Circling the protoplanet 210 kilometers (130 miles) beneath it every 4 hours, 21 minutes on average, Dawn is closer to the surface than the vast majority of Earth-orbiting satellites are to that planet. There are two primary scientific objectives of this low altitude mapping orbit (LAMO). With its gamma ray and neutron detector (GRaND), the probe is measuring the faint emanations of these subatomic particles from Vesta. Some are the by-products of the bombardment by cosmic rays, radiation that pervades space, and others are emitted through the decay of radioactive elements. Vesta does not glow brightly when observed in nuclear particles, so GRaND needs to measure the radiation for weeks at this low altitude. This is analogous to using a long exposure with a camera to photograph a dimly lit subject. If GRaND only detected the radiation, it would be as if it took a black and white picture, but this sophisticated instrument does more. It measures the energy of each particle, just as a camera can measure the color of light. The energies reveal the identities of the elements that constitute the uppermost meter (yard) of the surface. Dawn devotes most of its time now flying over Vesta to collecting the glimmer of radiation. It requires a long time, but this spacecraft has demonstrated tremendous patience in its use of the gentle but efficient ion propulsion system that made the mission possible, so it can be patient in making these measurements.

The second motivation for diving down so low is to be close enough that Vesta's interior variations in density affect the spacecraft's orbit discernibly. We have seen before that the distribution of mass inside the protoplanet reveals itself through the changing strength of its gravitational tug on Dawn. Exquisitely sensitive measurements of the ship's course can be translated into a three-dimensional map of the mass. In the plans discussed for LAMO one year ago, the delicate tracking of the spacecraft required pointing the main antenna to Earth. That provides a radio signal strong enough to achieve the required accuracy. Since then, navigators have determined that the radio signal received from one of the craft's auxiliary antennas, although far weaker, is sufficient. The main antenna broadcasts a tight beam, whereas the others emit over a much larger angle, exchanging signal strength for flexibility in pointing.

This allows an extremely valuable improvement. The spacecraft cannot aim GRaND at the surface and the main antenna at Earth concurrently, because both are mounted rigidly, just as you cannot simultaneously point the front of your car north and the back east. Therefore, in the original plan, gravity measurements and GRaND measurements were mutually exclusive. Now, as Dawn turns throughout its orbit to keep Vesta in GRaND's sights, it can transmit a weak radio signal that is just perceptible at Earth. This enables an even greater science return for the time in LAMO. Unlike the science camera and the visible and infrared mapping spectrometer (VIR), GRaND and gravity observations do not depend on the sun's illumination of the surface. Even as it orbits over a dark, cold, silent landscape, Dawn is fully capable of continuing to build its maps of elements and the interior structure.

The signal from the auxiliary antenna is just sufficient for the measurement of the spacecraft's motion, but it is not strong enough to carry data as well. So the spacecraft is still programmed to point its main antenna to Earth three times each week, allowing the precious GRaND observations that have been stored in computer memory to be transmitted. As always, the myriad measurements of temperatures, voltages, currents, pressures, and other parameters that engineers use to ensure the health of the ship are returned during these communications sessions as well.

Although the pictures of Vesta from survey orbit and the high altitude mapping orbit (HAMO) have exceeded scientists' expectations, not only in quality and quantity but also in the truly fascinating content, as enthusiastic explorers, the Dawn team could not pass up the opportunity for more. When GRaND is pointed at the surface, the camera is too, and already well over one thousand images have been returned, revealing detail three times finer than the spectacular images from HAMO. For readers who cannot go to Vesta on their own, go here for a selection of the best views, each showing surprising and captivating alien landscapes.

In addition to the bonus photography, beginning in January VIR will take observations. Although the instrument has already acquired nearly seven million spectra in the higher orbits, this new vantage point will allow sharper resolution, just as it does for the camera.

The ultra-long-distance communication between Dawn and Earth requires extraordinary technology on both ends. Even with all the sophistication, the amount of information that can be transmitted in a given time remains very limited. The remote spacecraft sends data at speeds significantly lower than a typical home Internet connection. Engineers use that precious communications link very carefully, judiciously selecting what information to instruct the probe to return. Because of the high priority given to GRaND, which needs to be pointed at the surface as long as possible, much of the limited time spent with the main antenna aimed at Earth is devoted to transmitting that instrument's findings (and the measurements of spacecraft subsystems). This restricts how much data from the camera and VIR can be communicated.

In the next log, we will see another limitation on the number of camera images and VIR spectra in LAMO. It is a consequence of another aspect of the complex operations in this low orbit around a massive body, and that is the small but real differences between the predicted orbit and the actual orbit. We will cover the first part of the explanation here.

Navigators use their best knowledge of the many forces acting on Dawn to chart an orbital course for it. The forces can be traced to three principal sources: gravity, light, and Dawn itself. We have discussed all of these before in detail (see, for example, this explication of the last two), but let's review them here. This is an involved story, so readers are advised to be in a comfortable orbit while following it. You can safely skip the next four paragraphs and no one ever need know.

Vesta has a complicated gravity field, and that leads to a complicated orbit. The spacecraft does not follow a perfectly circular, repetitive path because the gravitational pull on it changes according to where it is as the colossus beneath it rotates and it loops around. The map of the gravity field has been improving throughout Dawn's residence there, but its completion awaits the LAMO gravity measurements. In the meantime, unknown details of the variation of mass lead to small divergences in the orbit. All the other bodies in the solar system exert gravitational pulls on the spacecraft as well (just as they do on you), but those are more easily accounted for. The distances from Dawn are so great that the variations in their gravity fields don't matter. So although the effects of the faraway objects need to be accounted for, they do not contribute much to the discrepancies.

Dawn depends on sunlight for its power, using its large solar arrays to make electricity to run all systems. The sun also propels the spacecraft, because in the frictionless conditions of spaceflight, the ship recoils slightly in response to the miniscule but persistent pressure of the light. The force depends on whether the light is absorbed (whereupon it is converted to electrical power by the arrays or to heat by whatever component it illuminates) or reflected. If it is reflected, the angle makes a difference, so smooth shiny surfaces that act like mirrors cause different effects from the materials that present a matte finish or are curved or angled. As the spacecraft rotates to keep GRaND pointed at the ground below, different parts of the ship are presented to the sun, so the force from the light changes, and the orbit is constantly subjected to a variable disturbance.

Dawn itself adds to the complexity of its orbital path. The spacecraft carries reaction wheels, which are spun to help it control its orientation. These devices gradually spin faster, so every few days they need to be slowed down. That is accomplished by firing the small reaction control system thrusters during windows specified by mission controllers. In addition to the thrusters providing the needed torque on the craft to reduce the wheels' speeds, they impart a force that changes the orbit slightly.

The physical principles underlying all these phenomena that perturb Dawn's orbit are understood with exceptional clarity. Although the values of the myriad parameters involved are ascertained quite accurately, they are not known perfectly. As a result, navigators' prediction of the ship's course includes some degree of uncertainty. Even their ability to determine the present orbit is subject to a variety of small errors typical in sensitive physical measurements.

