On April 27, as it had done so many times earlier in the mission, the spacecraft executed a set of commands to rotate to point a thruster in a selected direction, perform the pre-start procedures, and undertake powered flight. With tests during the initial checkout phase plus all the thrusting in the interplanetary cruise phase designed to reshape the probe’s orbit around the Sun, Dawn had accumulated 123 days of operation with ion thruster #1 and 158 days with ion thruster #3. (The locations of the thrusters are explained in a previous log.) Thruster #2 has not had its turn yet for long-term thrusting, so it had been operated only for 22 hours. While much longer than the operating time for a propulsion system on a typical planetary spacecraft, for Dawn this is still considered brand new. Therefore, the 8.0 system thrust test used ion thruster #2, allowing it to accelerate xenon for the first time since November 16, 2007.

After thrusting for 4 hours, the spacecraft stopped and reoriented itself, aiming the main antenna to Earth again. On May 1, it conducted a similar exercise, differing only in the direction of thrust. These tests provided the final confirmation that 8.0 was ready to take the helm when the time came to resume propelling the craft to the asteroid belt.

These activities had a secondary benefit. During the initial checkout phase, some tests were conducted of how accurately thrusters #1 and #3 could be pointed. The equivalent test with #2 could not be run then because the spacecraft was too close to the hot Sun to point that thruster in the required direction, so it was planned for April 2008. That operation was interrupted by an event that led to safe mode being invoked. Because thruster #2 was not scheduled to be used until much later in the mission, the test was postponed to the coast period after Mars. The 8.0 system thrust verification test was executed in the directions needed to complete the thruster #2 pointing tests, thus accomplishing these additional objectives.

Thrusting at this time in the mission does not help reach the targets (if it did, Dawn would not spend so much time coasting), but the short thrust trials did not modify the trajectory appreciably. The ion propulsion system has changed the spacecraft’s speed by about 1.87 kilometers per second (4180 miles per hour) so far in the mission, and this latest work added only 2.3 meters per second (5.1 miles per hour) to the total.

Following these successful tests as well as several weeks of other operations, engineers were sufficiently pleased with 8.0’s admirable performance in both its software and its outreach functions that they were ready to commit to using it. In April, the software had been installed only in the primary location in the primary computer. On May 11, mission controllers powered on the backup computer and installed the primary and backup copies of 8.0. On May 13, they loaded the software to the backup area of the primary computer, so now 4 copies of 8.0 are stored onboard. Updating the software, even in a backup location, is a delicate operation, but all the procedures went according to plan. That brought the work on 8.0 to a successful and satisfying conclusion.

For the ion propulsion system to operate with its extraordinary efficiency, a gentle flow of xenon gas to the thruster needs to be accurately controlled. At full throttle, this amounts to slightly more than 3 milligrams per second (less than 10 ounces per day), and by the end of the mission, when limited electrical power necessitates a low throttle level, the flow will be reduced to less than 40% of that rate. A sophisticated labyrinth of tubes, electronically controlled valves, and other components feeds the propellant from the main tank to the selected thruster. Along the way, xenon is held temporarily in a pair of small, low-pressure tanks (each known as a “plenum” and the pair as “plena”). The main tank is pressurized to more than 1000 pounds per square inch (psi, and note we will spare readers the metric units for pressure), but one of the plena is charged to less than 67 psi and the other is never raised above 37 psi. Pressures will be still lower at lower throttle levels.

The ion propulsion system’s computer controller relies on pressure and temperature sensors on each plenum as it regulates the flow of xenon. The controller interprets any change in the pressure as a change in the amount of gas moving through the system, so a shift in the behavior of a sensor would cause the controller to adjust the rate at which xenon is delivered to the operating thruster.

The pressure sensors are designed to remain stable throughout the mission, but in order to keep the ship in good shape, ever-skeptical engineers want to determine how much (if at all) the devices have drifted in their measurements since they were calibrated well before launch. So here they are confronted with a conundrum: how can they verify the sensors on the distant spacecraft are giving accurate readings?

We solicit here ideas from readers for how to accomplish this “plenum pressure transducer calibration.” Please submit your ideas to the Dawn project before the next paragraph is written, and the best idea will be implemented. What a thrilling reward that will be for some lucky reader! Be sure to include thorough calculations, assessment of risks to the spacecraft and mitigations for those risks, a complete list of commands, predictions for subsystem telemetry, and any other pertinent details.

Thank you very much to all those who proffered suggestions; we received a surprising number of innovative methods, a few of which are even compatible with physical laws and the capabilities of the Dawn spacecraft. We now have an effective solution. (The winning entry came from a reader on Earth who does not want the prestige of being named here, but we offer our most sincere congratulations!)

By bringing the plena to a pressure that is ascertained without the use of the pressure sensors, engineers can compare the sensor readings with the independently determined pressures. So they commanded certain valves to open, thus allowing the xenon in these tanks to vent into space. Each plenum will evacuate to nearly 0 psi, so the report from the pressure sensors will provide a direct measurement of how much their behavior has changed, and any discrepancy can be accounted for by straightforward updates to the software.

The procedure of connecting the plena to the cosmic vacuum is known as a blowdown, perhaps connoting a gale-force wind of xenon as it begins rushing out of the system. The reality is somewhat different. When the valves were opened on May 4, the force from the zephyr of xenon leaving the spacecraft was less than 3% of the famously light thrust achieved when the xenon is ionized and accelerated at full throttle. The unionized xenon pushed on the spacecraft with about one tenth of the force you would feel holding a penny. As the plena pressures fell, the force diminished still further.

At the conclusion of the 8.0 system thrust test, one plenum held 90 grams (about 3 ounces) of gas and the other held 50 grams (less than 2 ounces). (For comparison, the main tank carried 425 kilograms, or 937 pounds, at launch and now holds 350 kilograms, or 772 pounds.) The expenditure of this small amount (about what is used in half a day of thrusting at maximum throttle level) is well within the mission’s overall xenon budget. The valves will be left open for several weeks to allow plenty of time for the xenon to find its way from the plena through the maze of hardware to space. Well before thrusting begins on June 8, the valves will be closed and the plena repressurized.

In the meantime, Dawn will pass another milestone in its solar system adventure. As we have seen in previous logs, the nature of orbits is that at greater distances, objects travel more slowly. Dawn and Earth both orbit the Sun, but the spacecraft, being more remote than the planet, travels more slowly. On June 3, Dawn will complete its first revolution around the Sun (a “Dawn year”) since its September 27, 2007 launch. Earth, along with its residents (constituting a small but enthusiastic fraction of our readership) as well as the moon and all other natural and human-made satellites in orbit around it, needs about 365 days for a revolution, so it will have made 1.68 loops while Dawn was in its first.

You may be tempted to think that Dawn takes longer to round the Sun than Earth because it has to travel a greater distance to do so in its larger orbit, but you are encouraged not to yield to this simple explanation. Earth has moved more than 1.58 billion kilometers (984 million miles) since letting go of the probe. In the same time, Dawn has traveled only 1.32 billion kilometers (817 million miles). When farther from the Sun, objects travel more slowly because the gravitational pull they need to resist is weaker. As it climbs atop its ion beam to still greater heights, Dawn will go even more slowly as it seeks to match orbits with its more remote destinations. Since the spacecraft departed from Earth, Vesta has completed only 0.44 revolutions, traveling 1.00 billion kilometers (622 million miles), and even more distant Ceres has made only 0.40 loops, covering 994 billion kilometers (617 million miles).

To keep our subscription fees low, we include here another subtle product placement for one of the many clocks in the Dawn gift shop, several of which have been featured in logs over the past 5 months. On this model, the Sun is at the center and there is a separate hand for each of these orbiting objects. Although they were not aligned at launch, suppose they were, with each hand pointed toward the 12. (Because of the northern hemisphere perspective with which astronomy was developed on Earth, our standard view of the solar system depicts the orbits going counterclockwise. Here we will ignore that familiar perspective, as our focus is on how many rounds each body completes, not the direction of their travel.) Since launch, Earth would have gone all the way around once and continued on clockwise to just past the 8. The spacecraft will return exactly to the 12 on June 3. Vesta will have leisurely advanced just beyond the 5, and Ceres will not quite have made it even to that number yet.

As Dawn continues to push outward from the Sun, with its sights set on alien worlds deep in the asteroid belt, we hope readers will continue to follow its progress. With (some of) the profits from the first Dawn Ponzi fund, we have added RSS feeds for these logs and other features at the Dawn website. To learn more, and to be among the first of your species to subscribe to automatic updates, visit http://dawn.jpl.nasa.gov/RSS/index.asp.

Dawn is 305 million kilometers (189 million miles) from Earth, or 845 times as far as the moon and 2.01 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 34 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 am PDT May 25, 2009

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

• 

  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.

Software 8.0 contains 34 changes. These range from improvements to make more efficient use of the extremely low rate of transmission of data from the spacecraft when it uses one of its small auxiliary antennas (as it does in safe mode and in some other circumstances), to methods to expedite recoveries from certain causes of safe mode, to increasing the robustness of the handling of some conditions that, while highly unlikely ever to arise, could be very serious. A few bugs, inevitable in software of this complexity, also were fixed. Indeed, readers who are about 7 weeks behind in these logs are just now learning that the bonus instrument calibrations during the crucial and successful gravity assist at Mars were interrupted by the combination of two bugs, neither of which by itself would have been sufficient to affect the activity. One of them, which prevented the backup star tracker from being called into use when needed quickly, was corrected in this version of the software.

