Blogs by Marc Rayman

Blogs by Marc Rayman

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.

Artist's concept of the Dawn spacecraft at Ceres

Dear Indawnspensable Readers,

Dawn is making good progress on the second segment of its cosmic travels. Following more than a year of arduous but sensationally productive and exciting work revealing the fascinating character of the giant protoplanet Vesta, it is now patiently pursuing its next target, the mysterious dwarf planet Ceres, which resides farther from the sun. For the second (and final) time in its interplanetary journey, however, Dawn is about to turn around, going closer to the sun rather than farther away.

In August 2008, we saw in detail how it could be that even as the bold explorer travels outward in the solar system from Earth, past Mars, to Vesta, and then on to Ceres, it could occasionally appear to reverse course temporarily. We present here a shorter explanation for those readers who did not memorize the log explaining this perplexing behavior (you know who you are, and we do as well, but your secret remains safe under the terms of our reader privacy agreement).

Dawn orbits the sun, as do Vesta, Ceres, the other residents of the main asteroid belt, and the planets. All orbits, whether of these objects around the star at the center of our solar system, artificial satellites or the moon in orbit around Earth, or even Dawn when it was in orbit around Vesta, are ellipses (like flattened circles). Earth, for example, orbits the sun at an average distance of 150 million kilometers (93.0 million miles), which astronomers call one astronomical unit (AU). During its year-long revolution, however, our planet comes in to 0.98 AU from the sun and goes out to 1.02 AU. Earthlings manage quite nicely with these small variations. (Note that the seasons are not caused by the changes in distance but instead are a result of the tilt of Earth's axis and thus the differing angles at which the warming rays of the sun arrive during the year. If the sun's distance were all that mattered, the northern and southern hemispheres would have the same seasons.) So, orbiting bodies move smoothly between a minimum and a maximum range from their gravitational masters rather than remaining at a constant distance.

When Dawn was in orbit around Vesta, it accompanied that world on its regular journey around the sun. The table last month showing the probe's progress over the five years of its deep space trek reminds us that Vesta's path brings it as close to the sun as 2.15 AU and takes it out to 2.57 AU.

If Dawn had remained in orbit around Vesta, it would have continued to follow the same elliptical course as its host in the asteroid belt. The pair would have reached their maximum solar distance next month and then would have fallen back to 2.15 AU in September 2014. While visiting Vesta was extremely gratifying, this explorer's ambitions are greater. It broke free of Vesta's grip, its sights set on a new and distant alien destination.

Now the spacecraft is in its own independent orbit around the sun, and the persistent but gentle pressure of its advanced ion propulsion system gradually reshapes that orbit. At any moment, the orbit is an ellipse, and an instant later, it is a slightly different ellipse, courtesy of the thrust. As Dawn departed from Vesta only last month, its orbit is not yet dramatically different, but over the course of the coming years, the effect of the thrusting will be to change the orbit tremendously. To reach Ceres in 2015, the ship will enlarge and tip its elliptical course to match the motion of the dwarf planet around the sun. (Some of the parameters characterizing each object's orbit are shown here.)

Although the ship's orbit is growing, it will reach the current high point on Nov. 1. It will then be 2.57 AU from the sun and, just as in 2008 (albeit at a smaller distance), it will begin moving closer, even as it continues to thrust.

If Dawn stopped thrusting on Nov. 1, its elliptical orbit would carry it down to 2.19 AU from the sun in September 2014. That's a higher orbit than Vesta's but still well below what it needs to be for the rendezvous with Ceres. Astute readers have already anticipated that the plan is not to stop thrusting but to continue reworking the trajectory, just as a ceramicist gradually achieves a desired shape to create the envisioned artistic result. The ongoing thrusting will raise the low point of the orbit, so if the ship follows the flight plan, it will descend only to 2.45 AU in October 2013 before sailing outward again. By May 2014 it will have risen to the same solar altitude as it is now. All the thrusting in the interim will have altered its course so much, however, that it will not turn around then; rather, it will continue ascending to keep its 2015 appointment with Ceres.

If not for its ion propulsion system, this extraordinary interplanetary expedition would be impossible. Conventional chemical propulsion does not have the requisite capability. The key to taking advantage of the unique performance of ion propulsion is patience, which Dawn demonstrates exemplarily. Whereas the vast majority of spacecraft spend almost all of their time coasting, Dawn devotes the preponderance of its time to powered flight, emitting a tenuous but very high velocity beam of blue-green xenon ions to propel itself. On Nov. 2, coincidentally around the same time it begins approaching the sun, the indefatigable robot will exceed three years of total thrust. By then, the gentle but persistent flow of ions will have imparted the equivalent of 7.28 kilometers per second (16,300 miles per hour) to the spacecraft. (As we have seen in many previous logs, such as here, this is not actually a measure of Dawn's speed. It is a convenient description of the effectiveness of a propulsion system that avoids the complications of orbital mechanics. The effect of the propulsion is not to increase velocity but rather to climb the solar system hill, just as pressing a car's accelerator while on a steep slope may not gain speed compared to driving on a level road, but it will gain elevation.)

As the ambitious adventurer forges through the asteroid belt, pushing tirelessly against the sun's incessant pull, the temporary reduction in heliocentric distance is just one element of the complex plan for a grand adventure to explore two of the last uncharted worlds in the inner solar system. Next month, we will see some of the changes the mission control team is making in how Dawn operates in order to ensure its work at Ceres is as richly productive as it was at Vesta.

Dawn is 708 thousand kilometers (440 thousand miles) from Vesta and 59 million kilometers (37 million miles) from Ceres. It is also 1.78 AU (267 million kilometers or 166 million miles) from Earth, or 660 times as far as the moon and 1.80 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 30 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 a.m. PDT October 31, 2012


  • Marc Rayman

Artist's concept of the Dawn spacecraft

Dear Dawnniversaries,

On the fifth anniversary of the beginning of its ambitious interplanetary adventure, Dawn can look back with great satisfaction on its spectacular exploration of the giant protoplanet Vesta and forward with great eagerness to reaching dwarf planet Ceres. Today Earth's robotic ambassador to the main asteroid belt is in quiet cruise, gradually reshaping its orbit around the sun so it can keep its appointment in 2015 with the mysterious alien world that lies ahead.

This anniversary resembles the first three more than the fourth. Its first years in space were devoted to spiraling away from the sun, ascending the solar system hill so it could gracefully slip into orbit around Vesta in time for its fourth anniversary. One year ago, Dawn was in the behemoth's gravitational grip and preparing to map its surface in stereo and make other measurements. The subsequent year yielded stunning treasures as Dawn unveiled the wondrous secrets of a world that had only been glimpsed from afar for over two centuries. While at Vesta, it spiraled around the massive orb to position itself for the best possible perspectives. Its final spiral culminated in its departure from Vesta earlier this month. Now for its fifth anniversary, it is spiraling around the sun again, climbing beyond Vesta so that it can reach 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 fifth 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, and fourth anniversaries.

In its five years of interplanetary travels, the spacecraft has thrust for a total of 1060 days, or 58 percent of the time (and about 0.000000021 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 267 kilograms (587 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The fraction of time the ship has spent in powered flight is lower than last year (when it was 68 percent), because Dawn devoted relatively little of the past year to thrusting. Although it did change orbits extensively at Vesta, most of the time it was focused on exactly what it was designed and built to do: scrutinize the ancient world for clues about the dawn of the solar system.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 7.14 kilometers per second (16,000 miles 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. Having accomplished slightly more than half 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 five revolutions around the sun, covering about 31.4 AU (4.70 billion kilometers or 2.92 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 23.4 AU (3.50 billion kilometers or 2.18 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. Since Dawn's launch, Vesta has traveled only 20.4 AU (3.05 billion kilometers or 1.90 billion miles) and the even more sedate Ceres has gone 18.9 AU (2.82 billion kilometers or 1.75 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.

Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family follow their 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 the journey is changing the inclination of its orbit, an energetically expensive task.)

Now we can see how Dawn has been doing 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 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 September 27, 2007, its orbit around the sun was exactly Earth's orbit. After launch, it was in its own solar orbit.

Minimum distance from the Sun (AU) Maximum distance from the Sun (AU) Inclination
Earth's orbit 0.98 1.02 0.0°
Dawn's orbit on Sept. 27, 2007 (before launch) 0.98 1.02 0.0°
Dawn's orbit on Sept. 27, 2007 (after launch) 1.00 1.62 0.6°
Dawn's orbit on Sept. 27, 2008 1.21 1.68 1.4°
Dawn's orbit on Sept. 27, 2009 1.42 1.87 6.2°
Dawn's orbit on Sept. 27, 2010 1.89 2.13 6.8°
Dawn's orbit on Sept. 27, 2011 2.15 2.57 7.1°
Vesta's orbit 2.15 2.57 7.1°
Dawn's orbit on Sept. 27, 2012 2.17 2.57 7.3°
Ceres's orbit 2.56 2.98 10.6°


For readers who are not overwhelmed by the number of numbers, the table may help to demonstrate how Dawn has patiently transformed its orbit during the course of its mission. Note that last year, 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 get into orbit around Vesta. While simply flying by Vesta would have been far easier, matching orbits with it required the unique capability of the ion propulsion system. Without it, NASA's Discovery Program would not have been able to afford a mission to explore this fascinating world, and a mission to both Vesta and Ceres would have been impossible.

