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.

As NASA’s Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA
› full image and caption

Dear Dawnt Look Backs,

Its long and daring interplanetary journey continuing smoothly, Dawn is making good progress in gradually reshaping its orbit around the sun. Its uniquely efficient ion propulsion system is gently bringing it closer to its next destination, dwarf planet Ceres, and ever farther from its previous one, Vesta. Although the robotic explorer’s sights are set firmly ahead, let’s take one last look back at the fascinating alien world it unveiled during its 14 months in orbit there.

Vesta, the second most massive resident of the main asteroid belt between Mars and Jupiter, was discovered in 1807. For more than two centuries thereafter, the mysterious object appeared as little more than a fuzzy patch of light among the stars. The only one of the millions of main belt asteroids to be bright enough to be visible to the naked eye, Vesta beckoned, but its invitation was not answered until Dawn arrived in July 2011, nearly four years after it left distant Earth. The cosmic ambassador is the only spacecraft ever to have orbited an object in the main asteroid belt, and its ambitious mission would have been impossible without ion propulsion.

Dawn found a complex and exotic place, and it returned a fabulously rich collection of pictures and other measurements that will continue to be analyzed for many, many years. For now, we will simply touch on a very few of the many insights that already have been illuminated by the light of Dawn.

Scientists recognize Vesta as being more like a mini-planet than like the chips of rock most people think of as asteroids. The behemoth is 565 kilometers (351 miles) wide at the equator and has a surface area more than twice that of California (although it is populated by far fewer eccentrics, billionaires, and other colorful characters found in that state). Dawn’s measurements of the gravity field provide good evidence that Vesta separated into layers, much like Earth did as the planet was forming. Vesta’s dense core, composed principally of iron and nickel, may be 200 to 250 kilometers (125 to 150 miles) across. Surrounding that is the mantle, which in turn is covered by the veneer of the crust, about 20 kilometers (12 miles) thick. The once-molten core is now solid (in contrast to Earth’s, which remains hot enough to be liquid), but the differentiation into layers gives Vesta a key distinction from most asteroids. Because it was likely still in the process of accumulating material to become a full-sized planet when Jupiter’s immense gravity terminated its growth, scientists often refer to Vesta as a protoplanet.

Among the most prominent features of the alien landscape is a huge gouge out of the southern hemisphere so large that its presence was inferred from observations with the Hubble Space Telescope. Dawn found this gigantic crater to be even deeper and wider than expected, penetrating about 19 kilometers (12 miles) and spanning more than 500 kilometers (310 miles), or nearly 90 percent of the protoplanet’s equatorial diameter.

The yawning hole is now known as Rheasilvia, after the Vestal Virgin who not only was the mythical mother of Romulus and Remus, but also surely would have been astounded by the spectacular sights on Vesta as well as the spacecraft’s capability to point any user-defined body vector in a time-varying inertial direction defined by Chebyshev polynomials. As Dawn has brought Vesta into focus, cartographers have needed labels for the myriad features it has discovered. The International Astronomical Union names Vestan craters for Vestal Virgins and other famous Roman women; mountains, canyons, and other structures are named for towns and festivals associated with the Vestal Virgins.

Vesta dates to the dawn of the solar system, more than 4.5 billion years ago, and its age shows. Myriad craters tell the story of a timeworn surface that has been subjected to the rough and tumble conditions of life in the asteroid belt ever since. A virtual rain of space rocks has fallen upon it. While Rheasilvia records the most powerful punch, from an object as much as 50 kilometers (30 miles) across, there are at least seven craters, some quite ancient indeed, more than 150 kilometers (nearly 100 miles) in diameter. As the eons pass, craters degrade and become more difficult to discern, their crisp shapes eroded by subsequent impacts large and small.

The long history of cratering is particularly evident in the startling difference between the northern and southern hemispheres. The north is very densely cratered, but the south is not. Why? The titanic blow that carved out Rheasilvia is estimated to have occurred over one billion years ago. It excavated a tremendous volume of material. Much of it fell back to the surface, wiping it clean, so the cratering record had to start all over again. Recall that the crater itself is 500 kilometers (310 miles) in diameter, and scientists estimate that 50 kilometers (30 miles) outside the rim, the debris may have piled about 5 kilometers (3 miles) high. Even at greater distances, preexisting features would have been partially or completely erased by the thick accumulation. The effect did not reach to the northern hemisphere, however, so it retained the craters than had formed before this enormous impact.

