Posts Tagged ‘chief engineer’

Dawn’s Downward Spiral to Reveal New World Views

Tuesday, July 1st, 2014

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 the Dawn spacecraft at Ceres
Artist’s concept of Dawn in its high altitude mapping orbit at dwarf planet Ceres. Credit: NASA/JPL

Dear Mastodawns,

Deep in the main asteroid belt, between Mars and Jupiter, far from Earth, far from the sun, far now even from the giant protoplanet Vesta that it orbited for 14 months, Dawn flies with its sights set on dwarf planet Ceres. Using the uniquely efficient, whisper-like thrust of its remarkable ion propulsion system, the interplanetary adventurer is making good progress toward its rendezvous with the uncharted, alien world in about nine months.

Dawn’s ambitious mission of exploration will require it to carry out a complex plan at Ceres. In December, we had a preview of the “fapproach phase,” and in January, we saw how the high velocity beam of xenon ions will let the ship slip smoothly into Ceres’s gravitational embrace. We followed that with a description in February of the first of four orbital phases (with the delightfully irreverent name RC3), in which the probe will scrutinize the exotic landscape from an altitude of 8,400 miles (13,500 kilometers). We saw in April how the spacecraft will take advantage of the extraordinary maneuverability of ion propulsion to spiral from one observation orbit to another, each one lower than the one before, and each one affording a more detailed view of the exotic world of rock and ice. The second orbit, at an altitude of about 2,730 miles (4,400 kilometers), known to insiders (like you, faithful reader) as “survey orbit,” was the topic of our preview in May. This month, we will have an overview of the plan for the third and penultimate orbital phase, the “high altitude mapping orbit” (HAMO).

(The origins of the names of the phases are based on ancient ideas, and the reasons are, or should be, lost in the mists of time. Readers should avoid trying to infer anything at all meaningful in the designations. After some careful consideration, your correspondent chose to use the same names the Dawn team uses rather than create more helpful descriptors for the purposes of these logs. What is important is not what the different orbits are called but rather what amazing new discoveries each one enables.)

It will take Dawn almost six weeks to descend to HAMO, where it will be 910 miles (1,470 kilometers) high, or three times closer to the mysterious surface than in survey orbit. As we have seen before, a lower orbit, whether around Ceres, Earth, the sun, or the Milky Way galaxy, means greater orbital velocity to balance the stronger gravitational grip. In HAMO, the spacecraft will complete each loop around Ceres in 19 hours, only one quarter of the time it will take in survey orbit.

In formulating the HAMO plans, Dawn’s human colleagues (most of whom reside much, much closer to Earth than the spacecraft does) have taken advantage of their tremendous successes with HAMO1 and HAMO2 at Vesta. We will see below, however, there is one particularly interesting difference.

As in all observation phases at Ceres (and Vesta), Dawn’s orbital path will take it from pole to pole and back. It will fly over the sunlit side as it travels from north to south and then above the side in the deep darkness of night on the northward segment of each orbit. This polar orbit ensures a view of all latitudes. As Ceres pirouettes on its axis, it presents all longitudes to the orbiting observer. The mission planners have choreographed the celestial pas de deux so that in a dozen revolutions, Dawn’s camera can map nearly the entire surface.

Graphic showing Dawn's spiral descent from survey orbit to HAMO
Dawn’s spiral descent from survey orbit to the high altitude mapping orbit. The trajectory progresses from blue to red over the course of the six weeks. The red dashed segments are where the spacecraft is not thrusting with its ion propulsion system (as explained in April). Credit: NASA/JPL

Rather than mapping once, however, the spacecraft will map Ceres up to six times. One of Dawn’s many objectives is to develop a topographical map, revealing the detailed contours of the terrain, such as the depths of craters, the heights of mountains, and the slopes and variations of plains. To do so, it will follow the same strategy employed so successfully at Vesta, by taking pictures at different angles, much like stereo imaging. The spacecraft will make its first HAMO map by aiming its camera straight down, photographing the ground directly beneath it. Then it will map the surface again with the camera pointed in a slightly different direction, and it will repeat this for a total of six maps, or six mapping “cycles.” With views from up to six different perspectives, the landscape will pop from flat images into its full three dimensionality. (As with all the plans, engineers recognize that complex and challenging operations in the forbidding, unforgiving depths of space do not always go as intended. So they plan to collect more data than they need. If some of the images, or even entire maps, are not acquired, there should still be plenty of pictures to use in revealing the topography.)

In addition to acquiring the photos, Dawn will make other measurements in HAMO. During some of the cycles, the camera will use its color filters to glean more about the nature of the surface. The visible and infrared mapping spectrometer will collect spectra to help scientists determine the composition of the surface, its temperature, and other properties.

Exquisitely accurate radio tracking of the spacecraft in its orbit, as indicated by the Doppler shift (the change in frequency, or pitch, as the craft moves toward or away from Earth) and by the time it takes radio signals to make the round trip from Earth, allows navigators to determine the strength of the gravitational tugging. That can be translated into not only the mass of Ceres but also how the mass is distributed in its interior. In August, when we look ahead to the fourth and final science phase of the Ceres mission, the low altitude mapping orbit, we will explain this in greater detail.

Although still too high for anything but the weakest indication of radiation from Ceres, the gamma ray and neutron detector will measure the radiation environment in HAMO. This will yield a useful reference for the stronger signals it will detect when it is closer.

There is a noteworthy difference between how Dawn will operate in HAMO and how it operated in HAMO1 and HAMO2 at Vesta and even how it will operate in survey orbit at Ceres.

› Continue reading Marc Rayman’s June 30, 2014, Dawn Journal


Survey Orbit: A Truly Extaordinary View

Tuesday, June 3rd, 2014

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.

