By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The south pole of the giant asteroid Vesta, as imaged by the framing camera on NASA’s Dawn spacecraft in September 2011. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA |
› Full image and caption

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.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

This 3-D video incorporates images from the framing camera instrument aboard NASA’s Dawn spacecraft from July to August 2011. The images were obtained as Dawn approached Vesta and circled the giant asteroid during the mission’s survey orbit phase. Survey orbit took place at an altitude of about 1,700 miles (2,700 kilometers). To view this video in 3-D use red-green, or red-blue, glasses (left eye: red; right eye: green/blue). Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
› See video

Dear Dawnward Spirals,

Continuing its ambitious campaign of exploration deep in the asteroid belt, Dawn has spent most of the past month spiraling ever closer to Vesta. Fresh from the phenomenal success of mapping the alien world in detail in October, the spacecraft and its human team members are engaged in one of the most complicated parts of the mission. The reward will be the capability to scrutinize this fascinating protoplanet further.

Thanks to the extraordinary performance of its ion propulsion system, Dawn can maneuver to different orbits that are best suited for conducting each of its scientific observations. The probe is now headed for its low altitude mapping orbit (LAMO), where the focus of its investigations will be on making a census of the atomic constituents with its gamma ray and neutron sensors and on mapping the gravity field in order to determine the interior structure of this protoplanet.

As secondary objectives, Dawn will acquire more images with its camera and more spectra with its visible and infrared mapping spectrometer. As we will see in a future log, these measurements will receive a smaller share of the resources than the high priority studies. The spectacular pictures obtained already will keep scientists happy for years, and you can continue to share in the experience of marveling at the astonishing discoveries by seeing some of the best views here, including scenes captured during the spiral to LAMO.

Planning the low altitude mapping orbit around massive Vesta, with its complicated gravity field, required a great deal of sophisticated analysis. Before Dawn arrived, mission designers studied a range of possible gravitational characteristics and honed the methods they would use for plotting the actual orbit once the details of the protoplanet’s properties were ascertained. In the meantime, the team used a tentative orbit at an altitude over the equator of 180 kilometers (110 miles). As explained in a previous log, the altitude varies both because the orbit is not perfectly circular and because Vesta displays such exceptional topography. The highest elevations turn out to be at the equator, and the average altitude of that orbit would be 200 kilometers (125 miles).

Now that navigators have measured Vesta’s gravity, they have the knowledge to refine the design for LAMO, and they decided to raise it by 10 kilometers (6 miles). The target then is an average altitude of 210 kilometers (130 miles). But there is more to the specification of the orbit than simply its height. To meet all of the scientific objectives, the orientation of this orbit needs to be different from the orientation of the previous orbits, the high altitude mapping orbit (HAMO) and survey orbit.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

NASA’s Dawn spacecraft obtained this image with its framing camera on September 20, 2011. This image was taken through the camera’s clear filter. The distance to the surface of Vesta is 673 km and the image resolution is about 66 meters per pixel. Image credit: NASA/ JPL-Caltech/ UCLA/ MPS/ DLR/ IDA
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Dear Dawnderfuls,

Dawn has completed another wonderfully successful phase of its exploration of Vesta, studying it in unprecedented detail during the past month. From the time of its discovery more than two centuries ago until just a few months ago, this protoplanet appeared as hardly more than a fuzzy blob, an indistinct fleck in the sky. Now Dawn has mapped it with exquisite clarity, revealing a fascinatingly complex alien world.

The high altitude mapping orbit (HAMO) includes the most intensive and thorough imaging of the entire year Dawn will reside at Vesta. Spectacular as the results from survey orbit were, the observations from HAMO are significantly better. From four times closer to the surface, Dawn’s sensors provided much better views of the extraordinary surface of craters large and small, tremendous mountains, valleys, towering cliffs, ridges, smooth and flat regions, gently rolling plains, systems of extensive troughs, many clusters of smaller grooves, immense landslides, enormous boulders, materials that are unusually bright and others that are unusually dark (sometimes adjacent to each other), and myriad other dramatic and intriguing features. There is no reason to try to capture in words what visual creatures like humans can best appreciate in pictures. To see the sites, which literally are out of this world, either go to Vesta or go here.