Although the probe left Vesta only three weeks ago, the effect of the ion thrusting is already evident. Dawn is no longer in the same orbit as Vesta. It is propelling itself along a different path, forging its own course through the asteroid belt. The journey will be long, and the exploration of Ceres will not commence until well after Dawn's seventh anniversary of venturing into space. Many exciting discoveries and many daunting challenges lie ahead, and some of them have yet even to be recognized. But this stalwart ship (supported by its crew on distant Earth) has proven itself capable of accomplishing remarkable feats in its quest to expand frontiers and reap the great rewards of new knowledge and exciting new perspectives on the solar system for the bold creatures whose passions and insightful creativity fuel its extraordinary cosmic adventure.

Dawn is 160 thousand kilometers (99 thousand miles) from Vesta and 62 million kilometers (38 million miles) from Ceres. It is also 2.18 AU (325 million kilometers or 202 million miles) from Earth, or 840 times as far as the moon and 2.17 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 36 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2012


  • Marc Rayman

The dwarf planet Ceres as imaged by the Keck Observatory

Dear Marvestalous Readers,

An interplanetary spaceship left Earth in 2007. Propelling itself gently and patiently through the solar system with a blue-green beam of xenon ions, it gradually spiraled away from the sun. It sailed past Mars in 2009, its sights set on more distant and exotic destinations. In July 2011, it gracefully and elegantly entered orbit around the second most massive resident of the main asteroid belt, Vesta. It spent more than 13 months there scrutinizing the gigantic protoplanet with all of its sensors and maneuvering to different orbits to optimize its investigations, making myriad marvelous discoveries. After they traveled together around the sun for 685 million kilometers (426 million miles), the ship left orbit in September 2012 and is now headed for dwarf planet Ceres, the largest body between the sun and Neptune not yet visited by a spacecraft. No other probe has ever been capable of the amazing feats Dawn is performing, exploring two of the largest uncharted worlds in the inner solar system.

The population of the main asteroid belt numbers in the millions. Vesta is such a behemoth that Dawn has now single-handedly examined about eight percent of the mass of the entire belt. And by the time it finishes at the colossus Ceres, it will have investigated around 40 percent.

The expedition to Vesta has produced riches beyond everyone's hopes. With 31,000 photos, 20 million visible and infrared spectra, and thousands of hours of neutron spectra, gamma ray spectra, and gravity measurements, Dawn has revealed to humankind a unique and fascinating member of the solar system family. More akin to Earth and the other terrestrial planets than to typical asteroids, Vesta is not just another chunk of rock. It displays complex geology and even has a dense iron-nickel core, a mantle, and a crust. Its heavily cratered northern hemisphere tells the story of more than 4.5 billion years of battering in the rough and tumble asteroid belt. Its southern hemisphere was wiped clean, resurfaced by an enormous impact at least two billion years ago and an even greater collision one billion years ago. These events excavated the 400-kilometer (250-mile) Veneneia and 500-kilometer (310-mile) Rheasilvia basins. The larger basin has a mountain at the center that towers more than twice the height of Mt. Everest; indeed, it soars higher than all but one of the mountains known in the solar system. The impacts were so forceful, they nearly destroyed Vesta. The fierce shock reverberated through the entire body and left as scars an extraordinary network of vast troughs near the equator, some hundreds of kilometers (miles) long and 15 kilometers (10 miles) wide.

The powerful impacts liberated tremendous amounts of material, flinging rocks far out into space, some of which eventually made it all the way to Earth. It is astonishing that more than one thousand meteorites found here came from Vesta. We have some meteorites from Mars, and we have some meteorites from the moon, but we have far, far more that originated in those impacts at Vesta, so distant in time and space. Vesta, Mars, and the moon are the only celestial bodies identified as the source of specific meteorites.

Scientists will spend years productively poring through Dawn's fabulous findings and learning what secrets they hold about the dawn of the solar system, and many more people will continue to marvel at the spectacular sights of this alien world. But the emissary from Earth has completed its assignment there and moved on. It has spent most of its time since the previous log using its ion propulsion system to climb higher and higher above Vesta. This departure spiral is the mirror image of the approach spiral the robotic adventurer followed last year. The unique method of entering and leaving orbit is one of the many intriguing characteristics of a mission that uses ion propulsion. Without that advanced technology, this ambitious deep space adventure would be impossible.

As Dawn ascended, Vesta's gravitational grip grew weaker and weaker. At some point along its spiral, the explorer was far enough and moving fast enough that Vesta could no longer hold it in orbit. As smoothly and tenderly as Vesta had taken Dawn in its embrace last year, it released its erstwhile companion, each to go its own way around the sun. The bond was severed at about 11:26 p.m. PDT yesterday, when they were 17,200 kilometers (10,700 miles) apart, separating at the remarkably leisurely speed of less than 33 meters per second (73 miles per hour). Many of our readers drove their cars that fast today (although we hope it was not in school zones).

Unlike missions that use conventional chemical propulsion, there was no sudden change on the spacecraft and no nail-biting on Earth. If you had been in space watching the action, you probably would have been hungry, cold, and hypoxic, but you would not have noticed anything unusual about the scene. Apart from a possible hint of self-satisfaction, Dawn would have looked just as it had for most of its interplanetary flight, a monument to humankind's ingenuity and passionate drive to know the cosmos perched atop a blue-green pillar of xenon ions. If, instead, you had been in Dawn mission control watching the action, you would have been in the dark and all alone (until JPL Security arrived). There was no need to have radio contact with the reliable spaceship. It had already thrust for almost 2.9 years, or 58 percent of its time in space. Thrusting during escape was no different. No one was tense or anxious; rather, all the drama is in the spectacular results of the bold mission at Vesta and the promise of what is to come at Ceres. When Dawn entered orbit, your correspondent was dancing. When Dawn left orbit, he was sleeping serenely.

A month earlier, on August 8, with the craft more than 2,100 kilometers (1,300 miles) above the surface, patiently powering its way up through Vesta's gravity field, one of the reaction wheels experienced an increase in internal friction. Reaction wheels are used to control a spacecraft's orientation in the frictionless, zero-gravity conditions of spaceflight. By electrically changing a wheel's spin rate, Dawn can rotate or stabilize itself. Protective software quickly detected the event and correctly responded by deactivating that wheel and the other two that were operating, switching to the small jets that are available for the same function, and reconfiguring other systems, including powering off the ion thrust and turning to point the main antenna to Earth.

A routine communications session the next day revealed to mission controllers what had occurred. They had planned long ago to turn the wheels off for the flight from Vesta to Ceres, so having them off a few weeks early was not a significant change. The team soon restored the spacecraft to normal operations and reformulated the departure plan, and on August 17 Dawn resumed its ascent. Because of the hiatus in thrusting, escape shifted from August 26 to September 4. The flexibility in the mission timeline provided by ion propulsion made this delay easy to accommodate.

In order to conserve the hydrazine propellant that the jets use, the bonus departure observations described before were curtailed, as they were not a high priority for the mission. Nevertheless, on August 25 and 26, at an altitude of around 6,000 kilometers (3,700 miles), the explorer did peer at Vesta once more with its camera and visible and infrared mapping spectrometer. The last time it had been this far away was July 21, 2011, during its descent to an unfamiliar destination. This time, 13 months later, the spacecraft turned back for a final gaze at the magnificent world it had unveiled during its remarkable time there, a world that prior to last year had appeared as little more than a tiny smudge among the stars for the two centuries it had been observed.

The delay in the departure schedule provided a convenient benefit. Vesta has seasons, just as Earth does, although they progress more slowly on that distant orb. August 20 was the equinox, when northern hemisphere spring began. Until then, the sun had been in Vesta's southern hemisphere throughout Dawn's residence there. While most of the northern hemisphere was revealed during the second high-altitude mapping orbit, the illumination of the landscape immediately around the north pole was even better for this last look. After radioing its parting shots to wistful mission controllers, the ship commenced its climb again.

And then, with an stunningly successful mission behind it, a newly explored world below it, and a mysterious dwarf planet ahead of it, the indomitable and indefatigable adventurer left Vesta forever.

Dawn is 18,500 kilometers (11,500 miles) from Vesta and 64 million kilometers (40 million miles) from Ceres. It is also 2.45 AU (367 million kilometers or 228 million miles) from Earth, or 910 times as far as the moon and 2.43 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 a.m. PDT September 5, 2012


  • Marc Rayman

near-true color image of the remarkable snowman feature on asteroid Vesta's surface

Dear Dawnpartures,

Dawn has completed the final intensive phase of its extraordinary exploration of Vesta, and it has now begun its gradual departure. Propelled by its uniquely efficient ion propulsion system, the probe is spiraling ever higher, reversing the winding path it followed into orbit last year.

In the previous log (which gained prominence last month by making it into the list of the top 78 logs ever written on this ambitious interplanetary adventure), we saw the plan for mapping Vesta from an altitude of 680 kilometers (420 miles). In this second high-altitude mapping orbit (HAMO2), the spacecraft circled the alien world beneath it every 12.3 hours. On the half of each orbit that it was on the day side, it photographed the dramatic scenery. As it passed over the night side, it beamed the precious pictures to the distant planet where its human controllers (and many of our readers) reside. Tirelessly repeating this strategy while Vesta rotated allowed Dawn's camera to observe the entirety of the illuminated land every five days.

The robot carried out its complex itinerary flawlessly, completely mapping the surface six times. Four of the maps were made not by pointing the camera straight down at the rocky, battered ground but rather at an angle. Combining the different perspectives of each map, scientists have a rich set of stereo images, allowing a full three dimensional view of the terrain that bears the scars of more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.

Dawn also mapped Vesta six times during the first high-altitude mapping orbit (HAMO1) in September and October 2011. The reason for mapping it again is that Vesta has seasons, and they progress more slowly than on Earth. Now it is almost northern hemisphere spring, so sunlight is finally reaching the high latitudes, which were under an impenetrable cloak of darkness throughout most of Dawn's residence here.