The process of loading software, particularly into the main computer, is complex and delicate. The spacecraft constantly relies upon that computer for keeping the solar arrays pointed at the Sun to generate electrical power for all systems, maintaining components at the correct temperatures despite the extreme conditions of spaceflight, aiming the antenna at Earth, and performing a great many other functions vital to Dawn’s operation. Should the loading of the new software go awry or introduce new bugs along with the intended changes, the consequences could be less than felicitous. A substantial effort was devoted to careful and thorough testing of the software in computer simulators on Earth, and the techniques and procedures for installing it on the most important computer, the one in deep space, were tested and rehearsed extensively as well.

As explained in December 2007, Dawn has a primary main computer, with primary and backup copies of the software, and an identical backup computer, also with primary and backup copies. On April 10, mission controllers verified that the backup copy in the primary computer and both copies residing in the backup computer were pristine, uncorrupted by radiation or any other anomalies since they had been loaded. If a problem developed while loading 8.0, the team was confident the spacecraft could resort to familiar and healthy software if need be.

On April 13, the new software was transmitted to the spacecraft. Dawn receives commands from large antennas of the Deep Space Network at 2000 bits per second. While this would seem agonizingly slow for our terrestrial readers located in the 21st century, that is the maximum rate for NASA’s interplanetary probes. Most sets of instructions bound for the spacecraft are short enough that they consume no more than a few minutes of transmission time (regardless of how long it takes the signals to travel to the spacecraft). The file containing the software was so large that the time required to transmit it was far longer than the nearly 18 minutes needed for radio signals (which, as regular readers know, travel at the universal limit of the speed of light) to traverse the vast distance to the craft. So, during the loading of the software, Earth and Dawn were connected by an uninterrupted bond, a radio link 320 million kilometers (199 million miles) long. Even as Dawn was accepting the signals, still more were being sent. An unbroken line of 0’s and 1’s, each bit 150 kilometers (93 miles) long, stretched through space, connecting the probe to its erstwhile home.

In addition to 8.0 onboard (and verifying that it did indeed arrive intact, with no dropouts, no errors, no miscommunications whatsoever), engineers conducted other checks and reconfigured systems to prepare for running the new software. Additional preparations and final verifications were conducted on April 14.

With all stations in mission control early that afternoon reporting their subsystems were ready, the final “go” was given, and the command to reboot the computer for the first time since November 2007 was transmitted. In addition to causing the computer to run the new software, this triggered the spacecraft to enter safe mode, as expected. (Somewhat less directly, it also triggered pizza to be delivered to mission control a few hours later.) Because safe mode does not use the main antenna but rather one of the smaller, auxiliary antennas, the operations team scheduled this delicate operation to occur under the watchful eye of the 70-meter-diameter (230-foot-diameter) radio antenna of the Deep Space Network complex near Canberra, Australia, rather than one of the 34-meter (112-foot) antennas. With the larger antenna, engineers could more quickly verify the new software was operating correctly and begin the long process of returning the spacecraft to its normal configuration.

The operations team took advantage of these activities to make another planned change. Safe mode employs all 4 of the reaction wheels, components which can be electrically spun faster or slower to help stabilize the spacecraft or rotate it. In normal operations, only 3 are used; wheel #2 had its turn to be dormant starting in October 2008. In commanding Dawn out of safe mode, controllers kept wheel #2 on and powered off #1.

By April 16, the spacecraft was fully reconfigured, and engineers continued to be happy with the functioning of the new software. While a great deal of work had gone into preparing for a wide variety of problems that could have occurred because of loading the software or that could have been more troublesome had they cropped up by coincidence during this sensitive operation, it all proceeded quite smoothly indeed.

We will report in the next log on installing the backup copies of the software and on further verifications that 8.0 is up to the job of continuing the mission that had been progressing so well with 7.0.3.

If you wish to download 8.0 for use at home (or on your spacecraft), you may obtain a copy directly from the main computer on Dawn. The computer is inside the box labeled “CEU A,” conveniently located under the -y panel of the spacecraft. Don’t forget to bring your own cable, and please be careful not to damage the delicate solar cells on the nearby wing.

Engineers have already begun work on the next version of the software, this one having the sleek and chic sobriquet “nine oh.” It will be sent to the spacecraft in 2010; and at that time, we surely will have occasion to link back to this log, so readers are advised to retain a copy for reference. (Some may wish to preserve this log purely as an investment. Dawn financial analysts predict that when it is cited in next year’s description of 9.0, its value might rocket to as much as 2% more than you paid for it.)

As it resumed normal operations with its new software, Dawn reversed one component of its course through space. Since August 8, 2008, it had been falling toward the Sun. Thanks to the principles of orbits, it was not in danger of reaching that star or even getting close enough to be singed. The ship is in an elliptical orbit, whose innermost point was reached on April 17 (at about 5:07 pm PDT, for those of you who wondered what seemed to change around that time). The probe was 205 million kilometers (127 million miles) from the Sun. We may recall a measurement unit more convenient than kilometers or miles, the astronomical unit (AU), in which 1 AU is the average distance between Earth and the Sun. With that handy ruler, Dawn was 1.37 AU from the star. After passing through that low point in its current orbit, momentum began carrying it farther from the Sun again.

Thanks to the effects of the ion thrusting in the next few years, it will continue to follow an outbound path until late 2012, when once more it will temporarily approach the Sun. It will never again be as close to the solar system’s center as it was this month, for the craft will have to climb to more than 2.2 AU to reach Vesta and to more than 2.9 AU as it orbits Ceres.

In preparation for its work there, on March 31, the spacecraft conducted another test of its primary science camera, which has performed flawlessly during the mission. It will be used not only to acquire exciting views of the alien surfaces of the unexplored worlds it is bound for, but also to help refine the location of those faraway and mysterious bodies so Dawn’s navigators can guide the probe into orbit. To plan for the most productive observations, the team needs to understand the detailed performance of the instrument under the full range of conditions in which it might be used. Can the camera point as close to the Sun as will be desired, without unwanted light compromising exquisite details in the pictures? Well, it is quite a coincidence that you, loyal reader, just now began thinking of that particular question, because the purpose of this test was to answer it.

The spacecraft started by pointing the camera 90 degrees from the Sun. It took a set of pictures, then rotated to aim a little closer to the Sun and took more, continuing through a range of 60 degrees. To imagine this, let’s take advantage once again of one of the many exotic Dawn clocks available in the Dawn gift shop (be sure to visit the shop on your planet soon to see the exciting new line of fashionable Dawnderwear). In this clock, the spacecraft is at the center, and the Sun is at the top, where the 12 would be. To begin, the craft pointed the camera toward the 9. After taking its pictures, it turned by 2.5 degrees, or about the same angle the second hand would move in just over 0.4 seconds. It stopped again and obtained more images. It continued with this pattern until it was directed at the 11, which was the closest it could point safely to the hot star at its current distance.

As with most clocks in the gift shop, this one might not be the best choice for something as mundane as telling time. Allowing time for each turn, stabilizing at each position, and then acquiring all the pictures, it took Dawn 12 hours to rotate from the 9 to the 11, or 6 times slower than the hour hand moves. This long test yielded 410 images at 25 different angles from the Sun, allowing engineers to determine if stray light could find its way to the camera’s detectors by any circuitous path, reflecting off components within the complex camera. Once again, the unit performed superbly. Even with exposures as long as 100 seconds, unwanted light did not reach appreciable levels.

Now flying under the control of new software and confident its camera will operate under all necessary conditions, Dawn is headed deeper into space again. Still more tasks lie ahead before it resumes its familiar routine of sustained thrusting with the ion propulsion system. The subsequent log, which we boldly predict will follow this one, will describe the next series of special activities.

Dawn is 315 million kilometers (196 million miles) from Earth, or 855 times as far as the moon and 2.09 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 35 minutes to make the round trip.

Dr. Marc D. Rayman
4:00 am PDT April 26, 2009

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

• 

  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.

While safe mode is the desired response to a wide range of unexpected or problematic conditions, Dawn’s engineers recognized that it was not necessary when space radiation hit that particular device. They reexamined the extensive analyses that had been conducted before launch and performed new studies as well, concluding that the mission could be interrupted by more such events because of the susceptibility of the component that was hit by the radiation. It was not possible to establish with high confidence how frequently it could occur (much less to know specifically when it would occur), but the data suggested that it might happen often enough that it would hamper the mission, interfering with too many activities and ultimately consuming too much of mission controllers’ limited time.

The team, therefore, formulated a change to the software in one of the spacecraft’s auxiliary computers that would allow it to accommodate another radiation hit without the subsequent cascade of events culminating in safe mode. While changing the software is challenging and time consuming, the project chose to undertake the work to avoid a repeat of the incident, judging the certain cost of the change to be less expensive to Dawn's mission than the possible cost of subsequent incidents.

On July 22, 2008, the new software (along with all the instructions to patch it in) was radioed to the remote spacecraft. Since then, it has been available should a radiation strike energize the component as it had 6 months earlier.

The modified part of the software had its first use in flight with the radiation impact on March 21, when nature repeated itself. The electronics were unchanged, of course, so the circuit responded the same way it had 14 months earlier, informing the software of a problem. This time though, the software responded simply by storing a short message to transmit along with all the other spacecraft health and status information during the next scheduled contact with Earth (by coincidence, later that day). The unnecessarily dramatic activation of safe mode is no longer part of the outcome, thus allowing the robotic probe and its human support team to continue their work without interruption or distraction.

