Posts Tagged ‘asteroids’

Dawn’s Journey: A Power Trip

Tuesday, July 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Dawn's solar arrays are folded to fit inside the nose cone in preparation for launch
The Dawn spacecraft’s solar array wings — pictured here in a folded position in preparation for launch — span 19.7 meters (nearly 65 feet) and are designed to keep the spacecraft powered even as it ventures further from the sun into the remote asteroid belt. Image credit: NASA/JPL-Caltech

Dear Megalodawniacs,

Powering its way through the main asteroid belt between Mars and Jupiter, Dawn continues on course and on schedule for its 2015 appointment with dwarf planet Ceres. After spending more than a year orbiting and scrutinizing Vesta, the second most massive object in the asteroid belt, the robotic explorer has its sights set on the largest object between the sun and Neptune that a spacecraft has not yet visited. This exotic expedition to unveil mysterious alien worlds would be impossible without the probe’s ion propulsion system.

Ion propulsion is not a source of power for this interplanetary spaceship. Rather, the craft needs a great deal of power to operate its ion propulsion system and all other systems. It needs so much that…

We crave power!!

The ion propulsion system is power-hungry. The process of ionizing xenon and then accelerating it to high velocity consumes a significant amount of electrical power, all of which is provided by the spacecraft’s huge solar arrays. With these two wings and its ion tail, Dawn resembles a celestial dragonfly. But this extraterrestrial odonate is a giant, with a wingspan of 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest tip-to-tip length of any probe NASA had ever dispatched on an interplanetary voyage. (Some such spacecraft have had flexible wire-like antennas that reach to greater lengths.) The large area of solar cells is needed to capture feeble sunlight in the remote asteroid belt to meet all of the electrical needs. Each solar array wing is the width of a singles tennis court, and the entire structure would extend 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.

To sail the ship to its intended destination, navigators plot a complex course on the solar system sea. The thrust delivered by the ion engine depends on the power level; higher power translates into higher (but still ever so gentle) thrust. The farther Dawn is from the luminous sun, the less power is available, so the thrust is lower. Therefore, to keep it on its itinerary, mission planners need to know the thrust at all times in the future. It would not be a recipe for success to propel the spacecraft to a position in space from which it could not achieve enough thrust to accomplish the rest of the carefully designed journey to Ceres.

To formulate the flight plan then requires knowing how much power will be available even as the probe ventures farther from the sun. Engineers make mathematical predictions of the power the solar arrays will generate, but these calculations are surprisingly difficult. Well, perhaps some readers would not be surprised, but it is more complicated than simply reducing the power in proportion to the intensity of the sunlight. As one example, at greater distances from the sun, the temperature of the arrays in the cold depths of space would be even lower, and the efficiency of the solar cells depends on their temperature. In 2008, the operations team devised and implemented a method to refine their estimates of the solar array performance, and that work enabled the deep-space traveler to arrive at Vesta earlier and depart later. Now they have developed a related but superior technique, which the faithful spacecraft executed flawlessly on June 24.

The only way to measure the power generation capability of the arrays is to draw power from them. With the ion thrust off, even with all other systems turned on, the spacecraft cannot consume as much power as the arrays can provide, so no meaningful measurement would be possible.

In typical operations, Dawn keeps its solar arrays pointed directly at the sun. For this special calibration, it rotated them so the incident sunlight came at a different angle. This reduced the total amount of light falling on the cells, effectively creating the conditions the spacecraft will experience when it has receded from the sun. As the angle increased, corresponding to greater distances from the brilliant star, the arrays produced less power, so the ion engine had to be throttled down. (The engines can be operated at 112 different throttle levels, each with a different input power and different thrust level.)

Engineers estimated what the maximum throttle level would be at each of the angles as well as the total power all other systems would consume during the test and then programmed it so the ion propulsion system would throttle down appropriately as the solar array angle increased. Of course, they could not know exactly what the highest throttle level at each angle would be; if they did, then they would already know the solar array characteristics well enough that the calibration would be unnecessary. Fortunately, however, they did not need to determine the perfect levels in advance. The sophisticated robot is smart enough to reduce by a few throttle levels if it detects that all systems combined are drawing more power than the solar arrays generate.

