Tag Search - All Blogs

Tag Search - All Blogs

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

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

Dear Indawnstructibles,

Dawn concludes 2011 more than 40 thousand times nearer to Vesta than it began the year. Now at its lowest altitude of the mission, the bold adventurer is conducting its most detailed exploration of this alien world and continuing to make thrilling new discoveries.

Circling the protoplanet 210 kilometers (130 miles) beneath it every 4 hours, 21 minutes on average, Dawn is closer to the surface than the vast majority of Earth-orbiting satellites are to that planet. There are two primary scientific objectives of this low altitude mapping orbit (LAMO). With its gamma ray and neutron detector (GRaND), the probe is measuring the faint emanations of these subatomic particles from Vesta. Some are the by-products of the bombardment by cosmic rays, radiation that pervades space, and others are emitted through the decay of radioactive elements. Vesta does not glow brightly when observed in nuclear particles, so GRaND needs to measure the radiation for weeks at this low altitude. This is analogous to using a long exposure with a camera to photograph a dimly lit subject. If GRaND only detected the radiation, it would be as if it took a black and white picture, but this sophisticated instrument does more. It measures the energy of each particle, just as a camera can measure the color of light. The energies reveal the identities of the elements that constitute the uppermost meter (yard) of the surface. Dawn devotes most of its time now flying over Vesta to collecting the glimmer of radiation. It requires a long time, but this spacecraft has demonstrated tremendous patience in its use of the gentle but efficient ion propulsion system that made the mission possible, so it can be patient in making these measurements.

The second motivation for diving down so low is to be close enough that Vesta's interior variations in density affect the spacecraft's orbit discernibly. We have seen before that the distribution of mass inside the protoplanet reveals itself through the changing strength of its gravitational tug on Dawn. Exquisitely sensitive measurements of the ship's course can be translated into a three-dimensional map of the mass. In the plans discussed for LAMO one year ago, the delicate tracking of the spacecraft required pointing the main antenna to Earth. That provides a radio signal strong enough to achieve the required accuracy. Since then, navigators have determined that the radio signal received from one of the craft's auxiliary antennas, although far weaker, is sufficient. The main antenna broadcasts a tight beam, whereas the others emit over a much larger angle, exchanging signal strength for flexibility in pointing.

This allows an extremely valuable improvement. The spacecraft cannot aim GRaND at the surface and the main antenna at Earth concurrently, because both are mounted rigidly, just as you cannot simultaneously point the front of your car north and the back east. Therefore, in the original plan, gravity measurements and GRaND measurements were mutually exclusive. Now, as Dawn turns throughout its orbit to keep Vesta in GRaND's sights, it can transmit a weak radio signal that is just perceptible at Earth. This enables an even greater science return for the time in LAMO. Unlike the science camera and the visible and infrared mapping spectrometer (VIR), GRaND and gravity observations do not depend on the sun's illumination of the surface. Even as it orbits over a dark, cold, silent landscape, Dawn is fully capable of continuing to build its maps of elements and the interior structure.

The signal from the auxiliary antenna is just sufficient for the measurement of the spacecraft's motion, but it is not strong enough to carry data as well. So the spacecraft is still programmed to point its main antenna to Earth three times each week, allowing the precious GRaND observations that have been stored in computer memory to be transmitted. As always, the myriad measurements of temperatures, voltages, currents, pressures, and other parameters that engineers use to ensure the health of the ship are returned during these communications sessions as well.

Although the pictures of Vesta from survey orbit and the high altitude mapping orbit (HAMO) have exceeded scientists' expectations, not only in quality and quantity but also in the truly fascinating content, as enthusiastic explorers, the Dawn team could not pass up the opportunity for more. When GRaND is pointed at the surface, the camera is too, and already well over one thousand images have been returned, revealing detail three times finer than the spectacular images from HAMO. For readers who cannot go to Vesta on their own, go here for a selection of the best views, each showing surprising and captivating alien landscapes.