Circling the colossus 680 kilometers (420 miles) beneath it in HAMO, the probe has spent most of its time over the illuminated side taking pictures and other scientific measurements and most of the time over the dark side beaming its precious findings back to eager Earthlings.

Dawn revolves in a polar orbit around Vesta, passing above the north pole, then traveling over the day side to the south pole, and then soaring north over the night side. Each circuit takes 12.3 hours. Meanwhile, Vesta completes a rotation on its axis every 5.3 hours. Mission planners choreographed this beautiful cosmic pas de deux by choosing the orbital parameters so that in 10 orbits, nearly every part of the lit surface would come within the camera’s field of view. (Because it is northern hemisphere winter on that world, a region around the north pole is hidden in the deep dark of night. Its appearance in Dawn’s pictures will have to wait for HAMO2.) A set of 10 orbits is known to Dawn team members (and now to you) as a mapping cycle.

Although the HAMO phase was extremely complex, it was executed almost flawlessly, following remarkably well the intricate plan worked out in great detail last year. It consisted of six mapping cycles, and they were conducted in order of their overall importance. In the first cycle, Dawn aimed its camera straight down and took pictures with all of the instrument’s color filters. In addition to showing the startling diversity of exotic features, the color images provide scientists some information about the composition of the surface materials, which display an impressive variation on this mysterious protoplanet. Cycle 1 yielded more than 2500 photos of Vesta, nearly as many as were acquired in the entire survey orbit phase. These observations were deemed so important that not only were they first, but cycle 6 was designed to acquire nearly the same data. This strategy was formulated so that if problems precluded the successful mapping in cycle 1, there would be a second chance without requiring the small and busy operations team to make new plans. As it turned out, there were only minor glitches that interfered with some of the pictures in cycle 1, but the losses were not important. Nevertheless, cycle 6 did fill in most of the missing views.

Cycles 2 through 5 were devoted to acquiring images needed to develop a topographical map. Instead of flying over the sunlit side with its camera pointed straight down, the spacecraft looked at an angle. Each direction was chosen to provide scientists the best combination of perspective and illumination to build up a three dimensional picture of the surface. Knowing the elevations of different features and the angles of slopes is essential to understanding the geological processes that shaped them.

In cycle 2, the camera constantly was directed at the terrain ahead and a little to the left of the point directly below the spacecraft. Cycle 3, in contrast, looked back and slightly to the left. Cycle 4 pointed straight ahead but by a smaller angle than in cycle 2. Cycle 5 did not look forward or backwards; it only observed the surface to the right. With the extensive stereo coverage in each of these 10-orbit mapping cycles, most of the terrain now has been photographed from enough different directions that the detailed shape of the alien landscape can be determined.

The HAMO observations constitute the most comprehensive visible mapping of Vesta for the mission. The survey orbit images were obtained from a higher altitude and so do not show as much detail. When Dawn flies down to its low altitude mapping orbit (LAMO), its primary objectives will be to measure the atomic constituents with the gamma ray and neutron detector (GRaND) and to map the gravitational field. While some images will be acquired, they will be a secondary objective. The principal resources, both for the spacecraft and for the operations team, will be devoted to the higher priority science. In addition, the probe will be too close in LAMO for its camera to collect enough pictures for a global map. The subsequent observations in HAMO2 will be designed mostly to glimpse some of the northern latitudes that are currently too dark to see.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

This image obtained by the framing camera on NASA’s Dawn spacecraft shows the south pole of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
› Full image and caption | › Read related news release

Dear Dawnniversaries,

Dawn’s fourth anniversary of being in space is very different from its previous ones. Indeed, those days all were devoted to reaching the distant destination the ship is now exploring. Celebrating its anniversary of leaving Earth, Dawn is in orbit around a kindred terrestrial-type world, the ancient protoplanet Vesta.