While Dawn happened to be quiescent when this radiation impact occurred, it might not have been, given the inherent unpredictability of the timing. It might not be when the next uninvited particle strikes the same component. Now, with the clear demonstration of the effectiveness of the software patch, it won’t matter. Controllers were gratified to receive the message on March 21 reporting the spacecraft's nonchalant response to the radiation, validating their decision to modify the software last year.

The spacecraft will be busier in the coming weeks than it has been since it was deflected by Mars last month. The next log, already available for readers in the future [Note to those readers: please send a copy to the author in his present to save him the time of writing it], will cover some of the upcoming special activities and will recognize another milestone (if not million-milestone) on this journey of scientific discovery and adventure to distant and unexplored worlds.

Dawn is 327 million kilometers (203 million miles) from Earth, or 875 times as far as the moon and 2.19 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 36 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 am PDT March 29, 2009

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

• 

  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 targeting of the encounter was well within acceptable limits. Before embarking on the mission, long before launch, engineers chose 500 kilometers (311 miles) as a convenient initial target altitude for planning purposes. With the extraordinary capability of the ion propulsion system, Dawn easily could accommodate significant deviations from the plan. To accomplish its mission, the probe needed to fly anywhere through a window shaped liked a croquet wicket, extending from more than 750 kilometers (466 miles) above the planet down to 300 kilometers (186 miles). (The lower limit was chosen for safety, maintaining a comfortable distance from the tenuous atmosphere and other threats the spacecraft was not designed to handle.) The width at the bottom of the wicket was almost 670 kilometers (415 miles).

In an effort to broaden our readership beyond only those sentient beings in the universe who share in the passion for the exploration of the solar system, the November and January logs included some material for anyone passionate about archery. We compared Dawn’s flight by Mars to shooting an arrow at a target 47 kilometers (29 miles) away. The objective was to hit a small region just outside the 30-centimeter (1-foot) red bull’s-eye. For the readers who have joined us because of that topic, we return to it once again here. (We do not plan to expand upon the croquet theme in future logs but hope our new readers attracted by references to that sport will remain with us anyway.)

Our original target altitude of 500 kilometers corresponded to about 17.2 centimeters (6 3/4 inches) from the center of the bull’s-eye. Using navigational data from January, we predicted Dawn would fly by at 543 kilometers (337 miles) above the surface, the arrow sinking into the target about 17.4 centimeters (less than 6 7/8 inches) from the center, just outside the red circle. Now we know that that prediction was in error by the equivalent of about 40 micrometers (less than 1/600 of an inch). The arrow was off from that expected location by about the thickness of a hair.

The location of the original target was at the 11:00 position, but the base of the wicket-shaped window extended from where the hour hand would be at 10:51:02 to 11:03:49. (The wicket is not as symmetrical as official croquet rules might require.) As a reminder, because we are concerned only about the hour hand, the tick marks adjacent to the 11:00 position correspond to 10:48 and 11:12, so this wicket is very narrow. In January, it appeared our arrow was headed for about 11:03:42. In fact, the actual trajectory took the spacecraft through the window at the 11:03:40 position.

The gravity assist was extremely accurate indeed. The archers hit their target and won the big prize: the continuation of the mission of exploration in the asteroid belt, seeking answers to questions about the dawn of the solar system. In addition to the gravity assist, which was essential to the success of the mission, the operations team had devised a plan to acquire some bonus data to aid in the calibration of the science instruments, as described last month.

In order to point its instruments at their calibration targets, the probe oriented itself for a short time in such a way that light reflected from Mars reached its “star tracker”. This unit (with a mystifying name, whose origin is lost in the dim mists of time) tracks stars in order to help the attitude control system establish the spacecraft’s orientation (or “attitude”) in the zero-gravity of spaceflight. The tracker’s camera images stars and its internal computer recognizes patterns, much as you might recognize some of the lovely constellations visible from your planet and use them to orient yourself at night. When Mars light entered the star tracker, the camera was dazzled, temporarily unable to see the stars. If you reside on a planet with a large moon, you may have experienced a similar phenomenon. It is much harder to see stars when the bright moon interferes.

Engineers had anticipated this behavior. Because they knew the star tracker likely would be unable to provide useful data to the attitude control system for a while, gyroscopes had been powered on well beforehand. Using these spinning masses, attitude control can sense turns and keep track of how the attitude changes even when the star tracker is not yielding accurate information.

As another preventive measure, commands stored on board temporarily precluded protective software, known as fault protection, from seeing any alerts indicating that the star tracker was not able to produce valid data. Because they expected the data to be invalid, engineers did not want fault protection to respond under the mistaken impression that the tracker was unhealthy.

As Dawn approached Mars, with instruments powered and beginning their calibrations, the light reached the star tracker, but it performed better than expected. After reaching its minimum distance, as the spacecraft rotated during its ascent, the star tracker’s line of sight moved closer still to Mars. Almost 2 minutes after the closest approach, the device finally was overwhelmed with light and reported that it could not track stars, declaring itself to be nonoperational and causing a software flag to be hoisted to alert interested parties. As planned, attitude control relied on gyros and fault protection remained blind to the alert.

About 11 minutes later, as the spacecraft’s attitude continued to change, the star tracker’s view moved far enough from Mars that the unit once again could discern stars. When it recognized patterns, it reported its data to attitude control, which readily used them. All was well, and the tracker had functioned better than anticipated, identifying stars with Mars closer to its line of sight than anticipated.

Another 7 minutes after that, as Dawn’s momentum continued to carry it away from Mars, the stored command to restore fault protection’s ability to see any star tracker problems was executed. The star tracker was indeed working well, but the compulsive reader will note that the chronology above does not include lowering the virtual flag that was raised when the tracker announced it had stopped tracking stars. A software bug, hardy enough to elude the operations team and survive the rigors of the deep-space environment, prevented the flag from coming back down when the tracker resumed normal operation. Although attitude control was making good use of the data, when fault protection saw the flag, it fulfilled its function; treating the tracker as if it were unhealthy, the protective software deactivated the unit.

Fault protection’s next step was to try to use the backup star tracker. Another bug, identified last year while the spacecraft was in flight, deprived the second tracker of the time necessary to complete its activation routine. (That bug has been fixed in a new version of the software scheduled to be transmitted to the spacecraft before ion thrusting resumes in June.) With no star tracker available, fault protection correctly followed the plan its designers had given it: it interrupted the calibrations, powered off the instruments, and put the spacecraft into “safe mode,” awaiting instructions from Earth on what to do next.

The operations team, following events on distant Earth (delayed by the more than 19 minutes it took radio signals to cross the separation), observed the signature of safe mode. There was no urgency in responding, however, because the instrument calibrations could not be restarted, and the crucial gravity assist was unaffected by the spacecraft’s activities. Mars would sling Dawn in the intended direction without regard for the probe’s actions.

The cause of the “safing” was quickly determined, and the team returned the spacecraft to its normal operational configuration within about 2 days. In addition, all the bonus calibration data that the instruments had transferred to the spacecraft’s main computer before they were deactivated were transmitted to Earth.

The gamma ray and neutron detector (GRaND) obtained excellent measurements of both gamma rays and neutrons from Mars. The instrument has been operated a number of times in flight to measure high-energy radiation that pervades space as it strikes the spacecraft, but, unlike the other science sensors onboard, it can detect bodies only when it is very close to them. Dawn's other instruments have observed distant planets and stars several times already, but that is not possible for GRaND, even with its suite of sophisticated detectors. Mars is the only occasion in the mission for it to observe a specific, well-characterized object. It was powered on in January in anticipation of this opportunity.

GRaND obtained a thorough set of data as Dawn traveled down to its lowest point, despite being over the night side of the planet part of that time, because it does not require illumination by the Sun to "see" its subject. It acquired additional valuable data as the spacecraft receded from Mars. This was GRaND's only measurement of a planetary body; and, apart from being quite brief, it was performed in much the same way it will be when Dawn orbits each of its protoplanetary destinations. It was especially good fortune that space radiation levels were relatively low during the encounter with Mars, making the radiation escaping from the atmosphere and surface particularly clear for GRaND. Scientists will compare GRaND's data with measurements of gamma rays and neutrons by the Mars Odyssey spacecraft (which has been studying the planet for more than 7 years), helping them prepare for interpreting the unique observations it will make of Vesta and Ceres to reveal many of the atomic constituents of those protoplanets.

The visible and infrared spectrometer’s data were not stored in the spacecraft’s main computer memory before the safing because the camera’s data had priority. One of the images the camera acquired is shown here.

With the benefit of an excellent gravity assist, the Dawn project is gratified to have Mars behind and Vesta now so clearly ahead. Having obtained even more than was needed from the red planet, Dawn is ever more eager to press on to its destinations in the asteroid belt.

Dawn is 4.1 million kilometers (2.5 million miles) from Mars. It is 336 million kilometers (209 million miles) from Earth, or 915 times as far as the moon and 2.26 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 37 minutes to make the round trip.

