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
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.
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
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.
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
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.