For most of the two centuries this mysterious orb had been studied from Earth, it was perceived as little more than a small fuzzy blob in the night sky. With the extensive imaging from HAMO1 and HAMO2, as well as from the low-altitude mapping orbit (LAMO, earthlings now know virtually all of the protoplanet's landscape in exquisite detail.

Among the prizes for the outstanding performance in HAMO2 are more than 4,700 pictures. In addition to the comprehensive mapping, Dawn collected nearly nine million spectra with its visible and infrared mapping spectrometer (VIR) to help scientists determine more about the nature of the minerals. This phenomenal yield is well over twice that of HAMO1, illustrating the great benefit of dedicating valuable observation time in HAMO2 to VIR before the mapping.

Dawn's measurements of the peaks and valleys, twists and turns of Vesta's gravity field, from which scientists can map the distribution of material in the interior of the behemoth, were at their best in LAMO. That low altitude also was where the gamma ray and neutron detector (GRaND) obtained its finest data, revealing the atomic constituents of the surface and subsurface. Indeed, the motivation for undertaking the challenging descent to LAMO was for those investigations, although the bonus pictures and spectra greatly enhanced the reward. Even in HAMO2, however, gravity and GRaND studies continued, adding to an already fabulous bounty.

Mission controllers have continued to keep the distant spacecraft very busy, making the most of its limited time at Vesta. Pausing neither to rest nor to marvel or delight in its own spectacular accomplishments, when the robot finished radioing the last of its HAMO2 data to Earth, it promptly devoted its attention to the next task: ion thrusting.

Missions that use conventional propulsion coast almost all of the time, but long-time readers know that Dawn has spent most of its nearly five years in deep space thrusting with its advanced ion propulsion system, the exotic and impressive technology it inherited from NASA's Deep Space 1. Without ion propulsion, the exploration already accomplished would have been unaffordable for NASA's Discovery Program and the unique exploit to orbit both Vesta and dwarf planet Ceres would have been quite impossible. Ion propulsion not only enables the spacecraft to orbit residents of the main asteroid belt, something no other probe has attempted, but it also allows the interplanetary spaceship to maneuver extensively while at each destination, thus tailoring the orbits for the different investigations.

On July 25 at 9:45 a.m. PDT, as it has well over 500 times before, the sophisticated craft began emitting a beam of high-velocity xenon ions. In powered flight once again, it is now raising its orbital altitude. On August 26, the ship will be too far and traveling too fast for Vesta's gravity to maintain its hold. Dawn will slip back into orbit around the sun with its sights set on Ceres.

Although HAMO2 is complete, the spacecraft will suspend thrusting four times to direct its instruments at Vesta during the departure phase, much as it did in the approach phase. The approach pictures aided in navigation and provided tantalizing views of the quarry we had been seeking for so long. This time, however, we will see a familiar world receding rather than an unfamiliar one approaching. But as the sun creeps north, advancing by about three quarters of a degree of latitude per week, the changing illumination around the north pole will continue to expose new features.

On August 15, the craft will interrupt its ascent for four and a half days. By then, Dawn will be at an altitude of about 5,000 kilometers (3,100 miles), but it will still be in orbit. Before it resumes thrusting, it will coast to as high as 6,400 kilometers (4,000 miles) and then descend again. Meanwhile, four times during this period it will photograph the giant asteroid throughout a full Vestan day of 5 hours, 20 minutes. This is a familiar activity for the spacecraft, as it watched Vesta rotate beneath it from a similar vantage point during its spiral descent in July 2011. With Vesta's weak gravitational grip at this distance, Dawn would take more than a week to complete one revolution, so it will be almost as if the probe hovers in place as Vesta pirouettes before its camera. The itinerary is planned so the explorer will begin its observations while flying over the highest northern latitudes, and subsequently it will take the opportunity to observe lower latitudes as it sails down to the equator. The ship will circle so slowly that there will be time between acquiring each set of rotation images to point its main antenna to Earth to transmit its findings. After the third session, while waiting for the orbit to carry it to the latitude needed for the final one, mission planners are squeezing in a routine calibration of the camera and VIR. Dawn will turn to aim them at Jupiter. It is much too far away to reveal any new or interesting details, because the sensors are designed for mapping from close orbit. The planet will appear to be little more than a speck. (Terrestrial observers can gain a better view with binoculars.) But Jupiter is bright and easily seen from there, and it is so well studied that it is a useful reference source to verify that the instruments are still performing in top condition as they continue their discoveries at Vesta.

On August 22, nearly 6,000 kilometers (3,700 miles) over the night side, the probe will halt thrusting again. With the sun on the other side of the protoplanet, Dawn will see only a thin glowing crescent against the deep blackness of space, like a new moon. This is a perspective we have not yet had for Vesta, and although not much of the terrain will be visible, a few pictures to measure the strength of the sunlight's reflection at this extreme angle will be useful for understanding certain properties of the surface material. As a bonus, the view may prove to be quite aesthetically appealing.

Dawn will be patiently and gently thrusting at the moment of escape from Vesta on August 26 and will not even notice a change. It will be as serene and uneventful for the spacecraft (and operations team) as the moment of capture was. Shortly after, when it is around 17,000 kilometers (over 10,000 miles) away, it will watch Vesta rotate once again. On September 1, at a distance of 38,000 kilometers (almost 24,000 miles), it will gaze upon Vesta for the last time. By then, the world it has scrutinized for more than a year will be shrinking rapidly and few details will be visible. Although scientists will spend many years delving into the data the probe has returned, learning more and more not only about Vesta but also what it reveals about the dawn of the solar system, Dawn will leave it behind as it journeys deeper into the main asteroid belt in search of another uncharted world to explore.

Dawn is 740 kilometers (460 miles) from Vesta. It is also 2.94 AU (439 million kilometers or 273 million miles) from Earth, or 1,185 times as far as the moon and 2.89 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
10:00 p.m. PDT July 25, 2012


  • Marc Rayman

Image of the giant asteroid Vesta taken by NASA's Dawn spacecraft

Dear Upside Dawn Readers,

Dawn is now seeing Vesta in a new light. Once again the probe is diligently mapping the ancient protoplanet it has been orbiting for nearly a year. Circling the alien world about twice a day, the ardent adventurer is observing the signatures of Vesta's tortured history, including the scars accumulated during more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.

Having successfully completed its orbital raising maneuvers to ascend to its second high-altitude mapping orbit (HAMO2), Dawn looks down from about 680 kilometers (420 miles). This is the same height from which it mapped Vesta at the end of September and October 2011. The lifeless rocky landscape has not changed since then, but its appearance to the spacecraft's sensors has. The first high-altitude mapping orbit (HAMO1) was conducted shortly after southern hemisphere summer began on Vesta, so the sun was well south of the equator. That left the high northern latitudes in the deep darkness of winter night. With its slower progression around the sun than Earth, seasons on Vesta last correspondingly longer. Thanks to Dawn's capability to linger in orbit, rather than simply conduct a brief reconnaissance as it speeds by on its way to its next destination, the probe now can examine the surface with different lighting.

Much of the terrain that was hidden from the sun, and thus the camera, during HAMO1 is now illuminated. Even the scenery that was visible then is lit from a different angle now, so new observations will reveal many new details. In addition to the seasonal northward shift in the position of the sun, Dawn's orbit is oriented differently in HAMO2, as described last month, so that makes the opportunity for new insights and discoveries even greater.

The strategy for mapping Vesta is the same in HAMO2 now as it was in HAMO1. Dawn's orbital path takes it nearly over the north pole. (As we saw last month, the orbit does not go exactly over the poles but rather reaches to 86 degrees latitude. That slight difference is not important for this discussion.) During the ship's southward passage over the sunlit side, the camera and the visible and infrared mapping spectrometer (VIR) acquire their precious data. After passing (almost) above the south pole, Dawn sails north over the night side. Instead of pointing its sensors at the deep black of the ground below, the probe aims its main antenna to the extremely distant Earth and radios its findings to the exquisitely sensitive receivers of the Deep Space Network. The pattern repeats as the indefatigable spacecraft completes loop after loop after loop around the gigantic asteroid every 12.3 hours.

As Dawn revolves, Vesta rotates on its axis beneath it, turning once every 5.3 hours. Just as in HAMO1, mission planners artfully choreographed this celestial pas de deux so that over the course of 10 orbits, lasting just over five days, the camera would be able to view nearly all of the lit surface. A set of 10 orbits is known to Dawn team members (and to you, loyal readers) as a mapping cycle.

Until a few months ago, HAMO2 was planned to be four cycles. Thanks to the determination in April that Dawn could extend its residence at Vesta and still meet its 2015 appointment with dwarf planet Ceres, HAMO2 has been increased to six mapping cycles (plus even a little more, as we shall see below), promising a yet greater scientific return.

In cycle 1, which began on June 23, the camera was pointed at the surface directly underneath the spacecraft. The same view will be obtained in cycle 6. In cycles 2 through 5, images are acquired at other angles, providing different perspectives on the complex and dramatic landscape. Scientists combine the pictures to formulate topographical maps, revealing Vesta's full three-dimensional character from precipitous cliffs and towering peaks of enormous mountains to gently rolling plains and areas with mysterious ridges and grooves to vast troughs and craters punched deep into the crust. Knowing the elevations of the myriad features and the angles of slopes is essential to understanding the geological processes and forces that shaped this exotic mini-planet. In addition to the exceptional scientific value, the stereo imagery provides realistic, exciting views for anyone who wants to visualize this faraway world. If you have not traveled there yourself, be sure to visit the Image of the Day regularly and the video gallery occasionally to see what you and the rest of humankind had been missing during the two centuries of Vesta's appearance being only that of a faint, tiny blob in the night sky.

