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NASA’s Dawn Fills Out its Ceres Dance Card

Saturday, November 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
This graphic shows the planned trek of NASA’s Dawn spacecraft from its launch in 2007 through its arrival at the dwarf planet Ceres in early 2015. Image credit: NASA/JPL-Caltech
› Full image and caption

Dear Hand-Me-Dawns,

Gliding smoothly through the main asteroid belt between Mars and Jupiter, Dawn continues to make good progress on its ambitious mission of exploration. It is patiently but persistently pursuing Ceres, the second destination on its interplanetary itinerary.

Protoplanets Ceres and Vesta, the two most massive residents of the asteroid belt, were discovered at the beginning of the 19th century, and they have tantalized astronomers and others curious about the nature of the universe ever since. (Indeed, Ceres was the first dwarf planet discovered, having been found 129 years before Pluto.) They have waited patiently for a visitor from Earth since the dawn of the solar system. Dawn’s objective is to turn these uncharted orbs from tiny smudges of light amidst the stars into richly detailed places. It succeeded spectacularly at Vesta in 2011 - 2012, and it remains on course and on schedule for doing so at Ceres in 2015.

Next month, the adventurer will pass an invisible milestone on its celestial journey. On Dec. 27, it will be equidistant from these behemoths of the asteroid belt as all three follow their own independent heliocentric paths. The spacecraft will be 0.21 AU (19.4 million miles, or 31.3 million kilometers) away from each world, the one already visited and the one yet to be reached. And as the indefatigable ship sails on the cosmic seas with its sights set on Ceres, our anticipation for glimpsing the alien landscape ahead grows and grows, while the now-familiar scenery of Vesta shrinks into the distance, fading over the horizon.

The next day, Dawn will be equidistant from two other solar system bodies, both of which have been known (to our human readers, at least) for somewhat longer than Ceres and Vesta have. On Dec. 28, our celestial ambassador will be 2.46 AU (229 million miles, or 368 million kilometers) from Earth and the sun. (We cannot specify in which century either of them was discovered.)

Its complex route through the solar system has already taken the spacecraft farther from each of these bodies before. In the current phase of the mission, it is receding from the sun again, climbing the solar system hill from Vesta to Ceres. (It approached the sun in late 2012 and 2013 as part of the strategy for arriving at Ceres’s orbit when Ceres itself was there.) Having attained its greatest distance from Earth for the year in August, the spacecraft is temporarily getting closer to its planet of origin. (More precisely, as we discussed then, Earth is currently moving toward Dawn, because Earth travels faster in its solar orbit than Dawn does in its much more remote orbit.)

Dawn will reach two more impressive milestones in December, although neither pertains to its location. Soon the craft will surpass four years of ion thrust. While most spacecraft rely on conventional propulsion and hence coast most of the time (just as planets, moons, and asteroids do), Dawn’s mission would be impossible if it did that. In order to orbit and explore two distant destinations, the only terrestrial probe ever to attempt such a feat, it must accomplish a great deal of maneuvering. It spends the majority of its time using its uniquely efficient and capable ion propulsion system, constantly putting a gentle pressure on its trajectory to gradually reshape it. Although the spacecraft has already accumulated far more time in powered flight than any other mission, it still has a great deal more ahead.

And in December, that thrusting will push the craft’s speedometer past an extraordinary 20,000 mph (8.94 kilometers per second). (As we have seen in many previous logs, such as this one, this measure of the speed does not represent the actual spacecraft velocity. Nevertheless, it is a useful metric that avoids the complicating effects of orbital mechanics.) That is more than twice the previous record for propulsive velocity change set by Deep Space 1, the first interplanetary mission to use ion propulsion.

Dawn spends most of its time emitting a lovely blue-green beam of high-velocity xenon ions to propel itself. As foretold in the prophecy commonly known as the October log, however, we are now in one of just two periods of the long mission in which coasting is better for the trajectory than thrusting. Mission controllers took advantage of this time to instruct the robot to perform some special activities that would have been less convenient during routine ion thrusting. The reliable ship completed all of them flawlessly.

For more than 27 hours on Nov. 12 and 13, Dawn operated in a mode that had not even been conceived of when it was designed and built. It controlled its orientation in the frictionless, zero-gravity conditions of spaceflight using a scheme that was developed long after it left Earth. This “hybrid” control method operated perfectly, validating the extensive work engineers have invested in it and verifying its readiness for use at Ceres.

When it embarked on its bold journey more than six years ago, the ship was outfitted with four reaction wheels. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can turn or stabilize itself. It generally used three at a time, with a fourth kept in reserve. For such a long and complex expedition, extending to well over one million times farther from Earth than the International Space Station, backup systems are essential.

One of the wheels experienced increased friction in June 2010, but the mission continued with the other three. A second met the same fate in August 2012, as Dawn was climbing away from Vesta. Other spacecraft have encountered similar issues with their reaction wheels as well, and the consequences can be dire.

We have described the operations team’s swift and productive responses to the regrettable behavior of the reaction wheels in a number of logs (see, as one example, here). As soon as the first wheel faltered, JPL and Orbital Sciences Corporation began working on a method to operate with fewer than three in case another one had difficulty. They developed software to operate in a hybrid mode of two wheels plus the small hydrazine-powered jets of the reaction control system and installed it in the craft’s main computer in April 2011 so it would be available at Vesta if needed.

