Полет АМС Dawn к Весте и Церере

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Dawn Journal: How does Dawn know where "down" is?

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.



http://www.planetary.org/image/IOTD-142-full.jpg

Bright and dark ejecta in one Vesta crater
A small, young, fresh crater has bright and dark ejecta rays extending from it. Dawn photographed it on October 22, 2011, during the High Altitude Mapping Orbit phase of the mission, from an altitude of 700 kilometers. This crater is approximately 5km wide and its ejecta extends for up to 15 kilometers. The ejecta rays outside of the crater are mostly bright. The dark ejecta rays mostly slump into the center of the crater, but there are some dark rays that extend for a few kilometers outside of the crater rim. This combination of bright and dark ejecta rays give the crater an impressively mottled appearance. There is dark and bright material located across Vesta but it is unusual to have a crater with both bright and dark ejecta rays. Credit: NASA / JPL / UCLA / MPS / DLR / IDA

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.

http://www.planetary.org/blog/article/00003348/
http://dawn.jpl.nasa.gov/mission/journal_01_27_12.asp

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http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120210

Markings of ejected material on Vesta's surface

February 10, 2012

This Dawn FC (framing camera) image shows many linear or sinuous grooves crisscrossing the surface of Vesta. These grooves are less than 1 kilometer (0.6 mile) in width. They were created when large pieces of debris, which were ejected when material from space hit Vesta, grazed and scoured the surface. The large, circular depressions in this image are old, heavily degraded craters whose rims are barely visible. These heavily degraded craters have been almost completely filled up by material called regolith. Regolith consists of numerous small particles that were deposited or created by various impacts into Vesta.

This image is located in Vesta's Gegania quadrangle and the center of the image is 10.8 degrees north latitude, 10.5 degrees east longitude. NASA's Dawn spacecraft obtained this image with its framing camera on Dec. 13, 2011. This image was taken through the camera's clear filter. The distance to the surface of Vesta is 189 kilometers (117 miles) and the image has a resolution of about 18 meters (59 feet) per pixel. This image was acquired during the LAMO (low-altitude mapping orbit) phase of the mission.

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http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120209

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http://dawnpub.igpp.ucla.edu/

Dawn images of Vesta! Released!! For everyone!!!
http://www.planetary.org/blog/article/00003377/

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A Valentine from Vesta

This image is based on a Dawn FC (framing camera) image that is overlain by a color-coded height representation of topography. The topography is calculated from a set of images that were observed from different viewing directions, which allows stereo reconstruction. The various colors correspond to the height of the area. The white and red areas in the top of the image are the highest areas and the blue, heart-shaped area in the bottom of the image is the lowest area. This heart-shaped hollow is roughly 10 kilometers (6 miles) across at its widest point.

This image is located in Vesta's Pinaria quadrangle and the center of the image is 36.0 degrees south latitude, 82.3 degrees east longitude. NASA's Dawn spacecraft obtained this image with its framing camera on Dec. 13, 2011. This image was taken through the camera's clear filter. The distance to the surface of Vesta is 272 kilometers (169 miles) and the image has a resolution of about 25 meters (82 feet) per pixel. This image was acquired during the LAMO (low-altitude mapping orbit) phase of the mission. This image is lambert-azimuthal map projected.

The Dawn mission to Vesta and Ceres is managed by NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, for NASA's Science Mission Directorate, Washington D.C. UCLA is responsible for overall Dawn mission science. The Dawn framing cameras have been developed and built under the leadership of the Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany, with significant contributions by DLR German Aerospace Center, Institute of Planetary Research, Berlin, and in coordination with the Institute of Computer and Communication Network Engineering, Braunschweig. The Framing Camera project is funded by the Max Planck Society, DLR, and NASA/JPL.