Dear Dawnosaurs,

Silently streaking through the main asteroid belt, emitting a blue-green beam of xenon ions, Dawn continues its ambitious interplanetary expedition. On behalf of creatures on distant Earth who seek not only knowledge and insight but also bold adventure, the spacecraft is heading toward its appointment with Ceres. In about 10 months, it will enter orbit around the ancient survivor from the dawn of the solar system, providing humankind with its first detailed view of a dwarf planet.

This month we continue with the preview of how Dawn will explore Ceres. In December we focused on the “approach phase,” and in January we described how the craft spirals gracefully into orbit with its extraordinary ion propulsion system. The plans for the first observational orbit (with a marvelously evocative name for a first examination of an uncharted world: RC3 — is that cool, or what?), at an altitude of 8,400 miles (13,500 kilometers), were presented in FebruaryLast month, we followed Dawn on its spiral descent from each orbital altitude to the next, with progressively lower orbits providing better views than the ones before. Now we can look ahead to the second orbital phase, survey orbit.

Survey orbit

This figure shows Dawn’s second observational orbit, “survey orbit,” at the same scale as the size of Ceres. At an altitude of 2,730 miles (4,400 kilometers), the spacecraft will make seven revolutions in about three weeks. Image credit: NASA/JPL-Caltech
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In survey orbit, Dawn will make seven revolutions at an altitude of about 2,730 miles (4,400 kilometers). At that distance, each orbit will take three days and three hours. Mission planners chose an orbit period close to what they used for survey orbit at Vesta, allowing them to take advantage of many of the patterns in the complex choreography they had already developed. Dawn performed it so beautifully that it provides an excellent basis for the Ceres encore. Of course, there are some adjustments, mostly in the interest of husbanding precious hydrazine propellant in the wake of the loss of two of the spacecraft’s four reaction wheels. (Although such a loss could have dire consequences for some missions, the resourceful Dawn team has devised a plan that can achieve all of the original objectives regardless of the condition of the reaction wheels.)

We had a preview of survey orbit at Vesta four years ago, and we reviewed the wonderfully successful outcome in September 2011. When we develop the capability to travel backwards in time, we will insert a summary of what occurred in survey orbit at Ceres here: _______…… Well, nothing yet. So, let’s continue with the preview.

As in all phases at Ceres (and Vesta), Dawn follows what space trajectory experts (and geeks) call a polar orbit. The ship’s course will take it above the north pole, and then it will sail south over the side bathed in the light of the sun. After flying over the south pole, Dawn will head north. Although the surface beneath it will be dark, the spacecraft will be high enough that it will not enter the dwarf planet’s shadow. The distant sun will constantly illuminate the large solar arrays.

The leisurely pace in survey orbit allows the explorer to gather a wealth of data during the more than 37 hours on the day side. It will train its science camera and visible and infrared mapping spectrometer (VIR) on the surface lit by the sun. The camera will collect hundreds of images using all seven of its color filters. It will reveal details three times finer than it observed in RC3 orbit and 70 times sharper than the best we have from the Hubble Space Telescope. VIR will acquire millions of spectra to help scientists determine the minerals present as well as the temperature and other properties of the surface. While the sensors are pointed at the surface, the main antenna cannot simultaneously be aimed at Earth, so the robot will store its pictures and spectra.

One Cerean day, the time it takes Ceres to rotate once on its axis, is a little over nine hours. (For comparison, Earth, as some of its residents and visitors know, takes 24 hours. Jupiter turns in just under 10 hours, Vesta in five hours and 21 minutes, and your correspondent’s cat Regulus in about 0.5 seconds when chasing a laser spot.) So as Dawn travels from the north pole to the south pole, Ceres will spin underneath it four times. Dawn will be close enough that even the wide field of view of its camera won’t capture the entire disc below, from horizon to horizon, but over the course of the seven orbits, the probe will see most of the surface. As in developing the plan for Vesta, engineers (like certain murine rodents and male humans) are keenly aware that as careful, as thorough, and as diligent as they are, their schemes don’t always execute perfectly. In the unknown, forbidding depths of space with a complex campaign to carry out, glitches can occur and events can go awry. The plan is designed with the recognition that some observations will not be achieved, but those that are promise great rewards.

Artist's concept of Dawn orbiting Ceres

Artist’s concept of Dawn in its survey orbit at dwarf planet Ceres. Image credit: NASA/JPL-Caltech

Most of the time, the spacecraft will gaze straight down at the alien terrain immediately beneath it. But on the first, second, and fourth passages over the day side of Ceres, it will spend some of the time looking at the limb against the blackness of space. Pictures with this perspective will not only be helpful for establishing the exact shape of the dwarf planet but they also will provide some very appealing views for eager sightseers on Earth.

In addition to using the camera and VIR, Dawn will measure space radiation with its gamma ray and neutron detector (GRaND). GRaND will still be too far from Ceres to sense the nuclear particles emanating from it, but recording the radiation environment will provide a valuable context for the sensitive measurements it will make at lower altitudes.

When Dawn’s orbit takes it over the dark side, it will turn away from the dwarf planet it is studying and toward the planet it left in 2007 where its human colleagues still reside. With its 5-foot (1.52-meter) main antenna, it will spend most of the day and a half radioing its precious findings across uncounted millions of miles (kilometers) of interplanetary space. (Well, you won’t have to count them, but we will.)

In addition to the instrument data it encodes, Dawn’s radio signal will allow scientists and engineers to measure how massive Ceres is. By observing the Doppler shift (the change in frequency caused by the spacecraft’s motion), they can determine how fast the ship moves in orbit. Timing how long the signals (traveling at the universal limit of the speed of light) take to make the round trip, navigators can calculate how far the probe is and hence where it is in its orbit. Combining these (and including other information as well) allows them to compute how strongly Ceres pulls on its orbital companion. The strength of its gravitational force reveals its heft.