In addition to the bonus photography, beginning in January VIR will take observations. Although the instrument has already acquired nearly seven million spectra in the higher orbits, this new vantage point will allow sharper resolution, just as it does for the camera.

The ultra-long-distance communication between Dawn and Earth requires extraordinary technology on both ends. Even with all the sophistication, the amount of information that can be transmitted in a given time remains very limited. The remote spacecraft sends data at speeds significantly lower than a typical home Internet connection. Engineers use that precious communications link very carefully, judiciously selecting what information to instruct the probe to return. Because of the high priority given to GRaND, which needs to be pointed at the surface as long as possible, much of the limited time spent with the main antenna aimed at Earth is devoted to transmitting that instrument's findings (and the measurements of spacecraft subsystems). This restricts how much data from the camera and VIR can be communicated.

In the next log, we will see another limitation on the number of camera images and VIR spectra in LAMO. It is a consequence of another aspect of the complex operations in this low orbit around a massive body, and that is the small but real differences between the predicted orbit and the actual orbit. We will cover the first part of the explanation here.

Navigators use their best knowledge of the many forces acting on Dawn to chart an orbital course for it. The forces can be traced to three principal sources: gravity, light, and Dawn itself. We have discussed all of these before in detail (see, for example, this explication of the last two), but let's review them here. This is an involved story, so readers are advised to be in a comfortable orbit while following it. You can safely skip the next four paragraphs and no one ever need know.

Vesta has a complicated gravity field, and that leads to a complicated orbit. The spacecraft does not follow a perfectly circular, repetitive path because the gravitational pull on it changes according to where it is as the colossus beneath it rotates and it loops around. The map of the gravity field has been improving throughout Dawn's residence there, but its completion awaits the LAMO gravity measurements. In the meantime, unknown details of the variation of mass lead to small divergences in the orbit. All the other bodies in the solar system exert gravitational pulls on the spacecraft as well (just as they do on you), but those are more easily accounted for. The distances from Dawn are so great that the variations in their gravity fields don't matter. So although the effects of the faraway objects need to be accounted for, they do not contribute much to the discrepancies.

Dawn depends on sunlight for its power, using its large solar arrays to make electricity to run all systems. The sun also propels the spacecraft, because in the frictionless conditions of spaceflight, the ship recoils slightly in response to the miniscule but persistent pressure of the light. The force depends on whether the light is absorbed (whereupon it is converted to electrical power by the arrays or to heat by whatever component it illuminates) or reflected. If it is reflected, the angle makes a difference, so smooth shiny surfaces that act like mirrors cause different effects from the materials that present a matte finish or are curved or angled. As the spacecraft rotates to keep GRaND pointed at the ground below, different parts of the ship are presented to the sun, so the force from the light changes, and the orbit is constantly subjected to a variable disturbance.

Dawn itself adds to the complexity of its orbital path. The spacecraft carries reaction wheels, which are spun to help it control its orientation. These devices gradually spin faster, so every few days they need to be slowed down. That is accomplished by firing the small reaction control system thrusters during windows specified by mission controllers. In addition to the thrusters providing the needed torque on the craft to reduce the wheels' speeds, they impart a force that changes the orbit slightly.

The physical principles underlying all these phenomena that perturb Dawn's orbit are understood with exceptional clarity. Although the values of the myriad parameters involved are ascertained quite accurately, they are not known perfectly. As a result, navigators' prediction of the ship's course includes some degree of uncertainty. Even their ability to determine the present orbit is subject to a variety of small errors typical in sensitive physical measurements.

