the surface in a dozen orbital loops.

When Dawn explored the giant protoplanet Vesta from comparable orbits (HAMO1 in 2011 and HAMO2 in 2012), it pointed its scientific instruments at the illuminated ground whenever it was on the dayside. Every time its orbit took it over the nightside, it turned to point its main antenna at Earth to radio its findings to NASA’s Deep Space Network. As we explained last year, however, that is not the plan at Ceres, because of the failure of two of the ship’s reaction wheels. (By electrically changing the speed at which these gyroscope-like devices rotate, Dawn can turn or stabilize itself in the zero-gravity conditions of spaceflight.)

We discussed in January that the flight team has excogitated innovative methods to accomplish and even exceed the original mission objectives regardless of the condition of the wheels, even the two operable ones (which will not be used until the final mapping orbit). Dawn no longer relies on reaction wheels, although when it left Earth in 2007, they were deemed indispensable. The spacecraft’s resilience (which is a direct result of the team’s resourcefulness) is remarkable!

One of the many ingredients in the recipe for turning the potentially devastating loss of the wheels into a solid plan for success has been to rotate the spacecraft less frequently. Therefore, sometimes Dawn will wait patiently for half an orbit (almost 9.5 hours) as it flies above ground cloaked in the deep darkness of night, its instruments pointed at terrain they cannot detect. Other times, it will keep its antenna fixed on Earth without even glancing at the sunlit scenery below, because it can capture the views on other revolutions. This strategy conserves hydrazine, the conventional rocket propellant used by the small jets of the reaction control system in the absence of the wheels. It takes more time, but because Dawn is in orbit, time is not such a limited resource. It will take 12 passages over the illuminated hemisphere, each lasting nearly 9.5 hours, to bring the entirety of the landscape within view of its camera, but we will need a total of 14 full revolutions, or 11 days (29 Cerean days, for those of you using that calendar), to acquire and transmit all the data. The Dawn team calls this 11-day period “11 days,” or sometimes a “cycle.”

In quite a change from the days that there simply didn’t seem to be enough hydrazine onboard to accomplish all of the mission’s ambitious objectives, engineers and the spacecraft itself have collaborated to be so efficient with the precious molecules that they now have some to spare. Therefore, mission planners have recently decided to spend a few more in this mapping orbit. They have added extra turns to allow the robot to communicate with Earth during more of the transits over the nightside than they had previously budgeted. This means Dawn can send the contents of its computer memory to Earth more often and therefore have space to collect and store even more data than originally planned. An 11-day mapping cycle is going to be marvelously productive.

But Dawn has goals still more ambitious than taking pictures and recording infrared and visible spectra of the lands passing underneath it. It will conduct six complete mapping cycles, each one looking at a slightly different angle. This will effectively yield stereo views, which when combined will make those flat images pop into full three dimensionality.

In its first mapping cycle, which is taking place now, the explorer aims its instruments straight down. For the second, it will keep the camera pointed a little bit back and to the left, making another full map but with a different perspective. For the third, it will look a little back and to the right. The fourth map will be viewing the scenery ahead and to the left. The fifth map will be of the terrain immediately ahead, and the sixth will be farther back than the third but not as far to the right.

In addition to the stereo pictures and the many spectra (which reveal the nature of the minerals as well as the surface temperature), Dawn will use the color filters in its camera to record the sights in visible and infrared wavelengths.

As always, mission planners schedule more observations than are needed, recognizing that glitches can occur on a complex and challenging expedition in the forbidding depths of space. So even if some data are not collected, the goals can still be accomplished.

The probe also will continue to acquire spectra both of neutrons and of gamma rays. It is unlikely to detect more than a whisper of neutrons from Ceres at this height, but the radiation coming from elsewhere in space now will serve as a useful calibration when it measures stronger nuclear emanations from one quarter the altitude starting in December, allowing scientists to inventory Ceres’ atomic constituents.

Precise measurements of Dawn’s radio signal will reveal more details of the dwarf planet’s gravitational field and hence the distribution of mass within. When the spacecraft is not aiming its main antenna at Earth, it will broadcast through one of its three auxiliary antennas, and the Deep Space Network will be listening (almost) continuously throughout the 84 orbits.

As at Vesta, Dawn’s polar orbits are oriented so that the craft always keeps the sun in view, never entering Ceres’ shadow, even when it is nighttime on the ground below. But its course will take the robot out of sight from Earth occasionally, and the behemoth of rock and ice will block the radio signal. Of course, Dawn is quite accustomed to operating in radio silence. It follows timed instructions (called sequences) that cover a full mapping cycle, so it does not require constant contact. And in budgeting how much data Dawn can collect and transmit, mission planners have accounted for the amount of time Ceres will eclipse its view of Earth.

Thanks to the uniquely efficient and exceptionally gentle thrust of the ion engines, as well as the flexibility inherent in being in orbit, Dawn operations generally can be more leisurely than those with conventional chemical propulsion or missions that only fly past their targets rather than stay for as long as needed. In that spirit, controllers had allowed the time in the spacecraft’s main computer to drift off, as there was no need to keep it particularly accurate. But recently the clock was off by so much that they decided to correct it, so before the mapping began, they adjusted it by a whopping 0.983 seconds, eliminating a large (but still tolerable) offset.

Many residents of Earth’s northern hemisphere are completing their leisurely summer vacations. As we saw in February, Dawn has measured the orientation of Ceres’ spin axis and found that it is tipped about four degrees (compared with Earth’s axial tilt of 23 degrees). The sun then never moves very far from the dwarf planet’s equator, so seasonal variations are mild. Nevertheless, northern hemisphere summer (southern hemisphere winter) began on Ceres on July 24. Because Ceres takes longer to revolve around the sun than Earth, seasons last much longer. The next equinox won’t occur until Nov. 13, 2016, so there is still plenty of time to plan a summer vacation.

Meanwhile, Dawn is working tirelessly to reveal the nature of this complex, intriguing world. Now seeing the exotic sights with a sharper focus than ever, the probe’s meticulous mapping will provide a wealth of new data that scientists will turn into knowledge. And everyone who has ever seen the night sky beckon, everyone who has heard the universe’s irresistible invitation, and everyone who has felt the overpowering drive for a bold journey far from Earth shares in the experience of this remarkable interplanetary adventure.

Dawn is 905 miles (1,456 kilometers) from Ceres. It is also 2.06 AU (191 million miles, or 308 million kilometers) from Earth, or 775 times as far as the moon and 2.03 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 34 minutes to make the round trip.

Dr. Marc D. Rayman
5:00 p.m. PDT August 21, 2015

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

• 

  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.