With 3-D movies and other familiar stereo pictures, only two angles are needed. That's sufficient to reproduce what our two eyes would perceive, but it does not tell the entire story. A left-right pair reveals nothing about the up-down dimension. Scientists chose the directions to point Dawn's camera that yield the best combinations of perspective and illumination to construct a complete contour map.

In cycle 2, the craft soars over the sunlit side with its camera pointed both ahead and to the left of the ground directly below. In cycle 3, the instrument will be targeted behind and slightly to the left. Cycle 4 will observe the surface farther back and to the right. Cycle 5 will look slightly ahead and to the right. Together these pictures will yield a fabulous sense of the detailed shape of Vesta, and combining them with the HAMO1 images will afford an extraordinarily comprehensive 3-D view.

The camera and VIR are mounted on the spacecraft so that they point in the same direction. During these six cycles, the direction is determined by what's needed for the topographic mapping, but VIR collects valuable spectra as well wherever it is aimed. A spectrum is a measure of the intensity of light at different wavelengths and is reminiscent of the rainbow you see when a glass prism or droplets of water separate white light into its constituent colors. The material on Vesta imprints its signature on the light it reflects from the sun, so VIR's measurements reveal the nature of the minerals. The sensor has already found that Vesta displays a highly varied composition, attesting to its complex geological history. VIR records light from ultraviolet through the entire visible range and into the infrared. Indeed, the instrument operates so far into the infrared that it can detect the meager heat emitted from the surface, thereby also functioning as a remote thermometer. Each VIR snapshot consists of the spectrum at 256 locations on the surface, providing a great richness of information.

Compared to the camera, VIR trades greater spectral coverage for smaller spatial coverage. VIR was the prime instrument in survey orbit, where it was high enough that even with its narrow view, it could observe most of the surface. At the lower altitude of HAMO1 and HAMO2, VIR cannot map all of Vesta in a single mapping cycle or even in six cycles. (And even with all the bonus data it collected during months of operation in the low-altitude mapping orbit (LAMO), the proximity to the surface allowed it to obtain excellent close-up views but only of small regions.) HAMO1 was so outstandingly productive that VIR did see much of the surface, and now the coverage is being increased significantly with HAMO2.

Because the mission has been going so well, mission planners decided to devote some extra time in HAMO2 to additional VIR measurements. From June 15 through 23, before the six mapping cycles commenced, VIR was the star of the celestial show again. Every orbit was dedicated exclusively to collecting as many spectra as could be transmitted to Earth. The telecommunications link that stretches across the solar system is very limited. By not splitting it between the camera's images and VIR's spectra, controllers could maximize the latter's coverage of Vesta.

Dawn's exceedingly productive exploration may make its accomplishments appear easy, but as with all such undertakings, the success is enabled by a group of people applying their collective expertise, discipline, creativity, and powerful drive to reveal the unknown. It is thanks to their extraordinary investment of time and energy that the distant probe is able to execute such an ambitious mission, unveiling an ancient world that previously had only been glimpsed from afar by telescopes.

When the previous log was unleashed upon readers of all dawnominations, Dawn was partway through its long spiral route from LAMO to HAMO2. (You can see the weekly progress in altitude by checking the May mission status reports.) Complex and challenging though it was, the flight went precisely as intended. Because maneuvering the spacecraft exactly to its targeted destination is so difficult, mission planners had scheduled a window to fine tune the orbit on June 9 and 10 after the main phase of ion thrusting was complete. This is very similar to the trajectory correction maneuvers planned before the swing past Mars. Nevertheless, upon carefully measuring the actual orbit following the end of thrusting on June 4, navigators determined that it was so good that no adjustments were needed.

Before the resumption of Vesta observations on June 15, engineers reversed some reconfigurations of the spacecraft they had made for operation at lower altitude. They also took advantage of the time to perform a routine verification of the health of the back-up camera, ensuring that it remained ready to take over if the primary camera encountered problems. Both instruments are in excellent condition.

As Dawn continues tirelessly to scrutinize Vesta and report its fascinating findings, the mission control team is putting the finishing touches on the plans for its departure. On July 25, the ship will begin climbing out of HAMO2, its sights set on Ceres. Just as during the approach phase, however, it will pause occasionally for some additional observations. As Vesta grows farther and smaller but sunlight touches more of the high northern latitudes, the instruments will take some parting shots. We will describe those plans in the next log. As we shall see, even as Dawn says goodbye to its companion of more than a year deep in the main asteroid belt, it will continue to discover new secrets to thrill and delight all the passionately curious and bold creatures who champion the eager explorer on its interplanetary voyage. Through this robot, they are transported far, far into space to behold sights and gain knowledge that otherwise would remain forever beyond their reach.

Dawn is 680 kilometers (420 miles) from Vesta. It is also 3.17 AU (474 million kilometers or 294 million miles) from Earth, or 1305 times as far as the moon and 3.12 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 53 minutes to make the round trip.

Dr. Marc D. Rayman
10:30 p.m. PDT June 30, 2012


  • Marc Rayman

Images of the giant asteroid Vesta taken by NASA's Dawn spacecraft in 2011 and 2012

Dear Readers of all Dawnominations,

Far from Earth, on the opposite side of the sun, deep in the asteroid belt, Dawn is gradually spiraling around the giant protoplanet Vesta. Under the gentle pressure of its uniquely efficient ion propulsion system, the explorer is scaling the gravitational mountain from its low-altitude mapping orbit (LAMO) to its second high-altitude mapping orbit (HAMO2).

Dawn spent nearly five months in LAMO, circling the rocky world at an average altitude of 210 kilometers (130 miles) as it acquired a fabulous bounty of pictures; visible, infrared, neutron, and gamma ray spectra; and measurements of the gravity field. As we saw last month, the probe was far more productive in each investigation than the ambitious team members had expected or had ever dared hope it would be. With that outstanding success behind it, it is looking ahead and up to its work in HAMO2, about 680 kilometers (420 miles) high.

Dawn is the first spacecraft to explore Vesta, the second most massive resident of the main asteroid belt between Mars and Jupiter. Indeed, this is the only craft ever to orbit a body in the asteroid belt. No other missions are currently on the books to visit this remote, exotic world, which is now appreciated to be more closely related to the terrestrial planets (including Earth) than to typical asteroids. And now Dawn is receding from it. On May 1, it began the slow ascent to its next observation orbit. It may well be decades before another robotic ambassador from Earth comes as close to Vesta as this bold traveler has.

Humankind's first exploration of Vesta has been exceptionally rewarding. A simple measure of that can be seen with just two photographs. More than two centuries after its discovery, this giant asteroid was first glimpsed by the approaching spaceship from Earth on May 3, 2011. From a distance of 1.2 million kilometers (750 thousand miles), or more than three times the separation between Earth and the moon, Dawn's mapping camera perceived Vesta as only five pixels across. Each pixel spanned more than 110 kilometers (70 miles), revealing nothing new compared to what astronomers' most powerful telescopes had shown (but the image was of importance for navigation purposes). Nevertheless, at the time, it was tremendously exciting to obtain the first views of a distant, unfamiliar shore after a voyage of more than 2.6 billion kilometers (1.6 billion miles) on the interplanetary ocean. Sighting our first celestial port of call more than three and a half years after this cosmic adventure began was thrilling indeed. But now, with more than 25 thousand spectacular photos in hand from much smaller distances, it is even more gratifying to acknowledge that first picture as one of the worst ever taken of Vesta. The Image of the Day from one year later was acquired in October 2011 from 1,700 times closer; and most of the images have been obtained from LAMO, about 5,700 times nearer than that first one. Dawn has rapidly transformed Vesta from a mere fleck among the stars into a fascinating, complex and splendidly detailed world.

Keeping the remote vessel on the planned spiraling course from one mapping orbit to another presents the crew with a set of formidable challenges, but this team has accomplished the maneuvers to successively reach survey orbit, the first high-altitude mapping orbit (HAMO1) and LAMO. The current orbital transfer is complex and demanding, but it is proceeding very well. Controllers update the flight profile every few days to ensure the probe stays close to the carefully designed trajectory to HAMO2. To gain a sense of the progress, go here for your correspondent's atypically succinct weekly summaries of the spiral status.

Imagine a globe of Vesta 30 centimeters (1 foot) in diameter. For the purpose of this illustration, you may be confident that no inhabitants (permanent or temporary) of the massive orb will object if we pretend that it does not rotate. We will use this to demonstrate the alignments of the orbits.

First, let's chose the position of the sun, because the orbits were chosen on the basis of their angles relative to its location. Even in this miniaturized cosmos, the sun today is 213 kilometers (134 miles) away. (Space is big!) What matters more, however, is the direction, so we will place the luminous master of the solar system over (albeit very, very far over) the prime meridian, the 0 degree longitude line on our stationary Vesta. Now we recall that Vesta, like Earth, has seasons because its axis is tipped. It is southern hemisphere summer there, so the sun is not over the equator; rather, it is currently at about 8 degrees south latitude. (On Nov. 29, 2011, when we last used the analogy of the globe, the sun was at 25 degrees south latitude. Since then, it has moved north because of the progression of seasons.) Although Earth's location is not pertinent to this discussion, we can accurately position it 285 kilometers (180 miles) away, high above a point at 5 degrees south latitude and 10 degrees east longitude.