For all of these reasons, the craft's actual orbit departs slightly from the plan, and the deviations tend to grow, albeit gradually. As designers expected, in survey orbit and HAMO, the differences were small enough that they did not affect the complex operations plans. Analysis well before Dawn arrived at Vesta predicted that the discrepancies in LAMO would be large enough that occasional adjustments of the orbit would be necessary. Therefore, mission controllers scheduled a window every week (on Saturdays, as it turned out) to use the ion propulsion system to fine-tune the spacecraft's trajectory, bringing it back to the intended orbit. These are known as "orbit maintenance maneuvers," and succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)

The LAMO phase began on December 12, and OMMs were performed on December 17 and 24. In contrast to the long periods of thrusting required with ion propulsion for other parts of the mission, the corrections needed were so small that each OMM needed less than 15 minutes. The whisper-like thrust changed the spacecraft's speed by less than five centimeters per second (one-tenth of a mph). But that was enough to nudge Dawn back to the planned orbit.

The ship was so close to the designated course that the OMMs for December 31 and even January 7 have already been canceled. Not executing the OMMs allows the probe to spend more time collecting neutrons and gamma rays from Vesta. The operations team productively uses the time saved in designing, checking, and transmitting the OMM commands to do other work to ensure LAMO proceeds smoothly and productively.

In the last log we discussed the complicated and dynamic spiral descent from HAMO to LAMO, which was still in progress. The flight required not only reducing the altitude from 680 kilometers (420 miles) to 210 kilometers (130 miles) but also twisting the plane of Dawn's orbit around Vesta. As with all orbiting bodies, whether around Vesta, Earth, or the sun, the lower the orbital altitude, the shorter the orbital period. Vesta's gravitational grip strengthened as Dawn closed in, forcing the spacecraft to make faster loops around it. This meant that as the probe performed the intricate choreography to align its ion thruster with the changing direction needed to alter its orbit, it had to pirouette faster.

When engineers command Dawn to rotate, they usually instruct it to use the same stately speed as the minute hand on a clock. The spacecraft may have to move a little faster however, as it pivots to keep its solar arrays pointed at the sun while accomplishing the required turn. Sometimes it knows that at the end of a turn, it will have to initiate another turn. For example, it may rotate to the orientation required to begin a session of ion thrusting. But while it is thrusting and curving around its orbit, it generally needs to steer the thruster to execute the maneuver. As a result, the robot may choose to turn at a slightly different rate from what its human team members command in order to make a smooth transition from the first turn to the second.

On Dec. 3, when preparing for one of the final thrust segments required to reach LAMO, the combination of all these factors caused the spacecraft to rotate faster than usual. That led to a temporary discrepancy between where it was pointed and where it expected to be pointed during the turn. When protective software detected the inconsistency, it interrupted the ongoing activities and put the spacecraft into safe mode.

When the safe mode signal was received by the Deep Space Network, the operations team responded with its usual calm and skill. They quickly determined that Dawn was fully healthy, diagnosed the cause of the safing, and began guiding the spacecraft back to its normal operational configuration. In addition, they devised a new flight profile that would compensate for the thrusting that was not completed. The team also determined how to prevent the same problem from recurring for subsequent maneuvers. While doing all this work, they were putting the finishing touches on the first LAMO science observation sequences. Controllers managed to complete everything flawlessly and even kept the mission on schedule, allowing LAMO to commence on Dec. 12.

The general plan for Dawn's three-month approach plus one year in orbit around Vesta was described in logs in 2010. The time was apportioned among the different science phases and the transfers between science orbits to ensure a comprehensive and balanced exploration of this mysterious and fascinating world. Fully appreciating that in such an exceedingly ambitious undertaking, some unexpected problems are inevitable, mission planners worked hard to devise an itinerary that left 40 days uncommitted. Their strategy was that as they recovered from anomalies, they would draw from that time and still not have to compromise any of their carefully designed activities. They also planned that any unspent margin would be used to extend LAMO.