Under normal circumstances, the spacecraft doesn’t need to adjust the ion throttle level on its own. Engineers know the solar array performance well enough that they can predict the correct setting with high accuracy for a typical four-week sequence of commands stored onboard. It is only for the much greater distances from the sun in the years ahead that the uncertainty becomes important. In addition, during regular operations, if the spacecraft temporarily needs to use more heaters than usual (more than 140 heaters are distributed around the ship, each turning on and off as needed), thereby increasing the power demand, its battery can make up for the difference. That avoids unnecessary throttle changes.

› Continue reading Marc Rayman’s Dawn Journal


For Dawn, a Little Push Goes a Long Way

Monday, July 1st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Dawnamic Readers,

The indefatigable Dawn spacecraft is continuing its extraordinary interplanetary flight on behalf of inquisitive creatures on distant Earth. Progressing ever farther from Vesta, the rocky and rugged world it so recently explored, the ship is making good progress toward its second port of call, dwarf planet Ceres.

We have seen in many logs that this adventure would be quite impossible without its advanced ion propulsion system. Even a mission only to orbit Vesta, which Dawn has accomplished with such stunning success, would have been unaffordable in NASA’s Discovery Program without ion propulsion. This is the only probe ever to orbit an object in the main asteroid belt between Mars and Jupiter. But now, thanks to this sophisticated technology, it is going beyond even that accomplishment to do something no other spacecraft has attempted. Dawn is the only mission ever targeted to orbit two extraterrestrial destinations, making it truly an interplanetary spaceship.

Ion Propulsion System Hot Fire Test for Deep Space 1
Ion Propulsion System Hot Fire Test for the Deep Space 1 spacecraft. Image credit: NASA/JPL-Caltech

Ion propulsion is 10 times more efficient than conventional chemical propulsion, so it enables much more ambitious missions. It uses its xenon propellant so parsimoniously, however, that the thrust is also exceptionally gentle. Indeed, the ion engine exerts about as much force on the spacecraft as you would feel if you held a single sheet of paper in your hand. At today’s thrust level, it would take more than five days to accelerate from zero to 60 mph. While that won’t rattle your bones, in the frictionless, zero-gravity conditions of spaceflight, the effect of the thrust gradually accumulates. Instead of thrusting for five days, Dawn thrusts for years. Ion propulsion delivers acceleration with patience, and patience is among this explorer’s many virtues.

To accomplish its mission, Dawn is outfitted with three ion engines. In the irreverent spirit with which this project has always been conducted, the units are fancifully known as #1, #2, and #3. (The locations of the thrusters were disclosed in a log shortly after launch, once the spacecraft was too far from Earth for the information to be exploited for tawdry sensationalism.) For comparison, the Star Wars TIE fighters were Twin Ion Engine ships, so now science fact does one better than science fiction. On the other hand, the TIE fighters employed a design that did seem to provide greater agility, perhaps at the expense of fuel efficiency. Your correspondent would concur that when you are trying to destroy your enemy while dodging blasts from his laser cannons, economy of propellant consumption probably isn’t the most important consideration.

At any rate, Dawn only uses one ion engine at a time. Since August 31, 2011, it has accomplished all of its thrusting with thruster #3. That thruster propelled Dawn along its complex spiral path down from an altitude of 2,700 kilometers (1,700 miles) to 210 kilometers (130 miles) above Vesta’s dramatic landscape and then back up again. Eventually, the engine pushed Dawn out of orbit, and it has continued to work to reshape the spacecraft’s heliocentric course so that it ultimately will match Ceres’s orbit around the sun.

Although any of the thrusters can accomplish the needed propulsion, and all three are still healthy, engineers consider many factors in deciding which to use at different times in the mission. Now they have decided to put #2 back to work. So on June 24, after its regular monthly hiatus in thrusting to point the main antenna to Earth for a communications session, the robotic explorer turned to aim that thruster, rather than thruster #3, in the direction needed to continue the journey to Ceres. Despite not being operated in nearly two years, #2 came to life as smoothly as ever. It is now emitting a blue-green beam of xenon ions as the craft has its sights set on the mysterious alien world ahead.

› Continue reading Marc Rayman’s Dawn Journal


Smooth Sailing: Dawn Spacecraft Passes Endurance Test

Monday, June 3rd, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Confidawnts,

Traveling from one alien world to another, Dawn is reliably powering its way through the main asteroid belt with its ion propulsion system. Vesta, the fascinating and complex protoplanet it explored in 2011 and 2012, falls farther and farther behind as the spacecraft gently and patiently reshapes its orbit around the sun, aiming for a 2015 rendezvous with dwarf planet Ceres.