By the end of survey orbit, Dawn will have given humankind a truly extraordinary view of a dwarf planet that has been cloaked in mystery despite more than 200 years of telescopic studies. As the exotic world of rock and ice begins to yield its secrets to the robotic ambassador from Earth, we will be transported there. We will behold new landscapes that will simultaneously quench our thirst for exploration and ignite our desire for even more. It is as humankind reaches ever farther into the universe that we demonstrate a part of what it means to be human, combining our burning need for greater understanding with our passion for adventure and our exceptional creativity, resourcefulness and tenacity. As we venture deeper into space, we discover much of what lies deep within ourselves.

Dawn is 7.2 million miles (12 million kilometers) from Ceres. It is also 1.87 AU (174 million miles, or 280 million kilometers) from Earth, or 695 times as far as the moon and 1.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 31 minutes to make the round trip.

› Continue reading Marc Rayman’s May 31, 2014, Dawn Journal


Riding the Spiral: Navigating a New World

Wednesday, April 30th, 2014

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 the Dawn spacecraft
Dawn will use its ion propulsion system to change orbits at Ceres, allowing it to observe the dwarf planet from different vantage points. Image credit: NASA/JPL-Caltech
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Dear Compedawnt Readers,

Less than a year from its rendezvous with dwarf planet Ceres, Dawn is continuing to make excellent progress on its ambitious interplanetary adventure. The only vessel from Earth ever to take up residence in the main asteroid belt between Mars and Jupiter, the spacecraft grows more distant from Earth and from the sun as it gradually closes in on Ceres. Dawn devotes the majority of its time to thrusting with its remarkable ion propulsion system, reshaping its heliocentric path so that by the time it nears Ceres, the explorer and the alien world will be in essentially the same orbit around the sun.

In December, we saw what Dawn will do during the “approach phase”; to Ceres early in 2015, and in January, we reviewed the unique and graceful method of spiraling into orbit. We described in February the first orbit (with the incredibly cool name RC3) from which intensive scientific observations will be conducted, at an altitude of 8,400 miles (13,500 kilometers). But Dawn will take advantage of the extraordinary capability of ion propulsion to fly to three other orbital locations from which it will further scrutinize the mysterious world.

Let’s recall how the spacecraft will travel from one orbit to another. While some of these plans may sound like just neat ideas, they are much more than that; they have been proven with outstanding success. Dawn maneuvered extensively during its 14 months in orbit around Vesta. (One of the many discussions of that was in November 2011.) The seasoned space traveler and its veteran crew on distant Earth are looking forward to applying their expertise at Ceres.

As long-time readers of these logs know so well, the ion thrust is uniquely efficient but also extremely low. Ion propulsion provides acceleration with patience. Ultimately the patience pays off, enabling Dawn to accomplish feats far beyond what any other spacecraft has ever had the capability to do, including orbiting two extraterrestrial destinations. The gentle thrust, comparable to the weight of a single sheet of paper, means it takes many weeks to maneuver from one observational orbit to another. Of course, it is worthwhile to spend that much time, because each of the orbital phases is designed to provide an exciting trove of scientific data.

Those of you who have navigated around the solar system, as well as others who have contemplated the nature of orbits without having practical experience, recognize that the lower the orbital altitude, the faster the orbital motion. This important principle is a consequence of gravity’s strength increasing as the distance between the massive body and the orbiting object decreases. The speed has to be higher to balance the stronger gravitational pull. (For a reminder of some of the details, be sure to go here before you go out for your next orbital expedition.)

While Dawn slowly reduces its altitude under the faint pressure of its ion engine, it continues circling Ceres, orbiting in the behemoth’s gravitational grip. The effect of combining these motions is that the path from one altitude to another is a spiral. And as Dawn descends and zips around Ceres faster and faster, the spirals get tighter and tighter.

Illustration of Dawn's orbits from RC3 to survey orbit
RC3 to survey: Dawn will make five spiral loops during the month it will take to fly from its RC3 orbit (at 8,400 miles, or 13,500 kilometers) to survey orbit (at 2,700 miles, or 4,400 kilometers). Image credit: NASA/JPL-Caltech
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The first coils around Ceres will be long and slow. After completing its investigations in RC3, the probe will spiral down to”survey orbit,”; about 2,700 miles (4,400 kilometers) above the surface. During that month-long descent, it will make only about five revolutions. After three weeks surveying Ceres from that new vantage point, Dawn will follow a tighter spiral down to the (misleadingly named) high altitude mapping orbit (HAMO) at 910 miles (1,470 kilometers). In the six-week trip to HAMO, the craft will wind around almost 30 times. It will devote two months to performing extensive observations in HAMO. And finally as 2015 draws to a close, it will fly an even more tightly wound course to reach its low altitude mapping orbit (LAMO) at 230 miles (375 kilometers), where it will collect data until the end of the mission. The ship will loop around 160 times during the two months to go from HAMO to LAMO. (We will preview the plans for survey orbit, HAMO and LAMO in May, July and August of this year, and if all goes well, we will describe the results in 2015 and 2016.)

Designing the spiral trajectories is a complex and sophisticated process. It is not sufficient simply to activate the thrust and expect to arrive at the desired destination, any more than it is sufficient to press the accelerator in 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 Ceres, it must constantly change the pointing of the blue-green beam of high velocity xenon ions to stay on precisely the desired winding route to the targeted orbit. The mission control team at JPL will program the ship to orient its thruster in just the right direction at the right time to propel itself on the intended spiraling course.

Illustration of Dawn's orbits from HAMO to LAMO
HAMO to LAMO: Dawn will complete 160 revolutions in two months as it follows a tight spiral from HAMO (at 910 miles, or 1,470 kilometers) to LAMO (at 230 miles, or 375 kilometers). Image credit: NASA/JPL-Caltech
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Aiming a thruster in the direction needed to spiral around Ceres requires turning the entire spacecraft. Each thruster is mounted on its own gimbal with a limited range of motion. In normal operation, the gimbal is positioned so that the line of thrust goes through the center of the ship. When the gimbal is swiveled to another direction, the gentle force from the ion engine causes the ship to rotate slowly. This is similar to the use of an outboard motor on a boat. When it is aligned with the centerline of the boat, the craft travels straight ahead. When the motor is turned, it continues to propel the boat but also turns it. In essence, Dawn’s steering of its thrust is accomplished by pivoting the ion engine.