To the great delight (and, to be honest, surprise) of all, not one day of the 40-day reserve has been needed. Although there have indeed been unanticipated difficulties, from the beginning of approach on May 3 to this point, the team has been able to resolve all of them without having to withdraw from that account. This is remarkable considering that Dawn is the first visitor from Earth to Vesta, with its many unknown physical properties. This expedition is the first ever in which humankind has sent a spacecraft to orbit such a massive body without first conducting a reconnaissance with a flyby spacecraft. Dawn has maintained a rapid pace of scrutinizing its enigmatic destination. Performing all of this so successfully without needing to use even a little of the spare time they provided for themselves was considered quite unlikely. And yet the entire 40 days remain available.

More ambitious operations lie ahead, with the rest of LAMO, the spiral ascent to HAMO2, HAMO2 itself, and the escape in July to begin the long interplanetary cruise to reach Ceres on schedule in February 2015. We will see in 2012 that each of these phases includes new challenges, and it is certain new problems will arise. Nevertheless, all 40 days are being used to extend LAMO. Therefore, the indomitable explorer will remain at this low altitude through the end of March, continuing to tease out secrets about the dawn of the solar system and revealing more startling and thrilling discoveries on behalf of everyone on distant Earth who yearns to reach out into the vastness of space.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 2.79 AU (418 million kilometers or 260 million miles) from Earth, or 1045 times as far as the moon and 2.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.

Dr. Marc D. Rayman
6:00 p.m. PST December 30, 2011


  • Marc Rayman

Diagram showing the solar arrays of the Rosetta and Dawn spacecraft

Recently, one of our fans on the NASAJPL Facebook page asked a good question about the efficiency of solar arrays on the Dawn and Rosetta spacecraft.

"A question about Dr. Marc D. Rayman's comment in his Dawn journal, saying that 'its tremendous solar arrays [are] the most powerful ever used on an interplanetary mission.' Is that really true? According to JPL's Dawn website, the solar arrays have a total span of 19.7 meters. By comparison, each of Rosetta's two arrays is 14 meters in length (28m total). Are Dawn's arrays so much more efficient? Thanks."

Here's an answer from Dawn Chief Engineer Marc Rayman:

Yes, this is really true. Dawn's solar arrays, although smaller than Rosetta's, could indeed produce more power because they are more efficient. Fortunately, it is not a competition! Both missions seek fascinating new insights into the complex history and character of the solar system as they take all of us on adventures to exotic destinations. To overcome some of the daunting challenges of traveling moderately far from the sun, engineers on each mission have turned to powerful solar arrays.

Rosetta is a fabulous mission, promising exciting results from comet Churyumov-Gerasimenko. It is with the greatest enthusiasm that I look forward to the astonishing discoveries that await its rendezvous with this solar system relict.

The spacecraft carries the largest solar arrays ever flown on an interplanetary mission. The two 14-meter (47-foot) arrays project in opposite directions from the main spacecraft itself, creating a structure about 32 meters (105 feet) tip-to-tip, and the total area of solar cells is 53 square meters (573 square feet). Composed of silicon, these cells could have produced somewhat in excess of seven kilowatts when at Earth's distance from the sun. Of course, Rosetta did not need that much power, but as it travels into the depths of space, every watt will be precious. When the spacecraft is more than five times Earth's distance from the sun and the light from our star is much weaker, the giant arrays still will generate 400 watts, just enough to keep the probe operating. (Rosetta will arc out to that distance on its way to the comet, but it will be closer to the sun, and hence able to produce more power, when it arrives and conducts its investigations of this mysterious body).

Dawn's solar arrays, while the largest used on a NASA interplanetary mission, are smaller than Rosetta's. This bird's wingspan is about 20 meters (65 feet), and the solar arrays, each more than eight meters (27 feet) in length, have a total of about 32 square meters (341 square feet) to capture sunlight. The panels are populated with advanced cells composed of three different materials that work together to convert a larger percentage of the incident light into electrical power. The combination of indium gallium phosphide, indium gallium arsenide, and germanium makes these cells so much more efficient that despite the smaller collecting area, together they produce higher power under the same conditions. These arrays could have generated more than 10 kilowatts at Earth's distance from the sun. Dawn not only did not need such tremendous power, but like Rosetta, it was not even capable of using it all. But it too ventures far from home to remote locations where sunlight is less abundant.