Image credit: NASA/ JPL-Caltech/ UCLA/ MPS/ DLR/ IDA

http://www.nasa.gov/mission_pages/dawn/multimedia/dawn20120214.html

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More Dawn Vesta approach images: first color views

I'm trying to savor the new Dawn data, and thought briefly about processing it in simulated real time -- which would have meant that, after the first Vesta approach animation I posted on Tuesday, I should have waited ten days before posting any more. Dawn didn't take images continuously as it approached. Unlike chemically propelled spacecraft, which spend most of their time coasting and consequently have free time to gawk about, ion-powered Dawn had to work her engines all the way in to orbit in order to catch up to Vesta at just the right speed and direction. Whenever Dawn took optical navigation images, the thrusting had to stop. So the optical navigation images that I posted before were taken in sets of about 20, once a week at first, and then, as Vesta got closer, twice a week.

But on June 28, when NASA got in touch with Dawn to receive the 11th expected set of approach optical navigation images, they got a rude surprise: Dawn was in safe mode due to a cosmic ray hit to one of the ion propulsion system computer controllers. As Marc Rayman explained in his July 18 Dawn Journal, the team restored the spacecraft to normal flight mode within the day. But they'd already missed the 11th optical navigation session, and because Dawn had not been thrusting during the time she was in safe mode, they were forced to make up for lost time and cancel the 12th set of planned optical navigation images, as well as two planned comm sessions, replacing all that time with thrusting. (Oddly, the compensatory thrusting wound up shifting Dawn's arrival at Vesta earlier than originally planned, by 15 hours.)

But they kept in a crucial observation, the first "rotation characterization" of Vesta. On June 30, Dawn stopped thrusting for a full Vestian day -- five hours and 20 minutes -- and just watched the asteroid rotate. But unlike the previous observations, they used all of Dawn's color filters to acquire the best-ever color photos of the lumpy world. (Best ever, that is, on the day they were taken. The best-ever images are being acquired as I write, with Dawn still in low-altitude mapping orbit at Vesta.)

Here's an animation composed of 12 color images made with red, green, and blue filters, so it's an attempt to approximate true color. Keep in mind that I can't vouch for the accuracy of the slightly greenish-grayish brown that Vesta appears here, because I didn't try to color-calibrate the images (and I'm not sure I would be able to). But I can vouch for the accuracy of the variations among the colors on the surface. There's dark colorful splats and bright splashes; and then there's that wrinkly nose of a mountain that sprouts from Vesta's south pole. This is the first set of Dawn's images where it's really obviously clear that we're looking at a lumpy, cratered mini-planet, and not just a fuzzy blob.



Dawn's first "rotation characterization" of Vesta, July 4, 2011
On July 4, 2011, Dawn performed its first "rotation characterization" of Vesta, watching the asteroid rotate underneath it while taking photos through all color filters. Dawn saw all longitudes of the asteroid, so the images could be used to make the first well-resolved color global map of Vesta. Credit: NASA / JPL / UCLA / MPS / DLR / IDA / color animation by Emily Lakdawalla

While playing with these photos it became clear that all the images in the data set are mirror-reversed. (This isn't a mistake on the Dawn team's part; mirroring happens multiple times from the moment that photons enter a camera's optics to the moment their charge gets read off of the CCD, and whether the images appear "right way around" in any instrument's raw data is up to chance.) I didn't notice it in the earlier, lower resolution images I posted, but it's obvious now, so I've flipped my versions to compensate.



Dawn approaches Vesta, May 3 - June 30, 2011 (animation)
This animation comprises 236 photos taken by Dawn as it approached Vesta over two months from May 3 to June 30, 2011. It includes ten sets of optical navigation photos and one set of "rotation characterization" photos. The images have been enlarged by a factor of two. Credit: NASA / JPL / UCLA / MPS / DLR / IDA / animation by Emily Lakdawalla

So here's an updated approach animation, with the clear frames from that first rotation characterization included. The jump in size to the last set of images is an artifact of that safe mode event. The next set of images was acquired four days later, on July 4. Shall I wait four days now to check them out?

http://www.planetary.org/blog/article/00003384/

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Dawn Journal: Bonus time at low altitude

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. The image below was taken near the lowest altitude in Dawn's orbit.