A crucial difference between the boat and our interplanetary ship is that with the former, the farther the motor is turned, the tighter the curving course. (Another difference is that the spacecraft wouldn’t float.) Dawn doesn’t have that liberty. For our craft, the gimballing of the thruster needs to be carefully coordinated with the orbital motion, as if the motorboat operator needed to compensate for a curving current. This has important implications at Ceres. Sophisticated as it is, Dawn knows its own location in orbit only by virtue of information mission controllers install onboard to predict where it will be at any time. That is based on their best computations of Ceres’ gravity, the planned operation of the ion propulsion system, and many other considerations, but it will never be perfectly accurate. Let’s take a look at two of the reasons.

Ceres, like Vesta, Earth, the moon, Mars, and other planets or planetary-type bodies, has a complex gravity field. The distribution of materials of different densities within the interior creates variations in the strength of the gravitational force, so Dawn will feel a slightly changing tug from Ceres as it travels in orbit. But there is a noteworthy difference between Ceres’ gravity field and the fields of those other worlds: Ceres’ field is unknown. We will have to measure it as we go. The subtle irregularities in gravity as Dawn descends will cause small deflections from the planned trajectory. Our ship will be traversing unknown, choppy waters.

Other phenomena will lead to slight discrepancies as well. The ion propulsion system will be responsible for changing the orbit, so even tiny deviations from the intended thrust eventually may build up to have a significant effect. This is no different from any realistic electrical or mechanical system, which is sure to have imperfections. If you planned a trip in which you would drive 60.0 miles (96.6 kilometers) at 60.0 mph (96.6 kilometers per hour), you could expect to arrive in exactly 60.0 minutes. (No surprises there, as it isn’t exactly rocket science.) But even if you maintained the speedometer as close to 60 as you could, it would not be accurate enough to indicate the true speed. If your actual speed averaged 60.4 mph (97.2 kilometers per hour), you would arrive 24 seconds early. Perhaps that difference wouldn’t matter to you (and if it did, you might consider replacing your car with a spaceship), but such minuscule errors, when compounded by Dawn’s repeated spirals around Ceres, would make a difference in achieving its carefully chosen orbit.

As a result of these and other effects, mission controllers will need to adjust the complex flight plan as Dawn travels from one observational orbit to another. So it will thrust for a few days and then stop to allow navigators to get a new fix on its position. When it points its main antenna to Earth, the Doppler shift of its radio signal will reveal its speed, and the time for radio signals (traveling, as all readers know so well, at the universal limit of the speed of light) to make the round trip will yield its distance. Combining those measurements with other data, mission controllers will update the plan for where to point the thruster at each instant during the next phase of the spiral, check it, double check it, and transmit it to the faraway robot, which will then put it into action. This intensive process will be repeated every few days as Dawn maneuvers to lower orbits.

The flight team succeeded brilliantly in performing this kind of work at Vesta, but they will encounter some differences at Ceres.

› Continue reading Marc Rayman’s April 30, 2014, Dawn Journal


A Preview of Upcoming Attractions: Dawn Meets Ceres

Friday, February 28th, 2014

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 the Dawn spacecraft at the protoplanet Ceres
This artist’s concept of NASA’s Dawn spacecraft shows the craft orbiting high above Ceres, where the craft will arrive in early 2015 to begin science investigations. Image credit: NASA/JPL-Caltech
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Dear Ardawnt Readers,

Continuing its daring mission to explore some of the last uncharted worlds in the inner solar system, Dawn remains on course and on schedule for its rendezvous with dwarf planet Ceres next year. Silently and patiently streaking through the main asteroid belt between Mars and Jupiter, the ardent adventurer is gradually reshaping its orbit around the Sun with its uniquely efficient ion propulsion system. Vesta, the giant protoplanet it unveiled during its spectacular expedition there in 2011-2012, grows ever more distant.

In December, and January, we saw Dawn’s plans for the “approach phase” to Ceres and how it will slip gracefully into orbit under the gentle control of its ion engine. Entering orbit, gratifying and historic though it will be, is only a means to an end. The reason for orbiting its destinations is to have all the time needed to use its suite of sophisticated sensors to scrutinize these alien worlds.

Illustration of Dawn's approach phase and RC3 orbit
Following its gravitational capture by Ceres during the approach phase, Dawn will continue to use its ion propulsion system to spiral to the RC3 orbit at an altitude of 8,400 miles (13,500 kilometers). Image credit: NASA/JPL-Caltech
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As at Vesta, Dawn will take advantage of the extraordinary capability of its ion propulsion system to maneuver extensively in orbit at Ceres. During the course of its long mission there, it will fly to four successively lower orbital altitudes, each chosen to optimize certain investigations. (The probe occupied six different orbits at Vesta, where two of them followed the lowest altitude. As the spacecraft will not leave Ceres, there is no value in ascending from its fourth and lowest orbit.) All of the plans for exploring Ceres have been developed to discover as much as possible about this mysterious dwarf planet while husbanding the precious hydrazine propellant, ensuring that Dawn will complete its ambitious mission there regardless of the health of its reaction wheels.

All of its orbits at Ceres will be circular and polar, meaning the spacecraft will pass over the north pole and the south pole, so all latitudes will come within view. Thanks to Ceres’s own rotation, all longitudes will be presented to the orbiting observer. To visualize this, think of (or even look at) a common globe of Earth. A ring encircling it represents Dawn’s orbital path. If the ring is only over the equator, the spacecraft cannot attain good views of the high northern and southern latitudes. If, instead, the ring goes over both poles, then the combined motion of the globe spinning on its axis and the craft moving along the ring provides an opportunity for complete coverage.