Dawn's ambitious mission to orbit the two most massive residents of the asteroid belt, Vesta and Ceres, would be quite impossible without its use of ion propulsion. The key to ion propulsion's extraordinary capability is its conversion of electrical power into thrust, so Dawn carries such powerful arrays to ensure that even when exploring dwarf planet Ceres at three times Earth's distance from the sun, it can produce sufficient power to thrust and operate all other systems. I describe more about the importance of power to the mission in my Dawn Journal of July 27, 2008.

I appreciate your interest in Dawn, and I hope you will continue to join us as we travel to two of the last unexplored worlds in the inner solar system. In only 10 months, Dawn will become the first spacecraft ever to orbit a resident of the main asteroid belt as it begins its exploration of protoplanet Vesta, and to put it quite simply, this is going to be really cool!

Join the Facebook conversation at http://www.facebook.com/NASAJPL.


  • Marc Rayman

Artist concept of NASA's Dawn spacecraft

Dear Daw9.0s,

A new version of the Dawn spacecraft is continuing the ambitious journey through the asteroid belt to uncharted distant worlds. Now holding a new solar system record, the probe is thrusting with its ion propulsion system, patiently and gently changing its orbit around the sun to match that of the immense protoplanet Vesta (and subsequently dwarf planet Ceres).

Even as Dawn continues pushing deeper into space, another spacecraft that used ion propulsion to conduct an exciting mission at a near-Earth asteroid has concluded. After traveling to and studying the diminutive Itokawa, Japan’s Hayabusa spacecraft returned to Earth on June 13. This was long one of your correspondent’s favorite missions, and he has joined many, many other enthusiasts in congratulating the team responsible for this impressive achievement.

When Dawn reaches each of its destinations, it will have a very full program of activities to acquire pictures and other scientific information. Brief overviews of some of its plans for Vesta were described in recent logs, and more will be presented later. To accomplish its mission of exploration, the spacecraft needs some enhancements to the capabilities it has been using for its travel through deep space to reach its targets. Those new capabilities are now onboard.

For the third time since it left Earth in September 2007, the spacecraft has received an upgrade of the software that runs in its primary computer. With a sense of grandeur and drama befitting this unique adventure, ever-poetical engineers fulfilled their dream of more than a year by denominating it OBC flight software version 9.0. Revealing their surprisingly cute and playful nature, however, most Dawn team members prefer the hypocorism “9.0” (or “nine oh”).

Engineers at JPL and Orbital Sciences Corporation began work on 9.0 almost immediately after 8.0 was installed on the spacecraft in April 2009. They continued with the careful and deliberate process of modifying the software until January, when the extensive test program commenced. It was crucial to verify not only that the new functions would work correctly but also that no unintended differences were introduced and that the existing capabilities were not compromised.

The latest software has 23 sets of changes from the previous version. Some of these are new methods of controlling the way the spacecraft will point its sensors at Vesta and Ceres in order to optimize the acquisition of data. Other modifications, based on experience gained in the ongoing operation of the spacecraft, improve its ability to handle certain potential anomalies on its own. In addition, just as 7.0 and 8.0 did, 9.0 corrects some bugs.

While it may seem quite elementary to load software into a computer that is in control of a spacecraft more than twice as far from Earth as the sun, it actually turns out to be somewhat complex and delicate. Even in “quiet cruise,” the computer is responsible for a great deal of activity onboard. The ion propulsion system was inactive, which is typical when the main antenna is pointed to Earth, but otherwise the computer was busy keeping all systems operating.