Dr. Marc D. Rayman
3:00 am PDT March 8, 2009

P.S. While the spacecraft is not concerned with changes between standard time and daylight saving time, its human colleagues are. Your correspondent, however, subscribes neither to the spacecraft’s indifference nor to the more traditional rigid adherence. He does follow the convention of advancing clocks, as we do today, but rather than setting his clocks back with everyone else late in the year, he saves that extra hour. Every 24 years, that should give him one extra day.

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

• 

  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.

To illustrate Dawn’s arc, let’s use the somewhat arbitrary speed of 4 kilometers/second (9000 miles/hour) as a reference. Over the coming days, Mars will attract Dawn, and the craft will have accelerated to that speed about 34 minutes before its closest point, still 5500 kilometers (3400 miles) away from the planet. After dipping still lower and falling still faster while it approaches Mars, the spacecraft will slow down as it departs. It will have decelerated to the same speed of 4 kilometers/second at 34 minutes after its closest encounter. Within a few days, the speed will have dropped to today’s value of 2.56 kilometers/second again. The arc of Dawn’s acceleration toward Mars matches the arc of its deceleration, displaying the same kind of symmetrical motion as a swing.

Where then is the effect of the gravity assist? Because Dawn is in orbit around the Sun, it is the distortion of that solar orbit caused by Mars that provides the advantage to the mission. In the previous log, we saw how the benefits of the gravitational interaction with Mars could be described as changes in the spacecraft speed. The 1.1 kilometers/second (2500 miles/hour) that represented the change in the shape of the orbit means that Dawn’s incoming speed and outgoing speed relative to the Sun are not the same, even though, as we saw above, they are the same relative to Mars. Before it reaches Mars, the probe will be traveling around the Sun at less than 25.5 kilometers/second (57,000 miles/hour). Thanks to the boost from Mars, the speed after the encounter will be more than 26.6 kilometers/second (59,500 miles/hour). The difference is precisely the effect described in the previous log.

The reshaping of Dawn’s orbit is different from the reorienting of the plane of the orbit, also explained in the previous log. The plane change from the gravity assist, of vital importance to the mission, does not manifest itself as a difference in the speed of the spacecraft around the Sun; it is a difference in the direction of motion. Nevertheless, the effect can be described as being equivalent to a change in speed (actually, in velocity), as it was in the last log. (For interested readers for whom these points are not already evident, please click here to go to the log that clarifies it. [Editor, I have not written such a log yet, although I will write an explanation in the future. In the meantime, please insert an acausal hyperlink for readers who do not want to wait.])

As Dawn plunges toward Mars, it will be coming in over the northern hemisphere, reaching above 60° latitude. As Mars bends the orbit, changing the direction the craft travels around the Sun, Dawn will depart over the southern hemisphere. Leaving the planet behind on its new path around the Sun, it will be above 60° south latitude.

The focus of the Dawn team’s work for the Mars encounter has been to achieve a trajectory that would deliver the probe to the target above Mars at the right time and traveling at the correct angle to accomplish the needed gravitational boost. As long as the spacecraft will be in the vicinity of such a familiar solar system site, albeit briefly, the team decided to take advantage of the occasion to calibrate the instruments that are designed to elucidate the nature of Vesta and Ceres. As mentioned at the end of the previous log, there is little Dawn can learn about Mars that is new. Indeed, it is flying higher and faster than spacecraft there right now. In addition, its instruments are intended for the initial examination of previously unexplored worlds, not the detailed investigation of a planet that already has been scrutinized by spacecraft for decades. That very scrutiny, however, means that there is a valuable database for use in comparing observations by Dawn’s instruments, helping to prepare them for their assigned tasks in the asteroid belt. Dawn’s observations at Mars, like the other, more distant measurements it has made of stars and planets since launch, help prepare for the real rewards of the mission. (To recapture the thrill of some of the previous in-flight observations, visit the logs of October 24, 2007, December 17, 2007, February 29 of any year, or April 22, 2008.)

Each instrument will acquire data to be used in comparisons with observations made by similar instruments on spacecraft dedicated to the study of Mars. Rather than using Dawn to learn about Mars, our focus is on using Mars to learn about the performance of our instruments. This will be helpful in doing science at Vesta and Ceres and in navigating there. As we will see in subsequent logs, the positions of Vesta and Ceres are not known accurately enough that Dawn could rendezvous with them using conventional radio navigation techniques alone. To improve the navigation, the craft will take images of the bodies as it is closing in on them, and analysis of those images by the Dawn navigation team will help pin down the location of the target. Tests at Mars will contribute to characterizing the camera not only for science, but for this separate function as well.

If Dawn’s encounter had represented a unique opportunity to conduct vital new science, the plans would have been different. As just one example, Dawn will not attempt to acquire the highest resolution visible images that it might be capable of achieving. Instead, by smearing the view of Mars across the camera’s line of sight, engineers will provide a relatively uniform illumination for the camera’s detectors, providing interesting engineering data that cannot be obtained when observing the pinpoint light of stars.

In preparation for the encounter, on January 20 controllers activated the gamma-ray and neutron detector (GRaND), which, despite its name, is a very modest (but wonderfully capable) member of the on-board instrumentation. The last time GRaND was operated was in April. It remains healthy and is continuing to work well, sensing the effects of cosmic radiation impinging on the spacecraft.

Dawn is approaching Mars from outside the planet’s orbit around the Sun. Therefore, from the spacecraft’s point of view, Mars and the Sun are close together, and Mars would appear to be a thin crescent. This geometry precludes directing its instruments toward the planet well before it arrives. Designed to operate in the more distant asteroid belt, the instruments would not be able to tolerate the heating from the Sun. Therefore, although GRaND is able to detect space radiation, there would be nothing for the other instruments to do while awaiting their calibrations at Mars.

These instruments will be activated on February 17 using instructions already stored onboard. Dawn will be pointing its main antenna to Earth, so the mission control team can observe telemetry, but there is no plan to send additional instructions then. The visible and infrared mapping spectrometer will be powered on at about 9:47 a.m. PST, and the primary science camera will be commanded on at about 1:21 p.m. PST. (The backup science camera has been used for other tests in flight, but it will not participate in the activities at Mars.) All the times presented here are as measured on the spacecraft. Mission controllers will have to wait more than 19 minutes, as radio signals traverse the great distance to Earth, to observe the associated telemetry. In essence, their entire view of events will be delayed by this “one-way light time.” (The Dawn project remains ready to advise the FCC on the use of such a system to provide a guaranteed delay in live broadcasting.)

At 1:57 p.m. PST, the spacecraft will begin turning to prepare for its calibration activities. The maneuver will move Earth out of the radio beam from the main antenna, so the spacecraft will switch to 1 of its 3 auxiliary antennas. Each of these antennas can emit a much broader beam, allowing communications over a wider range of orientations. The cost of spreading the signal over a much greater area is that when it is received at Earth, it is significantly weaker, so only a very limited amount of telemetry can be sent. For the subsequent day and a half, controllers will use this reduced flow of information to monitor Dawn’s work. Meanwhile, the instruments will attempt to record neutrons, gamma-rays, and ultraviolet, visible, and infrared light, all from Mars, all providing a bonus to the mission. (Dawn has previously conducted infrared observations of Mars. At that time the spacecraft was about 100,000 times farther from the planet than it will be when it swoops by next week.)

At 3:09 a.m. PST on February 19, Dawn will begin turning back to sight its main antenna on Earth once again. For more than a day, it will radio engineering data it stored during the time it could not transmit at high speed. At first, a Deep Space Network (DSN) antenna near Madrid, Spain, will receive the signals. As Earth rotates, the 34-meter (112-foot) dish will no longer be able to point to Dawn (as viewed from Madrid, Dawn will set in the west, just as the Sun, the moon, stars, and other celestial objects do), so a DSN antenna near Goldstone, California, will take over. After about 8 hours, the Goldstone facility will hand the responsibility over to a DSN antenna still farther west, near Canberra, Australia, which eventually will pass the baton once again to Madrid.

At 7:48 a.m. on February 20, the spacecraft will rotate again to bring its camera to bear on Mars. By then, the view of distant Mars will be similar to what the spacecraft will have as it navigates to Vesta, once again providing an opportunity to prepare for the visit to that mysterious world. At 11:28 a.m., it will turn away from Mars for the last time and resume transmitting data back to Earth 45 minutes later. The precious capabilities of the DSN are shared among all interplanetary spacecraft, so Dawn will return to more intermittent tracking on Friday. The return of data will be completed the following week.

All data that Dawn collects at Mars are considered a bonus of the mission. Indeed, when the mission was conceived, its launch was to be in 2006, and the mission to Vesta and Ceres then did not require the assistance of Mars. It was only in 2005, when the launch was rescheduled to 2007, that trajectory designers added Mars to the itinerary. The gravitational deflection is essential to the success of the mission, but the activities during the visit to Mars are secondary to the mission’s principal objectives.

Even before Dawn has paid a visit to Mars, engineers are already busy planning the details of the spacecraft’s next assignments. Following its brief divertissement at the planet, it will continue to coast in its orbit around the Sun until June. With Mars helping to reshape its orbit, there is no need for ion thrusting for a while. In the next few logs, we will follow along as Dawn engages in other work to be completed before the resumption of its most familiar function of reaching for the asteroid belt atop a blue-green pillar of xenon ions.

Dawn is 1.1 million kilometers (680 thousand miles) from Mars. It is 348 million kilometers (216 million miles) from Earth, or 910 times as far as the moon and 2.36 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.

Dr. Marc D. Rayman
8:00 pm PST February 12, 2009

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

• 

  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.