The adventurer spent August on Vesta’s shores and now it’s ready to dive in. Dawn devoted most of this month to working its way down from the 2,700-kilometer (1,700-mile) survey orbit to its current altitude of about 680 kilometers (420 miles) and changing the orientation of the orbit. (For a more detailed discussion of the altitude, go here.) The sensationally successful observing campaign in survey orbit produced captivating views, revealing a complex, fascinating landscape. Now four times closer to the surface, the probe is nearly ready for an even more comprehensive exploration from the high altitude mapping orbit (HAMO). The plans for HAMO have changed very little since it was described on the third anniversary of Dawn’s launch.

Dawn’s spiral descent went extremely well. We have seen before that bodies travel at higher velocities in lower altitude orbits, where the force of gravity is greater. For example, Mercury hurtles around the sun faster than Earth in order to balance the stronger pull of gravity, and Earth’s speed is greater than that of more remote Vesta. Similarly, satellites in close orbits around Earth, such as the International Space Station, race around faster than the much more distant moon. When it began its spiral on August 31, Dawn’s orbital speed high above Vesta was 76 meters per second (170 mph), and each revolution took nearly 69 hours. Under the gentle thrust of its ion propulsion system, the spacecraft completed 18 revolutions of Vesta, the loops getting tighter and faster as the orbital altitude gradually decreased, until it arrived at its new orbit on schedule on Sept. 18. In HAMO, Dawn orbits at 135 meters per second (302 mph), circling the world beneath it every 12.3 hours.

When Dawn’s itinerary called for it to stop thrusting, it was very close to HAMO but not quite there yet. As mission planners had recognized long beforehand, small differences between the planned and the actual flight profiles were inevitable. Extensive and sophisticated analysis has been undertaken in recent years to estimate the size of such discrepancies so the intricate plans for completing all the work at Vesta could account for the time and the work needed to deliver the robotic explorer to the intended destination. In order to accomplish the intensive program of observations with its scientific instruments, the spacecraft must follow an orbital path carefully matched to the sequences of commands already developed with painstaking attention to detail. The beauty of Dawn’;s artistically choreographed pas de deux with Vesta depends on the music and the movements being well synchronized.

During its descent, Dawn paused frequently to allow controllers to update the flight profile, accounting for some of the variances in its course along the way. Following the completion of thrusting, navigators tracked the ship more extensively as it sailed around Vesta, measuring its orbit with great accuracy. This revealed not only the details of the orbital parameters (such as size, shape, and orientation) but also more about the character of Vesta’s gravity field than could be detected at higher altitudes. With the new information, the team designed two short maneuvers to adjust the orbit. The first, lasting four hours, was executed last night, and the second, half an hour shorter, will be completed tonight. After further measurements to verify the final orbit, the month of HAMO observations will begin on Sept. 29.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

This anaglyph image of Vesta’s equator was put together from two clear filter images, taken on July 24, 2011 by the framing camera instrument aboard NASA’s Dawn spacecraft. The anaglyph image shows hills, troughs, ridges and steep craters. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
› Full image and caption | › Read related news release

Dear Magdawnificents,

Dawn has completed the first phase of its exploration of Vesta with tremendous success, and the peripatetic adventurer is now in powered flight again, on its way to a new location from which to scrutinize its subject. Meanwhile, scientists are deeply engaged in analyzing the magnificent views the stalwart surveyor has transmitted to Earth.

Most of August was devoted to survey orbit. At an altitude of about 2,700 kilometers (1,700 miles), the ship sailed slowly around the world beneath it, completing a loop every 69 hours. Vesta rotates faster, turning once on its axis each 5 hours, 20 minutes. As we saw in the previous log, the survey orbit phase of the mission consisted of seven revolutions around Vesta, providing ample opportunities to acquire the rich bounty of data that scientists yearned for.

As Dawn follows its course, it passes over the north pole, then heads south on the day side of Vesta. On each orbit, it trained its sensors on the illuminated surface and filled its memory with the spectacular sights. On the other half of its orbit, gliding high above the dark landscape, it radioed its findings to distant Earth.