To install 9.0, controllers used exactly the same processes they followed for 8.0 in April 2009. It went quite smoothly again this time, right down to the on-time delivery of pizza to mission control during the first day of returning the spacecraft to its normal configuration after rebooting the computer. We know almost all readers accepted the advice offered last year to retain a copy of the log that presented the details of the 8.0 installation, but we happily include a link here for the convenience of the sole reader who did not and wishes to recall what is involved. (For all other readers, congratulations on the handsome profit you have realized on your investment in that previous log.)

As last year, controllers had run a few tests to verify the integrity of some critical components during the normal weekly communications sessions in the weeks leading up to the loading of the new software. On June 15, the spacecraft stopped thrusting on schedule, turned to point its main antenna to Earth, and kept it there rather than returning to the thrust direction a few hours later. That allowed operators to perform the rest of these detailed checks. After confirming that both the primary and backup computers were fully healthy, they transmitted the files containing the new software.

On June 16, with all stations in mission control at JPL reporting all subsystems were healthy and stable, and all systems at the Deep Space Network performing equally well, the command to reset the computer was radioed to the distant ship. The computer dutifully rebooted for the first time since the installation of 8.0 and began running with version 9.0. Whenever the computer reboots, it puts the craft into safe mode. The team verified that the new software was running smoothly and then initiated the process of guiding the spacecraft out of safe mode and back to its normal interplanetary cruise configuration. The schedule had allowed until June 24, but by June 18, the robotic explorer was fully prepared to resume its normal duties.

Because the software upgrade went so well, the Dawn project has decided to present this exciting offer: we will install a functional copy of 9.0 on your computer or smartphone at no charge. Simply place your device in the asteroid belt, send us the coordinates, and we’ll do the rest.

On June 17, after the majority of reconfigurations had been completed and while all members of the team but the insomniacs and the spacecraft itself were slumbering, protective software that is always running onboard detected an increase in the internal friction in reaction wheel no. 4. Reaction wheels are devices used to control a ship’s orientation in the zero-gravity of spaceflight. By electrically controlling the speed of these spinning units, the spacecraft can hold steady or rotate as needed. Dawn is outfitted with four reaction wheels, although it only uses three during normal operations. As we have seen before, operators let each wheel have its turn at being off for a part of the mission. The software that detected the friction in no. 4 responded correctly by powering that unit off. If only three wheels had been in use, it would have activated the unused wheel; but it was unnecessary to do so this time because, by coincidence, all wheels were operating, as is normal when the spacecraft enters safe mode. The team had been planning to turn reaction wheel no. 1 off later on June 17 as part of the reconfiguration. Instead, after taking some time to reassess the spacecraft’s condition, they simply left wheel no. 4 off and continued with their plans, now using wheels 1, 2 and 3 instead of 2, 3 and 4.

Dawn resumed ion thrusting on schedule on June 24. As it continues propelling itself to Vesta, it does so with the recognition that it has accomplished a greater propulsive change in velocity than any other craft ever to leave Earth. Some spacecraft have experienced larger velocity changes through gravitational interactions with planets, but thanks to the extensive use of its extremely efficient ion propulsion system, Dawn surpassed the record for the greatest change in velocity under a ship’s own power on June 5.

The previous record holder, Deep Space 1, was the first interplanetary mission to use ion propulsion. In its 11-month primary mission of testing advanced technologies (including ion propulsion), its two-year extended mission devoted to the exploration of a comet, and its final three-month hyperextended mission of additional technology testing, DS1 accumulated so much thrust time that it achieved an effective change in speed of 4.3 kilometers per second (9,600 mph). (As we have seen in several earlier discussions, such as here, this “effective change in speed” is not the speed at which the craft travels. It is a very commonly used way to express the effectiveness of a spacecraft’s propulsion system that avoids the confounding effects of orbital mechanics.)

Having thrust now for 635 days, or 63 percent of its time in space, Dawn has attained a change of more than 4.4 kilometers per second (9,800 mph), and it has much, much more powered flight ahead.