For all of these reasons, the craft's actual orbit departs slightly from the plan, and the deviations tend to grow, albeit gradually. As designers expected, in survey orbit and HAMO, the differences were small enough that they did not affect the complex operations plans. Analysis well before Dawn arrived at Vesta predicted that the discrepancies in LAMO would be large enough that occasional adjustments of the orbit would be necessary. Therefore, mission controllers scheduled a window every week (on Saturdays, as it turned out) to use the ion propulsion system to fine-tune the spacecraft's trajectory, bringing it back to the intended orbit. These are known as "orbit maintenance maneuvers," and succumbing to instincts developed during their long evolutionary history, engineers refer to them by an acronym: OMM. (As the common thread among team members is their technical training and passion for the exploration of the cosmos, and not Buddhism, the term is spoken by naming the letters, not pronouncing it as a means of achieving inner peace. Instead, it may be thought of as a means of achieving orbital tranquility and harmony.)

The LAMO phase began on December 12, and OMMs were performed on December 17 and 24. In contrast to the long periods of thrusting required with ion propulsion for other parts of the mission, the corrections needed were so small that each OMM needed less than 15 minutes. The whisper-like thrust changed the spacecraft's speed by less than five centimeters per second (one-tenth of a mph). But that was enough to nudge Dawn back to the planned orbit.

The ship was so close to the designated course that the OMMs for December 31 and even January 7 have already been canceled. Not executing the OMMs allows the probe to spend more time collecting neutrons and gamma rays from Vesta. The operations team productively uses the time saved in designing, checking, and transmitting the OMM commands to do other work to ensure LAMO proceeds smoothly and productively.

In the last log we discussed the complicated and dynamic spiral descent from HAMO to LAMO, which was still in progress. The flight required not only reducing the altitude from 680 kilometers (420 miles) to 210 kilometers (130 miles) but also twisting the plane of Dawn's orbit around Vesta. As with all orbiting bodies, whether around Vesta, Earth, or the sun, the lower the orbital altitude, the shorter the orbital period. Vesta's gravitational grip strengthened as Dawn closed in, forcing the spacecraft to make faster loops around it. This meant that as the probe performed the intricate choreography to align its ion thruster with the changing direction needed to alter its orbit, it had to pirouette faster.

When engineers command Dawn to rotate, they usually instruct it to use the same stately speed as the minute hand on a clock. The spacecraft may have to move a little faster however, as it pivots to keep its solar arrays pointed at the sun while accomplishing the required turn. Sometimes it knows that at the end of a turn, it will have to initiate another turn. For example, it may rotate to the orientation required to begin a session of ion thrusting. But while it is thrusting and curving around its orbit, it generally needs to steer the thruster to execute the maneuver. As a result, the robot may choose to turn at a slightly different rate from what its human team members command in order to make a smooth transition from the first turn to the second.

On Dec. 3, when preparing for one of the final thrust segments required to reach LAMO, the combination of all these factors caused the spacecraft to rotate faster than usual. That led to a temporary discrepancy between where it was pointed and where it expected to be pointed during the turn. When protective software detected the inconsistency, it interrupted the ongoing activities and put the spacecraft into safe mode.

When the safe mode signal was received by the Deep Space Network, the operations team responded with its usual calm and skill. They quickly determined that Dawn was fully healthy, diagnosed the cause of the safing, and began guiding the spacecraft back to its normal operational configuration. In addition, they devised a new flight profile that would compensate for the thrusting that was not completed. The team also determined how to prevent the same problem from recurring for subsequent maneuvers. While doing all this work, they were putting the finishing touches on the first LAMO science observation sequences. Controllers managed to complete everything flawlessly and even kept the mission on schedule, allowing LAMO to commence on Dec. 12.

The general plan for Dawn's three-month approach plus one year in orbit around Vesta was described in logs in 2010. The time was apportioned among the different science phases and the transfers between science orbits to ensure a comprehensive and balanced exploration of this mysterious and fascinating world. Fully appreciating that in such an exceedingly ambitious undertaking, some unexpected problems are inevitable, mission planners worked hard to devise an itinerary that left 40 days uncommitted. Their strategy was that as they recovered from anomalies, they would draw from that time and still not have to compromise any of their carefully designed activities. They also planned that any unspent margin would be used to extend LAMO.

To the great delight (and, to be honest, surprise) of all, not one day of the 40-day reserve has been needed. Although there have indeed been unanticipated difficulties, from the beginning of approach on May 3 to this point, the team has been able to resolve all of them without having to withdraw from that account. This is remarkable considering that Dawn is the first visitor from Earth to Vesta, with its many unknown physical properties. This expedition is the first ever in which humankind has sent a spacecraft to orbit such a massive body without first conducting a reconnaissance with a flyby spacecraft. Dawn has maintained a rapid pace of scrutinizing its enigmatic destination. Performing all of this so successfully without needing to use even a little of the spare time they provided for themselves was considered quite unlikely. And yet the entire 40 days remain available.

More ambitious operations lie ahead, with the rest of LAMO, the spiral ascent to HAMO2, HAMO2 itself, and the escape in July to begin the long interplanetary cruise to reach Ceres on schedule in February 2015. We will see in 2012 that each of these phases includes new challenges, and it is certain new problems will arise. Nevertheless, all 40 days are being used to extend LAMO. Therefore, the indomitable explorer will remain at this low altitude through the end of March, continuing to tease out secrets about the dawn of the solar system and revealing more startling and thrilling discoveries on behalf of everyone on distant Earth who yearns to reach out into the vastness of space.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 2.79 AU (418 million kilometers or 260 million miles) from Earth, or 1045 times as far as the moon and 2.84 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.

TAGS: DAWN, CERES, VESTA, DWARF PLANETS, SOLAR SYSTEM, MISSION, SPACECRAFT

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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. To picture the different orbits, let’s recall our discussion of how the orbit shifted from survey to HAMO. Readers unconcerned about the details of the geometry may safely (and wisely) skip this explanation.

Think of Dawn’s orbit as a ring encircling Vesta, going over both poles and crossing the equator at a right angle. Globes of Earth often are supported by a ring like that, and it may be helpful to have a terrestrial globe in mind, or even in sight, as you ponder the celestial arrangement. For the purpose of this illustration, you may rest assured that no inhabitants (permanent or temporary) of Vesta will object if we pretend that the world does not rotate, so a ring that is aligned with a longitude line represents one of Dawn’s orbits. To start, let’s say survey orbit hovers over the 15 degree west longitude line (and, to make a complete circle, goes over the 165 degree east longitude line as well). HAMO would be shifted to 30 degrees west (and 150 degrees east on the other side of the globe). Dawn is now on its way to LAMO, which will be at about 46 degrees west (and 134 degrees east). The complex scheme for moving from HAMO to LAMO then involves not only lowering the altitude but also rotating the plane of the orbit by 16 degrees. We will delve into why this value was chosen after the spacecraft has arrived in LAMO.

These differing orientations are a crucial element of the strategy for gathering such a wealth of scientifically valuable data on Vesta. The ion propulsion system allows the probe to fly from one orbit to another without the tremendous penalty of carrying a massive supply of propellant. In general, it requires a great deal of maneuvering to change the plane of a spacecraft’s orbit. Indeed, one of the reasons traveling from Earth to Vesta (and later Ceres) requires ion propulsion is the challenge of tilting the orbit around the sun. (A more extensive discussion of this and a table showing Dawn’s progress in altering the angle of its solar orbit to match Vesta’s were presented on the fourth anniversary of launch.)