As we discussed last year, Vesta has seasons, just as your planet probably does. For readers on Earth, for example, it is summer in the northern hemisphere, and a region around the south pole is in constant darkness. On Vesta right now, the southern hemisphere is facing the sun, so everywhere between about 52 degrees north latitude and the north pole is in a long night. That ten percent of the surface is presently impossible to see. Because Dawn will stay in orbit around Vesta as together they travel around the sun, in 2012 it will be able to see some of this hidden scenery as the seasons advance.

The campaign of acquiring data in survey orbit was very complex. On the second, fourth, fifth, and sixth loops, the strategy included collecting more than Dawn’s memory could accommodate in the half of an orbit in which it was over sunlit terrain. Therefore, during those orbits, mission planners incorporated instructions to turn away from looking at Vesta to allow the spacecraft to point its main antenna to Earth for five to six hours. That provided time to transmit enough of its precious findings to make room for still more during the rest of the passage over the day side.

On the first and third revolutions, the computer in the visible and infrared mapping spectrometer (VIR) encountered an unexpected condition, so it stopped collecting data. When the spacecraft was next on the night side, controllers reconfigured the instrument so it could resume normal operation for the subsequent lap. Engineers and scientists from Italy who developed the complex device and from JPL are working closely together to establish the underlying cause. They have taken advantage of the extended periods in each orbit when the main antenna is pointing to Earth to run diagnostic tests on the unit. All indications are that it is healthy, and evidence points strongly to the glitches being related to some detail of the mode in which VIR collects and processes data. The team is confident that once they understand the behavior, they will be able to formulate plans to operate the spectrometer in ways that avoid triggering it.

Thanks to the strategy to perform more observations than needed, even with the interruptions, VIR accumulated a fantastic wealth of information. The principal scientific objective of survey orbit was to collect 5,000 sets of spectra or “frames.” A spectrum is the intensity of light at different wavelengths, and each frame consists of visible and infrared spectra at 256 locations on Vesta’s complex and mysterious surface. By the end of survey orbit, Dawn had obtained well in excess of 13,000 frames, or more than three million spectra. Acquiring more than one spectrum of the same location is valuable, as different angles of incident or reflected sunlight allow scientists to gain greater insight into the mineralogical composition and properties of the material. With an initial plan of observing 52 percent of the surface with VIR from survey orbit, the team is elated now to have spectra from about 63 percent.

The science camera has similarly overachieved. The intent was to photograph 60 percent of Vesta, but the entire 90 percent not in the darkness of northern winter has been captured at least five times. With pictures taken from multiple angles, stereo views can be constructed; and images at different times allow features to be observed under varied lighting conditions. All of the camera’s color filters were used, providing coverage in the near infrared and visible. Until recently, Vesta was known as little more than a smudge of light, but now scientists have more than 2,800 photos from Dawn’s survey.

A selection of stunning scenes of the latest world to come into the realm of humankind’s knowledge is here. As scientists pore through the treasure trove, they will continue to add their favorite views to that site.

This mission has already revealed far more about Vesta than a flyby mission could. While much more data will be obtained during the rest of Dawn’s residence there, the six gigabytes from VIR and the three gigabytes from the camera so far are enough to keep researchers busy (and extremely happy!) for a very long time as they tease out the nature of this alien world.

› Continue reading Marc Rayman’s September Dawn Journal

By Marc Rayman

NASA’s Dawn spacecraft has just arrived at its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

NASA’s Dawn spacecraft obtained this image of the giant asteroid Vesta with its framing camera on July 24, 2011. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
› See more images | › See related video

Dear Dawncredibles,

Dawn is now beginning intensive observations of the alien world it orbits. The approach phase, which began on May 3, is complete. Today Dawn is in its survey orbit around Vesta.

Following the previous log, the spacecraft continued using its ion propulsion system to spiral around Vesta, gradually descending to its present altitude of 2700 kilometers (1700 miles). Its flight plan included more observations of Vesta, each one producing incredible views more exciting than the last. Every image revealed new and exotic landscapes. Vesta is unlike any other place humankind’s robotic ambassadors have visited. To continue to share in the thrill of discovery, remember to visit here to see a new image every day during survey orbit. Your correspondent, writing with atypical brevity, also will continue to provide progress reports here at least once a week.