The record itself and even the total velocity change, while perhaps fun, really are not important, however. They are convenient measures of the progress this ship is making on its ambitious expedition, one that would not have been possible without ion propulsion and other innovations. The exploration of the cosmos is not a competition; it is a shared undertaking of all humankind. Each mission, each record, each accomplishment, each discovery builds on the successes (and even the failures) of those that preceded it and helps pave the way for those that will follow. Together they all contribute to the advancement of our understanding of the universe and our humble place within it.

Dawn is 0.32 AU (48 million kilometers or 30 million miles) from Vesta, its next destination. It is also 2.29 AU (342 million kilometers or 213 million miles) from Earth, or 855 times as far as the moon and 2.25 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 38 minutes to make the round trip.

Dr. Marc D. Rayman
10:30 p.m. PDT June 27, 2010

› Learn more about the Dawn mission


  • Marc Rayman

Artist concept of NASA's Dawn spacecraft

After more than 2.5 years of spaceflight, and more than 6 months in the asteroid belt, Dawn's interplanetary journey continues smoothly. The mission remains on course and schedule for this expedition to the dawn of the solar system.

Our Dawn is not the first spacecraft to use this name, although it is traveling farther from home than any other Dawn. This month 2 more craft traveled into space carrying that appellation, at least when translated into English. The Japanese Aerospace Exploration Agency sent Akatsuki to Earth's neighbor Venus, and Russia's Rassvet module was attached to the International Space Station in Earth orbit. The solar system is vast, however, and there is plenty of room for all such spacecraft. We send our best wishes for success to these other Dawns as they embark on their missions.

While our Dawn patiently and reliably thrusts with its ion propulsion system, gradually reshaping its path around the Sun to match orbits with the protoplanet Vesta, the human members of the team are very busy on distant Earth. Among their many activities is developing the sequences the robotic explorer will use when it begins studying that mysterious, alien world next year. We have seen recently what will occur during the “approach phase” and how Dawn will slip into orbit around Vesta. Now let's have a preview of what the ship will do once it has reached the first science orbit, known as “survey orbit.” Engineers are developing those sequences now, for execution in August 2011.

In survey orbit, the probe will be about 2700 kilometers (1700 miles) above the surface. During the approach phase, navigators will measure the strength of Vesta's gravitational tug on the spacecraft so they can compute the giant asteroid's mass with much greater accuracy than astronomers have yet been able to determine it. (The mass is calculated now using observations of how Vesta perturbs the orbits of other asteroids and even of Mars.) That knowledge will allow them to refine the survey orbit altitude, and they may target it to be somewhat higher or lower, depending on whether Vesta is more massive or less massive than the current calculations show. The sequences for acquiring science data are being designed to accommodate a reasonable range of masses.

Dawn will be in a near-polar orbit. Its trajectory will take it over the north pole (which will be in darkness, because it will be northern hemisphere winter at that time), then over the terminator (the boundary between the illuminated and unilluminated sides), down over the equator, over the south pole, and then across the terminator again to pass over Vesta's night side. Such an orbit allows the spacecraft to have a view of virtually every part of the lit surface at some time. Each revolution in survey orbit will take 2.5 to 3 days to complete. While this may seem like a leisurely pace, the spacecraft will be busy the entire time.

When on the day side of Vesta, Dawn will conduct an intensive campaign of observations. Vesta rotates on its axis in about 5 hours, 20 minutes (one Vestian “day”), which is faster than Dawn will be advancing in its orbit. So from the spacecraft's perspective, as it progresses slowly from north to south, the globe beneath it will complete several turns on its axis. That affords excellent opportunities for mapping the body.

During most of approach, Vesta will be so far away that it will fit comfortably in the fields of view of the science camera and the visible and infrared mapping spectrometer. Before Dawn reaches survey orbit, however, it will be too close to capture all of the expansive surface with its sensors in one glance. On each revolution, the sequences will command the spacecraft to point the instruments through profiles that will allow them to observe as much of the surface as possible.