Some of the rocks were ejected with so much energy that they broke free of Vesta’s gravitational grip, going into orbit around the sun. They then went their own way as they were yanked around by the gravitational forces of Jupiter and other bodies, and many of them eventually made it to the part of the solar system where your correspondent and some of his readers spend most of their time: Earth. When our planet’s gravity takes hold of one of these Vesta escapees, it pulls the rock into its atmosphere. Some lucky witness might even observe it as a meteor. Its blazing flight to the ground is not the end of its glory, however, for these rocks are prized by planetary geologists and other enthusiasts who want a souvenir from that impact.

Scientists now know that about 6 percent of the meteorites that have been found originated on Vesta. Six percent! One of every 16 meteorites! This is an astonishingly large fraction. Apart from Mars and the moon, Vesta is the only known source of specific meteorites. Although rocks from Vesta had to travel much farther, they far outnumber meteorites from these other two more familiar celestial bodies.

Combining laboratory studies of the numerous samples of Vesta with Dawn’s measurements at the source provides an extraordinary opportunity to gain insights into the nature of that remote world. Meteorites from Vesta are so common that they are often displayed in museums (occasionally even without the curators’ awareness of their special history) and can be obtained from many vendors. Anyone who has seen or held one surely must be moved by contemplating its origin, so distant in space and time, from well beyond Mars and long before animal or plant life arose on Earth.

› Continue reading Marc Rayman’s Dawn Journal for more Vesta history

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.

Artist’s concept of NASA’s Dawn spacecraft at its next target, the protoplanet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawndroids,

Dawn is continuing to gently and patiently change its orbit around the sun. In September, it left Vesta, a complex and fascinating world it had accompanied for 14 months, and now the bold explorer is traveling to the largest world in the main asteroid belt, dwarf planet Ceres.

Dawn has spent most of its time since leaving Earth powering its way through the solar system atop a column of blue-green xenon ions emitted by its advanced ion propulsion system. Mission controllers have made some changes to Dawn’s operating profile in order to conserve its supply of a conventional rocket propellant known as hydrazine. Firing it through the small jets of the reaction control system helps the ship rotate or maintain its orientation in the zero-gravity of spaceflight. The flight team had already taken some special steps to preserve this precious propellant, and now they have taken further measures. If you remain awake after the description of what the changes are, you can read about the motivation for such frugality.

Dawn’s typical week of interplanetary travel used to include ion thrusting for almost six and two-thirds days. Then it would stop and slowly pirouette to point its main antenna to Earth for about eight hours. That would allow it to send to the giant antennas of NASA’s Deep Space Network a full report on its health from the preceding week, including currents, voltages, temperatures, pressures, instructions it had executed, decisions it had made, and almost everything else save its wonderment at operating in the forbidding depths of space so fantastically far from its planet of origin. Engineers also used these communications sessions to radio updated commands to the craft before it turned once again to fire its ion thruster in the required direction.

Now operators have changed the pace of activities. Every turn consumes hydrazine, as the spacecraft expels a few puffs of propellant through some of its jets to start rotating and through opposing jets to stop. Instead of turning weekly, Dawn has been maintaining thrust for two weeks at a time, and beginning in January it will only turn to Earth once every four weeks. After more than five years of reliable performance, controllers have sufficient confidence in the ship to let it sail longer on its own. They have refined the number and frequency of measurements it records so that even with longer intervals of independence, the spacecraft can store the information engineers deem the most important to monitor.

Although contact is established through the main antenna less often, Dawn uses one of its three auxiliary antennas twice a week. Each of these smaller antennas produces a much broader signal so that even when one cannot be aimed directly at Earth, the Deep Space Network can detect its weak transmission. Only brief messages can be communicated this way, but they are sufficient to confirm that the distant ship remains healthy.

In addition to turning less often, Dawn now turns more slowly. Its standard used to be the same blinding pace at which the minute hand races around a clock (fasten your seat belt!). Engineers cut that in half two years ago but returned to the original value at the beginning of the Vesta approach phase. Now they have lowered it to one quarter of a minute hand’s rate. Dawn is patient, however. There’s no hurry, and the leisurely turns are much more hydrazine-efficient.

With these two changes, the robotic adventurer will arrive at Ceres in 2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply it had when it rocketed away from Cape Canaveral on a lovely September dawn in 2007. Mission planners will be able to make excellent use of it as they guide the probe through its exploration of the giant of the main asteroid belt.

Any limited resource should be consumed responsibly, whether on a planet or on a spaceship. Hydrazine is not the only resource that Dawn’s controllers manage carefully, but let’s recall why this one has grown in importance recently.