As the ship sailed ever closer to the massive protoplanet during the approach phase, the gravitational attraction grew stronger. We saw in previous logs that astronomers had estimated Vesta’s mass by observing the effect of the 530-kilometer (330-mile) diameter behemoth on distant bodies, including smaller residents of the asteroid belt and even Mars. Now that navigators can detect its pull on nearby Dawn, they are improving that value. Before the explorer’s arrival, Vesta’s mass was calculated to be about 262 billion billion kilograms (289 million billion tons). Now it is measured to be about 259 billion billion kilograms (286 million billion tons), well within the previous margin of error. It is impressive how accurately astronomers had been able to determine the heft of what had appeared as little more than a point of light among the myriad stars. Nevertheless, even this small change of 1.2 percent is important for planning the rest of Dawn’s mission.

› Continue reading Marc Rayman’s August Dawn Journal

By Marc Rayman

NASA’s Dawn spacecraft has just arrived at its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

This is the first image obtained by NASA’s Dawn spacecraft after successfully entering orbit around Vesta. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA › See more images

Dear Residawnts of Vesta,

Dawn has arrived!!

After covering 2.8 billion kilometers (1.7 billion miles) on its own, after traveling for nearly four years through the lonely emptiness of interplanetary space, after being bound by the gravity only of the sun, Dawn is finally in orbit around Vesta. To get here, it gently propelled itself with its ion propulsion system for 70% of its journey, or more than 2.6 years. Deep in the asteroid belt, far from its planet of origin, well beyond Mars (which it visited ever so briefly more than two years ago), where no spacecraft has ever been before, Dawn now resides with a giant.

While more detailed navigational analyses will be required to determine the exact time, around 10 p.m. PDT on July 15, as the spacecraft performed its familiar routine of ion thrusting, its orbit around the sun finally was so close to that of Vesta that the protoplanet’s gravity could take hold of it. Dawn was only about 16,000 kilometers (9,900 miles) above the ancient, scarred surface of the alien world. Traveling together around the sun at more than 20.5 kilometers per second (46,000 mph), their orbits were so similar that the cosmic craft was closing in at the leisurely speed of only 27 meters per second (60 mph). The last time it approached a nearby destination so slowly was in April 2007. At that time, it used more conventional propulsion technology: it rode on a truck from Washington, DC to Cape Canaveral, Florida.

That may be too many numbers for some readers (and too few for our good friends the Numerivores). But it all reduces to one cool fact: humankind has succeeded in delivering an interplanetary spaceship to orbit around one of the largest objects in the main asteroid belt between Mars and Jupiter. Indeed, Dawn is the first spacecraft to orbit any object in the main belt.

The probe slipped gently into orbit with the same grace it has displayed during its nearly 1000 days of ion thrusting through the solar system. Although the unusual nature of the spiral capture has been explained in detail before, there is one important difference (in addition to some minor ones) from previous descriptions: now it is history.

Dawn has orbited two other bodies. Shortly after it left Cape Canaveral atop a fiery rocket, the spacecraft spent about 45 minutes in Earth orbit, waiting for the proper orbital alignment to begin its ambitious deep-space voyage. Once the rocket gave it enough energy to leave the planet behind, Dawn orbited the sun as surely as Earth and the other planets do, although, of course, it spent most of its time reshaping that orbit. Now it is orbiting Vesta, as surely as the moon orbits Earth.

Entering orbit around the protoplanet is essential to Dawn’s plans for comprehensive studies of this exotic world, but simply being in orbit is not adequate. The craft did not miss a beat as it flew into Vesta’s grasp; it is spiraling around its new master as it aims for its first science orbit at an altitude of 2700 kilometers (1700 miles). The intensive scrutiny of Vesta from survey orbit will begin in the second week of August.