Dawn will orbit in the same direction it did at Vesta, traveling from north to south over the side illuminated by the distant Sun. After flying over the south pole, it will head north, the surface directly beneath it in the dark of night. When it travels over the north pole, the terrain below will come into sunlight and the ship will sail south again.

Dawn’s first orbital phase is distinguished not only by providing the first opportunity to conduct intensive observations of Ceres but also by having the least appealing name of any of the Ceres phases. It is known as RC3, or the third “rotation characterization” of the Ceres mission. (RC1 and RC2 will occur during the approach phase, as described in December.)

During RC3 in April 2015, Dawn will have its first opportunity for a global characterization of its new residence in the asteroid belt. It will take pictures and record visible and infrared spectra of the surface, which will help scientists determine its composition. In addition to learning about the appearance and makeup of Ceres, these observations will allow scientists to establish exactly where Ceres’s pole points. The axis Earth rotates around, for example, happens to point very near a star that has been correspondingly named Polaris, or the North Star. [Note to editors of local editions: You may change the preceding sentence to describe wherever the axis of your planet points.] We know only roughly where Ceres’s pole is from our telescopic studies, but Dawn’s measurements in RC3 will yield a much more accurate result. Also, as the spacecraft circles in Ceres’s gravitational hold, navigators will measure the strength of the gravitational pull and hence its overall mass.

RC3 will be at an orbital altitude of about 8,400 miles (13,500 kilometers). From there, the dwarf planet will appear eight times larger than the moon as viewed from Earth, or about the size of a soccer ball seen from 10 feet (3.1 meters). At that distance, Dawn will be able to capture the entire disk of Ceres in its pictures. The explorer’s camera, designed for mapping unfamiliar extraterrestrial landscapes from orbit, will see details more than 20 times finer than we have now from the Hubble Space Telescope.

Although all instruments will be operated in RC3, the gamma-ray and neutron detector (GRaND) will not be able to detect the faint nuclear emissions from Ceres when it is this far away. Rather, it will measure cosmic radiation. In August we will learn more about how GRaND will measure Ceres’s atomic composition when it is closer.

It will take about 15 days to complete a single orbital revolution at this altitude. Meanwhile, Ceres turns on its axis in just over nine hours (more than two and a half times faster than Earth). Dawn’s leisurely pace compared to the spinning world beneath it presents a very convenient way to map it. It is almost as if the probe hovers in place, progressing only through a short arc of its orbit as Ceres pirouettes helpfully before it.

When Dawn is on the lit side of Ceres over a latitude of about 43 degrees north, it will point its scientific instruments at the unfamiliar, exotic surface. As Ceres completes one full rotation, the robot will fill its data buffers with as much as they can hold, storing images and spectra. By then, most of the northern hemisphere will have presented itself, and Dawn will have traveled to about 34 degrees north latitude. The spacecraft will then aim its main antenna to Earth and beam its prized findings back for all those who long to know more about the mysteries of the solar system. When Dawn is between 3 degrees north and 6 degrees south latitude, it will perform the same routine, acquiring more photos and spectra as Ceres turns to reveal its equatorial regions. To gain a thorough view of the southern latitudes, it will follow the same strategy as it orbits from 34 degrees south to 43 degrees south.

When Dawn goes over to the dark side, it will still have important measurements to make (as long as Darth Vader does not interfere). While the surface immediately beneath it will be in darkness, part of the limb will be illuminated, displaying a lovely crescent against the blackness of space. Both in the southern hemisphere and in the northern, the spacecraft will collect more pictures and spectra from this unique perspective. Dawn’s orbital dance has been carefully choreographed to ensure the sensitive instruments are not pointed too close to the Sun.

› Continue reading Marc Rayman’s February 2014 Dawn Journal


For Dawn, a Time to Thrust and and a Time to Coast

Thursday, October 31st, 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.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear All Hallows’ Dawns,

Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.

The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn’s heliocentric orbit was the same as the rocky behemoth’s. Now they are very far apart, and by early 2015, the robotic explorer’s path will be close enough to Ceres’s that they will become locked in a gravitational embrace.

Without ion propulsion, Dawn’s unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA’s Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.

During its years of interplanetary travel, Dawn has reliably followed a carefully formulated flight plan from Earth past Mars to Vesta and now from Vesta to Ceres. We discussed some of the principles underlying the development of the complex itinerary in a log written when Dawn was still gravitationally anchored to Earth. To carry out its ambitious adventure, Dawn should thrust most of the time, but not all of the time. Indeed, at some times, thrusting would be unproductive.

We will not delve into the details here, but remember that Dawn is doing more than ascending the solar system hill, climbing away from the sun. More challenging than that is making its orbit match the orbit of its targets so that it does not fly past them for a brief encounter as some other missions do. Performing its intricate interplanetary choreography requires exquisite timing with the grace and delicacy of the subtly powerful ion propulsion.

Of course Dawn does not thrust much of the time it is in orbit at Vesta and Ceres; rather, its focus there is on acquiring the precious pictures and other measurements that reveal the detailed nature of these mysterious protoplanets. But even during the interplanetary flight, there are two periods in the mission in which it is preferable to coast. Sophisticated analysis is required to compute the thrusting direction and schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the mass of the sun and the masses and orbits of Earth, Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when mission planners will not allow thrusting (e.g., during spacecraft maintenance periods).

The first interval that interplanetary trajectory designers designated as “optimal coast” was well over four years and 1.8 billion miles (2.8 billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8, 2009. During that time, the ship took some of Mars’s orbital energy to help propel itself toward Vesta. (In exchange for boosting Dawn, Mars slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters, in 180 million years.)

The second and final interval when coasting is better than thrusting begins next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit around the sun without modifying it. The timing of this coast period is nearly as important to keeping the appointment with Ceres as is the timing of the thrusting. In next month’s log, we will describe some of the special assignments the sophisticated robot will perform instead of its usual quiet cruise routine of accelerating and emitting xenon ions. We also will look ahead to some interesting celestial milestones and alignments in December.