Vesta from only 190 kilometers away
Seen up close, Vesta's surface is dominated by craters. Dawn took this view during its Low Altitude Mapping Orbit on December 30, 2011, from an altitude of 190 kilometers. The view spans about 180 kilometers. Whether you are looking at a high-resolution or a low-resolution image, various types of impact craters dominate Vesta's surface. The relatively large circular depressions in this image are older, heavily degraded impact craters. The craters with sharper rims are fresher craters. Clusters of small secondary craters were created by the impact of material and boulders that were ejected when larger primary craters formed.

The center of the image is 18.6 degrees south latitude, 346.8 degrees east longitude. Credit: NASA / JPL / UCLA / MPS / DLR / IDA

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.

http://www.planetary.org/blog/article/00003396/
http://dawn.jpl.nasa.gov/mission/journal_02_29_12.asp

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Islands in the darkness
February 27, 2012

This Dawn FC (framing camera) image shows a part of Vesta's northern hemisphere, which is mostly in darkness. Since Dawn's arrival at Vesta the sun has not illuminated Vesta's most northerly latitudes. This is because the tilt of Vesta with respect to the sun has not been suitable for illuminating these parts of the asteroid. But, as Vesta progresses from its winter to its spring more of these northerly latitudes are being illuminated. As seen in this image the illumination begins by the low-angled sun revealing a few higher elevation parts of the surface, while the rest of the lower-lying surface is still covered in darkness. More of this area will need to be illuminated for any meaningful interpretations of it to take place.

This image is located in Vesta's Bellicia quadrangle and the center of the image is 50.6 degrees north latitude, 40.7 degrees east longitude. NASA's Dawn spacecraft obtained this image with its framing camera on Nov. 19, 2011. This image was taken through the camera's clear filter. The distance to the surface of Vesta is 275 kilometers (171 miles) and the image has a resolution of about 25 meters (82 feet) per pixel. This image was acquired during the transfer to LAMO (low-altitude mapping orbit) phase of the mission.

http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120227

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Dawn Sees New Surface Features on Giant Asteroid

PASADENA, Calif. – NASA's Dawn spacecraft has revealed unexpected details on the surface of the giant asteroid Vesta. New images and data highlight the diversity of Vesta's surface and reveal unusual geologic features, some of which were never previously seen on asteroids.

These results were discussed today at the Lunar and Planetary Science Conference at The Woodlands, Texas.

Vesta is one of the brightest objects in the solar system and the only asteroid in the so-called main belt between Mars and Jupiter visible to the naked eye from Earth. Dawn has found that some areas on Vesta can be nearly twice as bright as others, revealing clues about the asteroid's history.

"Our analysis finds this bright material originates from Vesta and has undergone little change since the formation of Vesta over 4 billion years ago," said Jian-Yang Li, a Dawn participating scientist at the University of Maryland, College Park. "We're eager to learn more about what minerals make up this material and how the present Vesta surface came to be."

Bright areas appear everywhere on Vesta but are most predominant in and around craters. The areas vary from several hundred feet to around 10 miles (16 kilometers) across. Rocks crashing into the surface of Vesta seem to have exposed and spread this bright material. This impact process may have mixed the bright material with darker surface material.

While scientists had seen some brightness variations in previous images of Vesta from NASA's Hubble Space Telescope, Dawn scientists also did not expect such a wide variety of distinct dark deposits across its surface. The dark materials on Vesta can appear dark gray, brown and red. They sometimes appear as small, well-defined deposits around impact craters. They also can appear as larger regional deposits, like those surrounding the impact craters scientists have nicknamed the "snowman."

"One of the surprises was the dark material is not randomly distributed," said David Williams, a Dawn participating scientist at Arizona State University, Tempe. "This suggests underlying geology determines where it occurs."

The dark materials seem to be related to impacts and their aftermath. Scientists theorize carbon-rich asteroids could have hit Vesta at speeds low enough to produce some of the smaller deposits without blasting away the surface.

Higher-speed asteroids also could have hit Vesta's surface and melted the volcanic basaltic crust, darkening existing surface material. That melted conglomeration appears in the walls and floors of impact craters, on hills and ridges, and underneath brighter, more recent material called ejecta, which is material thrown out from a space rock impact.