It’s a noteworthy coincidence that Earth and Vesta will happen to be very well aligned then. As they follow their independent orbits around the sun, occasionally their paths bring them relatively near to each other. So just as Dawn begins looking closely at Vesta, so too can residents of Earth. The protoplanet is the brightest object in the asteroid belt, and the only one ever visible to unaided terrestrial eyes, although binoculars or a telescope make it much easier to spot, especially under skies that are brightened by the lights of cities.

Even when their separation is at its minimum, Earth and Vesta will come only to within about 1.23 AU (184 million kilometers or 114 million miles) of each other. While their closest approach is late at night on July 31, the geometry changes slowly enough that there are good viewing opportunities well before and after. Go here for guidance on how to find Vesta in the constellation Capricornus. And if you are fortunate enough to glimpse that distant point of light, let your imagination add to the scene the recent immigrant from Earth, representing you and the rest of humankind on its mission of exploration. There, far from its erstwhile home and the beings who urge it on, this ambitious adventurer is translating that dot of light among the myriad stars into an exciting and fascinating account of the dawn of the solar system.

Dawn has spent most of its time since the last log thrusting as usual. The thrusting even at the time it was captured by Vesta’s gravity was no different. We have seen before that, in stark contrast to the tension when other missions enter orbit, with ion propulsion, the process is very calm indeed. For that matter, since May 2010, Dawn has thrust with its radio transmitter turned off, devoting that precious power to accelerating xenon ions rather than generating radio waves. The ship continued in silence when it went into orbit on Friday night. Mission control was empty, there being no need to monitor the probe’s operation. In fact, your correspondent was dancing, confident that the pas de deux being performed 188 million kilometers (117 million miles) away would be executed with graceful beauty and flawless precision.

Confirmation that Dawn was in orbit came shortly before 11:30 pm PDT on July 16 (more than 24 hours after it glided into orbit) when its radio signals were received at the Deep Space Network. Following its preprogrammed sequence of instructions, the spacecraft acquired more images of Vesta earlier in the evening and then initiated communications with Earth right on schedule. Observing that it was in good health and continuing to perform all of its functions demonstrated that it had achieved orbit. The choreography was beautiful!

Reliable as Dawn is, it did experience an unexpected interruption in thrust recently. On June 27, a cosmic ray, a high energy subatomic particle traveling through space, apparently managed to strike an electrical component on the spacecraft in an especially effective way. The component is used by the ion propulsion system computer controller to operate valves in the complex plumbing that transports xenon from the main tank to the operating thruster. The propellant needs to be delivered at just the right rate for optimal performance. When the cosmic ray deposited its energy in that device, it deprived the circuit of the ability to send signals to the valves, even when directed to do so by the computer. (A cosmic ray is the most likely culprit, but other explanations for the circuit’s inaction are still being considered.) As a result, when it was time to open valves to feed a little more xenon into the thruster, the controller was unable to comply. The computer detected the problem, followed the appropriate procedure for terminating thrust, and alerted the main spacecraft computer. That computer correctly responded by canceling other planned activities and commanding the ship into one of its safe modes. In this case, because all other systems were healthy, it was not necessary to invoke the normal safe mode. Rather, the robot properly chose to make fewer reconfigurations. It pointed its main antenna to Earth and transmitted its status, awaiting a response from controllers.

The Deep Space Network began a routine communications session early on June 28, and the Dawn team immediately understood the spacecraft condition. Before the end of the day, they had restored it to its normal flight mode and made preparations to activate the other controller.

Dawn had been using controller #1 and ion thruster #3 since December. With the controller unable to operate valves, engineers instructed the ship to switch to controller #2, which was in command for most of the thrusting in 2010. Its ability to operate the valves was not compromised. That unit can be used with thruster #2 and #3, but it was faster to formulate commands to use thruster #2, so in the interest of time, that was the choice.

Later this summer, engineers will conduct tests with controller #1 to assess its health and determine whether its valve signals can be restored. That controller operates thruster #1 and #3. Mission planners had long ago decided not to use venerable #1 for the rest of the mission, as it requires slightly more power than its siblings, so whether controller #1 will be fully functional or not, Dawn’s extraterrestrial expedition can be completed as planned with controller #2.