While the spacecraft courses through the asteroid belt, the flight team continues refining the plans for Ceres. In logs in December and several months in 2014, we will present extensive details of those plans so that by the time Dawn begins its mission there, you will be ready to ride along and share in the experience.

In the meantime, as the stalwart ship sails on, it is propelled not only by ions but also by the promise of exciting new knowledge and the prospects of a thrilling new adventure in exploring an uncharted alien world.

Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million miles, or 460 million kilometers) from Earth, or 1,200 times as far as the moon and 3.10 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

– Dr. Marc D. Rayman

P.S. This log is posted early enough to allow time for your correspondent to don his Halloween costume. In contrast to last year’s simple (yet outlandish) costume, this year’s will be more complex. He is going in double costume, disguised as someone who is only pretending to be passionate about the exploration of the cosmos and the rewards of scientific insight.

› Read more entries from Marc Rayman’s Dawn Journal


NASA’s Dawn Spacecraft Celebrates Six Years in Space

Friday, September 27th, 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.

Artist's concept of the Dawn spacecraft
Artist’s concept of NASA’s Dawn spacecraft between the giant asteroid Vesta and the dwarf planet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawnniversaries,

On the sixth anniversary of leaving Earth to embark on a daring deep-space expedition, Dawn is very, very far from its erstwhile planetary residence. Now humankind’s only permanent resident of the main asteroid belt between Mars and Jupiter, the seasoned explorer is making good progress toward the largest object in that part of the solar system, the mysterious dwarf planet Ceres. The voyage is long, and the intrepid but patient traveler will not reach its next destination until half a year after its seventh anniversary of departing Earth.

On its fifth anniversary, Dawn was still relatively close to Vesta, the giant protoplanet that had so recently held the craft in its gravitational grip. The only probe ever to orbit a main belt asteroid, Dawn spent 14 months (including its fourth anniversary) accompanying Vesta on its way around the sun. After more than two centuries of appearing to astronomers as little more than a fuzzy blob of light among the stars, the second most massive body in the asteroid belt has been revealed as a fascinating, complex, alien world more closely related to terrestrial planets (including Earth) than to typical asteroids.

Most of the ship’s first four years of spaceflight were devoted to using its ion propulsion system to spiral away from the sun, ascending the solar system hill from Earth to Vesta. Now it is working to climb still higher up that hill to Ceres.

For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the sixth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the logs from its first, second, third, fourth, and fifth anniversaries.

In its six years of interplanetary travels, the spacecraft has thrust for a total of 1,410 days, or 64 percent of the time (and about 0.000000028 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 318 kilograms (701 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 8.7 kilometers per second (19,500 mph). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished about three-quarters of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have completed six revolutions around the sun, covering about 37.7 AU (5.6 billion kilometers or 3.5 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 27.4 AU (4.1 billion kilometers or 2.5 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It will have to slow down still more to rendezvous with Ceres. Since Dawn’s launch, Vesta has traveled only 24.2 AU (3.6 billion kilometers or 2.2 billion miles), and the even more sedate Ceres has gone 22.8 AU (3.4 billion kilometers or 2.1 billion miles).

Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.

› Continue reading Marc Rayman’s Dawn Journal


Earth and Dawn on Opposite Sides Now

Friday, August 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.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear Antecedawnts,

Traveling confidently and alone, Dawn continues to make its way through the silent depths of the main asteroid belt. The only spacecraft ever to have orbited a resident of the vast territory between Mars and Jupiter, Dawn conducted a spectacular exploration of gigantic Vesta, revealing a complex place that resembles the terrestrial planets more than typical asteroids. Now the interplanetary adventurer is on its long journey to the uncharted dwarf planet Ceres, by far the largest of all asteroids (975 kilometers, or more than 600 miles, in equatorial diameter). In 2015, the mysterious world of rock and ice will begin to give up its ancient secrets to the immigrant from distant Earth.

Earth, Vesta, Dawn, and Ceres are following their own separate paths around the sun. The spacecraft is patiently reshaping its orbit, using its uniquely efficient ion propulsion system to accomplish a deep-space expedition that would be impossible with conventional propulsion.

As we have seen in many previous logs (including, for example, here), the higher an object’s orbit, the slower it needs to move in order to balance the gravitational pull, which diminishes with distance. Blistering Mercury orbits the sun faster than Venus, Venus goes faster than Earth, Earth goes faster than Mars, and Mars goes faster than the residents of the asteroid belt and the cold planets of the outer solar system. In the same way, satellites that orbit close to Earth, including the International Space Station, move faster than those at greater altitudes, and the moon travels even more slowly in its very high orbit.

Dawn is now a permanent inhabitant of the main asteroid belt. Therefore, the massive sun, the gravitational master of the solar system, has a weaker grip on it than on Earth. So as Dawn maneuvers from Vesta to Ceres, Earth revolves more rapidly around the sun. This month, their independent motions have taken them to their greatest separation of the year, as they are on opposite sides of the sun. How truly remarkable that humankind can accomplish such a feat!

On August 4, the planet and its robotic ambassador to the cosmos were an extraordinary 3.47 AU (519 million kilometers, or 322 million miles) apart. (To recapture the feeling of your position in the universe then, it may be helpful to know that the maximum range was attained at 4:16 a.m. PDT.) From the perspective of terrestrial observers, had they possessed the superhuman (and even supertelescopic) vision needed to descry the tiny ship far beyond the blindingly bright star, Dawn would have appeared to be very close to the sun but not directly behind it. To rendezvous with Vesta and then with Ceres, the spacecraft has tilted the plane of its solar orbit. Some of the time it is north of Earth’s orbital plane, sometimes it is south. August 4 was during the northern segment, so Dawn would have been a little north of the sun.