Now with the sun over the 0 degree longitude line, we can orient Dawn's orbits. Think of each orbit as a ring encircling Vesta, going over both poles and crossing the equator at a right angle. Globes of Earth often are supported within a ring like that, and it may be helpful to have a terrestrial globe in mind, or even in sight, as you ponder the celestial arrangement. Because our imaginary Vesta is not rotating, a ring that is aligned with a longitude line represents one of Dawn's orbits. (Of course, Vesta really does rotate, so as the spacecraft loops from pole to pole and back and the protoplanet turns beneath it, all parts come within view of its sensors.)

Survey orbit is a little more than 1.5 meters (5 feet) above the 15 degree west longitude line (and, to make a complete circle, it goes over the 165 degree east longitude line as well). It was from that vantage that the first thorough mapping was conducted in August. The ring representing HAMO1 is twisted to 30 degrees west (and 150 degrees east on the other side of the globe), only about 38 centimeters (15 inches) over the surface. The lower altitude of HAMO1 afforded much better views of the great variety of geological features than the reconnaissance from survey orbit. In addition, because the orbit was shifted farther from the sun, the angle of light on the landscape beneath the spacecraft was different, aiding in formulating a more complete portrait of the terrain. LAMO is rotated still farther from the sun, at 46 degrees west (and 134 degrees east), and is less than 12 centimeters (only 4.7 inches) high. The adventurer spent more time in this low orbit than anywhere else at Vesta.

Now the ship is on its way to HAMO2, which will be at exactly the same altitude as HAMO1 but not the same orientation. When its scientific scrutiny resumes on June 15, the orbit will be approximately aligned with the 35 degree west (and 145 degree east) longitude line on our globe. There is another important difference however. The HAMO2 ring does not quite extend to the poles this time; rather, it is tilted a little so that it goes only to 86 degrees north and south latitude. (Those familiar with orbital mechanics would describe the orbit as having an inclination of 94 degrees; those unfamiliar with orbital mechanics would not describe it that way. You all know who you are.) This tip allows the spacecraft to take advantage of Vesta's gravity field, which navigators have mapped with great accuracy, to gradually turn the orbit by about one degree every five days. As a consequence, by the time Dawn completes its observations in late July, the orbit will be above 27 degrees west (and 153 degrees east).

Using different orientations of the orbits relative to the sun is a crucial element of the strategy for gathering such a wealth of scientifically valuable data on Vesta. Each orbit provides views of the ground with different illumination angles.

In LAMO, the lighting was less important, as the primary objectives of that phase were to measure the protoplanet's nuclear radiation and changing gravitational tug as Dawn circled it, and neither of those depended on sunlight. (Although it was purely bonus, the operations team still managed to photograph most of the surface at high resolution.) But the LAMO angle was chosen in large part to ensure that the desired plane for HAMO2 would be within reach when the time came to undertake the orbital ascent. Moving the plane of an orbit is energetically very very expensive, and even with Dawn's extraordinary capabilities, only limited changes are practicable.

In contemplating the Vesta-centric universe we have just described, it may be evident that Dawn is not only enlarging its orbit from LAMO to HAMO2 but also twisting and tilting it. As with the descent to LAMO, described in more detail here, the team has designed a flight profile that relies principally on the extraordinary capability of ion propulsion but also rides Vesta's gravitational currents to help accomplish some of the shifts in the orbit plane.

The location of the sun described above suggests why HAMO2 is valuable. Orbiting farther from the sun than Earth, Vesta's year is equivalent to more than 3.6 terrestrial years. The seasons pass correspondingly slowly, lasting an average of 11 months each. In the time that will have passed from HAMO1 in October 2011 to HAMO2 in June and July, the sun will have moved northward, thus revealing some terrain that was in the deep shadow of northern winter during HAMO1. It is that landscape that is the principal target of HAMO2. As we will see in the next log, however, this phase will present opportunities for other investigations as well.

HAMO2 will be the final intensive campaign of observing Vesta. When it is complete, the craft will once again resume powered flight. It will escape from Vesta's gravitational grip in August and begin the next stage of its interplanetary voyage, aiming for dwarf planet Ceres in 2015 -- a new world explored, another world awaits!

Dawn is 610 kilometers (380 miles) from Vesta. It is also 3.37 AU (503 million kilometers or 313 million miles) from Earth, or 1,385 times as far as the moon and 3.32 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 56 minutes to make the round trip.

Dr. Marc D. Rayman
10:30 p.m. PDT May 31, 2012


  • Marc Rayman

Artist's concept of the Dawn spacecraft at asteroid Vesta

Dear Dawnright Spectacular Readers,

Dawn is wrapping up a spectacularly rewarding phase of its mission of exploration. Since descending to its low-altitude mapping orbit (LAMO) in December, the stalwart probe has circled Vesta about 800 times and collected a truly outstanding trove of precious observations of the protoplanet. Having far exceeded the plans, expectations, and even hopes for what it would accomplish when LAMO began, the ambitious explorer is now ready to begin its ascent. On May 1, atop its familiar blue-green pillar of xenon ions, the craft will embark upon the six-week spiral to its second high-altitude mapping orbit.

When the intricate plans for Dawn's one-year orbital residence at Vesta were developed, LAMO was to be 70 days, longer than any other phase. Because of the many daunting challenges of exploring an uncharted, alien world in the forbidding depths of the asteroid belt so far from home, mission planners could not be confident of staying on a rigid schedule, and yet they wanted to make the most of the precious time at the giant asteroid. They set aside 40 days (with no committed activities) to use as needed in overcoming problems during the unique approach and entry into orbit as well as the intensive observation campaigns in survey orbit and the first high-altitude mapping orbit plus the complex spiral flights from each science orbit to the next. To no one's surprise, unexpected problems did indeed arise on occasion, and yet in every case, the dedicated professionalism and expertise of the team (occasionally augmented with cortisol, caffeine, and carbohydrates) allowed the expedition to remain on track without needing to draw on that reserve. To everyone's surprise and great delight, by the beginning of LAMO on December 12, the entirety of the 40 days remained available. Therefore, all of it was used to extend the time the spacecraft would spend at low altitude studying the fascinating world beneath it.

Dawn's mission at Vesta, exciting and successful though it is, is not the craft's sole objective. Thanks to the extraordinary capability of its ion propulsion system, this is the first vessel ever planned to orbit two extraterrestrial destinations. After it completes its scrutiny of the behemoth it now orbits, the second most massive resident of the main asteroid belt, Dawn will set sail for dwarf planet Ceres, the largest body between the orbits of Mars and Jupiter.

Since 2009, the interplanetary itinerary has included breaking out of Vesta orbit in July 2012 in order to arrive at Ceres on schedule in February 2015. Taking advantage of additional information they have gained on the spacecraft's generation and consumption of electrical power, the performance of the ion propulsion system, and other technical issues, engineers have refined their analyses for how long the journey through the asteroid belt to Ceres will take. Their latest assessment is that they can shave 40 days off the previous plan, once again demonstrating the valuable flexibility of ion propulsion, and that translates into being able to stay that much longer at the current celestial residence. (This extension is different from the 40 days described above, because that was designed to ensure Dawn could complete its studies and still leave on schedule in July. For this new extension, the departure date is being changed.) Even though a larger operations team is required at Vesta than during the cruise to Ceres, the Dawn project has the wherewithal to cover the cost. Because operations at Vesta have been so smooth, no new funds from NASA are needed; rather, the project can use the money it had held in reserve in case of problems. In this new schedule, Dawn will gently free itself of Vesta's gravitational hold on August 26.

Most of the bonus time has been devoted to extending LAMO by a month, allowing the already richly productive investigations there to be even better. (Future logs will describe how the rest of the additional time at Vesta will be spent.) With all sensors fully operational, the robotic explorer has been making the best possible use of its precious time at Vesta, revealing more and more thrilling details of an exotic world deep in the asteroid belt.

One of the primary motivations of pushing down to the low altitude of 210 kilometers (130 miles) was to get close enough to measure the emission of radiation from the material in the uppermost meter (yard) of the surface of the rocky body. The gamma rays (a high energy version of electromagnetic radiation, beyond visible light, beyond ultraviolet, even beyond X-rays) and neutrons (nuclear particles that constitute most of the mass of atoms other than hydrogen in your correspondent and elsewhere in the universe) that emanate from Vesta carry the signature of the atoms they interacted with before they escaped from the surface and traveled into space. (Even though hydrogen nuclei contain only a single proton and no neutron, the free neutrons that have bounced off those nuclei can reveal their presence.) The gamma ray and neutron detector (GRaND), whose name belies its unpretentious demeanor, does more than detect them. It measures the energies of the gamma rays and neutrons to allow scientists to make an inventory of the major elements and thereby gain insight into the geochemistry of this world. As we have described in more detail before however, the signals are extremely faint. Just as you need a long exposure with a camera to record a picture of a dim object, GRaND needs a long exposure to make its picture of the atomic constituents of Vesta. Scientists had set a target exposure of about 56 days spread over the time at low altitude.

Planners knew that GRaND could not collect its data the entire time in LAMO, both because of conflicting spacecraft activities and because of the whims of nature. Whenever the spacecraft points its main antenna to Earth or its ion thruster in a direction needed to adjust the orbit, GRaND cannot simultaneously be pointed at the surface. The spacecraft entered safe mode in January and February, temporarily suspending the instrument's observations. In January and March, when the distant but powerful sun unleashed especially intense bursts of radiation that reached Dawn, it interfered with GRaND's measurements of the radiation from Vesta. Despite these interruptions, scientists now have about 91 days of beautiful GRaND data. They truly are grand data.