Vesta's dark materials suggest the giant asteroid may preserve ancient materials from the asteroid belt and beyond, possibly from the birth of the solar system.

"Some of these past collisions were so intense they melted the surface," said Brett Denevi, a Dawn participating scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "Dawn's ability to image the melt marks a unique find. Melting events like these were suspected, but never before seen on an asteroid."

Dawn launched in September 2007. It will reach its second destination, Ceres, in February 2015.

"Dawn's ambitious exploration of Vesta has been going beautifully," said Marc Rayman, Dawn chief engineer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "As we continue to gather a bounty of data, it is thrilling to reveal fascinating alien landscapes."

To view the new images, visit: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov .

Dawn's mission is managed by JPL for NASA's Science Mission Directorate in Washington. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Ala. UCLA is responsible for overall Dawn mission science. Orbital Sciences Corp. in Dulles, Va., designed and built the spacecraft. The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute are international partners on the mission team. JPL is managed for NASA by the California Institute of Technology in Pasadena.

http://www.nasa.gov/mission_pages/dawn/news/dawn20120321.html

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http://www.nasa.gov/mission_pages/dawn/multimedia/pia15495.html



http://www.nasa.gov/mission_pages/dawn/multimedia/pia15491.html



http://www.nasa.gov/mission_pages/dawn/multimedia/pia15496.html



http://www.nasa.gov/mission_pages/dawn/multimedia/pia15235.html

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Разрешение фото 19-25 м на пиксель

Blocks of ejected material and small craters near a crater rim
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120322



Lines of craters
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120326



Dark and bright material in a crater wall
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120328

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Dawn Journal[/size]

March 29, 2012

http://dawn.jpl.nasa.gov/mission/journal_03_29_12.asp

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Mission Status

March 29, 2012

Dawn's Investigations of Vesta Continue

Dawn has had another very productive month of using all sensors to investigate Vesta. The spacecraft is operating very smoothly in its low-altitude mapping orbit.

Meanwhile, as Earth and Dawn follow their separate orbits around the sun, the probe is now on the opposite side of the sun from Earth. As explained in the most recent Dawn Journal, you can find the ship's approximate location by using the sun as a reference. Only a few probes have ever operated so far from home.

http://dawn.jpl.nasa.gov/mission/status.asp

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Unusual bipolar crater
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120330

[sol]

Интересен механизм осыпей в условиях малой гравитации...
странные картинки

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Dark material in the ejecta of a small crater
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120405



Spots of dark material surrounding an impact crater
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120406



An old crater almost completely filled with regolith
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120409

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Dawn Gets Extra Time to Explore Vesta

PASADENA, Calif. – NASA's Dawn mission has received official confirmation that 40 extra days have been added to its exploration of the giant asteroid Vesta, the second most massive object in the main asteroid belt. The mission extension allows Dawn to continue its scientific observations at Vesta until Aug. 26, while still arriving at the dwarf planet Ceres at the same originally scheduled target date in February 2015.

"We are leveraging our smooth and successful operations at Vesta to provide for even more scientific discoveries for NASA and the world." said Robert Mase, Dawn project manager based at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "This extra time will allow us to extend our scientific investigation and learn more about this mysterious world."

The extension will not require any new funding, and will draw on financial reserves that have been carefully managed by the Dawn project. The flexibility provided by the spacecraft's use of efficient ion propulsion system allows it to maintain its originally planned Ceres arrival.

The extension allows for extra observations at Dawn's current low-altitude mapping orbit (average altitude 130 miles or 210 kilometers), which will now last until May 1. The additional time enables the gamma ray and neutron detector to build the best possible maps of the elemental composition of Vesta's surface and improve data for the gravity experiment, the two primary scientific investigations at the low-altitude orbit. The spacecraft's camera and spectrometer are also obtaining additional high-resolution images.

Additional time will also be spent in the planned second high-altitude mapping orbit later this summer. When Dawn arrived at Vesta in July 2011, much of the northern hemisphere was in shadow. But with the passage of time, more of that area will bask in sunshine.