The stalwart adventurer has recently completed its longest uninterrupted ion thrust period yet. As part of the campaign to conserve precious hydrazine propellant, Dawn now suspends thrusting once every four weeks to point its main antenna to Earth. (In contrast, spacecraft with conventional chemical propulsion spend the vast majority of time coasting.) Because of details of the mission operations schedule and the schedule for NASA’s Deep Space Network, the thrust durations can vary by a few days. As a result, the spacecraft spent 31.2 days thrusting without a hiatus. This exceeds Deep Space 1’s longest sustained powered flight of 29.2 days. While there currently are no plans to thrust for longer times, the unique craft certainly is capable of doing so. The principal limitation is how much data it can store on the performance of all subsystems (pressures, temperatures, currents, voltages, valve positions, etc.) for subsequent reporting to its terrestrial colleagues.

Thanks to the ship’s dependability, the operations team has been able to devote much of its energies recently to developing and refining the complex plans for the exploration of Ceres. You might be among the privileged readers who will get a preview when we begin describing the plans later this year.

Controllers also have devised some special activities for the spacecraft to perform in the near future, accounts of which are predicted to be in the next two logs.

In addition, team members have had time to maintain their skills for when the spacecraft needs more attention. Earlier this month, they conducted an operational readiness test (ORT). One diabolical engineer carefully configured the Dawn spacecraft simulator at JPL to behave as if a pebble one-half of a centimeter (one-fifth of an inch) in diameter shooting through the asteroid belt collided with the probe at well over twice the velocity of a high-performance rifle bullet.

When the explorer entered this region of space, we discussed that it was not as risky as residents of other parts of the solar system might assume. Dawn does not require Han Solo’s piloting skills to avoid most of the dangerous rocky debris.

The robot could tolerate such a wound, but it would require some help from operators to resume normal operations. This exercise presented the spacecraft team with an opportunity to spend several days working through the diagnosis and performing the steps necessary to continue the mission (using some of the ship’s backup systems). While the specific problem is extremely unlikely to occur, the ORT provided valuable training for new members of the project and served to keep others sharp.

One more benefit of the smooth operations is the time that it enables your correspondent to write his third shortest log ever. (Feel free to do the implied research.) Frequent readers can only hope he strives to achieve such a gratifying feat again!

Dawn is 13 million kilometers (7.9 million miles) from Vesta and 54 million kilometers (34 million miles) from Ceres. It is also 3.25 AU (486 million kilometers or 302 million miles) from Earth, or 1,275 times as far as the moon and 3.20 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 54 minutes to make the round trip.

› Read previous Dawn Journals by Marc Rayman


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.


Highs and Lows of Exploring the Giant Asteroid

Friday, March 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 soaring over the giant 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 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.

› Continue reading Marc Rayman’s Dawn Journal


A Look Inside Dawn’s Grand Asteroid Adventure

Wednesday, February 1st, 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.

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


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


Asteroid Vesta, All in the Details

Thursday, November 3rd, 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.

Image of the giant asteroid Vesta by Dawn
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
› Full image and caption

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


The Giant Asteroid, Up Close and Personal

Thursday, September 29th, 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.

Image of the giant asteroid Vesta by Dawn
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


Dawn Begins its Vesta Phase

Tuesday, May 3rd, 2011

By Marc Rayman

NASA’s Dawn spacecraft is less than three 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. The giant asteroid Vesta, Dawn’s next destination, is on the lower left. The largest body in the asteroid belt and Dawn’s second destination, dwarf planet Ceres, is on the upper right. Image credit: NASA/JPL-Caltech

Dear Dawntalizingly Close Readers,

Dawn is on the threshold of a new world. After more than three and a half years of interplanetary travel covering in excess of 2.6 billion kilometers (1.6 billion miles), we are closing in on our first destination. Dawn is starting its approach to Vesta.

The interplanetary cruise phase of the mission ends today and the 15-month Vesta phase begins. The first three months are the “approach phase,” during which the spacecraft maneuvers to its first science orbit. Many of the activities during approach were discussed in detail in March and April last year, and now we are about to see those plans put into action.