In the last log we saw that the mission operations team was preparing to adjust the probe's flight path to keep it on target for next month's flyby. Just getting to the vicinity of Mars is not sufficient, as the passage by the planet is only one short segment of a very long itinerary. Indeed, choreographing Dawn's trajectory is a complex matter of finding the most efficient route through the solar system to travel from the moving platform on which it started (Earth) to encounter Mars in just the right way to reach Vesta at the proper time to complete its work there before it has to begin the trek to Ceres to meet it on schedule, aided during most of the journey by the ion propulsion system. Dawn must arrive at Mars on time, traveling in the correct direction and at the necessary location, for the gravitational slingshot to yield the desired effect. Flying the spacecraft through that "window" at Mars is like threading a celestial needle.

Early this month, engineers determined that the craft's interplanetary route was so close to the one planned originally that it was unnecessary to refine it. In part, this is a reflection of how accurate the first (and now only) trajectory correction maneuver (TCM1) -- where do normally undemonstrative engineers come up with these incredibly cool names?) was when it was executed on November 20. In addition however, Dawn does not need to be aimed as accurately as a typical spacecraft might for such a gravity assist. The ion propulsion system will be used so extensively between Mars and Vesta to continue modifying the probe's orbit around the Sun that it has enough leverage to compensate for a significant range of deviations from the mathematically optimal bending of its course by Mars's gravity.

To understand the targeting at the red planet, at the request of readers on planets of all colors throughout the constellations Sagitta and Sagittarius, we shall extend the analogy of the archery problem described in an earlier log. We can follow both the spacecraft and the arrow as each travels to its goal. Of course, while the arrow's path ends when it produces a satisfying thunk at the target, Dawn's target is a region of space near Mars that is much more penetrable. There are other important differences between the two problems, not the least of which is that Dawn has thrust with its ion drive most of the way to Mars. (For the complete list of caveats that apply to the analogy, contact the Dawn legal department.)

When Dawn was fired into space, aiming for the window near Mars was analogous to shooting an arrow at a target 47 kilometers (29 miles) away. In the center of the target is a red circle 30 centimeters (almost 1 foot) in diameter, representing Mars. Of course, we don't want our arrow to hit the red bull's-eye! Rather, our goal is a spot about 2.2 centimeters (7/8 of an inch) outside the circle, near the 11:00 position.

By the time of TCM1 2 months ago, Dawn had traveled 880 million kilometers (550 million miles), corresponding to the arrow having sailed 39 kilometers (24 miles) from our bow. After flying that tremendous distance, our projectile was headed for the bull's-eye itself, so we applied a tiny adjustment to put it on course for the real aim point.

By January 15, when the mission operations team had scheduled TCM2, Dawn had put more than another 110 million kilometers (70 million miles) behind it, so our arrow would have streaked another 5 kilometers (3 miles) closer to its target. Even with so far still to go, the aim is so good that no further correction is needed.

Although controllers will continue to monitor Dawn's trajectory and refine the predictions of its course, as long as the arrow flies true, it will reach its target. With the latest estimate, instead of hitting the mathematically optimal spot, it will strike less than 2 millimeters (a little over 1/16 of an inch) farther from the bull's-eye. It won't quite land at the 11:00 position, but it will be less than a third of the way to the tick mark just after the 11:00 marker, where the hour hand would be pointing at about 11:04. This is comfortably within Dawn's allowance for accuracy.

These coordinates correspond to the spacecraft passing about 543 kilometers (337 miles) above the reddish surface of the planet rather than the original plan of 500 kilometers (311 miles). While that may seem like a large difference, the effect of a gravity assist largely depends on the distance from the center of a planet, not the surface. In that context, instead of passing about 3896 kilometers (2421 miles) from the center, Dawn will pass 3939 kilometers (2448 miles) from that reference location. (The arrow strikes 17.4 centimeters, or less than 6 7/8 inches, from the middle of the bull's-eye rather than 17.2 centimeters, or 6 ¾ inches.) The mission easily could accommodate much larger deviations from the original plan.

And what does the difference mean for the overall journey to the asteroid belt? Dawn's 4-month period of coasting between Mars and the resumption of ion thrusting will be shortened by about 4 days. As the spacecraft will thrust for most of the 2.5 years from Mars to Vesta, powering up the ion drive a few days earlier is virtually inconsequential.

Over the coming weeks, navigators will continue to gather data on Dawn's route through space to refine their computations, and some of the numbers above likely will change a little. It might be tempting to think that if we had truly perfect knowledge of the spacecraft's position and velocity at some time then we could predict the encounter details correspondingly accurately. After all, the gravitational forces from the Sun and Mars and even other planets and asteroids, far though they are from Dawn, are very well known indeed. It turns out, however, that there are other forces acting on the spacecraft as well. They are like a soft but variable breeze blowing on the arrow we have aimed at the distant target, making the already challenging archery problem still harder.

Light, as insubstantial as it feels to our corporeal readers, has momentum, so as the Sun shines on Dawn, it constantly modifies the trajectory. (The force from the light is far greater than from the solar wind, the stream of charged particles that our star blows into space. The effects of the solar wind are too tiny to matter.) The constant nudge from the Sun's light is small, but it needs to be incorporated into the targeting, and that is difficult because the size of the effect depends on the reflective properties of the spacecraft. Light bouncing off a polished piece of metal provides a different force from light absorbed by a dull black thermal blanket. As the spacecraft changes its orientation, different components are exposed to the light from the Sun, and the angles at which they are illuminated change. Navigators are able to account somewhat for the solar pressure, but the small uncertainty in it contributes to the uncertainty in the encounter details.

Dawn itself is responsible for an even larger variability in its own trajectory. As we have seen before, the spacecraft is equipped with reaction wheels, which are part of the attitude control system. (To achieve a certain mystique about their work, engineers use the term "attitude" to describe the orientation of the probe in the weightlessness of spaceflight; the system also happens to have a very sanguine attitude about its work.) By electrically spinning the wheels faster or slower, the spacecraft can control its attitude (and sometimes even its outlook). The reaction wheels are used to start and stop turns as well as to hold the craft's attitude steady, right now, for example, keeping the main antenna pointed to Earth. The solar pressure not only alters the spacecraft's trajectory but it can rotate the spacecraft as well, just as wind can both propel and turn what it blows against. Attitude control automatically changes the wheels' speeds to counteract this effect. The consequence, however, is that the units gradually speed up as they hold their own against the incessant solar pressure.

The wheels cannot increase their speeds indefinitely, and they lose effectiveness when they reach their limit. To allow them to slow down, attitude control calls upon its colleague the reaction control system to fire small thrusters, which use the rocket propellant hydrazine. The force essentially reverses the influence of the solar pressure that has caused the wheels to spin up, thus allowing them to spin down without the spacecraft ever having to change its attitude. Indulging their usually secretive poetical impulses, engineers refer to this process as "desaturating the reaction wheels."

In addition to slowing the wheels, the thrust from the hydrazine pushes the spacecraft, modifying its trajectory. During the current phase of the mission, the thrusters are operated twice a week to desaturate the reaction wheels, so engineers need to quantify the ramifications of firing the reaction control thrusters prior to the encounter with Mars. Here again, while the phenomena are understood, there remains some level of uncertainty in how accurately they can be predicted, given imperfect knowledge of the actual effect of the solar pressure and details of how the spacecraft will operate its 6 thrusters to reduce the speeds of the 3 operating wheels.

The uncertainty in the change in the spacecraft's speed from the desaturation of the reaction wheels is around 1 millimeter/second (12 feet/hour). Except to our readers in the planetary nebula NGC 6543 (sometimes known as the Snail Nebula), this speed may seem very slow, but consider a simplified example of how this affects the calculation of exactly where Dawn will be as it sails past Mars. Suppose the planetary encounter is 5 weeks away, and all present and future trajectory parameters are known perfectly except the result of the desaturation scheduled for 4 weeks before the encounter. (This example disregards all the other desaturations yet to occur, treating them as if they would be known exactly.) The best prediction of the effect on the spacecraft's trajectory of that desaturation still a week away is included in the analysis to determine where Dawn will be when it reaches Mars, but that prediction is off by just 1 millimeter/second. That speed, when maintained for the more than 2.4 million seconds in the 4 weeks between the desaturation and the flyby of Mars, leads to a distance of more than 2.4 kilometers (1.5 miles). So, if the actual effect of the desaturation is 1 millimeter/second different from what is included in the computations, Dawn's actual position will wind up 2.4 kilometers away from the calculated position. Of course, all other parameters are not known perfectly, and there are more desaturations before Mars, so the actual encounter conditions can change by much more than in this illustration. Nevertheless, there has been great effort to establish how unpredictable all the factors are, and engineers are confident the trajectory is well within the required window to achieve the needed assistance from Mars.

The powerful tug exerted by the planet will bend Dawn's path by about 78 degrees. To picture that angle, suppose Mars is a dot at the center of a clock, and Dawn flies toward it (or, more accurately, toward the required window very nearby) from the 12. If the planet had no gravity, the spacecraft would continue in a straight path, exiting the face of the clock by the 6. Instead, the probe takes a sharp turn at the center of the clock and heads out between the 3 and the 4. (This is not the same clock used in the discussion of solar conjunction in the previous log. Be sure to check out the full selection of celestial timepieces in your planet's Dawn gift shop.)