As long as we have used the globe to illustrate the orientation of the orbits, we can enhance the picture for those readers who want to sharpen their mental images of the geometry. Suppose Vesta, which Dawn has transformed from a smudge of light into a richly detailed world, is 30 centimeters (1 foot) in diameter. Even in this miniaturized universe, the sun is 199 kilometers (124 miles) away. (Space is big!) To get the alignments right, we will place the sun over (albeit very, very far over) the prime meridian, the 0 degree longitude line on our stationary Vesta. Setting the longitude of the sun is important here because Dawn’s orbits were chosen on the basis of their angles relative to the sun. Now we recall that Vesta, like Earth, has seasons because its axis is tipped. It is southern hemisphere summer there, so the sun is not over the equator; rather, it is at about 25 degrees south latitude. (On Sept. 1, when we first used the analogy of the globe, the sun was at 27 degrees south latitude. Since then, it has moved a little bit north because of the progression of seasons.) Although Earth’s location doesn’t matter for this discussion, we can accurately position it 204 kilometers (127 miles) away, high above a point at 26 degrees south latitude and 26 degrees west longitude.

Survey orbit in this Vesta-centric universe is a hoop a little more than 1.5 meters (5 feet) above the 15 degree west longitude line (and, again, the 165 degree east longitude line on the night side). It was from that vantage point that the first thorough mapping was conducted in August. The ring representing HAMO is only about 38 centimeters (15 inches) over the 30 degree west (and 150 degree east) longitude line. The lower altitude of HAMO afforded much better views of the great variety of surface features than the reconnaissance from survey orbit. In addition, because the orbit was shifted farther from the sun, the angle of light on the landscape beneath the spacecraft was different, aiding in formulating a more accurate portrait of the terrain. Dawn is now nearing LAMO, less than 12 centimeters (only 4.7 inches) above the 46 degree west (and 134 degree east) longitude line.

Although the ion propulsion system accomplishes the majority of the orbit change, Dawn’s navigators are employing another novel propulsion system as well: they are enlisting Vesta itself. Some of the ion thrusting was designed in part to put the spacecraft in certain locations from which Vesta would twist its orbit toward the target LAMO angle. As Dawn rotates and the world underneath it revolves (unlike the static picture we used to visualize the orientation of the orbits), the spacecraft feels a changing pull. There is always a tug downward, but because of Vesta’s heterogeneous interior structure, the product of its complex geologic history, sometimes there is also a slight force to one side or another. With their knowledge of the gravity field, the team plotted a course that took advantage of these variations to get a free ride. This is akin to experienced sailors not only relying on their ships’ engines but also following routes that use known currents to let nature do some of the work. Of course, sailors benefit from knowledge of currents measured by those who plied the waters before them. Dawn is the first, venturing boldly into mysterious seas never visited before. But the measurements of the gravity field in HAMO, even though it was at a higher altitude, gave navigators enough information about what lay ahead on the horizon that our vessel could safely and productively ride the gravitational currents. The flight plan from HAMO to LAMO then is a complex affair of carefully timed thrusting and equally carefully timed coasting. Under ion thrust, the spacecraft flies to a certain location in a certain orbit at a certain time, waits a certain interval as Vesta propels it to the next waypoint, and then it resumes thrusting.

This itinerary was worked out in exquisite detail when Dawn was still in HAMO, but it is impossible to follow the mathematically perfect path. Although navigators measured the gravity field, they were not able to detect all of its convolutions, so the ship is subjected to slightly different currents from those expected. Occasional firings of the spacecraft’s small jets and tiny discrepancies between the planned strength of the ion thrust and the actual value contribute more to the deviation of the trajectory. Mission planners studied these and other effects thoroughly and have been well prepared to account for all of them.

Every few days during the spiral transit from HAMO to LAMO, the spacecraft points its main antenna to Earth so navigators can get an accurate fix on it. The time for radio signals (traveling, as all readers can attest, at the universal limit of the speed of light) to make the round trip allows them to measure the probe’s distance. The slight change in the frequency of the signal, known as the Doppler shift, reveals how fast the craft is moving. Combining these with their best mathematical description of the gravity field and other data, as well as the plan that is onboard for upcoming thrusting and coasting, they determine Dawn’s orbit and calculate where it will be at a certain time in the future. For most of the way between HAMO and LAMO, that time is three days, which is just long enough for the entire operations team to perform an intricate set of carefully choreographed steps.

Following the computation of where the spacecraft will be, trajectory designers develop an update to the ideal reference they formulated while Dawn was still in HAMO, now accounting for the observed discrepancies. They devise a new profile of thrusting and coasting. Next, others on the operations team translate that into the sequence of timed commands that the robotic probe will execute in order to accomplish the maneuvers. (Some of the principal challenges faced in guiding the ship through these ever tightening spiral loops were described in February. As we saw then, the spacecraft even simulated a portion of this complex flight; now it is really doing it, and it is doing it extremely well.)

While all of that is taking place at JPL, Dawn is continuing to fly around Vesta, carrying out the previous set of instructions it received to thrust and coast. Right on schedule, the team completes the work, including all the diligent checks to make sure every detail is right, in time to transmit the new sequence to the adventurer when the one already onboard calls for it to turn its main antenna to Earth once again. Almost as soon as the process is finished, it starts again, with a measurement of Dawn’s latest orbit.

In some of the more dynamic parts of the transfer from HAMO to LAMO, three days is too long to let the spacecraft fly on its own without an update to the plan, so the team had to do all that work in two days. Thanks to the meticulous planning, preparation, and rehearsing, this complex work has been conducted with the smooth professionalism that has characterized the entire mission. As a result, Dawn remains on course and on schedule to begin its scientific observations in LAMO on December 12.

Very far from home, the spacecraft is making excellent progress in its expedition at a fascinating world that, until a few months ago, had never seen a probe from Earth. This unique mission to a place unlike any that has been visited before has already produced a tremendous bounty of fabulous pictures and other important scientific data. Soon it will be ready to undertake a new phase of exploration, revealing ever more about this behemoth of the asteroid belt. Both patient and ambitious, Dawn plumbs the depths of alien waters, stirring the imagination and nourishing the spirit and the mind of everyone on distant Earth who ever gazes at the sky in wonder or who longs to know the cosmos.

Dawn is 240 kilometers (150 miles) from Vesta. It is also 2.41 AU (361 million kilometers or 224 million miles) from Earth, or 935 times as far as the moon and 2.45 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 40 minutes to make the round trip.

TAGS: DAWN

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  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

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

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

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

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

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

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

The more than 7000 images Dawn acquired in HAMO will form a very firm basis for years of productive study. Combined with the other data the explorer returns from Vesta, we will follow the global maps to find exciting new insights about the solar system. What better destination could the maps guide us to?

Included in the “other data” is a collection of fabulous new observations with the visible and infrared mapping spectrometer (VIR). A spectrum is the intensity of light at different wavelengths. When you enjoy the sight of light dispersed through a prism, which decomposes white light into its constituent colors, you have a similar view. The material on Vesta leaves its signature in the spectrum of light it reflects from the sun. Each VIR snapshot produces a “frame,” which consists of a full spectrum, from a portion of ultraviolet through the entire visible range into infrared, at 256 distinct locations on the surface. Every frame contains a richness of information.