› Continue reading Marc Rayman’s Dawn Journal

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.

Artist’s concept of NASA’s Dawn spacecraft at its next target, the protoplanet Ceres. Image credit: NASA/JPL-Caltech

Dear Indawnspensable Readers,

Dawn is making good progress on the second segment of its cosmic travels. Following more than a year of arduous but sensationally productive and exciting work revealing the fascinating character of the giant protoplanet Vesta, it is now patiently pursuing its next target, the mysterious dwarf planet Ceres, which resides farther from the sun. For the second (and final) time in its interplanetary journey, however, Dawn is about to turn around, going closer to the sun rather than farther away.

In August 2008, we saw in detail how it could be that even as the bold explorer travels outward in the solar system from Earth, past Mars, to Vesta, and then on to Ceres, it could occasionally appear to reverse course temporarily. We present here a shorter explanation for those readers who did not memorize the log explaining this perplexing behavior (you know who you are, and we do as well, but your secret remains safe under the terms of our reader privacy agreement).

Dawn orbits the sun, as do Vesta, Ceres, the other residents of the main asteroid belt, and the planets. All orbits, whether of these objects around the star at the center of our solar system, artificial satellites or the moon in orbit around Earth, or even Dawn when it was in orbit around Vesta, are ellipses (like flattened circles). Earth, for example, orbits the sun at an average distance of 150 million kilometers (93.0 million miles), which astronomers call one astronomical unit (AU). During its year-long revolution, however, our planet comes in to 0.98 AU from the sun and goes out to 1.02 AU. Earthlings manage quite nicely with these small variations. (Note that the seasons are not caused by the changes in distance but instead are a result of the tilt of Earth’s axis and thus the differing angles at which the warming rays of the sun arrive during the year. If the sun’s distance were all that mattered, the northern and southern hemispheres would have the same seasons.) So, orbiting bodies move smoothly between a minimum and a maximum range from their gravitational masters rather than remaining at a constant distance.

When Dawn was in orbit around Vesta, it accompanied that world on its regular journey around the sun. The table last month showing the probe’s progress over the five years of its deep space trek reminds us that Vesta’s path brings it as close to the sun as 2.15 AU and takes it out to 2.57 AU.

If Dawn had remained in orbit around Vesta, it would have continued to follow the same elliptical course as its host in the asteroid belt. The pair would have reached their maximum solar distance next month and then would have fallen back to 2.15 AU in September 2014. While visiting Vesta was extremely gratifying, this explorer’s ambitions are greater. It broke free of Vesta’s grip, its sights set on a new and distant alien destination.

Now the spacecraft is in its own independent orbit around the sun, and the persistent but gentle pressure of its advanced ion propulsion system gradually reshapes that orbit. At any moment, the orbit is an ellipse, and an instant later, it is a slightly different ellipse, courtesy of the thrust. As Dawn departed from Vesta only last month, its orbit is not yet dramatically different, but over the course of the coming years, the effect of the thrusting will be to change the orbit tremendously. To reach Ceres in 2015, the ship will enlarge and tip its elliptical course to match the motion of the dwarf planet around the sun. (Some of the parameters characterizing each object’s orbit are shown here.)

Although the ship’s orbit is growing, it will reach the current high point on Nov. 1. It will then be 2.57 AU from the sun and, just as in 2008 (albeit at a smaller distance), it will begin moving closer, even as it continues to thrust.

If Dawn stopped thrusting on Nov. 1, its elliptical orbit would carry it down to 2.19 AU from the sun in September 2014. That’s a higher orbit than Vesta’s but still well below what it needs to be for the rendezvous with Ceres. Astute readers have already anticipated that the plan is not to stop thrusting but to continue reworking the trajectory, just as a ceramicist gradually achieves a desired shape to create the envisioned artistic result. The ongoing thrusting will raise the low point of the orbit, so if the ship follows the flight plan, it will descend only to 2.45 AU in October 2013 before sailing outward again. By May 2014 it will have risen to the same solar altitude as it is now. All the thrusting in the interim will have altered its course so much, however, that it will not turn around then; rather, it will continue ascending to keep its 2015 appointment with Ceres.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

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

Three impact craters of different sizes, arranged in the shape of a snowman, make up one of the most striking features on Vesta, as seen in this view from NASA’s Dawn mission. In this view the three “snowballs” are upside down, so that the shadows make the features easily recognizable. North is to the lower right in the image, which has a resolution of 230 feet (70 meters) per pixel. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dear Dawnpartures,

Dawn has completed the final intensive phase of its extraordinary exploration of Vesta, and it has now begun its gradual departure. Propelled by its uniquely efficient ion propulsion system, the probe is spiraling ever higher, reversing the winding path it followed into orbit last year.