The deflection from Mars changes Dawn's orbit around the Sun (as does thrusting with the ion propulsion system). To enter orbit around Vesta, Dawn needs to match its orbit around the Sun to the one that Vesta is in, and the Mars encounter is designed to help accomplish that, bringing Dawn's plane into closer alignment with Vesta's. (We saw in the previous log that part of the journey requires changing the plane of Dawn's solar orbit.) The gravity of Mars will alter Dawn's orbital plane by about 5.2 degrees, a seemingly modest angle. Yet, if it were up to the spacecraft to accomplish such a change on its own, it would require a velocity change of more than 2.3 kilometers/second (5200 miles/hour).

Thanks to the design of the Mars encounter, there is another benefit as well. In addition to tilting Dawn's orbit around the Sun, Mars changes its shape, enlarging the elliptical orbit and sending the probe farther from the Sun.

In considering the size of solar system orbits, it often is convenient to measure lengths with the "astronomical unit," which is simply the average distance between the Sun and Earth. So 1 astronomical unit (AU) is 150 million kilometers (93 million miles), conveniently helping your correspondent locate not only how far he lives from the Sun, but also how far he works from it.

If Mars had no gravity and thus could not divert Dawn on its travels, the craft's current elliptical orbit would take it as close as about 1.23 AU to the Sun and as far as 1.69 AU away. (For comparison, Mars orbits between about 1.38 AU and 1.67 AU.) As we saw in a log last summer, Dawn is temporarily heading in toward the Sun now. Without the effect of Mars, and with no additional ion thrusting, our interplanetary robot would reach that minimum distance in June 2009, and by early May 2010 it would have swung out to the greatest distance, only to begin falling back again as it followed its elliptical loop.

After the boost from Mars, if Dawn undertook no further thrusting, it would come no closer than 1.37 AU to the Sun, reaching that distance in April 2009. Then it would head out to 1.84 AU, arriving there in April 2010. In this new orbit, Dawn still will be a long way from Vesta (which never dips even quite to 2.1 AU from the Sun) and still farther from Ceres (which travels out to 3.0 AU), yet it will be much closer than it would have been without Mars's help.

Reshaping Dawn's orbit, quite a separate effect from reorienting it, would require more than 1.1 kilometers/second (2500 miles/hour). The combination of these two benefits is equivalent to the planet imparting about 2.6 kilometers/second (5800 miles/hour) to the spacecraft.

Dawn's unique propulsion system allows it to change its own speed by well more than this during its mission. Yet the famously gentle ion thrust means it would take quite a while to achieve these changes, and the mission itinerary, fit between the September 2007 launch and the February 2015 arrival at Ceres, does not afford enough uncommitted time. There are other technical reasons as well that making these changes only with its built-in capabilities would be impractical. The encounter with Mars is a free way to get significant help.

Is it really free? Well, in these difficult economic times, there is a cost we are obligated to divulge in the interest of full disclosure. The changes to Dawn's orbit come at the expense of Mars's orbit. Just as when you throw a ball forward, you feel a "reaction" force backward, in pushing the spacecraft one way, Mars reacts by moving the other. Mars exerts a force on Dawn, but Dawn exerts an opposite force on Mars. As the planet's mass is nearly 600 million million million times that of the spacecraft, the effect on our probe is far larger than on the fourth planet from the Sun. The cost of helping Dawn is that Mars will slow in its orbit enough that after 1 year, its position will be off by about the width of an atom. Adding up the growing deficit, it would take 180 million years for Mars to be out of position by 2.5 centimeters (1.0 inches). That is the cost, and, on behalf of Dawn and all who share in the eager anticipation of the mysteries it will reveal in the asteroid belt, we express our gratitude to Mars for its upcoming sacrifice!

The control team's best estimate now is that Dawn's closest encounter with Mars will occur at about 4:28 pm PST on February 17. In the next log, we will cover some of the spacecraft's activities during its short visit, but we conclude this one with a note about the plans. Many spacecraft have visited this intriguing planet already, and it has been studied extensively from orbit and from the surface for years and years. During its brief passage, Dawn cannot learn much that is new. On the other hand, Mars is so well characterized and familiar that it provides a useful reference for calibrating Dawn's scientific instruments. As we will see, the calibration plan precludes obtaining the highest resolution images that might otherwise be attempted. Such a strategy may seem surprising to all those who appreciate spectacular pictures of Mars, but, of course, there are already many many such images, and future missions will produce even more that will continue to captivate and inspire us.

The instrument calibrations will be a nice bonus, but if Dawn ended up flying by Mars and conducting no calibrations whatsoever, as long as the trajectory targeting were correct, everyone on the project would consider it a success, because the gravity assist is all that is required for the mission to forge ahead to its goal of unlocking exciting secrets deep in the asteroid belt. Dawn's focus is on preparing for the unknowns of Vesta and Ceres rather than the knowns of Mars.

Dawn is 4.6 million kilometers (2.9 million miles) from Mars. It is 358 million kilometers (222 million miles) from Earth, or 900 times as far as the moon and 2.41 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 40 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 pm PST January 27, 2009

TAGS: DAWN, CERES, VESTA, SPACECRAFT, MISSION

• 

  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 Earth, the Sun, and the spacecraft are on a straight line, such as at 6:00, the Sun and spacecraft would appear to overlap from the perspective of an observer on Earth, near the bottom of the clock. As we noted last month, Dawn would not pass directly behind the Sun, because it does not orbit in the same plane as Earth. Therefore, the precisely linear arrangement of hands at exactly 6:00:00 never occurs. Pushing the clock analogy beyond its limits of usefulness, the minute hand would be bent toward the clock face, so it does not circle in quite the same plane as the hour hand. We shall ignore that enhancement for now but return to this point below. In the meantime, let’s consider the arrangements that have occurred.

On December 12, when the angle between the Sun and the spacecraft was at its minimum, it would be analogous to the alignment of the hands about 10 seconds from the hour, or the arrangement at 6:00:10. (Remember, this clock only has hour and minute hands; your correspondent types too slowly to be able to construct a useful analogy with a clock that includes a second hand.) When most modern interplanetary craft are within about 2 degrees of the Sun, normal communications may be less reliable. This limitation, which lasted about 2 weeks for Dawn, would correspond to half a minute on either side of 6:00, or between about 5:59:30 and 6:00:30.

Despite the powerful interference caused by radio signals passing through the distorting environment of the Sun on their way from the spacecraft to Earth, enough of the transmissions made it through for engineers to confirm that the spacecraft remained healthy throughout the conjunction period. Dawn was programmed to modify its radio transmissions to account for the angle between it and the Sun. Operators chose to accept a reduced return of information from the ship’s systems in exchange for boosting the quality of the signals used for navigation because of the upcoming flight by Mars. Some usable navigation data were obtained every day, but, as expected, most of the data, particularly during the 4 days when the spacecraft was nearest the Sun, were too degraded to be useful in refining the parameters of Dawn’s orbit.

Now, as Earth and the spacecraft have progressed in their separate travels around the Sun (making an angle today equivalent to about 6:01:45 on our Dawn clock), the radio waves traverse a less tortuous path, so the signal quality has improved. After collecting and analyzing more navigational data, engineers will determine what refinement is needed to the trajectory to guarantee Dawn encounters Mars in just the right way to provide the needed gravitational deflection. Following the same procedure applied to the design of Dawn’s first trajectory correction maneuver (TCM), the team will begin designing the second TCM early next month for the spacecraft to perform on January 15. In fact, the creative process has already begun; the maneuver has been given the imaginative appellation TCM2. Using those 4 characters (and perhaps a few others as well), the next log will report on the maneuver and provide some details on the nature of Dawn’s gravitational interaction with Mars and how it affects the trajectory.

The only reason for Dawn to travel to the vicinity of Mars is for the help to reach its targets in the asteroid belt. Nevertheless, as the probe races by, the team will take advantage of the opportunity to accomplish some bonus goals. Some of the plans will be covered in an upcoming log.

In the meantime, as the thrill of conjunction begins to fade, our vast staff has yet to sort through all the data on how many terrestrial readers used this convenient alignment to guide their mental eyes toward the spacecraft. The Dawn project sincerely hopes all observers reaped the maximum possible inspiration and joy from solar conjunction, as the mission will not offer another like it. Our destinations, 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 plane of Earth’s orbit, and no spacecraft has ventured as far out of that plane to orbit another body as Dawn will.) While the probe is already in a slightly different plane from Earth’s orbit now, the gravity of Mars and subsequent ion thrusting will propel it to still a greater angle. As a result, when Dawn and Earth find themselves on opposite sides of the Sun in the future, the alignment will not be as close as it was this month. Dawn’s next apparent encounter with the Sun will be in November 2010, but it will appear to pass far enough north of the Sun that communications should not be significantly compromised. Following that, there will be 3 more times before the primary mission ends in 2015 that Earth and the spacecraft will be on opposite sides of the Sun, but in each case Dawn’s path through Earth’s skies will take it farther north or south of the brilliant landmark than in the 2008 conjunction. Nevertheless, each will be close enough that it may provide a visual reference once again to stir meditation upon the magnificence of a journey far away in the depths of space.

Dawn is 11 million kilometers (7 million miles) from Mars. It is 372 million kilometers (231 million miles) from Earth, or 930 times as far as the moon and 2.53 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
6:01:45 pm PST December 30, 2008

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

• 

  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.