The direction in which Dawn pointed its instruments in HAMO was dictated by the plan for thoroughly mapping Vesta with the camera. Nevertheless, VIR was used to acquire a sumptuous set of spectra which are the principal means by which scientists will determine the nature of the minerals on its surface. The spectra extend far enough into the infrared that they also measure the meager heat radiated from the surface, thereby acting as a thermometer.

As with the pictures from the camera, the VIR frames in HAMO benefit from the lower altitude by showing more detail than those acquired in survey orbit. VIR acquired 15 thousand frames in HAMO, yielding full spectra on nearly four million regions over Vesta’s vast surface.

The spectra from VIR provide greater chromatic detail than the seven color filters in the camera. The camera is able to cover larger areas with finer resolution in a single image. The two instruments are complementary, and together they contribute to developing a detailed picture of the geology of Vesta.

Although the main gravity and GRaND measurements will be made in LAMO, they have already begun. The gravity investigation is revealing hints of the interior structure even before the probe travels to its lowest altitude. The nuclear radiation GRaND will measure is so weak that Dawn will need to go closer to allow that instrument to do its primary work. Nevertheless, even in HAMO it has been detecting neutrons emitted by Vesta.

After GRaND was reactivated at the end of September as part of the recovery from safe mode, scientists observed that one of its sophisticated gamma ray detectors showed an increase in noise. Scientists and engineers worked together in analyzing the telemetry from the instrument and establishing the best way to remedy it. They proceeded carefully and gradually over the course of about 10 days, first powering selected internal components off, then deactivating the entire instrument, rebooting it, and reconfiguring it. They were rewarded by seeing GRaND back to its normal performance, once again ready to contribute fully to this grand mission of exploration.

Dawn collected slightly more data during most of its transits over the lit side of Vesta than it could transmit over the subsequent dark side passes. Therefore, by the end of HAMO, it still had precious information that had accumulated in its memory. Following the completion of mapping cycle 6 early this morning, the spacecraft is spending another two days beaming the last of its findings back to the distant planet it left more than four years ago.

Engineers are taking advantage of this time with the main antenna pointed to Earth to perform some reconfigurations of the spacecraft. They are preparing it for operating closer to Vesta, where more reflected light from the surface will reach some of the sensors that keep track of the location of the sun and the vast rocky body will cause fewer stars to be visible to the star trackers. Other changes are scheduled to occur at lower altitudes.

With a truly amazingly productive HAMO phase complete and the spacecraft ready to venture on, plans are all in place for what may be the most arduous phase of the mission. On November 2 at 5:20 a.m. PDT, the ship will set sail again with its ion propulsion system to push down to its lowest orbit. It will take more than five weeks to reach LAMO. In the next log we will check in on the probe’s progress and consider some of the obstacles to be overcome in dipping so low.

In the meantime, your correspondent was inspired by the splendid results from HAMO to devise his best Halloween costume in a long time. This year, he pretended to be someone who is unmoved by the stunning views of Vesta and the extraordinary accomplishments of the indefatigable spacecraft that acquired them. He appeared not to feel the deep gratification of being a member of a species that works together to extend beyond our limitations to undertake bold adventures in search of some of the greatest rewards of all: new knowledge and new perspectives on the cosmos. He disguised himself as one who does not feel the powerful longing to share in the unveiling of an alien world. He acted as if he has not hungered for the wonderful feast Dawn is now serving. He was not only unrecognizable, but in this costume he surely would not be mistaken for any of the dawnderful readers of these logs.

Dawn is 680 kilometers (420 miles) from Vesta. It is also 2.03 AU (303 million kilometers or 184 million miles) from Earth, or 795 times as far as the moon and 2.04 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 34 minutes to make the round trip.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

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

When the main portion of the thrusting was finished on Sept. 18, there was still more for Dawn to do than let navigators determine its exact orbit. It trained its sensors on Vesta, acquiring more exciting and valuable data. Although these observations are not part of the meticulously orchestrated and systematic mapping in HAMO, they contribute to the overall effort to squeeze as much as possible out of the precious time at Vesta. Engineers also performed more tests with the visible and infrared mapping spectrometer. In addition, they acquired images with the backup science camera, confirming that it is still fully functional and ready for action should its perfectly healthy twin ever become infirm.

Following instructions loaded earlier, on Sept. 21 the spacecraft reconfigured its memory to prepare for the great volume of data it will collect in HAMO. One of the software functions took longer than expected, causing the main computer to reset. The robot is designed to enter safe mode after a reboot, so it dutifully powered off nonessential systems, turned to point at the sun (the only celestial reference easily detectable anywhere along Dawn’s long route through the solar system), and waited for further instructions. Controllers detected the condition late that night and quickly identified the cause. With calm professionalism they swiftly executed all the steps necessary to return Dawn to its normal flight configuration less than two days later, and operations have continued smoothly.

As the spacecraft flies around Vesta, its altitude is constantly changing. Indeed, it would vary even if the orbit were a perfect circle, because Vesta is not a perfect sphere. This is similar to flying in a plane over Earth. If the plane maintains a constant altitude above sea level, the distance above the ground can change because the elevation of the ground itself varies, coming closer to the aircraft on mountains and farther in valleys. The topography on Vesta is even more pronounced, reflecting the tortured history it has experienced during 4.5 billion years in the rough and tumble asteroid belt.

The event that created the huge gouge centered near the south pole, now officially known as Rheasilvia (after the vestal virgin who was the mythical mother of Romulus and Remus, a weird story unlikely to be clarified by Dawn’s investigations), has left Vesta not only with astounding and jumbled terrain but also with an overall shape that is very peculiar. Indeed, although this world is smaller than Earth, it displays some of the most extreme topography in the solar system. The tremendous mountain in the center of Rheasilvia towers about twice as high above the surrounding plains as Mt. Everest does above sea level. Despite their being widely separated, the difference in elevation between the highest features near the equator and the lowest points deep in craters punched into Rheasilvia is more than 60 kilometers (37 miles).

Even if we imagined Dawn as being stationary while Vesta rotated underneath it, the altitude would change as the misshapen surface surges and subsides. In addition, the craft’s path is not a perfect circle, and the lower the orbit, the more it will deviate, as the irregular gravity field tugs on it with changing strength depending on where in that complex field the spacecraft is. When Dawn pushes down to closer orbits, we will discuss more about the actual height above the surface. In the meantime, for simplicity, these logs will continue to present the altitude as an average value, measured with respect to the average distance from the center of Vesta to its rocky surface. This is analogous to using sea level on Earth for the reference to describe altitude for aircraft and satellites. It is on that basis that the altitude in HAMO is given as 680 kilometers (420 miles).

On the last three September 27s, we have summarized Dawn’s progress on its journey. Now that it is at its first destination, the best measure of its progress is the stunning images and other scientific results it has transmitted to Earth. In addition to special announcements at press conferences in the coming months, beautiful and intriguing views continue to be posted here every day.

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 fourth 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 first parts of the logs from its first, second, and third anniversaries. (On this special day, members of the operations team will further reflect upon the mission with the help of cupcakes decorated with the Dawn/Vesta logo.)

In its four years of interplanetary travels, the spacecraft has thrust for a total of about 988 days, or 68% of the time (and about 0.000000020% 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 254 kilograms (561 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 6.85 kilometers per second (15,300 miles per hour). 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 barely half 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 four revolutions around the sun, covering about 25.1 AU (3.76 billion kilometers or 2.34 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 19.4 AU (2.91 billion kilometers or 1.81 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. Since Dawn’s launch, Vesta has traveled only 16.4 AU (2.46 billion kilometers or 1.53 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.

Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family follow their paths around the sun, they sometimes move closer and sometimes move farther from it.

In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth’s orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body’s orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and no spacecraft has ventured as far out of that plane in order to achieve orbit around another body as Dawn has.)

Now we can see how Dawn has been doing by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts’ websites when their readership extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft had terminated thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on September 27, 2007, its orbit around the sun was exactly Earth’s orbit. After launch, it was in its own solar orbit.

For readers who are not overwhelmed by the number of numbers, the table may help to demonstrate how Dawn has transformed its orbit during the course of its mission. Note that now the spacecraft's orbit around the sun is the same as Vesta’s. Achieving that was, of course, the objective of the long flight that started in the same solar orbit as Earth. While simply flying by Vesta would have been far easier, matching orbits with it has required the extraordinary capability of the ion propulsion system. Without it, NASA’s Discovery Program would not have been able to afford a mission to explore this fascinating world, and a mission to both Vesta and Ceres would be impossible.

Amazing and inspiring as its extraordinary trek has been, climbing the solar system hill atop a blue-green pillar of xenon ions, gently reshaping its orbit with the finesse of a sculptor creating a cosmic masterpiece, traveling far from its home planet through the forbidding and lonely depths of interplanetary space, it is the destination, and not the journey, that provides the grand prize. For most of the two centuries prior to Dawn’s arrival, Vesta was known as little more than a small fuzzy patch of light amidst the stars. The sharply focused picture that we are developing now of a complex alien world, with a dramatic history and a truly unique character, is the great reward for the long years and the billions of kilometers (miles) to get there. As the expedition continues, how can anyone not thrill to the experience of Vesta simultaneously becoming both more familiar and yet more mysterious?

Now as the operations team completes preparations for the next phase of its scrutiny of Vesta, Dawn embarks on its fifth year of spaceflight doing what it was designed to do. At the limits of human ingenuity, powered by the zeal of those who seek to perceive and to understand the beauty of the cosmos, the stalwart ship forges ahead with its exploration of a relict from the dawn of the solar system.

Dawn is 680 kilometers (420 miles) from Vesta. It is also 1.59 AU (238 million kilometers or 148 million miles) from Earth, or 665 times as far as the moon and 1.59 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 26 minutes to make the round trip.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

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

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

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

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

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

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

Even before the outstandingly successful survey orbit had begun, navigators were starting to plan the flight to the next science orbit. Throughout Dawn’s approach and survey orbit, they have been refining measurements of Vesta’s mass and therefore its gravitational strength. The closer Dawn has come, the better they have been able to detect variations in the gravity field that are due to the uneven distribution of mass within the protoplanet. With their improved charts of the waters around Vesta, they plotted the ship’s course, and it is now under sail. Thrusting with the ion propulsion system began on August 31 at 4:05 p.m. PDT, and this trip to the high altitude mapping orbit will take a month.

In survey orbit, Dawn was 2,700 kilometers (1,700 miles) above Vesta. Its next orbital target lies at an altitude of about 680 kilometers (420 miles). The separation between them may seem relatively small, but maneuvering in orbit requires far more work than may be evident simply from the distance. In addition, Dawn is doing even more than flying down to a lower altitude. Each of the observation orbits at Vesta is designed to optimize a set of scientific investigations. Scientists want to shift the plane of Dawn’s orbit in going from survey to HAMO in order to change the illumination presented to the sensors.

To visualize the nature of the shift, picture the orbit as a ring around the world, going over both poles and crossing the equator at right angles. Many globes are supported within a ring like this. Now for this explanation, we have the permission of the residents to ignore Vesta’s rotation, so the ring is like a circle of constant longitude hovering in space. For the purpose of illustration, let’s say survey orbit is at a longitude of 15 degrees. (The distant sun would be at a longitude of 0 degrees. Shining south of the equator at a latitude of 27 degrees, the star is more than 2.25 times farther from Vesta than it is from Earth.) HAMO is not only four times closer to the surface than survey but also rotated so it is at 30 degrees longitude. Of course, Vesta will continue to turn on its axis, but with the plane of Dawn’s orbit changed, the angle of sunlight falling on the surface below will be different. (Once again, “longitude” is used here only to illustrate the relative orientation of the orbit planes; it is not intended to describe a relationship to specific coordinates on the ancient, battered, rocky world.)

To travel from survey to HAMO, Dawn will have to accomplish the equivalent of a change in speed of around 65 meters per second (145 mph). Compared to the 6.8 kilometers per second (15,200 mph) it achieved in its trek from Earth to Vesta, this is very modest indeed. Nevertheless, as the spacecraft spirals lower, the flight plan is much more complex than it was during the interplanetary flight. The outcome will be described in the next log.

Even as Dawn ventures closer to the giant that holds it in orbit, the splendid results of its first detailed survey of Vesta will continue to dazzle and excite us. The images and other data beamed to Earth are filled not only with scientific value but also with the exhilaration of discovery and the thrill of exploration. The drama upon beholding some of humankind’s first views of an alien world is something everyone can experience. After all, it is the collective passion for extending our reach beyond the confines of our terrestrial neighborhood and the shared hunger for knowledge of the cosmos that enabled an emissary from Earth to take up residence far from home, deep in the asteroid belt. There, our species’ yearning for noble adventures is now being fulfilled.

Dawn is 2,600 kilometers (1,600 miles) from Vesta. It is also 1.35 AU (202 million kilometers or 125 million miles) from Earth, or 555 times as far as the moon and 1.34 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 22 minutes to make the round trip.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

With this superior knowledge of the strength of Vesta's gravity, navigators now estimate that the giant asteroid took Dawn tenderly in its hold at 9:48 p.m. PDT on July 15. They will spend the next year together.

From Vesta's point of view, its visitor approached from the south. Shortly after the previous log, Dawn passed over the south pole and then arced north as its orbit carried it over the unilluminated side of Vesta. It continued its graceful, spiraling descent until July 22 when it was about 5200 kilometers (3200 miles) above the rocky surface and coming within view of the day side in the northern hemisphere. As it curved south and flew over Vesta's illuminated face, it stopped thrusting to conduct its most extensive set of observations yet.

At that altitude, it would take a full week for the spacecraft to complete one orbit, so the pace was not rushed. During four sessions over the next three days, Dawn viewed the world beneath it, turning after each one to beam its findings back to Earth.

The first occurred in the northern hemisphere, which was of particular interest because the preceding pictures were centered deep in the southern hemisphere. In the original plan for this time late in approach, Dawn would observe Vesta three times during its passage over the bright side. The second of the sessions would be devoted to collecting images throughout a complete Vestan day of 5 hours, 20 minutes, just as the probe had done twice before during the approach phase from greater distances. Because watching Vesta for an entire rotation was so important, and recognizing the many challenges in acquiring such data, mission planners decided to schedule a backup observation, bringing the total to four during this coast period on the way to survey orbit. The first rotation observation was performed with Dawn near Vesta's equator. The backup occurred 15 hours later, by which time the spacecraft had moved to latitudes about 30 degrees farther south. That was followed more than 30 hours later on July 25 by additional photography over the far southern regions.