The other principal objective of LAMO was to learn about the interior structure of Vesta by making extremely accurate measurements of the spacecraft's orbit. Gravity's weakness is one of the fascinating mysteries of the universe. It feels strong to us (well, most of us anyway), because we don't so easily sense the strong and weak nuclear forces, and we tend not to recognize the electromagnetic force. In addition, with both positive and negative electric charges, attractive and repulsive electromagnetic forces often cancel. Not so with gravity. All matter exerts attractive gravity, and it can all add up. The reason gravity is even as strong as it is for our readers on Earth is because there is such a vast amount of matter in the planet, all of it pulling together to hold you down. (The electromagnetic force is sufficient to resist the pull, preventing you from sinking into the surface.) The gravitational pull on Dawn is the cumulative effect of all the matter in Vesta.

Gravity diminishes with distance, and the spacecraft is subjected to a changing force as the inhomogeneous protoplanet rotates and the ship revolves around it. When Dawn is closer to locations with greater density, it experiences a stronger tug and when it is near regions with less powerful gravity, the attraction is weaker. By carefully mapping the exquisitely small variations in the probe's orbital motion, navigators can calculate how the mass is distributed within Vesta. This has already enabled the discovery of a dense iron core, one of the reasons scientists believe it has a complex geological history more akin to planets than to typical asteroids.

The orbit is calculated with astonishing accuracy using several methods, with the principal one being the measurement of the Doppler shift of Dawn's radio signal, in which the frequency changes as the spacecraft's speed changes. To map the complex shape of the gravity field, the team had wanted to accumulate a total of about 26 days worth of Doppler measurements at the Deep Space Network using the main antenna when it was pointed to Earth and one of the auxiliary antennas some other times. Again, thanks to the combination of favorable operations and the extension to LAMO, the mission has achieved 80 days of valuable radio tracking.

As we explored in some depth in the logs in December, January, and February, observations with the science camera and the visible and infrared mapping spectrometer (VIR) in LAMO were considered a bonus. Survey orbit and HAMO were dedicated to the acquisition of images to show the appearance and topography of the surface and spectra to reveal the nature of the minerals and even the temperature. The successes of those phases allowed the development of near global maps of many characteristics of the alien world. Nevertheless, the closer view from LAMO was irresistible, where the detail visible is more than three times better than from HAMO. A few such close-up pictures would have been intriguing and tantalizing. The actual reward from LAMO far exceeds all expectations, with well over 13,000 photos, covering most of the surface, and more than 2.6 million spectra. (As recently as October, one of these otherwise trustworthy logs stated that it would not be possible to collect enough images in LAMO to make a global map. The wonderful opportunity to spend so much time in LAMO and the truly extraordinary success of the bonus imaging program were not foreseen.) If you haven't been to Vesta to see the sights as well as Dawn can, then be sure to visit the Dawn Image of the Day for some of the best views.

This mysterious world, descried during more than two centuries of telescopic observations and perceived as little more than a smudge among the stars before last summer, now has yielded myriad secrets to the robotic ambassador from distant Earth. Scientists are thrilling to the experience of turning Dawn's fantastic bounty of data into knowledge. As they discover and become more familiar with the features on what was so recently an entirely uncharted world, they are naming more and more of them. The growing list of landmark names approved by the International Astronomical Union is here, and you can see them on a map here.

Although Dawn will begin gradually receding from Vesta on May 1, many more observations are planned before it leaves for Ceres on August 26. Meanwhile, that still more distant world, another relict from the dawn of the solar system, waits patiently. Dawn and Vesta now are 2.5 AU from the sun, but Ceres is even more remote, and the ship will have a long journey to reach it in 2015. The craft has been in flight for more than four and a half years, so Earth has revolved around the sun more than four and a half times since it dispatched Dawn on its interplanetary adventure. The spacecraft itself has completed just over two heliocentric revolutions during that time (some of it while accompanying Vesta). Following a more leisurely pace around the sun than Vesta, Earth, and all the other objects under a tighter grip of the master of the solar system, Ceres will complete its first loop since Dawn's launch later this week. Well before it finishes its subsequent revolution, the dwarf planet will become the host of this remarkable probe, which will continue to unveil secrets of the solar system on behalf of the passionately curious and bold creatures on the faraway planet where its voyage began.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.47 AU (518 million kilometers or 322 million miles) from Earth, or 1,380 times as far as the moon and 3.44 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 58 minutes to make the round trip.

Dr. Marc D. Rayman
11:00 p.m. PDT April 30, 2012


  • Marc Rayman

Layered young crater as imaged by NASA's Dawn spacecraft

Dear Dawnscoverers,

On March 29, Vesta spent the 205th anniversary of its discovery by treating Dawn to more spectacular vistas, as it does so often these days. When Heinrich Wilhelm Matthäus Olbers first spotted Vesta, he could hardly have imagined that the power of the noble human spirit for adventure and the insatiable hunger for knowledge would propel a ship from Earth to that mysterious point of light among the stars. And yet today our spacecraft is conducting a detailed and richly rewarding exploration of the world that Olbers found.

Dawn is continuing its intensive low-altitude mapping orbit (LAMO) campaign, scrutinizing the protoplanet 210 kilometers (130 miles) beneath it with all instruments. The primary objectives of the craft's work here are to measure the atomic composition and the interior distribution of mass in this geologically complex world. In addition, this low orbit provides the best vantage point for high resolution pictures and visible and infrared spectra to reveal the nature of the minerals on the surface.

Ever since it left its home planet behind in September 2007, the robotic adventurer has pursued its own independent course through the solar system. As Earth and its orbiting retinue (including the moon and many artificial satellites) followed their repetitive annual loop around the sun, Dawn used its ion propulsion system to spiral outward to rendezvous with Vesta in July 2011. When the gigantic asteroid's gravity gently took hold of the visiting craft, the two began traveling together around the sun, taking the same route Vesta has since long before humans gazed in wonder at the nighttime sky.

As we have discussed before, the speed of an object in orbit, whether around Earth, the sun, the Milky Way (either my cat or the galaxy of the same name) or anything else, decreases as its orbital altitude increases. Farther from the sun than Earth is, and hence bound to it by a weaker gravitational grip, Vesta moves at a more leisurely pace, taking more than 3.6 years per revolution. When Dawn travels to the more remote Ceres, it will orbit the sun even more slowly, eventually matching Ceres' rate of 4.6 years for each loop.

Just as the hour hand and minute hand of a clock occasionally are near each other and at other times are on opposite sides of the clock face, Earth and Dawn sometimes are relatively close and other times are much farther apart. Now their orbits are taking them to opposite sides of the sun, and the distance is staggering. They have been on opposite sides of the sun twice before (albeit not as far apart as this time), in November 2008 and November 2010. We used both occasions to explain more about the nature of the alignment as well as to contemplate the profundity of such grand adventures.

On April 18, Dawn will attain its greatest separation yet from Earth, nearly 520 million kilometers (323 million miles) or more than 3.47 astronomical units (AU). Well beyond Mars, fewer than a dozen spacecraft have ever operated so far from Earth. Those interested in the history of space exploration (such as your correspondent) will enumerate them, but what should be more rewarding is marveling at the extent of humanity's reach. At this extraordinary range, Dawn will be nearly 1,400 times farther than the average distance to the moon (and 1,300 times farther than the greatest distance attained by Apollo astronauts 42 years ago). The deep-space ship will be well over one million times farther from Earth than the International Space Station and Tiangong-1.

Vesta does not orbit the sun in the same plane that Earth does. Indeed, a significant part of the challenge in matching Dawn's orbit to Vesta's was tipping the plane of its orbit from Earth's, where it began its journey, to Vesta's, where it is now. As a result, when they are on opposite sides of the sun this time, Dawn will not appear to go directly behind the sun but rather will pass a little south of it. In addition, because the orbits are not perfectly circular, the greatest separation does not quite coincide with the time that Dawn and the sun appear to be most closely aligned. The angular separation will be at its minimum of less than five degrees (about 10 times the angular size of the sun itself) on April 9, but the sun and Dawn appear to be within ten degrees of each other from March 23 until April 27. For our human readers, that small angle is comparable to the width of your palm at arm's length, providing a handy way to find the approximate position of the spacecraft in the sky. Earth's robotic ambassador to the cosmos began east of the salient celestial signpost and progresses slowly to the west over the course of those five weeks. Readers are encouraged to step outside and join your correspondent in raising a saluting hand to the sun, Dawn, and what we jointly accomplish in our efforts to gain a perspective on our place in the universe.

For those awestruck observers who lack the requisite superhuman visual acuity to discern the faraway spacecraft amidst the dazzling light of the sun, this alignment provides a convenient occasion to reflect once again upon missions deep into space. Formed at the dawn of the solar system, Vesta, arguably the smallest of the terrestrial planets, has waited mostly in patient inconspicuousness for a visit from the largest terrestrial planet. For the entire history of life on Earth, the inhabitants remained confined to the world on which they have lived. Yet finally, one of the millions upon millions of species, inspired by the splendor of the universe, applied its extraordinary talents and collective knowledge to overcome the limitations of planetary life and strove to venture outward. Dawn is the product of creatures fortunate enough to be able to combine their powerful curiosity about the workings of the cosmos with their impressive abilities to explore, investigate and ultimately understand. While its builders remain in the vicinity of the planet upon which they evolved, their emissary now is passing on the far side of the sun! This is the same sun that is more than 100 times the diameter of Earth and a third of a million times its mass. This is the same sun that has been the unchallenged master of our solar system for more than 4.5 billion years. This is the same sun that has shone down on Earth throughout that time and has been the ultimate source of so much of the heat, light and other energy upon which the planet's residents have been so dependent. This is the same sun that has so influenced human expression in art, literature, mythology and religion for uncounted millennia. This is the same sun that has motivated scientific studies for centuries. This is the same sun that is our signpost in the Milky Way galaxy. And humans have a spacecraft on the far side of it. We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.45 AU (516 million kilometers or 321 million miles) from Earth, or 1,290 times as far as the moon and 3.45 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 57 minutes to make the round trip.