It’s time to refer to one of those novel clocks available in the Dawn gift shop on your planet (although if you already have such a clock, it probably doesn’t tell you that it’s time — we stand by our policy of full refunds within 24 hours, as measured by our Dawn clocks). With the sun at the center of the clock, Earth’s motion would be like that of a short minute hand. Dawn, both farther from the sun and moving more slowly, would be following the path of a longer hour hand. If we ignore the effect of the ion thrust, which is constantly changing the orbit, and the slight misalignment of the hour hand representing Dawn’s being in a different plane, the conditions on August 4 were like those at 6:00.

As time progresses and Earth continues circling the sun, it will come closer to Dawn until April 2014 (like 12:00). Even then, however, they will be over 1.55 AU (232 million kilometers, or 144 million miles) apart, and they will never be that close again. The spacecraft will continue climbing higher and higher from the sun toward Ceres, so by the time Earth loops around once more, Dawn will be even farther from it. In the meantime, when next the arrangement is like 6:00, in December 2014, the separation will be more than 3.78 AU (565 million kilometers, or 351 million miles), even greater than the remarkable range a few weeks ago.

› Continue reading Marc Rayman’s Dawn Journal


The Giant Asteroid, Near and Far

Thursday, January 10th, 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.

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

Dawn concluded 2012 almost 13,000 times farther from Vesta than it began the year. At that time, it was in its lowest orbit, circling the alien world at an average altitude of only 210 kilometers (130 miles), scrutinizing the mysterious protoplanet to tease out its secrets about the dawn of the solar system.

To conduct its richly detailed exploration, Dawn spent nearly 14 months in orbit around Vesta, bound by the behemoth’s gravitational grip. In September they bid farewell, as the adventurer gently escaped from the long embrace and slipped back into orbit around the sun. The spaceship is on its own again in the main asteroid belt, its sights set on a 2015 rendezvous with dwarf planet Ceres. Its extensive ion thrusting is gradually enlarging its orbit and taking it ever farther from its erstwhile companion as their solar system paths diverge.

Meanwhile, on faraway Earth (and all the other locations throughout the cosmos where Dawnophiles reside), the trove of pictures and other precious measurements continue to be examined, analyzed, and admired by scientists and everyone else who yearns to glimpse distant celestial sights. And Earth itself, just as Vesta, Ceres, Dawn, and so many other members of the solar system family, continues to follow its own orbit around the sun.

Thanks to a coincidence of their independent trajectories, Earth and Dawn recently reached their smallest separation in well over a year, just as the tips of the hour hand and minute hand on a clock are relatively near every 65 minutes, 27 seconds. On Dec. 9, they were only 236 million kilometers (147 million miles) apart. Only? In human terms, this is not particularly close. Take a moment to let the immensity of their separation register. The International Space Station, for example, firmly in orbit around Earth, was 411 kilometers (255 miles) high that day, so our remote robotic explorer was 575 thousand times farther. If Earth were a soccer ball, the occupants of the orbiting outpost would have been a mere seven millimeters (less than a third of an inch) away. Our deep-space traveler would have been more than four kilometers (2.5 miles) from the ball. So although the planet and its extraterrestrial emissary were closer than usual, they were not in close proximity. Dawn remains extraordinarily far from all of its human friends and colleagues and the world they inhabit.

As the craft reshapes its solar orbit to match Ceres’s, it will wind up farther from the sun than it was while at Vesta. (As a reminder, see the table here that illustrates Dawn’s progress to each destination on its long interplanetary voyage.) We saw recently, however, that the route is complex, and the spacecraft is temporarily approaching the sun. Before the ship has had time to swing back out to a greater heliocentric range, Earth will have looped around again, and the two will briefly be even a little bit closer early in 2014. After that, however, they will never be so near each other again, as Dawn will climb higher and higher up the solar system hill, its quest for new and exciting knowledge of distant worlds taking it farther from the sun and hence from Earth.

› Continue reading Marc Rayman’s Dawn Journal to learn how to approximate Dawn’s position in the sky on Jan. 21 and 22


Short Puffs Keep Dawn Chugging Along

Tuesday, December 4th, 2012

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 the Dawn spacecraft at Ceres
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


Dawn’s Split from Asteroid Vesta - Mission Insider Explains

Wednesday, September 5th, 2012

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.

The dwarf planet Ceres as imaged by the Keck Observatory
NASA’s Dawn spacecraft departed the giant asteroid Vesta on Sept. 04, 2012 PDT to begin its journey to a second destination, the dwarf planet Ceres, which is seen in this image from the Keck Observatory on Mauna Kea, Hawaii. Image credit: NASA/JPL-Caltech, Keck Observatory, C. Dumas

Dear Marvestalous Readers,

An interplanetary spaceship left Earth in 2007. Propelling itself gently and patiently through the solar system with a blue-green beam of xenon ions, it gradually spiraled away from the sun. It sailed past Mars in 2009, its sights set on more distant and exotic destinations. In July 2011, it gracefully and elegantly entered orbit around the second most massive resident of the main asteroid belt, Vesta. It spent more than 13 months there scrutinizing the gigantic protoplanet with all of its sensors and maneuvering to different orbits to optimize its investigations, making myriad marvelous discoveries. After they traveled together around the sun for 685 million kilometers (426 million miles), the ship left orbit in September 2012 and is now headed for dwarf planet Ceres, the largest body between the sun and Neptune not yet visited by a spacecraft. No other probe has ever been capable of the amazing feats Dawn is performing, exploring two of the largest uncharted worlds in the inner solar system.

The population of the main asteroid belt numbers in the millions. Vesta is such a behemoth that Dawn has now single-handedly examined about eight percent of the mass of the entire belt. And by the time it finishes at the colossus Ceres, it will have investigated around 40 percent.