In the previous log (which gained prominence last month by making it into the list of the top 78 logs ever written on this ambitious interplanetary adventure), we saw the plan for mapping Vesta from an altitude of 680 kilometers (420 miles). In this second high-altitude mapping orbit (HAMO2), the spacecraft circled the alien world beneath it every 12.3 hours. On the half of each orbit that it was on the day side, it photographed the dramatic scenery. As it passed over the night side, it beamed the precious pictures to the distant planet where its human controllers (and many of our readers) reside. Tirelessly repeating this strategy while Vesta rotated allowed Dawn’s camera to observe the entirety of the illuminated land every five days.

The robot carried out its complex itinerary flawlessly, completely mapping the surface six times. Four of the maps were made not by pointing the camera straight down at the rocky, battered ground but rather at an angle. Combining the different perspectives of each map, scientists have a rich set of stereo images, allowing a full three dimensional view of the terrain that bears the scars of more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.

Dawn also mapped Vesta six times during the first high-altitude mapping orbit (HAMO1) in September and October 2011. The reason for mapping it again is that Vesta has seasons, and they progress more slowly than on Earth. Now it is almost northern hemisphere spring, so sunlight is finally reaching the high latitudes, which were under an impenetrable cloak of darkness throughout most of Dawn’s residence here.

For most of the two centuries this mysterious orb had been studied from Earth, it was perceived as little more than a small fuzzy blob in the night sky. With the extensive imaging from HAMO1 and HAMO2, as well as from the low-altitude mapping orbit (LAMO, earthlings now know virtually all of the protoplanet’s landscape in exquisite detail.

Among the prizes for the outstanding performance in HAMO2 are more than 4,700 pictures. In addition to the comprehensive mapping, Dawn collected nearly nine million spectra with its visible and infrared mapping spectrometer (VIR) to help scientists determine more about the nature of the minerals. This phenomenal yield is well over twice that of HAMO1, illustrating the great benefit of dedicating valuable observation time in HAMO2 to VIR before the mapping.

Dawn’s measurements of the peaks and valleys, twists and turns of Vesta’s gravity field, from which scientists can map the distribution of material in the interior of the behemoth, were at their best in LAMO. That low altitude also was where the gamma ray and neutron detector (GRaND) obtained its finest data, revealing the atomic constituents of the surface and subsurface. Indeed, the motivation for undertaking the challenging descent to LAMO was for those investigations, although the bonus pictures and spectra greatly enhanced the reward. Even in HAMO2, however, gravity and GRaND studies continued, adding to an already fabulous bounty.

Mission controllers have continued to keep the distant spacecraft very busy, making the most of its limited time at Vesta. Pausing neither to rest nor to marvel or delight in its own spectacular accomplishments, when the robot finished radioing the last of its HAMO2 data to Earth, it promptly devoted its attention to the next task: ion thrusting.

Missions that use conventional propulsion coast almost all of the time, but long-time readers know that Dawn has spent most of its nearly five years in deep space thrusting with its advanced ion propulsion system, the exotic and impressive technology it inherited from NASA’s Deep Space 1. Without ion propulsion, the exploration already accomplished would have been unaffordable for NASA’s Discovery Program and the unique exploit to orbit both Vesta and dwarf planet Ceres would have been quite impossible. Ion propulsion not only enables the spacecraft to orbit residents of the main asteroid belt, something no other probe has attempted, but it also allows the interplanetary spaceship to maneuver extensively while at each destination, thus tailoring the orbits for the different investigations.

On July 25 at 9:45 a.m. PDT, as it has well over 500 times before, the sophisticated craft began emitting a beam of high-velocity xenon ions. In powered flight once again, it is now raising its orbital altitude. On August 26, the ship will be too far and traveling too fast for Vesta’s gravity to maintain its hold. Dawn will slip back into orbit around the sun with its sights set on Ceres.