  • Marc Rayman

Artist's concept of the Dawn spacecraft soaring over the giant asteroid Vesta

Dear Ups and Dawns,

Dawn is continuing its exploits at Vesta, performing detailed studies of the colossal asteroid from its low altitude mapping orbit (LAMO). The robotic ambassador is operating extremely well on behalf of the creatures it represents on a distant planet. On this second intercalary day of its ambitious adventure, the spacecraft is doing exactly what it was designed to do: exploring a previously uncharted alien world.

Although we usually describe LAMO as being at an average altitude of 210 kilometers (130 miles), that does not mean it is at a constant altitude. As we saw on the fourth anniversary of Dawn's departure from Earth, there are two reasons the spacecraft's height changes. One is that the elevation of the surface itself changes, so if the probe flew in a perfect circle around Vesta, its altitude would vary according to the topography. Like the planet from which Dawn embarked upon its deep space journey in 2007 (and even some of the residents there), Vesta is broadest near its equator, and that is where the ground generally reaches its greatest distance from the center. In addition, the ancient surface, battered over billions of years in the rough and tumble of the asteroid belt, displays remarkable variations in shape. The giant Rheasilvia basin is a scar from an extraordinary impact that excavated a region encompassing the south pole more than 500 kilometers (over 300 miles) in diameter. This immense gouge has left that part of Vesta at a much lower elevation than elsewhere. In the center of the enormous depression is the second tallest mountain known in the solar system, soaring to well over twice the height of Mt. Everest. The vertical range from the highest locations near the equator to the bottoms of the deepest craters within Rheasilvia is more than 60 kilometers (37 miles). So as Dawn loops around in just over four hours, the surface underneath it rises and falls dramatically.

The second reason is that the orbit itself is not exactly a circle. Let's ignore for a moment the effect of the topography and focus solely on the shape of the craft's path around Vesta. As Vesta rotates and Dawn revolves, the gravitational forces acting on the orbiter are always changing because of the irregular distribution of material inside the geologically complex protoplanet. This effect occurred at the higher altitudes as well, but it was much less pronounced there. Now that the adventurer is deep in the gravity field, the peaks and valleys of its own motion are magnified.

Navigators were very careful in choosing the parameters for LAMO, recognizing that the orbital waters were turbulent. Nevertheless, their mapping of the gravitational currents proved quite accurate, and the spacecraft has followed the planned course quite well. The lengthy and relatively technical discussions in the two previous logs described why the ship drifts off a little, but operators occasionally nudge it back with the ion propulsion system.

Orbits usually are best described by ellipses, like flattened circles. Now Vesta's bumpy gravity field does not allow perfectly smooth, regular orbits at low altitude. Moreover, the variations in the strength of the gravitational attraction transform the orbits. Sometimes, the difference between the high point of a loop and the low point is less than 16 kilometers (10 miles). As the changing forces reshape the orbit, the ellipse gets more exaggerated, with the low points going lower and the high points going higher. The differences within one revolution grow to be more than 75 kilometers (47 miles). Thanks to the ingenious design of the orbital trajectory however, those same forces then will gradually attenuate the profile, causing it to become more round again. This pattern repeats every 11.5 days in LAMO. It is almost as if the orbit breathes slowly, its envelope expanding and contracting.

This evolution of the orbit occurs above the rugged shape of Vesta itself. These two effects have conspired so that Dawn has been less than 170 kilometers (106 miles) from the rocky surface on several occasions when it was over equatorial regions. At its greatest altitude in LAMO, Dawn occasionally reaches to more than 290 kilometers (180 miles). This happens when it is deep in the southern hemisphere, soaring over the low elevation terrain of Rheasilvia.

These changes in the distance to the ground were known before Dawn arrived in LAMO, and they do not compromise the ongoing campaign to learn as much as possible about this survivor from the dawn of the solar system. As it revolves around the behemoth beneath it, the spacecraft uses its gamma ray and neutron detector (GRaND) to record these subatomic particles, which carry the signature of the elements within the top meter (yard) of the surface. Navigators' extraordinarily accurate measurements of the ship's orbital motion reveal subtleties in the gravity field and hence the distribution of material throughout the gigantic asteroid. Controllers have taken advantage of the low altitude and smooth operations to collect more observations with the camera and the visible and infrared mapping spectrometer (VIR). More than 7500 pictures have been acquired so far in LAMO, and VIR has returned nearly one million spectra. These provide a fabulous scientific bonus, affording scientists a much more detailed view of Vesta than had been planned with survey orbit and the high altitude mapping orbit (HAMO).

The acquisition of science data was interrupted on February 21 when the main computer was temporarily overloaded with tasks. The system correctly responded by rebooting the computer, which put the spacecraft into safe mode. Because this occurred during a communications session, controllers observed the event (albeit delayed by the long travel time for radio signals to reach Earth). They quickly diagnosed the problem and began the meticulous commanding to bring the robot back its normal configuration. Within a few days, it had resumed its normal schedule of observations.

In some sense, even the GRaND and gravity measurements now are a bonus. When the detailed timeline for Dawn's residence at Vesta was formulated, mission planners allowed 70 days in LAMO, which began on December 12 and so would have concluded on February 20. As we saw at the end of 2011, because the unique approach, the intensive observations in survey orbit and HAMO, and the complex spiral flights from each science orbit to the next have all been accomplished so well (perhaps even unexpectedly well), the 40 days that were held in reserve to overcome problems are now being used to prolong the studies at low altitude. With all sensors fully operational, the robotic explorer is making the best possible use of its precious time at Vesta, revealing more and more exciting details of a mysterious world deep in the asteroid belt.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.33 AU (498 million kilometers or 309 million miles) from Earth, or 1240 times as far as the moon and 3.36 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 55 minutes to make the round trip.

Dr. Marc D. Rayman
8:00 a.m. PST February 29, 2012


  • Marc Rayman

Image of asteroid Vesta taken by NASA's Dawn spacecraft from low altitude mapping orbit, or LAMO

Dear Asdawnished Readers,

Dawn is scrutinizing Vesta from its low-altitude mapping orbit (LAMO), circling the rocky world five and a half times a day. The spacecraft is healthy and continuing its intensive campaign to reveal the astonishing nature of this body in the mysterious depths of the main asteroid belt.

Since the last log, the robotic explorer has devoted most of its time to its two primary scientific objectives in this phase of the mission. With its gamma ray and neutron detector (GRaND), it has been patiently measuring Vesta's very faint nuclear emanations. These signals reveal the atomic constituents of the material near the surface. Dawn also broadcasts a radio beacon with which navigators on distant Earth can track its orbital motion with exquisite accuracy. That allows them to measure Vesta's gravity field and thereby infer the interior structure of this complex world. In addition to these top priorities, the spacecraft is using its camera and its visible and infrared mapping spectrometer (VIR) to obtain more detailed views than they could in the higher orbits.

As we have delved into these activities in detail in past logs, let's consider here some more aspects of controlling this extremely remote probe as it peers down at the exotic colossus 210 kilometers (130 miles) beneath it.

Well, the first aspect that is worth noting is that it is incredibly cool! Continuing to bring this fascinating extraterrestrial orb into sharper focus is thrilling, and everyone who is moved by humankind's bold efforts to reach into the cosmos shares in the experience. As a reminder, you can see the extraordinary sights Dawn has by going here for a new image every weekday, each revealing another intriguing aspect of the diverse landscape.

The data sent back are providing exciting and important new insights into Vesta, and those findings will continue to be announced in press releases. Therefore, we will turn our attention to a second aspect of operating in LAMO. Last month, we saw that various forces contribute to Dawn moving slightly off its planned orbital path. (That material may be worth reviewing, either to enhance appreciation of what follows or as an efficacious soporific, should the need for one ever arise.) Now let's investigate some of the consequences. This will involve a few more technical points than most logs, but each will be explained, and together they will help illustrate one of the multitudinous complexities that must be overcome to make such a grand adventure successful.

Far away, traveling through the vast expanse of (mostly) empty space, Dawn only knows where it is because of information the mission control team installs in it. This is typical for interplanetary spacecraft. Earth-orbiting satellites may be able to use the Global Positioning System (GPS) constellation or other means to find their own location, but only a few spacecraft that have gone far from Earth have the means to independently establish their own location. This should not be confused with a spacecraft's ability to determine its own orientation, which Dawn does with its star trackers, gyros, and sun sensors. In the same way, if you were in a dark and unidentified place on your planet, you could determine the direction you were looking by recognizing patterns of stars, but that would not help you ascertain your position.

Throughout the mission, controllers regularly transmit to the spacecraft a mathematical description of its location in the solar system at any instant over a given period of time. They also provide it with the information needed to calculate where Earth is. That's how it is able to point its main antenna in the correct direction when it needs to do so. During the Vesta phase of the mission, the probe is given the additional means it needs to determine its location relative to Vesta. All the information sent to the spacecraft is based on navigators' best prediction of where the spacecraft will be in the future. Dawn remains unaware of any deviations from its expected course, so it always behaves as if it were exactly where it would be if its motion matched the team's projections perfectly, without the discrepancies that are sure to occur. For the majority of the mission, both in interplanetary cruise and at higher altitude orbits at Vesta, the effects of being slightly off the predicted trajectory are insignificant. In LAMO, they are not.