Suppose you want to shoot an arrow at a target. Unlike typical archers, you are so far from the target that you can only barely see it. In that case, aiming for the bull’s-eye is essentially out of the question. Adding to the problem may be a variable breeze that could nudge the arrow off course. Shooting sufficiently accurately to get the arrow even to the vicinity of the target would be challenging enough; hitting the precise point you want on the target is just too difficult.

For readers who are principally interested in archery, this concludes our in-depth analysis of the sport.

Now let’s consider how to change the situation to make it more similar to an interplanetary mission. If the arrow had a tiny radio locator mounted on it, you would be able monitor its progress as it flew closer to the target. This would be like watching it on a radar screen. You might see your arrow miss the target entirely or, if you had made a particularly good shot, hit somewhere on it. Now if you could occasionally send a signal to the arrow, perhaps to change the angles of the feathers, you might not be able to alter its course drastically, but you could change it a little. So if your initial shot had been good enough, you could guide the arrow to the desired destination. (To buy your radio controlled archery set, visit the Dawn gift shop on your planet. The set may be found between the display case with xenon ion beam jewelry and the shelves and shelves and shelves and shelves of really cool new Dawn Journal reader action figures -- be sure to buy the one that looks just like you!)

Shooting the arrow is akin to launching a spacecraft, and its flight to the target represents the interplanetary journey, although operating a spacecraft involves far greater precision (and fun!). Our knowledge of where the spacecraft is and where it is heading is amazingly, fantastically, incredibly accurate, but it is not perfect. This point is essential. Keeping most spacecraft on course is a matter of frequently recalculating the position, speed, and direction of travel and then occasionally fine-tuning the trajectory through burns of the propulsion system.

Dawn’s near-constant use of its advanced ion propulsion system for most of 2008 changes the story, but only a little. The thrust plan was calculated before launch and then updated once our arrow was free of the bow. Throughout the interplanetary cruise phase, a new thrust plan was transmitted to the spacecraft about every 5 weeks, each time with slight updates to account for the latest calculations of Dawn’s orbit around the Sun. With this method, the small adjustments to the trajectory have been incorporated into the large, preplanned changes.

The mission control team requires about 5 weeks to design, develop, check, double-check, transmit, and activate a 5-week set of commands. By the time the spacecraft is executing the final part of those instructions, it is following a flight plan that is based on information from 10 weeks earlier. During most of the mission, when there are months or even years of thrusting ahead of it, subsequent opportunities to adjust the trajectory are plentiful. In contrast, for the last period of preplanned thrusting before Mars, controllers modified their normal process for formulating the commands, making a fast update for the final few days of thrusting. By including the latest navigational data in the computations for the direction and duration of the concluding segment of powered flight, the mission control team put Dawn on a more accurate course for Mars than it otherwise would have been.

Even with this strategy, navigators recognized long ago that subsequent adjustments would be required. The plan for approaching Mars has always included windows for trajectory correction maneuvers (which engineers are physiologically incapable of calling anything other than TCMs). Dawn’s first TCM occurred on November 20.

As navigators refined their trajectory calculations after thrusting finished on October 31, they determined that the spacecraft was quite close to the aim point they wanted, but still not exactly on target. In fact, rather than being on a course to sail a few hundred kilometers above Mars, the probe’s path would have taken it to the surface of the planet. Despite the power of the ion propulsion system, Dawn does not have the capability to bore through the rocky planet and continue on its way to Vesta.

Such a situation is not surprising. Suppose in the archery, the bull’s-eye were 30 centimeters (1 foot) in diameter, but we preferred to hit a point 2.2 centimeters (7/8 inch) outside the bull’s-eye, near the 11:00 position (corresponding to where we want Dawn to fly past Mars). As our arrow approached the target, it might turn out that it was going to miss the target entirely, it might be headed for some other point on the target, and it just might be that it was headed for the bull’s-eye itself. Dawn’s case was this last one, so TCM1 put it on track for the destination we desired.

Amazing sports analogies for the fantastic accuracy of interplanetary navigation usually fail to account for TCMs, as most arrows, balls, and other projectiles do not include active control after they are on their way. Your correspondent has presented his own simile for the astonishing accuracy with which a spacecraft can reach a faraway destination, but most such analogies neglect TCMs, without which deep-space missions could not be accomplished. (Note that the accuracy is impressive with or without TCMs. We shall extend our archery example in a future log, making it more quantitative. It will be important, however, to keep in mind that the ion propulsion system provides so much maneuvering flexibility that Dawn does not need to achieve the degree of accuracy in its gravity assist that a mission using conventional chemical propulsion might.)

For reasons we will not divulge, Dawn’s first TCM has been designated TCM1. On November 20, just as it had for all of its previous thrusting, the spacecraft pointed a thruster (TCM1 used thruster #1) in the required direction and resumed emitting the familiar blue-green beam of xenon ions to alter course. While typical thrusting during the mission has lasted for almost 7 days at a time (followed by a hiatus of 7 to 8 hours), in this case only a short burn was necessary. Propelling itself from about 4:31 pm to 6:42 pm PST was just enough to fine-tune its course and change its speed by a bit more than 60 centimeters per second (1.3 miles per hour). This adjustment was modest indeed, as at that time Dawn was traveling around the Sun at more than 22.5 kilometers per second (50,400 miles per hour). Dawn and Mars, following their separate orbits that will (almost!) intersect on February 17, 2009, were moving relative to each other at 3.17 kilometers per second (7100 miles per hour).

Dawn’s second TCM window (inexplicably named TCM2) is in January. Traveling two-thirds of the way from here to Mars, the navigational accuracy then will be still better, with smaller deviations from the planned target point being detectable, so another refinement in the trajectory then is likely. In the meantime, Dawn will follow its orbital path with its ion thrusters idle.

As Dawn travels through space on its own, its path has been essentially independent of Earth’s. We saw in a previous log that the weaker grasp exerted by the Sun at Dawn’s greater distance means that it travels more slowly around the solar system. While Earth has completed more than 1 full revolution (each revolution requiring 1 year) since launch, Dawn has not yet rounded the Sun once. After receding from the Sun until early August, the spacecraft began falling back, albeit only temporarily.

The probe attained its maximum distance from Earth on November 10. (For anyone who was on Earth on that date and plans to use this information in an alibi, it may be helpful to know that the greatest range was reached at about 3:07 am PST.) The spacecraft was more than 384 million kilometers (239 million miles) from its one-time home. Although it will make substantial progress on its journey in the meantime, Dawn’s distance to Earth will continue to decrease until January 2010, when it will be less than one-third of what it is today. In the summer of that year, however, as Earth maintains its repetitive annual orbital motion and the explorer climbs away from the Sun, it will surpass this month’s distance to Earth. (Readers are encouraged to memorize the contents of this log for reference in 2010 in case we fail to include a link to this paragraph.)

The complex choreography of the solar system’s grand orbital dance rarely calls for a circular orbit; rather, the dancers follow ellipses (ovals in which the ends are of equal size) around the Sun. Thanks to the details of the shapes of their orbits, the greatest separation between Earth and Dawn did not occur when they were precisely on opposite sides of the Sun, although the alignment was close to that.

On December 12, their dance steps will take them to points almost exactly on opposite sides of the Sun. For observers on Earth, this is known as solar conjunction, because the spacecraft and the Sun will appear to be in the same location. (Similarly, from Dawn’s point of view, Earth and the Sun will be almost coincident.) In reality, of course, Dawn will be much farther away than Earth’s star. It will be 147 million kilometers (91.5 million miles) from Earth to the Sun but 379 million kilometers (236 million miles) from the planet to its cosmic envoy.

Its apparent proximity to the Sun presents a helpful opportunity for terrestrial readers to locate Dawn in the sky. On December 9 - 15, the spacecraft will be less than 1 degree from the Sun, progressing from east to west and passing just 1/3 degree south of that brilliant celestial landmark on December 12. (As Dawn does not orbit in the same plane as Earth, it will not pass directly behind the Sun.) The Sun itself is 1/2 degree across, so this is close indeed; the spacecraft will sneak in to less than 1 solar diameter from the disk. To demonstrate how small the separation is, if you blocked the Sun with your thumb at arm’s length during this week around conjunction (and you are exhorted to do so), you also would cover Dawn.

For those interested observers who lack the requisite superhuman visual acuity to discern the remote spacecraft amidst the dazzling light of the Sun, conjunction still may provide a convenient occasion to reflect upon this most recent of humankind’s missions far into the solar system. This small probe is the product of creatures fortunate enough to be able to combine their powerful curiosity about the workings of the cosmos with their impressive abilities to explore, investigate, and ultimately understand. While its builders remain in the vicinity of the planet upon which they evolved, their robotic ambassador now is passing on the far side of the extraordinarily distant Sun. This is the same Sun that has been the unchallenged master of our solar system for 4.5 billion years. This is the same Sun that has shone down on Earth throughout that time and has been the ultimate source of so much of the heat, light, and other energy upon which the planet’s inhabitants have been so dependent. This is the same Sun that has so influenced human expression in art, literature, and religion for uncounted millennia. This is the same Sun that has motivated scientific studies for centuries. This is the same Sun that acts as our signpost in the Milky Way galaxy. This is the same Sun that is more than 100 times the diameter of Earth and a third of a million times the planet’s mass. And humans have a spacecraft on the far side of it. We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty.