All planned observations were completed successfully, providing magnificent views of the complex surface and showing once again that further exploration promises many more exciting rewards. The rotation observations included not only the standard black and white images but also the use of all of the science camera's color filters, providing tantalizing hints of how varied the surface is. In addition, the visible and infrared mapping spectrometer (VIR) collected a rich set of spectra throughout both Vestan days.

Spectacular as the pictures and spectra are, they are not the objective of the mission; still better views are desired and will be obtained soon. With that in mind, thrusting resumed on July 28.

Since June 30, Dawn has been propelling itself with ion thruster #2. On June 27, while operating with thruster #3, an ion propulsion system electronics unit that controls the valves used to regulate the flow of xenon to the thruster stopped working. The operations team responded with swift professionalism to keep the mission on course and schedule. In addition to reconfiguring the spacecraft from its safe mode, swapping to another control unit and thruster #2, and devising a new flight plan, the team conducted a thorough analysis of the circuit that misbehaved. The best explanation for its inability to send electrical signals to the valves, as well as for all the other detailed telemetry the spacecraft provided to engineers, was that a component in the circuit had been struck by a cosmic ray, a high energy particle of space radiation. Experts also concluded that the circuit would operate correctly again once the unit was powered off and powered back on.

After careful consideration of whether there might be any risk to the electronics or to other systems onboard in reactivating the controller, the team devised a plan to turn it on and test it. On July 20, when Dawn was conducting a routine communications session over the night side of Vesta, they transmitted the instructions to the spacecraft and were rewarded with a fully functional control unit that operated as well as ever. Although the mission certainly could have been completed without restoring this device, having it available again provides greater robustness and makes flying the spacecraft easier. When ion thrusting resumes on August 31 to begin spiraling from survey orbit to the next science orbit, this controller will be used again with thruster #3.

At 12:07 a.m. PDT on August 2, Dawn reached its targeted orbit. Although entering orbit around Vesta was an extremely important milestone for the mission, reaching survey orbit is of even greater significance. It is here that Dawn will focus on doing what it was conceived for: exploring an ancient protoplanet to provide new insight into the dawn of the solar system and its present nature.

To get here from Earth, the spacecraft traveled more than 2.8 billion kilometers (1.7 billion miles) in nearly four years. It accumulated two years, eight months of ion thrusting, during which it expended 252 kilograms (556 pounds) of its xenon propellant. That enabled the spacecraft to achieve the equivalent of 6.8 kilometers per second (15,200 mph) after leaving its Delta rocket behind. This is almost two thirds more than any other spacecraft has achieved under its own power.

Orbiting Vesta, which Dawn has now accomplished, would have been unaffordable within NASA's Discovery Program with conventional propulsion, and a mission to both Vesta and Ceres would be impossible. The exquisitely gentle touch of the ion thruster that allowed the patient spacecraft to reshape its orbit around the sun, and then around Vesta, was as silent as space itself, but if we imagined that it made a sound, it would be the faintest of whispers, the softest of sighs. Yet it tells us the secret of making an interplanetary spaceship that can travel to and explore distant, alien worlds, carrying with it the dreams of those on Earth who long to know the cosmos.

Even after the spacecraft arrived in the planned orbit, mission controllers continued preparing for the survey orbit phase. Navigators tracked the probe as it looped slowly around Vesta, refining their measurements of the orbit for use in timing the observation sequences and telling the spacecraft exactly where to aim its sensors. With the latest approach results in hand, engineers and scientists updated parameters for the camera and VIR, essentially tuning the instruments to ensure they will perform at their best. For example, a few areas of Vesta were found to be somewhat brighter than anticipated, so the effective shutter speed of the camera needed to be reduced slightly.

While controllers were putting the finishing touches on the survey sequences, Dawn acquired its final set of approach phase observations with the camera and VIR on August 5 - 6, returning the best images and spectra yet. On August 8 the team set the spacecraft's main clock to the correct time. Reflecting the more casual nature of operations during interplanetary cruise and approach, they had allowed the clock to drift off by 0.37 seconds, but such insouciance is no longer appropriate.

The detailed surveying of Vesta was designed to begin when Dawn passed over the boundary between night and day in the northern hemisphere, known as the terminator. Following the completion of all the exacting work by the mission control team, the sequences were transmitted to the spacecraft, where they waited until the probe was correctly positioned at 9:13 a.m. PDT on August 11 to delve into its work.

Each survey orbit lasts about 69 hours, or nearly three days. Dawn will spend most of the time over the lit half aiming its instruments at the surface to acquire data. While over the dark half, it will point its main antenna to Earth to radio its findings back to the Deep Space Network. When the plan was described last year, the survey orbit phase lasted for six revolutions, which included acquiring much more data than needed. As with the rotation observations, mission planners had scheduled backups so that even if some are not obtained, it is likely there will be enough to avoid complex and rapid replanning in the event of glitches. The principal change since the previous description is that they have added one orbit, ensuring an even more resilient plan for this vitally important phase of the mission.

Dawn is beginning to fulfill its destiny in service of humankind's collective passion for knowledge. With the amazing images and other data returned so far, the Dawn project is gratified to say, "Earth, meet Vesta!" Now we are about to really get to know our new acquaintance.

Dawn is 2700 kilometers (1700 miles) from Vesta. It is also 1.24 AU (186 million kilometers or 115 million miles) from Earth, or 480 times as far as the moon and 1.22 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.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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 25 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.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

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

There are several ways you can have the same spectacular views as Dawn while it closes in on this immense protoplanet. You could build your own ion-propelled interplanetary spaceship and travel to Vesta. Just imagine how exciting that would be! But if you did that, the time to write all the logs describing your adventure probably would make the undertaking impractical. Another option would be to build your own telescope. To see Vesta as clearly as Dawn sees it today, you would need a telescope only three times the size of the Hubble Space Telescope. Regrettably, then you would have to wait until Earth’s and Vesta’s orbits around the sun brought them into closer alignment. In the meantime, Dawn’s camera would draw much closer to its subject. By the beginning of August, it will see Vesta with more than 100 times the clarity that Hubble could ever obtain. Still another alternative would be to go here to see some of the best views that humankind’s robotic ambassador to the asteroid belt has returned. New pictures will be added regularly.

A relict that bears witness to the dawn of the solar system, a colossus that outweighs all other residents of the main asteroid belt save Ceres (Dawn’s second destination), a fuzzy point of light among the stars that has intrigued and enticed astronomers for more than two centuries, a mysterious orb that has stirred the passions of creatures on a distant planet, inspiring them to dispatch an emissary to scrutinize it, is finally being revealed as a unique and fascinating world. Dawn’s images delight and tantalize us, and the wait for still better views now is brief indeed.

After more than 950 days of ion thrusting, with an effective change in velocity of more than 6.6 kilometers per second (15,600 mph), Dawn’s orbit is so much like Vesta’s that their paths around the sun are quite similar. Less than a month ago, they were coming together at 240 meters per second (540 mph). Today, the relative speed has declined to only 110 meters per second (250 mph). By July 16, Dawn will be traveling no faster toward Vesta than you can drive in a car. The probe will be close enough and slow enough that the protoplanet’s gravity will tenderly take the approaching explorer in its grasp. The next log will be posted shortly after Dawn is in a high, loose orbit around Vesta, still spiraling down toward survey orbit, where it will begin making detailed studies of its home for the subsequent year.