The primary objective of survey orbit is to get a broad overview of Vesta with color pictures and with ultraviolet, visible, and infrared spectra. The camera will obtain views with 250 meters (820 feet) per pixel, about 150 times sharper than the best images from the Hubble Space Telescope. The mapping spectrometer will reveal much of the surface at better than 700 meters (2300 feet) per pixel. While subsequent science orbits will yield more detail, these first, new perspectives of this ancient world will represent an exciting step in the exploration of the solar system.

Throughout the year at Vesta, gamma-ray spectra and neutron spectra will be recorded with GRaND, and ultrasensitive measurements of the spacecraft's motion using the radio signal will reveal ever greater details of the protoplanet's gravity field and hence its internal structure. Although such information will be acquired in survey orbit, these investigations will benefit most from the lower altitude orbits.

Survey orbit is planned to last for 6 revolutions, or about 17 days. For most of the time it is on the day side, Dawn will fill its memory buffers with images and spectra. For most of the other half of each orbit, as it travels over the night side, the spacecraft will transmit those precious data through its main antenna to eager scientists and all others curious about the cosmos who reside on Earth. (Even when the surface below the spacecraft is in darkness, Dawn itself will be high enough that it will remain in sunlight, so its solar arrays will continue to provide electrical power.) There is so much to see at Vesta, and the instruments generate so much data, that a simple strategy of filling the memory on the day side and emptying it on the night side would be too limiting. Therefore, in the middle of its second, fourth, and fifth passes over the sunlit side, Dawn will halt its acquisition of data to spend a few hours radioing some of its findings to Earth, making more room for subsequent measurements.

Because the program of activities during the residence at Vesta is so full, and it all has to be planned in detail long before Dawn arrives, the project needs plans that are resilient to the inevitable problems, both large and small, that arise in such complex and challenging endeavors. While every observation in survey orbit is of interest, many more are scheduled than are necessary to fulfill the scientific objectives. Therefore, even if some are missed because of glitches in systems on the spacecraft or on Earth, as long as others are acquired, the mission will proceed. With the extremely rich set of measurements planned, there is no intention of repeating some that are lost.

After it has completed its survey of Vesta, Dawn will resume thrusting, spiraling down to its next science orbit for an even closer view. We will learn more about that in a subsequent log.

Meanwhile, as the craft continues to propel itself toward its destination, traveling farther and longer than ever, it will pass 3 milestones on its journey next month. Look for a NASA news release soon on a record it will set as it keeps thrusting with its ion propulsion system. We will describe that in the next log.

On June 23, Dawn will have been in flight for 1000 days. No doubt readers will enjoy taking a minute (at least, for those who read 61,000 words per minute) to reread all the logs since launch to recall some of what has occurred so far during the mission. While much has already been accomplished, the great rewards lie ahead, as Dawn pushes deeper into the asteroid belt, where it will explore faraway new worlds.

On June 3, Dawn will be exactly twice as far from Earth as Earth is from the Sun. Of course, the distance between the planet and the star does not matter for the spacecraft; it is on its own independent journey through the solar system. Nevertheless, such an occasion may provide some terrestrial readers with another opportunity to reflect upon the nature of such a journey. Dawn's trek is not simply that of a robot in space. Although in a narrow sense the ship is sailing the cosmic seas on its own, there is much more to the voyage than that. Such a mission represents a journey by a remarkable species that does not allow its physical confinement to the vicinity of its home planet to keep it from reaching ever farther in its pursuit of knowledge and its quest for grand and noble adventures.

Dawn is 1.96 AU (293 million kilometers or 182 million miles) from Earth, or 760 times as far as the Moon and 1.93 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 33 minutes to make the round trip.

Dr. Marc D. Rayman
9:00 pm PDT May 27, 2010


  • Marc Rayman