Given the problems with reaction wheels on Dawn and other spacecraft, engineers do not have high confidence that the two remaining units will operate for long (although it certainly is possible they will). Thanks to their remarkable ingenuity and resourcefulness, the team has devised a detailed plan that should allow Dawn to complete its extraordinary mission using only the hydrazine thrusters, achieving all of its objectives in exploring Ceres regardless of the condition of its wheels. (Note that it is not even obvious that doing so is possible, but then again, it isn’t obvious that sending probes so far from our home planet is possible either. Part of the thrill of a solar system adventure is overcoming the extremely daunting challenges.) So now, hybrid control would provide an enhancement, extending the supply of precious hydrazine propellant and giving the spacecraft the opportunity to operate even longer at Ceres than it would without the two functioning wheels. When the hydrazine is exhausted, the mission will conclude.

Dawn will use hybrid control only in its lowest altitude orbits at Ceres, the final phase of the mission. (Beginning in December and continuing in 2014, we will describe all phases of the Ceres plan in detail.) Hybrid control will be called upon to perform three kinds of tasks for the spacecraft: train the suite of sophisticated sensors at the mysterious world beneath it, point the main antenna to distant Earth to transmit its findings and receive updated instructions, and rotate from one orientation to another. The innovative system has now unerringly demonstrated its capability to accomplish all three by executing exactly those functions earlier this month.

The confirmation that hybrid control works as intended is not the only task Dawn is carrying out during this coasting period. All of its scientific instruments (including even the backup camera) are being powered on and given thorough health checks, verifying that they remain fully functional and ready to reveal Ceres’s secrets. Engineers also conducted some tests with the ion engine that has operated the longest of the three to confirm expectations of how it will perform at Ceres.

On Dec. 9, Dawn’s four-week coast period will end. Once again it will turn to point an ion engine in the direction needed to push forward to its rendezvous with the distant and exotic world ahead. As the probe nears and then passes the halfway point on its remarkable journey from Vesta to Ceres, it is pulled by forces even more powerful than ion propulsion: the attraction of discovery, the lure of the unknown, and the draw of tremendous new insights and profound new understandings to be gained in a daring adventure far, far from home.

Dawn is 18 million miles (29 million kilometers) from Vesta and 22 million miles (35 million kilometers) from Ceres. It is also 2.78 AU (258 million miles, or 415 million kilometers) from Earth, or 1,125 times as far as the moon and 2.82 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.

– Dr. Marc D. Rayman

› Read more entries from Marc Rayman’s Dawn Journal


For Dawn, a Time to Thrust and and a Time to Coast

Thursday, October 31st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear All Hallows’ Dawns,

Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.

The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn’s heliocentric orbit was the same as the rocky behemoth’s. Now they are very far apart, and by early 2015, the robotic explorer’s path will be close enough to Ceres’s that they will become locked in a gravitational embrace.

Without ion propulsion, Dawn’s unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA’s Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.

During its years of interplanetary travel, Dawn has reliably followed a carefully formulated flight plan from Earth past Mars to Vesta and now from Vesta to Ceres. We discussed some of the principles underlying the development of the complex itinerary in a log written when Dawn was still gravitationally anchored to Earth. To carry out its ambitious adventure, Dawn should thrust most of the time, but not all of the time. Indeed, at some times, thrusting would be unproductive.

We will not delve into the details here, but remember that Dawn is doing more than ascending the solar system hill, climbing away from the sun. More challenging than that is making its orbit match the orbit of its targets so that it does not fly past them for a brief encounter as some other missions do. Performing its intricate interplanetary choreography requires exquisite timing with the grace and delicacy of the subtly powerful ion propulsion.

Of course Dawn does not thrust much of the time it is in orbit at Vesta and Ceres; rather, its focus there is on acquiring the precious pictures and other measurements that reveal the detailed nature of these mysterious protoplanets. But even during the interplanetary flight, there are two periods in the mission in which it is preferable to coast. Sophisticated analysis is required to compute the thrusting direction and schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the mass of the sun and the masses and orbits of Earth, Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when mission planners will not allow thrusting (e.g., during spacecraft maintenance periods).

The first interval that interplanetary trajectory designers designated as “optimal coast” was well over four years and 1.8 billion miles (2.8 billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8, 2009. During that time, the ship took some of Mars’s orbital energy to help propel itself toward Vesta. (In exchange for boosting Dawn, Mars slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters, in 180 million years.)

The second and final interval when coasting is better than thrusting begins next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit around the sun without modifying it. The timing of this coast period is nearly as important to keeping the appointment with Ceres as is the timing of the thrusting. In next month’s log, we will describe some of the special assignments the sophisticated robot will perform instead of its usual quiet cruise routine of accelerating and emitting xenon ions. We also will look ahead to some interesting celestial milestones and alignments in December.

While the spacecraft courses through the asteroid belt, the flight team continues refining the plans for Ceres. In logs in December and several months in 2014, we will present extensive details of those plans so that by the time Dawn begins its mission there, you will be ready to ride along and share in the experience.

In the meantime, as the stalwart ship sails on, it is propelled not only by ions but also by the promise of exciting new knowledge and the prospects of a thrilling new adventure in exploring an uncharted alien world.

Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million miles, or 460 million kilometers) from Earth, or 1,200 times as far as the moon and 3.10 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

P.S. This log is posted early enough to allow time for your correspondent to don his Halloween costume. In contrast to last year’s simple (yet outlandish) costume, this year’s will be more complex. He is going in double costume, disguised as someone who is only pretending to be passionate about the exploration of the cosmos and the rewards of scientific insight.

› Read more entries from Marc Rayman’s Dawn Journal


NASA’s Dawn Spacecraft Celebrates Six Years in Space

Friday, September 27th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft
Artist’s concept of NASA’s Dawn spacecraft between the giant asteroid Vesta and the dwarf planet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawnniversaries,

On the sixth anniversary of leaving Earth to embark on a daring deep-space expedition, Dawn is very, very far from its erstwhile planetary residence. Now humankind’s only permanent resident of the main asteroid belt between Mars and Jupiter, the seasoned explorer is making good progress toward the largest object in that part of the solar system, the mysterious dwarf planet Ceres. The voyage is long, and the intrepid but patient traveler will not reach its next destination until half a year after its seventh anniversary of departing Earth.

On its fifth anniversary, Dawn was still relatively close to Vesta, the giant protoplanet that had so recently held the craft in its gravitational grip. The only probe ever to orbit a main belt asteroid, Dawn spent 14 months (including its fourth anniversary) accompanying Vesta on its way around the sun. After more than two centuries of appearing to astronomers as little more than a fuzzy blob of light among the stars, the second most massive body in the asteroid belt has been revealed as a fascinating, complex, alien world more closely related to terrestrial planets (including Earth) than to typical asteroids.

Most of the ship’s first four years of spaceflight were devoted to using its ion propulsion system to spiral away from the sun, ascending the solar system hill from Earth to Vesta. Now it is working to climb still higher up that hill to Ceres.

For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the sixth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the logs from its first, second, third, fourth, and fifth anniversaries.

In its six years of interplanetary travels, the spacecraft has thrust for a total of 1,410 days, or 64 percent of the time (and about 0.000000028 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 318 kilograms (701 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 8.7 kilometers per second (19,500 mph). 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. Having accomplished about three-quarters of the thrust time planned for its entire mission, Dawn has already 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.)

Since launch, our readers who have remained on or near Earth have completed six revolutions around the sun, covering about 37.7 AU (5.6 billion kilometers or 3.5 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 27.4 AU (4.1 billion kilometers or 2.5 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It will have to slow down still more to rendezvous with Ceres. Since Dawn’s launch, Vesta has traveled only 24.2 AU (3.6 billion kilometers or 2.2 billion miles), and the even more sedate Ceres has gone 22.8 AU (3.4 billion kilometers or 2.1 billion miles).

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 a few 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. 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.

› Continue reading Marc Rayman’s Dawn Journal


Earth and Dawn on Opposite Sides Now

Friday, August 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear Antecedawnts,

Traveling confidently and alone, Dawn continues to make its way through the silent depths of the main asteroid belt. The only spacecraft ever to have orbited a resident of the vast territory between Mars and Jupiter, Dawn conducted a spectacular exploration of gigantic Vesta, revealing a complex place that resembles the terrestrial planets more than typical asteroids. Now the interplanetary adventurer is on its long journey to the uncharted dwarf planet Ceres, by far the largest of all asteroids (975 kilometers, or more than 600 miles, in equatorial diameter). In 2015, the mysterious world of rock and ice will begin to give up its ancient secrets to the immigrant from distant Earth.

Earth, Vesta, Dawn, and Ceres are following their own separate paths around the sun. The spacecraft is patiently reshaping its orbit, using its uniquely efficient ion propulsion system to accomplish a deep-space expedition that would be impossible with conventional propulsion.

As we have seen in many previous logs (including, for example, here), the higher an object’s orbit, the slower it needs to move in order to balance the gravitational pull, which diminishes with distance. Blistering Mercury orbits the sun faster than Venus, Venus goes faster than Earth, Earth goes faster than Mars, and Mars goes faster than the residents of the asteroid belt and the cold planets of the outer solar system. In the same way, satellites that orbit close to Earth, including the International Space Station, move faster than those at greater altitudes, and the moon travels even more slowly in its very high orbit.

Dawn is now a permanent inhabitant of the main asteroid belt. Therefore, the massive sun, the gravitational master of the solar system, has a weaker grip on it than on Earth. So as Dawn maneuvers from Vesta to Ceres, Earth revolves more rapidly around the sun. This month, their independent motions have taken them to their greatest separation of the year, as they are on opposite sides of the sun. How truly remarkable that humankind can accomplish such a feat!

On August 4, the planet and its robotic ambassador to the cosmos were an extraordinary 3.47 AU (519 million kilometers, or 322 million miles) apart. (To recapture the feeling of your position in the universe then, it may be helpful to know that the maximum range was attained at 4:16 a.m. PDT.) From the perspective of terrestrial observers, had they possessed the superhuman (and even supertelescopic) vision needed to descry the tiny ship far beyond the blindingly bright star, Dawn would have appeared to be very close to the sun but not directly behind it. To rendezvous with Vesta and then with Ceres, the spacecraft has tilted the plane of its solar orbit. Some of the time it is north of Earth’s orbital plane, sometimes it is south. August 4 was during the northern segment, so Dawn would have been a little north of the sun.