Solar conjunction means even more to Dawn mission controllers than the opportunity to meditate upon what magnificent feats our species can achieve. As Earth, the Sun, and the spacecraft come closer into alignment, radio signals that go back and forth must pass near the Sun. The solar environment is fierce indeed, and it causes interference in those radio waves. While some signals will get through, communications will be less reliable. Therefore, controllers plan to send no messages to the spacecraft from December 5 through December 18; all instructions needed during that time will be stored onboard beforehand. Deep Space Network antennas, pointing near the Sun, will listen through the roaring noise for the faint whisper of the spacecraft, but the team will consider any signals to be a bonus.

There is plenty of other work to do while waiting to resume communications after conjunction. In addition to preparing for the visit to Mars, engineers will continue to interpret the results of election day. On November 4, the Dawn team voted unanimously for more power. They commanded the spacecraft to execute a set of steps to yield data that will reveal the full potential of the enormous solar arrays to generate electrical power. The method was tested first on July 21, and then refined for a test on September 22. For this month’s measurement, the commands were identical to those used for the second test with one exception that had been planned from the beginning: the solar arrays were rotated to point 60 degrees away from the Sun instead of 45 degrees. The solar arrays are so powerful that when they are pointed directly at the Sun, the spacecraft could not draw enough power to measure their full capability.

The data collected show the electrical behavior of the arrays as the ion propulsion system was commanded through its start-up, drawing more and more power. Unlike the two tests, this calibration was designed so that with the arrays pointed so far from the Sun, they would not be able to provide as much power as was requested. Engineers wanted to find the point at which the arrays would no longer be able to satisfy the demands. They were not disappointed; power climbed up and up until no more was available. The prospect of having a spacecraft not be able to meet its own power demands may seem risky, but the procedure was carefully designed, analyzed, and simulated, and it executed perfectly. When the ion propulsion system asked for more power than the arrays could deliver, in the language of the trade, the solar arrays “collapsed.” Now to some (including even some engineers unfamiliar with the terminology), this suggests something not entirely desirable, such as 2 bent and twisted wings, each with 5 warped panels, and 11,480 shattered solar cells, the fragments sparkling in the sunlight as they tumbled and floated away from the powerless probe. In this case though, “collapse” is an electrical, not a mechanical, phenomenon and hence would be somewhat less visually spectacular and quite reversible -- a key attribute for a mission with well over 6 years of space exploration ahead of it. Once all the data are analyzed, controllers will have a better prediction for how much power the arrays will be able to generate for the rest of the voyage.

Dawn is 20 million kilometers (12 million miles) from Mars. It is 383 million kilometers (238 million miles) from Earth, or 950 times as far as the moon and 2.59 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 43 minutes to make the round trip.

Dr. Marc D. Rayman
7:00 am PST November 26, 2008

TAGS: DAWN, VESTA, CERES, DWARF PLANET, 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.

The Wide-field Infrared Explorer (WISE) mission, an infrared telescope launching in about a year, will observe hundreds of near-Earth asteroids, offering unique insights into this question. The risk posed by hazardous asteroids is critically dependent on how many there are of different sizes. We know that there are more small asteroids than large ones, but how many more, and what are they made of?

Asteroids reflect sunlight (about half of which is the visible light that humans see), but the sun also warms them up, making them glow brightly in infrared light. The problem with observing asteroids in visible light alone is that it is difficult to distinguish between asteroids that are small and highly reflective, or large and dark. Both types of objects, when seen as distant points of light, can appear equally bright in visible light. However, by using infrared light to observe asteroids, we obtain a much more accurate measurement of their size. This is because the infrared light given off by most asteroids doesn’t depend strongly on reflectivity.

WISE will give us a much more accurate understanding of how many near-Earth asteroids there are of different sizes, allowing astronomers to better assess the hazard posed by asteroids. The danger posed by a near-Earth asteroid depends not only on its size, but also on its composition. An asteroid made of dense metals is more dangerous than one of the same size made mostly of less dense silicates. By combining infrared and visible measurements, we can determine how reflective the asteroids are, which gives us some indication of their composition.

TAGS: WISE, ASTEROIDS & COMETS, SPACECRAFT, MISSION, UNIVERSE

• 

  ABOUT THE AUTHOR

  Amy Mainzer, Scientist

  Amy Mainzer is an astronomer specializing in astrophysical instrumentation and infrared astronomy. She is currently the principal investigator for the NEOWISE comet-hunting mission.

Some of the work during this week was to verify that the contents of the computer memory in certain components remained intact. On September 30, engineers confirmed that the memory in each of the 2 ion propulsion computer control units was in good condition. On October 2, the backup star tracker was tested, and it also remains healthy and ready for use whenever needed. A star tracker helps the attitude control system determine the orientation of the spacecraft by imaging groups of stars and recognizing patterns, much as you might orient yourself on a dark, cloudless night if you were familiar with the constellations. (Readers who travel frequently, and hence must keep track of where they are in their galaxy in order to know what the arrangement of stars should be, have a more difficult problem than Dawn’s star trackers face. The solar system is so tiny compared to interstellar distances that the views of the stars remain essentially unaffected by where the spacecraft is, just as the shapes of constellations are the same for observers anywhere on Earth.)

In addition to performing maintenance on software, the mission control team needs to keep Dawn’s hardware in peak condition. The 3 ion thrusters are mounted on separate mechanical apparatuses that allow each 8.9-kilogram (19.5-pound) thruster to be pointed accurately. These thruster gimbal assemblies, known as TGAs to team members who find themselves too busy to use entire words (such people are themselves known as being TBTUEW), need to have lubricant in their bearings redistributed occasionally. Even when a TGA is in use for an operating thruster (thruster #1 has been the active one since June), the usual motion is not enough to accomplish the needed spreading of lubricant. Therefore, all 3 TGAs were moved through a prescribed pattern, ensuring that they will be able to continue to operate smoothly and point correctly.

Dawn is outfitted with 4 reaction wheels, devices whose spin is controlled electrically. Changing a wheel’s spin rate allows the attitude control system to rotate the spacecraft. The wheels are mounted in different orientations, but any 3 are sufficient for normal operations. Wheel #3 has been off since May. On October 2, it was powered on again and wheel #2 was deactivated, beginning its turn as the backup.

Gyroscopes, which will help attitude control perform the accurate pointing of science instruments at the 2 protoplanetary destinations, normally are turned off, as they are not needed for most of Dawn’s assignments along the way. A few times each year they do need to be operated to ensure they remain in good condition. The last such time was in May. On September 29, the units were activated again, and they remained powered on until October 3.

With all maintenance completed successfully, normal interplanetary thrusting resumed on October 3. Soon however, interplanetary thrusting will no longer be the norm. Some of the unusual principles of an interplanetary journey driven with ion propulsion were considered in a log written while Dawn was still gravitationally anchored to Earth. One essential characteristic of such missions is the long periods of thrusting, familiar now to those fortunate enough to have followed Dawn’s progress since the beginning of the interplanetary cruise phase. But, thrusting is not required for the entire voyage; indeed, at some times thrusting is helpful to the mission and at other times it would be detrimental. Extensive analysis is devoted to computing the thrusting schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the masses and orbits of Earth, Mars, Vesta, Ceres, and myriad other bodies) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when thrusting is not permitted (e.g., during spacecraft maintenance periods).

As hinted obscurely only a second ago, the period in which thrusting is beneficial for reaching Vesta on schedule is drawing to a temporary close. For nearly all of the next 7 months, Dawn will coast in its orbit around the Sun (just as do most objects in the solar system, including other spacecraft and planets), no longer mounted atop a bluish-green pillar of xenon ions. Still, its orbit will change dramatically during this interval, as its flight by Mars in February will deflect its path through the solar system. As we shall see in the next log, to achieve exactly the gravitational bending needed, the spacecraft will execute some special thrusting in November and again in January, but very little indeed.

The interplanetary cruise phase has gone so smoothly that the completion of thrusting is being reached somewhat sooner than had been expected earlier in the mission. Commands already stored in Dawn’s central computer will terminate the thrust on October 31 at 3:22 pm PDT. In the next log, we will discuss a bit about the process the team used to determine that time, as it bears on another activity planned for November; contrary to what you might conclude however, leaving enough time for team members to don their costumes in preparation for going door to door to collect Halloween treats was not a factor. (Your correspondent, who disguises himself in costumes at JPL most days, won’t need any extra time at all tomorrow to outfit himself for perfectly frightening appearances on Halloween.)

Although thrusting will be uncommon over the coming months, there will be plenty of other news to look forward to in these logs, including the reversal of Dawn’s departure from Earth, the first attempt to measure the total power generating capability of the solar arrays, passage of the spacecraft nearly behind the Sun, plans for and results of the brief visit to Mars, a dramatic increase in the quality of writing [Note from writer to sponsor: Now that I’ve made such a promise to our readers, I hope you’ll come through with that generous raise I’ve been requesting. Note from sponsor to writer: OK, you win. We agree to a 2% raise from the current $0.00 per log, and we will pay 1% of your tuition if you can buckle down, gain readmission to GUFF, and finally receive your degree.], and much more.

Dawn is 384 million kilometers (238 million miles) from Earth, or 950 times as far as the moon and 2.58 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 43 minutes to make the round trip.

Dr. Marc D. Rayman
9:00 pm PDT October 30, 2008

TAGS: DAWN, VESTA, CERES, DWARF PLANET, MISSION, SPACECRAFT

• 

  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.