Although HAMO2 is complete, the spacecraft will suspend thrusting four times to direct its instruments at Vesta during the departure phase, much as it did in the approach phase. The approach pictures aided in navigation and provided tantalizing views of the quarry we had been seeking for so long. This time, however, we will see a familiar world receding rather than an unfamiliar one approaching. But as the sun creeps north, advancing by about three quarters of a degree of latitude per week, the changing illumination around the north pole will continue to expose new features.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

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

This image, from NASA’s Dawn spacecraft, shows rock material that has moved across the surface and flowed into a low area in the ridged floor of the Rheasilvia basin on Vesta. The image shows how impacts and their aftermath constantly reshape the landscape. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Dear Upside Dawn Readers,

Dawn is now seeing Vesta in a new light. Once again the probe is diligently mapping the ancient protoplanet it has been orbiting for nearly a year. Circling the alien world about twice a day, the ardent adventurer is observing the signatures of Vesta’s tortured history, including the scars accumulated during more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.

Having successfully completed its orbital raising maneuvers to ascend to its second high-altitude mapping orbit (HAMO2), Dawn looks down from about 680 kilometers (420 miles). This is the same height from which it mapped Vesta at the end of September and October 2011. The lifeless rocky landscape has not changed since then, but its appearance to the spacecraft’s sensors has. The first high-altitude mapping orbit (HAMO1) was conducted shortly after southern hemisphere summer began on Vesta, so the sun was well south of the equator. That left the high northern latitudes in the deep darkness of winter night. With its slower progression around the sun than Earth, seasons on Vesta last correspondingly longer. Thanks to Dawn’s capability to linger in orbit, rather than simply conduct a brief reconnaissance as it speeds by on its way to its next destination, the probe now can examine the surface with different lighting.

Much of the terrain that was hidden from the sun, and thus the camera, during HAMO1 is now illuminated. Even the scenery that was visible then is lit from a different angle now, so new observations will reveal many new details. In addition to the seasonal northward shift in the position of the sun, Dawn’s orbit is oriented differently in HAMO2, as described last month, so that makes the opportunity for new insights and discoveries even greater.

The strategy for mapping Vesta is the same in HAMO2 now as it was in HAMO1. Dawn’s orbital path takes it nearly over the north pole. (As we saw last month, the orbit does not go exactly over the poles but rather reaches to 86 degrees latitude. That slight difference is not important for this discussion.) During the ship’s southward passage over the sunlit side, the camera and the visible and infrared mapping spectrometer (VIR) acquire their precious data. After passing (almost) above the south pole, Dawn sails north over the night side. Instead of pointing its sensors at the deep black of the ground below, the probe aims its main antenna to the extremely distant Earth and radios its findings to the exquisitely sensitive receivers of the Deep Space Network. The pattern repeats as the indefatigable spacecraft completes loop after loop after loop around the gigantic asteroid every 12.3 hours.

As Dawn revolves, Vesta rotates on its axis beneath it, turning once every 5.3 hours. Just as in HAMO1, mission planners artfully choreographed this celestial pas de deux so that over the course of 10 orbits, lasting just over five days, the camera would be able to view nearly all of the lit surface. A set of 10 orbits is known to Dawn team members (and to you, loyal readers) as a mapping cycle.

Until a few months ago, HAMO2 was planned to be four cycles. Thanks to the determination in April that Dawn could extend its residence at Vesta and still meet its 2015 appointment with dwarf planet Ceres, HAMO2 has been increased to six mapping cycles (plus even a little more, as we shall see below), promising a yet greater scientific return.

In cycle 1, which began on June 23, the camera was pointed at the surface directly underneath the spacecraft. The same view will be obtained in cycle 6. In cycles 2 through 5, images are acquired at other angles, providing different perspectives on the complex and dramatic landscape. Scientists combine the pictures to formulate topographical maps, revealing Vesta’s full three-dimensional character from precipitous cliffs and towering peaks of enormous mountains to gently rolling plains and areas with mysterious ridges and grooves to vast troughs and craters punched deep into the crust. Knowing the elevations of the myriad features and the angles of slopes is essential to understanding the geological processes and forces that shaped this exotic mini-planet. In addition to the exceptional scientific value, the stereo imagery provides realistic, exciting views for anyone who wants to visualize this faraway world. If you have not traveled there yourself, be sure to visit the Image of the Day regularly and the video gallery occasionally to see what you and the rest of humankind had been missing during the two centuries of Vesta’s appearance being only that of a faint, tiny blob in the night sky.

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

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

This image 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 majesty 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.

By Marc Rayman

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

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

Dear Dawnward Spirals,

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

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

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

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

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

› Continue reading Marc Rayman’s Dawn Journal

By Marc Rayman

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

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

Dear Residawnts of Vesta,

Dawn has arrived!!

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

› Read Marc Rayman’s previous Dawn Journals