It’s time to refer to one of those novel clocks available in the Dawn gift shop on your planet (although if you already have such a clock, it probably doesn’t tell you that it’s time — we stand by our policy of full refunds within 24 hours, as measured by our Dawn clocks). With the sun at the center of the clock, Earth’s motion would be like that of a short minute hand. Dawn, both farther from the sun and moving more slowly, would be following the path of a longer hour hand. If we ignore the effect of the ion thrust, which is constantly changing the orbit, and the slight misalignment of the hour hand representing Dawn’s being in a different plane, the conditions on August 4 were like those at 6:00.

As time progresses and Earth continues circling the sun, it will come closer to Dawn until April 2014 (like 12:00). Even then, however, they will be over 1.55 AU (232 million kilometers, or 144 million miles) apart, and they will never be that close again. The spacecraft will continue climbing higher and higher from the sun toward Ceres, so by the time Earth loops around once more, Dawn will be even farther from it. In the meantime, when next the arrangement is like 6:00, in December 2014, the separation will be more than 3.78 AU (565 million kilometers, or 351 million miles), even greater than the remarkable range a few weeks ago.

› Continue reading Marc Rayman’s Dawn Journal


Dawn’s Journey: A Power Trip

Tuesday, July 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Dawn's solar arrays are folded to fit inside the nose cone in preparation for launch
The Dawn spacecraft’s solar array wings — pictured here in a folded position in preparation for launch — span 19.7 meters (nearly 65 feet) and are designed to keep the spacecraft powered even as it ventures further from the sun into the remote asteroid belt. Image credit: NASA/JPL-Caltech

Dear Megalodawniacs,

Powering its way through the main asteroid belt between Mars and Jupiter, Dawn continues on course and on schedule for its 2015 appointment with dwarf planet Ceres. After spending more than a year orbiting and scrutinizing Vesta, the second most massive object in the asteroid belt, the robotic explorer has its sights set on the largest object between the sun and Neptune that a spacecraft has not yet visited. This exotic expedition to unveil mysterious alien worlds would be impossible without the probe’s ion propulsion system.

Ion propulsion is not a source of power for this interplanetary spaceship. Rather, the craft needs a great deal of power to operate its ion propulsion system and all other systems. It needs so much that…

We crave power!!

The ion propulsion system is power-hungry. The process of ionizing xenon and then accelerating it to high velocity consumes a significant amount of electrical power, all of which is provided by the spacecraft’s huge solar arrays. With these two wings and its ion tail, Dawn resembles a celestial dragonfly. But this extraterrestrial odonate is a giant, with a wingspan of 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest tip-to-tip length of any probe NASA had ever dispatched on an interplanetary voyage. (Some such spacecraft have had flexible wire-like antennas that reach to greater lengths.) The large area of solar cells is needed to capture feeble sunlight in the remote asteroid belt to meet all of the electrical needs. Each solar array wing is the width of a singles tennis court, and the entire structure would extend from a pitcher’s mound to home plate on a professional baseball field, although Dawn is engaged in activities considerably more inspiring and rewarding than competitive sports.

To sail the ship to its intended destination, navigators plot a complex course on the solar system sea. The thrust delivered by the ion engine depends on the power level; higher power translates into higher (but still ever so gentle) thrust. The farther Dawn is from the luminous sun, the less power is available, so the thrust is lower. Therefore, to keep it on its itinerary, mission planners need to know the thrust at all times in the future. It would not be a recipe for success to propel the spacecraft to a position in space from which it could not achieve enough thrust to accomplish the rest of the carefully designed journey to Ceres.

To formulate the flight plan then requires knowing how much power will be available even as the probe ventures farther from the sun. Engineers make mathematical predictions of the power the solar arrays will generate, but these calculations are surprisingly difficult. Well, perhaps some readers would not be surprised, but it is more complicated than simply reducing the power in proportion to the intensity of the sunlight. As one example, at greater distances from the sun, the temperature of the arrays in the cold depths of space would be even lower, and the efficiency of the solar cells depends on their temperature. In 2008, the operations team devised and implemented a method to refine their estimates of the solar array performance, and that work enabled the deep-space traveler to arrive at Vesta earlier and depart later. Now they have developed a related but superior technique, which the faithful spacecraft executed flawlessly on June 24.

The only way to measure the power generation capability of the arrays is to draw power from them. With the ion thrust off, even with all other systems turned on, the spacecraft cannot consume as much power as the arrays can provide, so no meaningful measurement would be possible.

In typical operations, Dawn keeps its solar arrays pointed directly at the sun. For this special calibration, it rotated them so the incident sunlight came at a different angle. This reduced the total amount of light falling on the cells, effectively creating the conditions the spacecraft will experience when it has receded from the sun. As the angle increased, corresponding to greater distances from the brilliant star, the arrays produced less power, so the ion engine had to be throttled down. (The engines can be operated at 112 different throttle levels, each with a different input power and different thrust level.)

Engineers estimated what the maximum throttle level would be at each of the angles as well as the total power all other systems would consume during the test and then programmed it so the ion propulsion system would throttle down appropriately as the solar array angle increased. Of course, they could not know exactly what the highest throttle level at each angle would be; if they did, then they would already know the solar array characteristics well enough that the calibration would be unnecessary. Fortunately, however, they did not need to determine the perfect levels in advance. The sophisticated robot is smart enough to reduce by a few throttle levels if it detects that all systems combined are drawing more power than the solar arrays generate.