The expedition to Vesta has produced riches beyond everyone’s hopes. With 31,000 photos, 20 million visible and infrared spectra, and thousands of hours of neutron spectra, gamma ray spectra, and gravity measurements, Dawn has revealed to humankind a unique and fascinating member of the solar system family. More akin to Earth and the other terrestrial planets than to typical asteroids, Vesta is not just another chunk of rock. It displays complex geology and even has a dense iron-nickel core, a mantle, and a crust. Its heavily cratered northern hemisphere tells the story of more than 4.5 billion years of battering in the rough and tumble asteroid belt. Its southern hemisphere was wiped clean, resurfaced by an enormous impact at least two billion years ago and an even greater collision one billion years ago. These events excavated the 400-kilometer (250-mile) Veneneia and 500-kilometer (310-mile) Rheasilvia basins. The larger basin has a mountain at the center that towers more than twice the height of Mt. Everest; indeed, it soars higher than all but one of the mountains known in the solar system. The impacts were so forceful, they nearly destroyed Vesta. The fierce shock reverberated through the entire body and left as scars an extraordinary network of vast troughs near the equator, some hundreds of kilometers (miles) long and 15 kilometers (10 miles) wide.

The powerful impacts liberated tremendous amounts of material, flinging rocks far out into space, some of which eventually made it all the way to Earth. It is astonishing that more than one thousand meteorites found here came from Vesta. We have some meteorites from Mars, and we have some meteorites from the moon, but we have far, far more that originated in those impacts at Vesta, so distant in time and space. Vesta, Mars, and the moon are the only celestial bodies identified as the source of specific meteorites.

Scientists will spend years productively poring through Dawn’s fabulous findings and learning what secrets they hold about the dawn of the solar system, and many more people will continue to marvel at the spectacular sights of this alien world. But the emissary from Earth has completed its assignment there and moved on. It has spent most of its time since the previous log using its ion propulsion system to climb higher and higher above Vesta. This departure spiral is the mirror image of the approach spiral the robotic adventurer followed last year. The unique method of entering and leaving orbit is one of the many intriguing characteristics of a mission that uses ion propulsion. Without that advanced technology, this ambitious deep space adventure would be impossible.

As Dawn ascended, Vesta’s gravitational grip grew weaker and weaker. At some point along its spiral, the explorer was far enough and moving fast enough that Vesta could no longer hold it in orbit. As smoothly and tenderly as Vesta had taken Dawn in its embrace last year, it released its erstwhile companion, each to go its own way around the sun. The bond was severed at about 11:26 p.m. PDT yesterday, when they were 17,200 kilometers (10,700 miles) apart, separating at the remarkably leisurely speed of less than 33 meters per second (73 miles per hour). Many of our readers drove their cars that fast today (although we hope it was not in school zones).

Unlike missions that use conventional chemical propulsion, there was no sudden change on the spacecraft and no nail-biting on Earth. If you had been in space watching the action, you probably would have been hungry, cold, and hypoxic, but you would not have noticed anything unusual about the scene. Apart from a possible hint of self-satisfaction, Dawn would have looked just as it had for most of its interplanetary flight, a monument to humankind’s ingenuity and passionate drive to know the cosmos perched atop a blue-green pillar of xenon ions. If, instead, you had been in Dawn mission control watching the action, you would have been in the dark and all alone (until JPL Security arrived). There was no need to have radio contact with the reliable spaceship. It had already thrust for almost 2.9 years, or 58 percent of its time in space. Thrusting during escape was no different. No one was tense or anxious; rather, all the drama is in the spectacular results of the bold mission at Vesta and the promise of what is to come at Ceres. When Dawn entered orbit, your correspondent was dancing. When Dawn left orbit, he was sleeping serenely.

A month earlier, on August 8, with the craft more than 2,100 kilometers (1,300 miles) above the surface, patiently powering its way up through Vesta’s gravity field, one of the reaction wheels experienced an increase in internal friction. Reaction wheels are used to control a spacecraft’s orientation in the frictionless, zero-gravity conditions of spaceflight. By electrically changing a wheel’s spin rate, Dawn can rotate or stabilize itself. Protective software quickly detected the event and correctly responded by deactivating that wheel and the other two that were operating, switching to the small jets that are available for the same function, and reconfiguring other systems, including powering off the ion thrust and turning to point the main antenna to Earth.

A routine communications session the next day revealed to mission controllers what had occurred. They had planned long ago to turn the wheels off for the flight from Vesta to Ceres, so having them off a few weeks early was not a significant change. The team soon restored the spacecraft to normal operations and reformulated the departure plan, and on August 17 Dawn resumed its ascent. Because of the hiatus in thrusting, escape shifted from August 26 to September 4. The flexibility in the mission timeline provided by ion propulsion made this delay easy to accommodate.

In order to conserve the hydrazine propellant that the jets use, the bonus departure observations described before were curtailed, as they were not a high priority for the mission. Nevertheless, on August 25 and 26, at an altitude of around 6,000 kilometers (3,700 miles), the explorer did peer at Vesta once more with its camera and visible and infrared mapping spectrometer. The last time it had been this far away was July 21, 2011, during its descent to an unfamiliar destination. This time, 13 months later, the spacecraft turned back for a final gaze at the magnificent world it had unveiled during its remarkable time there, a world that prior to last year had appeared as little more than a tiny smudge among the stars for the two centuries it had been observed.

The delay in the departure schedule provided a convenient benefit. Vesta has seasons, just as Earth does, although they progress more slowly on that distant orb. August 20 was the equinox, when northern hemisphere spring began. Until then, the sun had been in Vesta’s southern hemisphere throughout Dawn’s residence there. While most of the northern hemisphere was revealed during the second high-altitude mapping orbit, the illumination of the landscape immediately around the north pole was even better for this last look. After radioing its parting shots to wistful mission controllers, the ship commenced its climb again.

And then, with an stunningly successful mission behind it, a newly explored world below it, and a mysterious dwarf planet ahead of it, the indomitable and indefatigable adventurer left Vesta forever.

Dawn is 18,500 kilometers (11,500 miles) from Vesta and 64 million kilometers (40 million miles) from Ceres. It is also 2.45 AU (367 million kilometers or 228 million miles) from Earth, or 910 times as far as the moon and 2.43 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 a.m. PDT September 5, 2012

› Read previous Dawn Journals by Marc Rayman