For Dawn to aim its scientific sensors at Vesta, controllers instruct it to point straight "down." Again, it knows how to compute where "down" is because of the information it was given by navigators. Any disparity between where the craft was predicted to be and where it really is along its orbit causes it to point in a slightly different direction, not quite truly straight down. This does not compromise the observations; it could tolerate larger pointing errors and still capture the desired targets in the field of view of the instruments.

Dawn is a very large spacecraft. Indeed, the wingspan from one solar array tip to the other is 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest span of any probe NASA had ever dispatched on an interplanetary journey. The large area of solar cells is needed to capture faint sunlight in the asteroid belt to meet all of the electrical power needs. Each solar array wing is the width of a singles tennis court, and the whole spacecraft would reach 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.

Now consider that when Dawn is looking precisely down, directly toward the center of Vesta, its wings are level. If it is pointed off even a little, then one of those long extensions is slightly closer to the massive body it is circling and one is slightly farther away. Because gravity diminishes with increasing distance, the one that is closer is subject to a very slightly stronger pull than the farther other. If unchecked, that lower side would gently be pulled down even more, thus increasing the difference in gravitational attraction between the two wings still more. Eventually, this would cause Dawn to be oriented so that one wing points straight down toward the ancient surface below and the other points straight up, back into the depths of space. Because this phenomenon depends on the change in gravity from the lower point to the higher one, it is known as "gravity gradient." Some satellites that orbit Earth are designed to take advantage of the gravity gradient to align their long axis with the planet below, but Dawn (and most other spacecraft) need greater flexibility in where they point.

Rather than accepting the passive method of orienting provided by the gravity gradient, Dawn uses its reaction wheels to train its science instruments on Vesta. By electrically changing the rate at which these devices spin, the ship can control its orientation in the zero-gravity, frictionless conditions of spaceflight. When a small deviation from the perfect orbit causes it to tip its wings a little when pointing to where it calculates "down" to be, the spacecraft's reaction wheels work to prevent it from succumbing to the gravity gradient, countering the tendency of the wings to deviate still more from being level. As a consequence, the ship remains stable and the wheels gradually spin faster and faster as it conducts its observations.

To reduce the wheels' speeds, mission planners schedule a period almost every day in LAMO during which the spacecraft fires its reaction control system thrusters, a function known as "desaturating the wheels." Indeed, the principal reason Dawn is outfitted with these small thrusters and a modest supply of conventional rocket propellant known as hydrazine is to manage the speed of the wheels.

The thruster firings not only provide the torque needed to reduce the rotation rate of the wheels, but they also have the incidental effect of propelling the spacecraft slightly. The push is small, changing the orbital speed by no more than about one centimeter per second (around one fiftieth of a mph, or about 120 feet per hour). But that causes Dawn to deviate from its planned orbit, and the accumulated force from all the firings is the largest source of trajectory discrepancies in LAMO.

To summarize so far, once Dawn has any variance at all between the predicted orbital motion that mission controllers have radioed to it and its actual path, its long wings will be tipped a little while it observes Vesta. In opposing the resultant gravity gradient effect, the reaction wheels will accelerate. When the reaction control system thrusters fire to decelerate the wheels, they will nudge Dawn still more off course, and the cycle will continue.

Of course, engineers have devised strategies to accommodate this contribution (and others) to deviations from the plan. In LAMO, they frequently measure the ship's trajectory and revise their estimates of the future course. They transmit to the spacecraft a new prediction for the orbit twice a week, so the main computer usually has a very good estimate of where it is relative to Vesta and hence how to orient itself so that its long solar arrays remain level as it acquires its fabulous pictures and other scientific information. With the updated knowledge of its position, Dawn can aim its sensors accurately and keep the thruster firings from being excessive, even when it is not following its orbit perfectly. This solution works well, but let's continue delving into the consequences of the orbital perturbations.

While the operations team has the capability to provide the ship regularly with a good description of where it will be, it is much more difficult to make such frequent adjustments to its detailed itinerary. The schedule of its myriad activities has to be planned longer in advance. The sequences of commands, which are timed to the second, are very complicated to develop and verify, and the operations team does not have the resources to refine the timing as often as they can send updates on the craft's predicted location.

Engineers took many factors into account in selecting the orbits Dawn uses for its science observations. We saw in November that the orbits are characterized not only by the altitude but also by the orientation of the orbital plane. A subsequent log will explain the choices for the planes more fully, but for now, what matters is that, among other considerations, the orbits were designed to ensure Dawn remains in constant sunlight. It always has the sun in sight, never entering Vesta's shadow. Keeping Earth in view at all times was not part of the design, and on every one of the more than 600 revolutions around the gigantic rocky body since August 28 (the seventh circuit in survey orbit), the spacecraft has been temporarily behind Vesta from the geocentric point of view. In its present orbit, these occultations last for about half an hour in every 4.3-hour loop.

When Dawn is observing Vesta, that doesn't matter. When it is using its ion propulsion system to transfer from one orbit to another, it also doesn't matter. It does matter, however, when it is in contact with Earth, because Vesta blocks the radio signal. Controllers give the spacecraft a detailed schedule of which data to transmit and when, making the best possible use of the precious communications link that stretches across the solar system. The timed plan tells it not to send high priority data during the radio blackout, but the timing of the occultations can shift a little as the orbit departs from the plan.

The strategy to deal with the slight deviations in the timing of the interruption in the radio link principally involves including some padding in the plan. The schedule for the transmission of the highest priority data places it well away from the expected gap, so no important losses occur if Dawn is a little ahead in its orbit or a little behind (causing the gap to occur a little earlier or a little later).

But what is there to do during and near the time the craft is predicted to be blocked by Vesta while conducting a communications session? Dawn rotates too slowly to make it worth turning to point its sensors at the surface just for these periods. Of course, it could simply transmit nothing at all. Instead, the team has it transmit data that otherwise would be lost. There is never enough time to send to Earth all the information the probe generates and collects. So most of the time it is behind Vesta, it broadcasts many of the measurements of its own subsystems that cannot be stored and sent later. And during the periods immediately before and after the expected occultation, when there is a chance that the signal will reach Earth, it sends bonus pictures and VIR spectra. If the deviations from the planned orbit are small, then the antenna will have an unobstructed view of Earth, and these data will make it home. And if the spacecraft enters the blackout period late (or early), then it will exit late (or early) as well, so the bonus results sent before (or after) the occultation will be received. But in the rest of the cases, well, Dawn will transmit those bits right back where they came from, sending the photos and spectra into the vast rocky surface between the spacecraft and Earth.

Last month we described one of the limitations in how much bonus information could be obtained from LAMO. Now we have another. In summary, because the probe can acquire more images and other data than it is possible to return, it radios some of them during times that it is possible they will make it to Earth. Because of realistic causes of variation from its predicted orbital path, however, some of these measurements will be transmitted when, from Dawn's perspective, Vesta blocks Earth, thus preventing the broadcast signals from getting through. The GRaND observations (as well as essential telemetry on the health of the ship) are scheduled to be sent during times that, even with the reasonable range of orbit discrepancies, the communications link will not be obstructed. In this way, mission planners return as much data as possible, taking maximum advantage of the time Dawn points its main antenna to Earth. Having a sophisticated robot in orbit around the second most massive resident of the asteroid belt presents truly unique opportunities for the exploration of the solar system, and the team has devised every strategy they could to use the time as productively as possible.

The spacecraft aims GRaND at Vesta most of the time in order to develop a good picture of the weak nuclear glow. Controllers schedule three periods per week, each about eight hours, in which it directs its antenna to Earth. The orbit predictions have been extremely good, matching the actual motion quite well. Moreover, some time is allocated to return the camera and VIR data apart from the times that Vesta might be in the way. As a result, the team has been rewarded with more than 3200 photos from LAMO so far. Every one is bonus, and every one is neat!

After well over four years of travel in deep space and already half a year in orbit around Vesta, engineers recently encountered a bug lurking in the spacecraft's software. As with most bugs, this one had waited silently until just the right circumstances occurred to provoke it. The combination of conditions was achieved late in the day on January 13, and the bug caused the main computer to reboot. Dawn correctly responded by going into safe mode. The mission control team observed this the next day, and promptly began investigating the reason. They soon determined the nature of the bug (as well as ways to ensure it would never be activated again) and restored the spacecraft to its usual operating configuration for LAMO. Even with the slow communications in safe mode, the long time for radio signals to travel between Earth and Dawn, and the frequent interruptions by the regular occultations by Vesta, they had fully restored all systems by January 19. It took a few more days to configure GRaND, but it, along with the other instruments, is now back to its intensive inspection of Vesta.

We saw last month that the mission has been progressing so well that the time originally allocated to deal with anomalies had not been needed, so it is being applied to extend the duration of LAMO. This allows even more scientific observations to be conducted in this lowest altitude. Far from the planet it left in 2007, in a region of the solar system in which no other spacecraft has ever taken up residence, Dawn will continue its exploration of Vesta, alternating between examining the alien world below and transmitting its discoveries to Earth. Meanwhile, everyone who ponders what undiscovered lands lie beyond our sight, everyone who hungers for exciting challenges and noble adventures, and everyone who values turning the unknown into the known profits from the great treasures this stalwart cosmic ambassador sends to its erstwhile home, a faraway place it will never visit again.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.08 AU (461 million kilometers or 286 million miles) from Earth, or 1155 times as far as the moon and 3.13 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.


  • Marc Rayman