Under normal circumstances, the spacecraft doesn’t need to adjust the ion throttle level on its own. Engineers know the solar array performance well enough that they can predict the correct setting with high accuracy for a typical four-week sequence of commands stored onboard. It is only for the much greater distances from the sun in the years ahead that the uncertainty becomes important. In addition, during regular operations, if the spacecraft temporarily needs to use more heaters than usual (more than 140 heaters are distributed around the ship, each turning on and off as needed), thereby increasing the power demand, its battery can make up for the difference. That avoids unnecessary throttle changes.

› Continue reading Marc Rayman’s Dawn Journal


For Dawn, a Little Push Goes a Long Way

Monday, July 1st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Dawnamic Readers,

The indefatigable Dawn spacecraft is continuing its extraordinary interplanetary flight on behalf of inquisitive creatures on distant Earth. Progressing ever farther from Vesta, the rocky and rugged world it so recently explored, the ship is making good progress toward its second port of call, dwarf planet Ceres.

We have seen in many logs that this adventure would be quite impossible without its advanced ion propulsion system. Even a mission only to orbit Vesta, which Dawn has accomplished with such stunning success, would have been unaffordable in NASA’s Discovery Program without ion propulsion. This is the only probe ever to orbit an object in the main asteroid belt between Mars and Jupiter. But now, thanks to this sophisticated technology, it is going beyond even that accomplishment to do something no other spacecraft has attempted. Dawn is the only mission ever targeted to orbit two extraterrestrial destinations, making it truly an interplanetary spaceship.

Ion Propulsion System Hot Fire Test for Deep Space 1
Ion Propulsion System Hot Fire Test for the Deep Space 1 spacecraft. Image credit: NASA/JPL-Caltech

Ion propulsion is 10 times more efficient than conventional chemical propulsion, so it enables much more ambitious missions. It uses its xenon propellant so parsimoniously, however, that the thrust is also exceptionally gentle. Indeed, the ion engine exerts about as much force on the spacecraft as you would feel if you held a single sheet of paper in your hand. At today’s thrust level, it would take more than five days to accelerate from zero to 60 mph. While that won’t rattle your bones, in the frictionless, zero-gravity conditions of spaceflight, the effect of the thrust gradually accumulates. Instead of thrusting for five days, Dawn thrusts for years. Ion propulsion delivers acceleration with patience, and patience is among this explorer’s many virtues.

To accomplish its mission, Dawn is outfitted with three ion engines. In the irreverent spirit with which this project has always been conducted, the units are fancifully known as #1, #2, and #3. (The locations of the thrusters were disclosed in a log shortly after launch, once the spacecraft was too far from Earth for the information to be exploited for tawdry sensationalism.) For comparison, the Star Wars TIE fighters were Twin Ion Engine ships, so now science fact does one better than science fiction. On the other hand, the TIE fighters employed a design that did seem to provide greater agility, perhaps at the expense of fuel efficiency. Your correspondent would concur that when you are trying to destroy your enemy while dodging blasts from his laser cannons, economy of propellant consumption probably isn’t the most important consideration.

At any rate, Dawn only uses one ion engine at a time. Since August 31, 2011, it has accomplished all of its thrusting with thruster #3. That thruster propelled Dawn along its complex spiral path down from an altitude of 2,700 kilometers (1,700 miles) to 210 kilometers (130 miles) above Vesta’s dramatic landscape and then back up again. Eventually, the engine pushed Dawn out of orbit, and it has continued to work to reshape the spacecraft’s heliocentric course so that it ultimately will match Ceres’s orbit around the sun.

Although any of the thrusters can accomplish the needed propulsion, and all three are still healthy, engineers consider many factors in deciding which to use at different times in the mission. Now they have decided to put #2 back to work. So on June 24, after its regular monthly hiatus in thrusting to point the main antenna to Earth for a communications session, the robotic explorer turned to aim that thruster, rather than thruster #3, in the direction needed to continue the journey to Ceres. Despite not being operated in nearly two years, #2 came to life as smoothly as ever. It is now emitting a blue-green beam of xenon ions as the craft has its sights set on the mysterious alien world ahead.

› Continue reading Marc Rayman’s Dawn Journal


Smooth Sailing: Dawn Spacecraft Passes Endurance Test

Monday, June 3rd, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Confidawnts,

Traveling from one alien world to another, Dawn is reliably powering its way through the main asteroid belt with its ion propulsion system. Vesta, the fascinating and complex protoplanet it explored in 2011 and 2012, falls farther and farther behind as the spacecraft gently and patiently reshapes its orbit around the sun, aiming for a 2015 rendezvous with dwarf planet Ceres.

The stalwart adventurer has recently completed its longest uninterrupted ion thrust period yet. As part of the campaign to conserve precious hydrazine propellant, Dawn now suspends thrusting once every four weeks to point its main antenna to Earth. (In contrast, spacecraft with conventional chemical propulsion spend the vast majority of time coasting.) Because of details of the mission operations schedule and the schedule for NASA’s Deep Space Network, the thrust durations can vary by a few days. As a result, the spacecraft spent 31.2 days thrusting without a hiatus. This exceeds Deep Space 1’s longest sustained powered flight of 29.2 days. While there currently are no plans to thrust for longer times, the unique craft certainly is capable of doing so. The principal limitation is how much data it can store on the performance of all subsystems (pressures, temperatures, currents, voltages, valve positions, etc.) for subsequent reporting to its terrestrial colleagues.