The beginning of the phase is marked by the first images of the alien world Dawn has been pursuing since it left Earth. Vesta will appear as little more than a smudge, a small fuzzy blob in the science camera’s first pictures. But navigators will analyze where it shows up against the background stars to help pin down the location of the spacecraft relative to its target. To imagine how this works, suppose that distant trees are visible through a window in your house. If someone gave you a photo that had been taken through that window, you could determine where the photographer (Dawn) had been standing by lining up the edge of the window (Vesta) with the pattern of the background trees (stars). Because navigators know the exact position of each star, they can calculate where Dawn and Vesta are relative to each other. This process will be repeated as the craft closes in on Vesta, which ultimately will provide a window to the dawn of the solar system.

Even though the mysterious orb is still too far away to reveal new features, it will be exciting to receive these first pictures. During the approach phase, images will be released in periodic batches, with priority viewing for residents of Earth. The flow will be more frequent thereafter. For most of the two centuries that Vesta has been studied, it has been little more than a pinpoint of light. Interrupting thrusting once a week this month to glimpse its protoplanetary destination, Dawn will watch it grow from about five pixels across to 12. By June, the images should be comparable to the tantalizing views obtained by the Hubble Space Telescope. As the approach phase continues and the distance diminishes, the focus will grow still sharper and new details will appear in each subsequent set of images.

› Continue reading Marc Rayman’s May 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 …


Dawn Spacecraft Creeping Up on Vesta

Wednesday, March 9th, 2011

By Marc Rayman

NASA’s Dawn spacecraft is less than five 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 Dawn at Vesta
Artist’s concept of NASA’s Dawn spacecraft at the large asteroid Vesta. The mission is less than five months away from getting into orbit around the large asteroid, its first target.

Dear Pleasant Dawnversions,

Deep in the asteroid belt, Dawn continues thrusting with its ion propulsion system. The spacecraft is making excellent progress in reshaping its orbit around the sun to match that of its destination, the unexplored world Vesta, with arrival now less than five months away.

We have considered before the extraordinary differences between Dawn’s method of entering orbit and that of planetary missions employing conventional propulsion. This explorer will creep up on Vesta, gradually spiraling closer and closer. Because the probe and its target already are following such similar routes around the sun, Dawn is now approaching Vesta relatively slowly compared to most solar system velocities. The benefit of the more than two years of gentle ion thrusting the spacecraft has completed so far is that now it is closing in at only 0.7 kilometers per second (1600 mph). Each day of powered flight causes that speed to decrease by about 7 meters per second (16 mph) as their orbital paths become still more similar. Of course, both are hurtling around the sun much faster, traveling at more than 21 kilometers per second (47,000 mph), but for Dawn to achieve orbit around Vesta, what matters is their relative velocity.

It may be tempting to think of that difference from other missions as somehow being a result of the destination being different, but that is not the case. The spiral course Dawn will take is a direct consequence of its method of propelling itself. If this spacecraft were entering orbit around any other planetary body, it would follow the same type of flight plan. This unfamiliar kind of trajectory ensues from the long periods of thrusting (enabled by the uniquely high fuel efficiency of the ion propulsion system) with an extremely gentle force.

Designing the spiral trajectories is a complex and sophisticated process. It is not sufficient simply to turn the thrust on and expect to arrive at the desired destination, any more than it is sufficient to press the accelerator pedal on your car and expect to reach your goal. You have to steer carefully (and if you don’t, please don’t drive near me), and so does Dawn. As the ship revolves around Vesta in the giant asteroid’s gravitational grip, it has to change the pointing of the xenon beam constantly to stay on precisely the desired winding route to the intended science orbits.

Dawn will scrutinize Vesta from three different orbits, known somewhat inconveniently as survey orbit, high altitude mapping orbit (HAMO), and low altitude mapping orbit (LAMO). Upon concluding its measurements in each phase, it will resume operating its ion propulsion system, using the mission control team’s instructions for pointing its thruster to fly along the planned spiral to the next orbit.

› Continue reading Dawn Spacecraft Creeping Up on Vesta


Rocks and Stars with Amy: This Year I Saw the Universe

Tuesday, February 1st, 2011

By Amy Mainzer

Rocks and Stars with Amy

With WISE, I roamed the skies — seeing everything from the closest asteroids to the most distant galaxies. When I was a kid, maybe 6 or 7, I remember reading the encyclopedia about Andromeda, Mars and Jupiter. After that, I spent a lot of my free time (and a fair amount of gym class) wishing that I could be “out there” exploring the stars, imagining what it must be like to get close to a black hole or the lonely, cold surface of a moon. Fast-forwarding several decades, I’ve just spent a tremendously satisfying and delightful year using some of our most sophisticated technology to see “out there” for real. It’s pretty cool when your childhood dreams come true!