Dawn is 152 thousand kilometers (95 thousand miles) from Vesta, or 40 percent of the average distance between Earth and the moon. It is also 1.39 AU (208 million kilometers or 129 million miles) from Earth, or 515 times as far as the moon and 1.37 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 23 minutes to make the round trip.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

The bending of Dawn's path will depend on exactly what Vesta's mass is, so navigators will continue to refine the flight profile as they measure the strength of the pull it exerts. As a result, the exact date of arrival in survey orbit will not be known until the mass is determined more accurately. Indeed, although the altitudes of survey orbit, the high altitude mapping orbit, and the low altitude mapping orbit have been presented in previous logs, mission planners may target somewhat different altitudes depending on what they discover Vesta's gravity to be.

There is another reason that the beginning of survey orbit cannot be specified precisely. Ion propulsion tends to afford much greater flexibility to missions than conventional propulsion does. One example of that was evident in the original schedule for the mission. When the first log (whose current price on the black market is reportedly well in excess of 2.4 percent of its original price) was written, the launch was planned for June 2007. Because of schedule changes, including those in the preparation of the rocket and the inflexibility of another deep-space mission (which did not use ion propulsion), Dawn's launch moved to September of that year. This unique mission to Vesta and Ceres, ambitious as it is, would have been possible with a launch on any day from 2005 through late 2007 because of the ion propulsion. A typical interplanetary mission has a period of a few weeks in which it must depart Earth.

In many cases, controllers use this flexibility to allow the dates of key events to move based on details of the progress of the thrusting and other subtleties of spaceflight. That choice permits Dawn to squeeze still more out of the mission by, for example, spending a little extra time at Vesta. So instead of pinning down the date of survey orbit (assuming a particular value of Vesta's mass), they let it change as the mission proceeds. This strategy also makes it easier to update the flight profile as the spacecraft closes in on its destination.

Navigators steer the ship toward a survey orbit with certain geometrical characteristics, such as its altitude (depending on Vesta's mass) and the angle of the orbit relative to the sun. They do not choose a specific entry point in the orbit when performing their trajectory calculations; rather, they let the trajectory calculations determine the arrival point. With each refinement, that location shifts slightly. The survey orbit observations are designed to begin after the craft has attained the correct orbit and then passes over the north pole, traveling from the night side to the day side of Vesta. Regardless of where the spacecraft arrives along the circle of the intended orbit, controllers will program it to begin surveying at that moment. Because one revolution will take Dawn almost three days, the freedom given to the mathematics of the trajectory design computer programs to determine where the probe will enter orbit can introduce a shift of that much in the timing of survey orbit.

To picture this, let's take a look at one of the new clocks now available in the Dawn gift shop on a planet near you. Imagine Vesta in the middle of the clock face and survey orbit as the perimeter, where the numbers are. Dawn spirals in from a greater distance and eventually reaches that orbit, circling around as if on the tip of a clock hand. But if the sun is far to the right of the 3, then the observations of the protoplanet at the center begin when the spacecraft loops past the 12. Now if it enters the orbit at the 11, it only has a short distance to go before it is ready to initiate the observations. If it enters at the 1, we will have to wait for it to travel all the way around to the 12. So until we finalize where it enters survey orbit, we cannot specify when it will undertake its activities there. (This complex problem is a result of flying in from deep space to Vesta orbit. When traveling from one orbit to another, such as from survey to the high altitude mapping orbit, engineers will not let the mathematics establish where in the orbit the spacecraft will arrive; rather, they will tell the trajectory programs where they want it to arrive.)

Based on the current approach trajectory, survey orbit will begin some time from August 8 to 11. The date and time will be established firmly in July. The sequences of commands to operate in Vesta orbit were designed last year with that in mind (just as they were for the approach phase), so they can easily be adjusted once the exact initial time is known.

To help target the probe for survey orbit, controllers have commanded it to observe Vesta once a week since the beginning of the approach phase on May 3. As we saw that day, the pictures allow navigators to gain a better fix on Dawn's trajectory relative to Vesta. So far, the images reveal little more than the desired important information of where Vesta appears against the background of stars. And yet, in a sense they show much more. After its long and lonely voyage through the vast emptiness of interplanetary space, most of the time far from anything but bits of dust and the occasional insignificant rock, an alien world is finally coming into view. Although too far now to do more than illuminate a handful of pixels in the camera, the small disc of Vesta stands out as the brightest and largest object visible to the explorer except the master of the solar system, the sun. The pictures are visible proof of Dawn's progress from an intriguing concept not so many years ago to a distant spaceship about to orbit an uncharted protoplanet, the second most massive body between Mars and Jupiter.

Dawn has traveled 2.7 billion kilometers (1.7 billion miles) since leaving Earth. Today it is only 580 thousand kilometers (360 thousand miles) from Vesta, just 1.5 times the distance between Earth and the moon. (Note: If you are in doubt about these numbers, you may confirm them by seeing the standard closing paragraph below.) Yet as the spacecraft continues to thrust during the approach phase, making the final adjustments to its solar orbit, it will travel more than 88 million kilometers (55 million miles) around the sun before Vesta captures it in a month and a half.

Dawn's orbit is already so similar to Vesta's that today it is closing in at only 240 meters per second (540 mph), not even as fast as many commercial aircraft fly. That is unusually slow for the speeds typical of interplanetary travel. Meanwhile the two of them rush around the sun at nearly 21 kilometers per second (more than 46,000 mph). This is similar to what you would experience if you tried to match velocities with a car on the freeway (you probably would want to use a car yourself for such a demonstration). The two cars may be traveling at high speed, but their relative speed could be quite modest.

Early in the evening of June 6, if you gaze at Earth's moon before it sinks below the horizon, you might consider that the moon is about as far from you as Dawn is from Vesta. On its way out into the solar system after lifting off from Cape Canaveral, Dawn passed the orbit of the moon in less than 29 hours. The tremendous push imparted by the Delta rocket to start the probe on its mission is quite unlike the gentle approach to Vesta. As you enjoy the sight of the moon, Dawn will have two months of flight ahead of it to cover that same distance to survey orbit, where it will begin reaping the rewards of its long journey.

Dawn is 580 thousand kilometers (360 thousand miles) from Vesta, or 1.5 times the average distance between Earth and the moon. (See?) It is also 1.64 AU (246 million kilometers or 153 million miles) from Earth, or 610 times as far as the moon and 1.62 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 28 minutes to make the round trip.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.

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

The visible and infrared mapping spectrometer (VIR) will join the camera in spying Vesta on May 10 and again later in the approach phase. At the end of June, Dawn will watch Vesta for a full Vestian day of 5 hours, 20 minutes. When the camera searches for moons on July 9 and 10, it will also enjoy another full pirouette. By the third and final time the spacecraft observes Vesta throughout a complete rotation on its axis, during a set of observations from July 23 to 25, Dawn will be in orbit.

On July 16, when the ship is at an altitude of around 15,500 kilometers (9,600 miles) and propelling itself with its ion propulsion system in the same way it has been for more than 900 days of interplanetary travel, Vesta will gently take hold. For the first time since September 27, 2007, when Dawn rode atop the second and third stages of the Delta rocket for a short time in Earth orbit, it will be bound to a planetary body.