Thanks to the ship’s dependability, the operations team has been able to devote much of its energies recently to developing and refining the complex plans for the exploration of Ceres. You might be among the privileged readers who will get a preview when we begin describing the plans later this year.

Controllers also have devised some special activities for the spacecraft to perform in the near future, accounts of which are predicted to be in the next two logs.

In addition, team members have had time to maintain their skills for when the spacecraft needs more attention. Earlier this month, they conducted an operational readiness test (ORT). One diabolical engineer carefully configured the Dawn spacecraft simulator at JPL to behave as if a pebble one-half of a centimeter (one-fifth of an inch) in diameter shooting through the asteroid belt collided with the probe at well over twice the velocity of a high-performance rifle bullet.

When the explorer entered this region of space, we discussed that it was not as risky as residents of other parts of the solar system might assume. Dawn does not require Han Solo’s piloting skills to avoid most of the dangerous rocky debris.

The robot could tolerate such a wound, but it would require some help from operators to resume normal operations. This exercise presented the spacecraft team with an opportunity to spend several days working through the diagnosis and performing the steps necessary to continue the mission (using some of the ship’s backup systems). While the specific problem is extremely unlikely to occur, the ORT provided valuable training for new members of the project and served to keep others sharp.

One more benefit of the smooth operations is the time that it enables your correspondent to write his third shortest log ever. (Feel free to do the implied research.) Frequent readers can only hope he strives to achieve such a gratifying feat again!

Dawn is 13 million kilometers (7.9 million miles) from Vesta and 54 million kilometers (34 million miles) from Ceres. It is also 3.25 AU (486 million kilometers or 302 million miles) from Earth, or 1,275 times as far as the moon and 3.20 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 54 minutes to make the round trip.

› Read previous Dawn Journals by Marc Rayman


To Be in the Right Place, Dawn Catches Up With Time

Wednesday, May 1st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear Dawnscerning Readers,

Nearly three times as far from Earth as the sun is, the Dawn spacecraft is making very good progress on its ambitious trek from Vesta to Ceres. After a spectacular adventure at the second most massive resident of the main asteroid belt between Mars and Jupiter, Dawn used its extraordinary ion propulsion system to leave it behind and undertake the long journey to a dwarf planet.

Ceres orbits the sun outside Vesta’s orbit, yet Dawn is now closer to the sun than both of these alien worlds. How can it be that as the probe climbs from one to the other, it seems to be falling inward? Perhaps the answer lies in the text below; let’s venture on and find out!

On Halloween we discussed why Dawn is heading in toward the sun, but this question is different. Vesta also is getting closer to the sun, but what’s of interest now is that Dawn, despite its more remote destination, has been approaching the sun more quickly. That earlier log stands out as the best one ever written on this exciting mission in the entire history of October 2012, but if you prefer not to visit it now, we can summarize here the explanation for the spacecraft moving toward the sun. Like all members of the sun’s entourage, Vesta and Ceres follow elliptical orbits, their distances from the master of the solar system growing and shrinking as they loop around it. Even Earth’s orbit, although nearly round, certainly is not perfectly circular. Our planet is a little closer to the sun in the northern hemisphere winter (southern hemisphere summer) than it is in the summer (southern hemisphere winter). Dawn’s orbit is elliptical as well, so it naturally moves nearer to the sun sometimes, and now is such a time. But that does not address why it is currently closer to the sun than Vesta, even though it is seeking out the more distant Ceres.

Because it will orbit Ceres, and not simply fly past it (which would be significantly easier but less valuable), Dawn must make its own orbit around the sun be identical to its target’s. But that is not the entire story. After spending 14 months orbiting Vesta, Dawn’s challenge is more than to change the shape of its orbit to match Ceres’s. The spacecraft also must be at the same place in Ceres’s heliocentric orbit that Ceres itself is.

It would not be very rewarding to follow the same looping path around the sun but always be somewhere else on that path. You can visualize this if you have one of the many defective — er, exotic clocks from the Dawn gift shop on your planet that have two minute hands. If the clock starts with one hand pointed at 12 and another pointed at 1, they will take the same repetitive route, but neither hand will ever catch up with the other. For Dawn’s goal of exploring Ceres, this would not prove satisfying. Therefore, part of the objective of the ion thrusting is to ensure the spacecraft arrives not only on the same heliocentric course as Ceres but is there when Ceres is also.

This is a problem familiar to all readers who have maneuvered in orbit, where the principles of orbital mechanics are the rules of the road. To solve it, we rely on one of the laws that we have addressed many times in these logs: objects in a lower orbit travel faster. We described this in more detail in February, and we can recall the essential idea here. The gravitational attraction of any body, whether it is the sun, Earth, a black hole, or anything else, is greater at shorter ranges. So to balance that strong inward pull, an orbiter is compelled to race around quickly. At higher orbits, where gravity is weaker, a more leisurely orbital pace suffices.

We can take advantage of this characteristic of orbits. If we drop to a slightly lower orbit, we travel along more swiftly. That is precisely what Dawn needs to do in order to ensure that when it finishes expanding and tilting its orbit in 2015 so that it is the same as Ceres’s, it winds up at the same location as its target. This would be like speeding up the minute hand that had begun at the 12, allowing it to catch up with the hand that would otherwise always be leading it.