Today, the operations team sent the command to kill the survey sequence and put WISE into a deep sleep. While I’m sad to see the survey stop, the real voyage of discovery is just getting started as we unpack the treasures that our spacecraft beamed back to us. Although I’m going to miss waking up to see a new slew of pictures fresh from outer space, what I’ve looked at so far is only a tiny fraction of the millions of images we’ve garnered. My colleagues and I are working nonstop now to begin the decades-long process of interpreting the data. But I can already say for certain that we’re learning that the universe is a weirder, more wonderful place than any science fiction I’ve ever read. If I could go back in time to when I was kid, I’d tell myself not to worry and to hang in there through the tough parts — it was all worth it.

A cast of hundreds, maybe thousands, of people have worked on WISE and deserve far more credit than they get. The scientists will swoop in and write papers, but all those results are squarely due to the brilliance, stubborn persistence and imagination of the technicians, managers, engineers of all stripes (experts in everything from the optical properties of strange materials to the orbital perturbations of the planets), and administrative staff who make sure we get home safely from our travels. Although we may not be able to fly people around the galaxy yet, one thing Star Trek got right is the spirit of camaraderie and teamwork that makes projects like WISE go. For the opportunity to explore the universe with such fine friends and teammates, I am truly grateful.


Rocks and Stars with Amy: This Asteroid Inspected by #32

Monday, November 15th, 2010

By Amy Mainzer

Rocks and Stars with Amy

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.

Image of the first near-Earth asteroid discovered by WISE
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.
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.


Rocks and Stars with Amy: Milestones

Tuesday, July 20th, 2010
Rocks and Stars with Amy
By Amy Mainzer

It’s hard to believe that we’ve just crossed the six-month mark on WISE — seems like just yesterday when we were all up at Vandenberg Air Force Base, near Santa Barbara, shivering in the cold at night while watching the countdown clock. But the time is flying (literally!) as WISE whips by over our heads. We’re analyzing data ferociously now, trying to get the images and the data ready for the public release next May. Even though the mission’s lifetime is short, we’ve gotten into a semblance of a routine. We receive and process images of stars, galaxies and other objects taken by the spacecraft every day, and we’re running our asteroid-hunting routine on Mondays and Thursdays. We’ve got a small army (well, okay, three — but they do the work of a small army!) of extremely talented students who are helping us verify and validate the asteroid detections, as well as hunt for new comets in the data. Plus, there is an unseen, yet powerful, cadre of observers out there all over the world following up our observations.

asteroids and comets detected by WISEThis plot shows asteroids and comets observed by NASA’s Wide-field Infrared Survey Explorer, or WISE. Image credit: NASA/JPL-Caltech/ULCA/JHU   |   ›See related video

And so it’s come to pass that we’ve achieved some milestones. We completed our first survey of the entire sky on July 17 — and we just discovered our 100th new near-Earth object! That’s out of the approximately 25,000 new asteroids we’ve discovered in total so far; most of these hang out in the main belt between Mars and Jupiter and never get anywhere near Earth’s orbit. These new discoveries will allow us to conduct an accurate census of both the near-Earth and main belt asteroid populations. We’re really busy chewing on the data right now and calculating what it all means.

Because it’s so short, this mission reminds me a little bit of what the first days of college felt like — a tidal wave of new ideas, new sights and new thoughts. The pace of learning has been incredibly quick, whether I’m trying to get up to speed on asteroid evolution theories or tinkering with the software we use to write papers.

Speaking of papers, we’re in the process of preparing to submit several to science journals; in fact, I’ve already submitted one. The gold standard of science, of course, is the peer-review process. We submit our paper to a journal, and the scientific editor assigns another scientist who is an expert in the field but not involved in the project (and who usually remains anonymous) to read it and offer comments. The referee’s job is to “kick the tires,” so to speak, and ask tough questions about the work to make sure it’s sound. We get a chance to respond, and the referee gets a chance to respond to our responses, and then when everybody’s convinced the results are right, the paper is accepted and can be published. So stay tuned — we should have some of the first papers done soon telling us what these milestones mean for asteroid science.

› Read more from “Rocks and Stars with Amy”