Once the spacecraft had deviated from its intended flight plan by not thrusting, navigators had to devise a new plan to fly to Vesta. To ensure there would be enough time to make up for the lost thrust, they removed one of the navigation imaging sessions (and the communications period that followed it) from the schedule and another routine communications session. Of course, as experienced interplanetary explorers, Dawn’s mission team had always recognized that glitches could interfere with any activity, so more imaging and more communications had been planned than truly were required. Doing without a few to allow time for some compensatory thrusting was easily accommodated.

In order to resume thrusting quickly, controllers chose not to optimize the plan but rather simply to devise a plan that was adequate. The consequence was that they ended up giving Dawn more time to thrust than it really even needed. The entire episode beginning with the balky controller cost 1.2 days of thrust, and the revised plan added 1.8 days of thrust at other times. As a result, the insertion into orbit shifted 15 hours earlier. Such flexibility is another of the many differences between missions that use ion propulsion and those that use conventional propulsion.

Before restarting its powered flight, however, the team was eager to allow Dawn to conduct its first planned observation of Vesta throughout one full rotation of the protoplanet on its axis, a Vestian day of 5 hours 20 minutes. (This and other activities during the approach phase were described last year.) Thanks to the fleet and flawless work of the team, that was carried out on schedule on June 29-30, and all the planned images were acquired. The visible and infrared mapping spectrometer (VIR) also peered at Vesta to provide additional information for use in setting instrument parameters for the science observations in survey orbit. After it acquired two excellent sets of data, its internal computer detected an unexpected condition, so it did not complete the rest of its activities. As the camera’s images were beaming back to Earth on June 30, engineers verified that VIR was in good condition, and they will study its telemetry further as they continue to plan for its important measurements of the minerals that compose Vesta’s surface.

In the original itinerary, ion thrusting would recommence after the communications session on June 30. And that is exactly what occurred, even with the unplanned thrusting hiatus in the preceding days. Dawn continued closing in on Vesta with the gentle pressure of thruster #2, just as it still is today.

As a reminder, an easy way you can have the same otherworldly view of Vesta as Dawn is to visit here. These logs generally will not provide interpretations of the rich bounty of images (but they are fantastic, aren’t they?) or other fascinating measurements. As the data are assessed by Dawn’s team of planetary scientists from four countries, news of the results will be distributed by NASA’s and JPL’s news organizations. And for more frequent updates on the progress of the mission than are provided in these logs, readers may want to go here, where your correspondent abandons his idiolect to provide extremely brief reports much more often (with much less, ahem, color).

On July 9-10, the spacecraft’s agenda included another pause in thrusting. This time, in addition to acquiring its second set of images while Vesta completed a full rotation, Dawn photographed the space around Vesta in search of moons. Remote observations with the Hubble Space Telescope and other observatories on Earth had not found any, but that did not rule out their presence. As no moons had been detected yet, however, they would have to be small and therefore faint. In order to try to discover whether there might be any, the camera used different exposures, some as long as 4.5 minutes. (For photographers, the effective shutter speed for the pictures of Vesta that reveal its surface features is 1/125 of a second.) The spacecraft pointed its camera around Vesta and acquired 72 images. Three hours later, it imaged the same locations, and then another nine hours after that, it repeated the sequence once again. The pictures are being scrutinized for points of light that shift position from one set of images to another, betraying the orbital motion of natural satellites of Vesta.

Although those results are not yet available, we now know with certainty that Vesta does have a moon. Its name is Dawn!

Dawn is 11,000 kilometers (6,800 miles) from Vesta, closer than many terrestrial satellites are to Earth. It is also 1.25 AU (187 million kilometers or 116 million miles) from Earth, or 470 times as far as the moon and 1.23 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 21 minutes to make the round trip.