› Continue reading Marc Rayman’s Dawn Journal


While Dawn Keeps Cruising, Engineers Carry On

Friday, March 29th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Indawnstrious Readers,

In the depths of the main asteroid belt between Mars and Jupiter, far from Earth, far even from any human-made object, Dawn remains in silent pursuit of dwarf planet Ceres. It has been more than six months since it slipped gracefully away from the giant protoplanet Vesta. The spacecraft has spent 95 percent of the time since then gently thrusting with its ion propulsion system, using that blue-green beam of high velocity xenon ions to propel itself from one alien world to another.

The ship set sail from Earth more than two thousand days ago, and its voyage on the celestial seas has been wonderfully rewarding. Its extensive exploration of Vesta introduced humankind to a complex and fascinating place that had only been tantalizingly glimpsed from afar with telescopes beginning with its discovery 206 years ago today. Thanks to the extraordinary capability of ion propulsion, Dawn was able to spend 14 months orbiting Vesta, observing dramatic landscapes and exotic features and collecting a wealth of measurements that scientists will continue to analyze for many years.

When it was operating close to Vesta, the spacecraft was in frequent contact with Earth. It took Dawn quite a bit of time to beam the 31,000 photos and other precious data to mission control. In addition, engineers needed to send a great many instructions to the distant adventurer to ensure it remained healthy and productive in carrying out its demanding work in the unforgiving depths of space.

Dawn is now more than 20 times farther from Vesta than the moon is from Earth. Alone again and on its long trek to Ceres, it is not necessary for the ship to be in radio contact as often. As we saw in November, the spacecraft now stops ion thrusting only once every four weeks to point its main antenna to Earth. This schedule conserves the invaluable hydrazine propellant the explorer will need at Ceres. But communicating less frequently does not mean the mission operations team is any less busy. Indeed, as we have explained before, “quiet cruise” consists of a considerable amount of activity.

Each time Dawn communicates with Earth, controllers transmit a second-by-second schedule for the subsequent four weeks. They also load a detailed flight profile with the ion throttle levels and directions for that period. It takes about three weeks to calculate and formulate these plans and to analyze, check, double check, and triple check them to ensure they are flawless before they can be radioed to Dawn.

In addition to all the usual information Dawn needs to keep flying smoothly, operators occasionally include some special instructions. As one example, over the last few months, they have gradually lowered the temperatures of some components slightly in order to reduce heater power. When Dawn stretched out its solar array wings shortly after separating from the Delta rocket on September 27, 2007, its nearly 65-foot wingspan was the longest of any NASA interplanetary probe. The large area of solar cells is needed to collect enough light from the distant sun to power the ion propulsion system and all other spacecraft systems. Devoting a little less power to heaters allows more power to be applied to ionizing and accelerating xenon, yielding greater thrust. With two and a half years of powered flight required to travel from Vesta to Ceres, even a little extra power can make a worthwhile difference to a mission that craves power.

Most temperature adjustments are only two degrees Celsius (3.8 degrees Fahrenheit) at a time, but even that requires careful analysis and investigation, because lowering the temperature of one component may affect another. Xenon and hydrazine propellants need to be maintained in certain ranges, and the lines they flow through follow complicated paths around the spacecraft, so the temperatures all along the way matter. Most of the hardware onboard, from valves and switches to electronics to structural mounts for sensitively aligned units, needs to be thermally regulated to keep Dawn shipshape.

It can take hours for a component to cool down and stabilize at a new setting, and sometimes the change won’t even occur until the spacecraft has turned away to resume thrusting, when the faint warmth of the sun and the deep cold of black space affect different parts of the complex robot. Then it will be another four weeks until engineers will receive a comprehensive report on all the temperatures, so they need to be cautious with each change.

› Continue reading Marc Rayman’s Dawn Journal


A Hard Day’s Flight: Dawn Achieves Orbital Velocity

Friday, March 1st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Impordawnt Readers,

The indefatigable Dawn spacecraft is continuing to forge through the main asteroid belt, gently thrusting with its ion propulsion system. As it gradually changes its orbit around the sun, the distance to dwarf planet Ceres slowly shrinks. The pertinacious probe will arrive there in 2015 to explore the largest body between the sun and Neptune that has not yet been glimpsed by a visitor from Earth. Meanwhile, Vesta, the fascinating alien world Dawn revealed in 2011 and 2012, grows ever more distant. The mini-planet it orbited and studied in such detail now appears only as a pinpoint of light 15 times farther from Dawn than the moon is from Earth.

Climbing through the solar system atop a column of blue-green xenon ions, Dawn has a great deal of powered flight ahead in order to match orbits with faraway Ceres. Nevertheless, it has shown quite admirably that it is up to the task. The craft has spent more time thrusting and has changed its orbit under its own power more than any other ship from Earth. While most of the next two years will be devoted to still more thrusting, the ambitious adventurer has already accomplished much more than it has left to do. And now it is passing an interesting milestone on its interplanetary trek.

With all of the thrusting Dawn has completed, it has now changed its speed by 7.74 kilometers per second (17,300 mph), and the value grows as the ion thrusting continues. For space enthusiasts from Earth, that is a special speed, known as “orbital velocity.” Many satellites, including the International Space Station, travel at about that velocity in their orbits. So does this mean that Dawn has only now achieved the velocity necessary to orbit Earth? The short answer is no. The longer answer constitutes the remainder of this log.