"Dawn has beamed back to us such dazzling Vestan vistas that we are happy to stay a little longer and learn more about this special world," said Christopher Russell, Dawn's principal investigator at UCLA. "While we have this one-of-a-kind opportunity to orbit Vesta, we want to make the best and most complete datasets that we can."

Dawn's mission to Vesta and Ceres is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., for NASA's Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena. Dawn is a project of the directorate's Discovery Program, managed by NASA's Marshall Space Flight Center in Huntsville, Ala. UCLA is responsible for overall Dawn mission science. Orbital Sciences Corp. in Dulles, Va., designed and built the spacecraft. The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute are international partners on the mission team.

For more information about the Dawn mission, visit: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov .

http://www.nasa.gov/mission_pages/dawn/news/dawn20120418.html

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Smooth, grooved surface
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120412



Grooved surface and area of boulders
http://dawn.jpl.nasa.gov/multimedia/imageoftheday/image.asp?date=20120418

[Space Alien]

Dawn Gets Extra Time to Explore Vesta

Статья на русском:

НАСА продлило на 40 дней работу зонда Dawn у астероида Веста

Американское аэрокосмическое агентство НАСА официально продлило на 40 дней время пребывания автоматического зонда Dawn возле астероида Веста, что позволит ученым изучать это небесное тело вплоть до 26 августа, сообщает пресс-служба Лаборатории реактивного движения НАСА.

"Мы пытаемся извлечь большее из удачного и беспроблемного течения миссии во время пребывания у Весты для того, чтобы получить больше научных сведений для мира и НАСА. Это дополнительное время поможет нам расширить научную программу и узнать больше об этом таинственном небесном теле", - заявил руководитель миссии Dawn Роберт Мейз (Robert Mase) из Лаборатории реактивного движения НАСА в городе Пасадена (США).

Как отмечает пресс-служба лаборатории, расширение миссии не потребует дополнительных средств, так как руководству проекта удалось сохранить часть изначально выделенных средств. Кроме того, высокая эффективность ионных двигательных установок зонда позволяет не беспокоиться о том, что зонд не сможет вовремя достигнуть точки "рандеву" со следующим небесным телом - астероидом Церерой.

Зонд пробудет на текущей низкой орбите до начала мая, продолжая составлять минеральную и гравитационную карту астероида. После этого он перейдет на высокую орбиту, где пробудет до конца августа. Это позволит изучить северное полушарие Весты, которое было скрыто тенью в июле 2011 года, в момент встречи Dawn и астероида.

"Dawn передал на Землю столь завораживающие пейзажи Весты, что мы решили остаться еще на чуть-чуть и узнать больше об этом уникальном астероиде. И пока у нас есть уникальная возможность изучать Весту с орбиты, мы хотим извлечь наиболее полные наборы данных о ней", - добавил Кристофер Рассел (Christopher Russell) из университета Калифорнии в Лос-Анджелесе.

http://ria.ru/science/20120419/629889044.html[/quote]

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Dawn Reveals Secrets of Giant Asteroid Vesta

...

PASADENA, Calif. – Findings from NASA's Dawn spacecraft reveal new details about the giant asteroid Vesta, including its varied surface composition, sharp temperature changes and clues to its internal structure. The findings were presented today at the European Geosciences Union meeting in Vienna, Austria, and will help scientists better understand the early solar system and processes that dominated its formation.

Images from Dawn's framing camera and visible and infrared mapping spectrometer, taken 420 miles (680 kilometers) and 130 miles (210 kilometers) above the surface of the asteroid, show a variety of surface mineral and rock patterns. Coded false-color images help scientists better understand Vesta's composition and enable them to identify material that was once molten below the asteroid's surface.

Researchers also see breccias, which are rocks fused during impacts from space debris. Many of the materials seen by Dawn are composed of iron- and magnesium-rich minerals, which often are found in Earth's volcanic rocks. Images also reveal smooth pond-like deposits, which might have formed as fine dust created during impacts settled into low regions.

"Dawn now enables us to study the variety of rock mixtures making up Vesta's surface in great detail," said Harald Hiesinger, a Dawn participating scientist at M