The precise time and distance at which Vesta gains control of its visitor depend not only on subtleties of the thrusting until then but also on the strength of the giant asteroid's gravity. Among the many characteristics of Vesta yet to be known well is its mass. Astronomers have estimated it by detecting the tiny changes Vesta induces in the orbits of other asteroids and even of Mars, but those measurements yield only approximate values. One of Dawn's objectives is to determine Vesta's mass and to map its gravitational field.

The approach phase concludes when Dawn is ready to commence its survey orbit in the second week of August. We will consider the timing of the beginning of this next phase in a subsequent log.

While the start of the approach phase is defined by the beginning of the navigation imaging, other changes are being made today as well, both in procedures used by the operations team and in the configuration of the spacecraft. Let's consider just one subsystem: attitude control. (To achieve a certain mystique about their work, engineers use the term "attitude" to describe the orientation of the probe in the weightless conditions of spaceflight; the system also happens to have a very enthusiastic attitude about its work.) Since August Dawn has controlled its attitude with its reaction control system, the small thrusters that operate with hydrazine propellant. (When the craft is using the ion propulsion system, which is most of the time, the ion thruster helps control the attitude.)

At the beginning of the approach phase, the ship returns to using reaction wheels, gyroscope-like devices which, when electrically spun faster or slower, rotate (or stop the rotation of) the spacecraft. During Vesta operations, Dawn will turn much more frequently, as it points its sensors at the alien world it is exploring, aims its main antenna to Earth frequently to transmit its precious findings, and follows a complex flight profile to travel from one science orbit to another. The reaction wheels will be used until Dawn has departed from Vesta in July 2012, providing more accurate control of the attitude while conserving hydrazine.

To enable the explorer to point its camera and VIR even more delicately, the ship's gyroscopes are powered on. Not to be confused with the reaction wheels, these devices help determine exactly what the attitude is so that the system can command the wheels to achieve the desired attitude. The gyroscopes are not needed for most of Dawn's activities during the interplanetary cruise phase of its mission, so they have been off for most of the mission so far.

The gyroscopes serve another purpose at Vesta, which we discussed in more detail in January. The probe usually relies on star trackers for sensing its attitude. Each tracker takes pictures of the stars. Its internal computer processes the images, finding familiar patterns of stars to determine where it is pointed, just as you might use some of the constellations visible from your planet to orient yourself at night. When some component (such as the main antenna or an ion thruster) needs to be oriented in such a way that the star trackers happen to point at Vesta, the gyroscopes will take over so the spacecraft doesn't lose track of its attitude. There will be much to discover about the enigmatic 530-kilometer-diameter (330-mile) rocky world, but its ability to block starlight is not in doubt.

While the science camera and VIR will be turned on and off as needed during the Vesta phase, the gamma-ray and neutron detector (GRaND) is being activated today and will remain on until the departure next year. Most of that time, the majority of the signals it detects will be from space radiation known as cosmic rays. But the closer it gets to Vesta, the more gamma rays and neutrons it will receive from the surface, gradually allowing scientists to formulate a census of the atomic constituents. GRaND's greatest ability to sense the faint radiation will be in the low altitude mapping orbit.

The instruments were tested during a planned coast period in March, and each was in excellent condition. Dawn had another scheduled hiatus in thrusting from April 11 to 19, but this one was not intended for calibrations or tests. Rather, controllers had planned this for an upgrade to the software in the craft's main computer.

When version 9.0 of the software was installed last year, it was intended to be used at Vesta. By coincidence, the day after they rebooted the computer to start running with 9.0, the operations team began thinking about adding a new capability to the software. The motivation was the development of excessive friction in reaction wheel no. 4. While Dawn performs perfectly well with the other three wheels, the unavailability of one wheel meant that there was no longer a spare. Since then, three tests of wheel no. 4 have shown that it cannot be restored as a backup prior to Vesta and probably not for the rest of the mission. Therefore, to regain the robot's resilience to the loss of almost any component, work began immediately at Orbital Sciences Corporation and JPL on new software that would allow safe and stable attitude control with only two wheels. (Of course, the spacecraft can function with all wheels powered off, relying on the reaction control system, but ever-cautious engineers wanted the two-wheel option to reduce the hydrazine expenditure for complex Vesta and Ceres operations.)

The installation of software on our probe flying in deep space is a delicate task. To begin running with the new version, the computer has to be rebooted. That same computer constantly performs such essential functions as maintaining a steady attitude and acceptable temperatures. Controllers followed the same intricate procedures they used successfully to load new software in November 2007, April 2009, and June 2010. Preparing the spacecraft, radioing the new software to it, rebooting the computer, and commanding the craft back to its normal flight configuration all went exactly according to plan. Although more than a week was allocated, it only took three days.

Dawn is now running what the team officially designates OBC flight software version 10.0, but what the more zany team members refer to as 10.0 or "ten oh." It may be surprising that even with the complex and rigorous work to overcome myriad challenges of operating the first explorer from Earth to take up residence in the main asteroid belt, normally dispassionate engineers can display such frivolity.

Now with new software, the spacecraft is beginning the approach phase. Its journey has been long, but the reward is almost in view. Since leaving Earth in September 2007, Dawn has made about one and three quarters circuits around the sun as it spirals outward. Earth itself (along with your correspondent and some readers) has completed more than three and a half orbits in that time. But on May 14, Vesta will finish its first revolution around the sun since Dawn has been in flight; the mission will then have been under way for exactly one Vestian year.

We have seen before that objects travel more slowly in more distant orbits, where the force of gravity holding them is weaker. Dawn has been climbing the solar system hill, traveling farther and farther from the sun at the bottom. It began its journey on Earth, partway up the hill. Now far above Mars, the probe is closing in on Vesta. As the adventurer and the mysterious world each race around the sun at nearly 21 kilometers per second (47,000 mph), Dawn is gradually closing in for its rendezvous. Two months ago, the spacecraft's course was bringing it toward Vesta at 0.7 kilometers per second (1,600 mph). Today, having completed more thrusting to bring its orbit into a closer and closer match with Vesta's, the craft is approaching at about 0.37 kilometers per second (830 mph). The speed will continue to diminish as Dawn gradually reshapes its flight path to be exactly the same as Vesta's. Soon, they will travel together around the sun.

Meanwhile, the distance between them continues to shrink. Since the middle of March, Vesta has outshone everything in Dawn's sky save the sun. By the middle of April, a sharp-eyed passenger would notice that Vesta is more than a pinpoint of light like the myriad stars and distant planets; it would appear as a tiny disk, hinting of the exciting adventure ahead. (The passenger also might notice that his luggage was left back on Earth, more than 320 million kilometers or 200 million miles away.) Now, with Dawn's interplanetary cruise ending and the approach beginning, Vesta is coming into its sights, as the ship prepares to sail into port after an extraordinarily long journey across the lonely emptiness of the vast interplanetary seas.

Dawn is 1.2 million kilometers (760,000 miles) from Vesta, or 3.2 times the average distance between Earth and the moon. It is also 1.90 AU (284 million kilometers or 177 million miles) from Earth, or 715 times as far as the moon and 1.89 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 32 minutes to make the round trip.

TAGS: DAWN

• 

  ABOUT THE AUTHOR

  Marc Rayman, Dawn Mission Director and Chief Engineer

  Marc Rayman is the director and chief engineer for NASA's Dawn mission, which was launched in 2007 on a mission to orbit the two most massive bodies in the main asteroid belt between Mars and Jupiter to characterize the conditions and processes that shaped our solar system.