We have discussed some of these principles before, but they are counterintuitive and questions continue to arise. Rather than send our readers on a trajectory through the history of these logs even more complicated than Dawn’s flight through the asteroid belt, we will revisit a few of the ideas here. (After substantial introspection, your correspondent granted and was granted permission to reuse not only past text but also future text.)

While marking Dawn’s progress in terms of its speed is a convenient description of the effectiveness of its maneuvering, it is not truly a measure of how fast it is moving. Rather, it is a measure of how fast it would be moving under very special (and unrealistic) circumstances. To understand this, we need to look at the nature of orbits in general and Dawn’s interplanetary trajectory in particular.

The overwhelming majority of craft humans have sent into space have remained in the vicinity of Earth, accompanying that planet on its annual revolutions around the sun. All satellites of Earth (including the moon) remain bound to it by its gravity. (Similarly, Dawn spent much of 2011 and 2012 as a satellite of distant Vesta, locked in the massive body’s gravitational grip.) As fast as satellites seem to travel compared to terrestrial residents, from the larger solar system perspective, their incessant circling of Earth means their paths through space are not very different from Earth’s itself. Consider the path of a car racing around a long track. If a fly buzzes around inside the car, to the driver it may seem to be moving fast, but if someone watching the car from a distance plotted the fly’s path, on average it would be pretty much like the car’s.

Everything on the planet and orbiting it travels around the sun at an average of 30 kilometers per second (67,000 mph), completing one full solar orbit every year. To undertake its interplanetary journey and travel elsewhere in the solar system, Dawn needed to break free of Earth’s grasp, and that was accomplished by the rocket that carried it to space more than five years ago. Dawn and its erstwhile home went their separate ways, and the sun became the natural reference for the spacecraft’s position and speed on its voyage in deep space.

Despite the enormous push the Delta II rocket delivered (with affection!) to Dawn, the spacecraft still did not have nearly enough energy to escape from the powerful sun. So, being a responsible resident of the solar system, Dawn has remained faithfully in orbit around the sun, just as Earth and the rest of the planets, asteroids, comets, and other members of the star’s entourage have.

Whether it is for a spacecraft or moon orbiting a planet, a planet or Dawn orbiting the sun, the sun orbiting the Milky Way galaxy, or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to a sizeable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight path. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its path; if you let go of the string, the weight’s natural motion would carry it away in a straight line (ignoring the effect of Earth’s gravity).

The force of gravity diminishes with distance, so the sun’s pull on a nearby body is greater than on a more distant one. Therefore, to remain in orbit, to balance the relentless tug of gravity, the closer object must travel faster, fighting the stronger pull. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, around 400 kilometers, or 250 miles, from the surface) must streak around the planet at about 7.7 kilometers per second (17,000 mph) to keep from being pulled down. The moon, orbiting almost 1000 times farther above, needs only to travel at about 1.0 kilometers per second (less than 2300 mph) to balance Earth’s weaker hold at that distance.

Notice that this means that for an astronaut to travel from the surface of Earth to the International Space Station, it would be necessary to accelerate to quite a high speed to rendezvous with the orbital facility. But then once in orbit, to journey to the much more remote moon, the astronaut’s speed eventually would have to decline dramatically. Perhaps speed tells an incomplete story in describing the travels of a spacecraft, just as it does with another example of countering gravity.

A person throwing a ball is not that different from a rocket launching a satellite (although the former is usually somewhat less expensive and often involves fewer toxic chemicals). Both represent struggles against Earth’s gravitational pull. To throw a ball higher, you have to give it a harder push, imparting more energy to make it climb away from Earth, but as soon as it leaves your hand, it begins slowing. For a harder (faster) throw, it will take longer for Earth’s gravity to stop the ball and bring it back, so it will travel higher. But from the moment it leaves your hand until it reaches the top of its arc, its speed constantly dwindles as it gradually yields to Earth’s tug. The astronaut’s trip from the space station to the moon would be accomplished by starting with a high speed “throw” from the low starting orbit, and then slowing down until reaching the moon.

The rocket that launched Dawn threw it hard enough to escape from Earth, sending it well beyond the International Space Station and even the moon. Dawn’s maximum speed relative to Earth on launch day was so high that Earth could not pull it back. As we saw in the explanation of the launch profile, Dawn was propelled to 11.46 kilometers per second (25,640 mph), well in excess of the space station’s orbital speed given three paragraphs above. But it has remained under the sun’s control.

Now we can think of the general problem of flying elsewhere in space as similar to climbing a hill. For terrestrial hikers, the rewards of ascent come only after doing the work of pushing against Earth’s gravity to reach a higher elevation. Similarly, Dawn is climbing a solar system hill with the sun at the bottom. It started part way up the hill at Earth; and its first rewards were found at a higher elevation, where Vesta, traveling around the sun at only about two thirds of Earth’s speed, revealed its fascinating secrets to the visiting ship. The ion thrusting now is propelling it still higher up the hill toward Ceres, which moves even more slowly to balance the still-weaker pull of the sun.

› Continue reading Marc Rayman’s Dawn Journal for more on how Dawn achieved orbital velocity