› Read Marc Rayman’s previous Dawn Journals

By Amy Mainzer

Over the course of the nine months we’ve been operating WISE, we’ve observed over 150,000 asteroids and comets of all different types. We had to pick all of these moving objects out of the hundreds of millions of sources observed all over the sky — so you can imagine that sifting through all those stars and galaxies to find the asteroids is not easy!

We use a lot of techniques to figure out how to distinguish an asteroid from a star or galaxy. Even though just about everything in the universe moves, asteroids are a whole lot closer to us than your average star (and certainly your average galaxy), so they appear to move from place to place in the WISE images over a timescale of minutes, unlike the much more distant stars. It’s almost like watching a pack of cyclists go by in the Tour de France. Also, WISE takes infrared images, which means that cooler objects like asteroids look different than the hotter stars. If you look at the picture below, you can see that the stars appear bright blue, whereas the sole asteroid in the frame appears red. That’s because the asteroid is about room temperature and is therefore much colder than the stars, which are thousands of degrees. Cooler objects will give off more of their light at longer, infrared wavelengths that our WISE telescope sees. We can use both of these unique properties of asteroids — their motion and their bright infrared signatures — to tease them out of the bazillions of stars and galaxies in the WISE images.

The first near-Earth asteroid discovered by WISE (red dot) stands out from the stars (blue dots). The asteroid is much cooler than the stars, so it emits more of its light at the longer, infrared wavelengths WISE uses. This makes it appear redder than the stars. Image credit: NASA/JPL-Caltech/UCLA |   › Full image and caption

 
Thanks to the efforts of some smart scientists and software engineers, we have a very slick program that automatically searches the images for anything that moves at the longer, infrared wavelengths. With WISE, we take about a dozen or so images of each part of the sky over a couple of days. The system works by throwing out everything that appears again and again in each exposure. What’s left are just the so-called transient sources, the things that don’t stay the same between snapshots. Most of these are cosmic rays — charged particles zooming through space that are either spat out by our sun or burped up from other high-energy processes like supernovae or stars falling into black holes. These cosmic rays hit our detectors, leaving a blip that appears for just a single exposure. Also, really bright objects can leave an after-image on the detectors that can persist for many minutes, just like when you stare at a light bulb and then close your eyes. We have to weed the real asteroid detections out from the cosmic rays and after-images.

The data pipeline is smart enough to catch most of these artifacts and figure out what the real moving objects are. However, if it’s a new asteroid that no one has ever seen before, we have to have a human inspect the set of images and make sure that it’s not just a collection of artifacts that happened to show up at the right place and right time. About 20 percent of the asteroids that we observe appear to be new, and we examine those using a program that we call our quality assurance (QA) system, which lets us rapidly sift through hundreds of candidate asteroids to make sure they’re real. The QA system pops up a set of images of the candidate asteroid, along with a bunch of “before” and “after” images of the same part of the sky. This lets us eliminate any stars that might have been confused for the asteroids. Finally, since the WISE camera takes a picture every 11 seconds, we take a look at the exposures taken immediately before the ones with the candidate asteroid — if the source is really just an after-image persisting after we’ve looked at something bright, it will be there in the previous frame. We’ve had many students — three college students and two very talented high school students — work on asteroid QA. They’ve become real pros at inspecting asteroid candidates!

This is a screenshot from the WISE moving-object quality assurance system, which helps weed out false asteroid candidates. The top two rows show an asteroid candidate detected in 16 different WISE snapshots, at two different infrared wavelengths. The lower rows show the same patch of sky at different times — they let the astronomers make sure that stars or galaxies haven’t been confused for the asteroid. Image credit: NASA/JPL-Caltech/UCLA

 
Meanwhile, the hunt continues — we’re still trekking along through the sky with the two shortest-wavelength infrared bands, now that we’ve run out of the super-cold hydrogen that was keeping two of the four detectors operating. Even though our sensitivity is lower, we’re still observing asteroids and looking for interesting things like nearby brown dwarfs (stars too cold to shine in visible light because they can’t sustain nuclear fusion). Our dedicated team of asteroid inspectors keeps plugging away, keeping the quality of the detections very high so that we leave the best possible legacy when our little telescope’s journey is finally done.