Posts Tagged ‘asteroids and comets’

Dawn Goes Over ‘n’ Out

Monday, June 4th, 2012

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.

Images of the giant asteroid Vesta taken by NASA's Dawn spacecraft in 2011 and 2012
On May 3, 2011, the mapping camera on NASA’s Dawn spacecraft captured its first image (left) of the giant asteroid Vesta. Only 5 pixels across, the image didn’t provide any new information about the asteroid, but it was important for navigation purposes and provided an exciting first look at Dawn’s eventual target. About five months later, Dawn snapped the much more detailed image on the right from only 700 kilometers (435 miles) from the surface of Vesta and has since provided unparalleled views of the mysterious world. Image credit: NASA/JPL-Caltech

Dear Readers of all Dawnominations,

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

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

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

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

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

› Continue reading Marc Rayman’s Dawn Journal


Dawn Ascends Over Asteroid Vesta

Wednesday, May 2nd, 2012

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.

Artist's concept of the Dawn spacecraft at asteroid Vesta
This artist’s concept shows NASA’s Dawn spacecraft orbiting the giant asteroid Vesta. The depiction of Vesta is based on images obtained by Dawn’s framing cameras. Image credit: NASA/JPL-Caltech |
› Full image and caption

Dear Dawnright Spectacular Readers,

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

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

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

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

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

› Continue reading Marc Rayman’s Dawn Journal


All Eyes on Asteroid Vesta

Friday, March 30th, 2012

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.

Layered young crater as imaged by NASA's Dawn spacecraft
This image from NASA’s Dawn spacecraft shows a young crater on Vesta that is 9 miles (15 kilometers) in diameter. Layering is visible in the crater walls, as are large boulders that were thrown out in the material ejected from the impact. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA |
› Full image and caption

Dear Dawnscoverers,

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

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

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

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

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

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

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

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

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


Getting the Lowdown on Asteroid Vesta

Monday, December 5th, 2011

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.

Still from a 3-D video incorporating images from NASA's Dawn spacecraft
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


Taken In by the Giant Asteroid Vesta

Monday, July 18th, 2011

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.

Latest Image of Vesta captured by Dawn on July 17, 2011
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


Dawn Sets its Sights, and Lens, on Vesta

Friday, June 24th, 2011

By Marc Rayman

NASA’s Dawn spacecraft is less than one month away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

Image of the giant asteroid Vesta from Dawn's approach
NASA’s Dawn spacecraft obtained this image on its approach to the protoplanet Vesta, the second-most massive object in the main asteroid belt. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/PSI. › See more images

Dear Dawnstinations,

Vesta beckons, and Dawn responds. Now more than halfway through its approach to Vesta, Dawn continues creeping up on the destination it has been pursuing since it began its interplanetary travels. The separation between them gradually shrinks as the probe’s ion thrusting brings its orbit around the sun into a closer and closer match with Vesta’s. At the same time, the giant protoplanet’s gravity tugs gently on the approaching ship, luring it into orbit.

Starting at the beginning of the approach phase on May 3, Dawn interrupted thrusting once a week to photograph Vesta against the background stars. These images help navigators determine exactly where the probe is relative to its target. This technique does not replace other means of navigation but rather supplements them. One of the principal methods of establishing the spacecraft’s trajectory relies on accurately timing how long it takes radio signals, traveling, as all readers know, at the universal limit of the speed of light, to make the round trip between Earth and Dawn. Another uses the Doppler shift of the radio waves, or the slight change in pitch caused by the craft’s motion. These sensitive measurements remain essential to navigating the faraway ship as it sails the interplanetary seas.

Despite the very slow approach, the distance is small enough now that observing Vesta weekly is no longer sufficient. To achieve the navigational accuracy required to reach the intended orbit in early August, last week the frequency of imaging was increased to twice per week. In each session, half of the pictures are taken with long exposures to ensure many stars are detectable, thus overexposing the much brighter disc of the nearby Vesta. The other half use short exposures to ensure that the rocky world shows up correctly so its precise location can be measured. The visible and infrared mapping spectrometer has been commanded to observe Vesta during three of these sessions, each time providing valuable information that will help scientists select instrument settings for when Dawn is close enough to begin its detailed scientific measurements.

In addition to the regular campaign of imaging for navigation, mission controllers have other plans in store for the approach phase that were laid out more than a year ago. Twice in the next few weeks, the spacecraft will watch Vesta throughout its complete 5.3-hour rotation on its axis, revealing exciting new perspectives on this uncharted body. The explorer also will search for moons of the alien world.

› Continue reading Marc Rayman’s June 23, 2011 Dawn Journal


Dawn Spacecraft Getting Ready for Vesta

Friday, April 1st, 2011

By Marc Rayman

NASA’s Dawn spacecraft is less than four months away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

Artist's concept of the Dawn spacecraft
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Conndawnsseurs,

Three and a half years after launch, Dawn continues its travels around the sun, maneuvering to take the same orbital path as Vesta. Following its usual pattern, the spacecraft has spent most of the past month gently thrusting with its ion propulsion system. Some of the thrusting this month, however, was not designed to propel Dawn to Vesta. In addition, mission controllers stopped the thrusting to conduct other planned activities.

Spacecraft that use conventional propulsion coast through space most of the time, just as the moon coasts around Earth, and the planets and asteroids coast around the sun. In contrast, Dawn is in powered flight most of the time, using its ion propulsion system to change its orbit. The flight plan requires pointing the ion thruster in just the right direction to deliver the adventurer to its destination. The spacecraft orientation needed to aim the thruster ends up pointing the main antenna in an arbitrary direction. We have seen before that the robotic craft interrupts thrusting for about eight hours each week to direct the antenna toward Earth for communications.

Ever since Dawn’s trajectory was first being designed, long before launch, it has included coast periods for activities that require orientations incompatible with routine thrusting. One such period was the week of March 14; the previous was in July 2010.

Engineers and scientists operate the science instruments about twice each year to ensure they remain in good condition. This time was the last scheduled use of the sensors prior to their observations of Vesta. All tests showed they are in excellent condition and ready to expose the mysteries of the world they are about to visit.

Controllers transmitted upgraded software to each of the two identical science cameras, containing a few improvements over the version installed in July. The procedure went as smoothly as it had for previous software updates, including the first time such an operation was performed. After each camera received its new software, it performed its standard routine of exercises, just as it did only three weeks after reaching space. The tests confirmed that each camera’s electronics, optics, detector, cover, and filter wheel are in perfect condition.

Sometimes the spacecraft is turned to aim the cameras at carefully selected astronomical targets for their tests; other times, they take pictures of whatever stars happen to be in their field of view. This month’s tests were of the latter type, in which the orientation of the spacecraft was set to keep the antenna pointed at Earth. That put stars from a region near the border between Pisces and Cetus in the grasp of the cameras, quite appropriate for a ship voyaging across the cosmic ocean on its way to a distant and unfamiliar land.

Continue reading this entry from Marc Rayman’s Dawn journal …