Three and a half years after launch, Dawn continues its travels around the sun, maneuvering to take the same orbital path as Vesta. Following its usual pattern, the spacecraft has spent most of the past month gently thrusting with its ion propulsion system. Some of the thrusting this month, however, was not designed to propel Dawn to Vesta. In addition, mission controllers stopped the thrusting to conduct other planned activities.
Spacecraft that use conventional propulsion coast through space most of the time, just as the moon coasts around Earth, and the planets and asteroids coast around the sun. In contrast, Dawn is in powered flight most of the time, using its ion propulsion system to change its orbit. The flight plan requires pointing the ion thruster in just the right direction to deliver the adventurer to its destination. The spacecraft orientation needed to aim the thruster ends up pointing the main antenna in an arbitrary direction. We have seen before that the robotic craft interrupts thrusting for about eight hours each week to direct the antenna toward Earth for communications.
Ever since Dawn’s trajectory was first being designed, long before launch, it has included coast periods for activities that require orientations incompatible with routine thrusting. One such period was the week of March 14; the previous was in July 2010.
Engineers and scientists operate the science instruments about twice each year to ensure they remain in good condition. This time was the last scheduled use of the sensors prior to their observations of Vesta. All tests showed they are in excellent condition and ready to expose the mysteries of the world they are about to visit.
Controllers transmitted upgraded software to each of the two identical science cameras, containing a few improvements over the version installed in July. The procedure went as smoothly as it had for previous software updates, including the first time such an operation was performed in the mission. After each camera received its new software, it performed its standard routine of exercises, just as it did only three weeks after reaching space.
The tests confirmed that each camera’s electronics, optics, detector, cover, and filter wheel are in perfect condition.Sometimes the spacecraft is turned to aim the cameras at carefully selected astronomical targets for their tests; other times, they take pictures of whatever stars happen to be in their field of view. This month’s tests were of the latter type, in which the orientation of the spacecraft was set to keep the antenna pointed at Earth. That put stars from a region near the border between Pisces and Cetus in the grasp of the cameras, quite appropriate for a ship voyaging across the cosmic ocean on its way to a distant and unfamiliar land.
The visible and infrared mapping spectrometer was powered on and put through its paces as well. The instrument demonstrated that it too is in top condition. When next these instruments are operated, they will behold Vesta.
The gamma ray and neutron detector (GRaND) showed that it also is healthy. As with the cameras, the GRaND activities included some improvements. One of the gamma-ray sensors is based on a crystal that gradually loses its sensitivity when exposed to the radiation in space, but the performance can be restored by carefully heating it. This behavior is well understood, so the instrument was designed with the necessary capabilities. GRaND aficionados recall that a test in July proved that the crystal could be heated to the required temperature. So this month, with Vesta just ahead, controllers raised the crystal to 56 degrees Celsius (133 degrees Fahrenheit) for five days. Following that, as with other times GRaND has been tested, it was configured to measure cosmic radiation. It remained on until March 28.
The revitalized GRaND will be operated continuously from the beginning of the Vesta approach phase in early May through the departure in July 2012. Most of that time, the majority of the signals it detects will be from cosmic rays. But the closer it gets to Vesta, the more gamma rays and neutrons it will receive from the rocky surface, gradually allowing scientists to build up a census of the atomic constituents. GRaND’s greatest ability to sense the faint radiation will be in the low altitude mapping orbit.
In addition to using the coasting period to do the final checks of all the science instruments prior to reaching Vesta, engineers recalibrated the two ion thrusters planned for the greatest use in orbit. Last month, we discussed that even tiny differences between the planned thrust and the actual thrust can be important as Dawn spirals closer and closer to Vesta.
Even occasional readers know that the thrust from the ion propulsion system is very gentle. In fact, the thruster pushes on the spacecraft about as hard as a single sheet of paper would push on your hand. When Dawn is maneuvering at Vesta, the acceleration from the thruster will be around 200,000 times less than the acceleration from gravity on Earth. Thanks to the extraordinary persistence of the soft push, however, this crushing 5 micro-g acceleration gradually enables the spacecraft to conduct its comprehensive exploration of Vesta. But the thrust is so light that it cannot be measured accurately with sensors onboard.
To establish what the actual thrust is, engineers took advantage of the same method they used shortly after launch to verify the performance of the propulsion. The technique relies on exquisitely accurate measurements of the change in the frequency of Dawn’s radio signal as the ion thruster accelerates the craft. Just as the pitch of a siren changes as its speed toward or away from the listener changes, the pitch of the radio changes as the spacecraft’s speed relative to Earth changes. This familiar phenomenon is known as the Doppler effect. All interplanetary missions use it for navigation. In addition to applying Doppler measurements to determine the ion thrust, Dawn will use them to map the gravity of Vesta and hence the protoplanet’s interior structure.
The calibration differed in several ways from our familiar interplanetary thrusting. Even as Dawn is climbing away from the sun to reach Vesta, Vesta’s elliptical orbit is taking it farther from the sun as well. By the time Dawn catches up with it and begins maneuvering in orbit, both of them will be farther from the sun than they are now. In the course of the year they are together, the explorer and its target will travel from 2.22 AU from the sun to 2.54 AU. During that time, as the sunlight on the ship’s solar arrays weakens, the ion thrust level will be reduced step by step. Therefore, the calibration was conducted with the throttle levels that Dawn will operate with at Vesta, not the higher one it could have achieved at its solar distance the week of March 14. The system has 112 throttle levels (each corresponding to a specific power consumption and thrust), and these tests focused on 13 of the levels in the range that will be used at Vesta.
Since May 2010, Dawn has thrust with its radio transmitter turned off in order to provide as much power as possible to the ion propulsion system. For the calibration, the transmitter was kept on to provide the signal for the Doppler measurements. There was still sufficient power, however, because the throttle levels chosen were relatively low.
The ship is equipped with three thrusters, although it only uses one at a time. Thrusters 2 and 3 will be used the most at Vesta, so they were the subjects of the calibration. The Doppler method only works for motion toward or away from the observer, not across the line of sight. Therefore, to provide the best measurement, the thrust was aligned with Earth. When thruster 2 was calibrated, it was pointed directly at that distant planet; when thruster 3 was calibrated, it was pointed directly away from it. Both orientations were chosen on the basis of what parts of the spacecraft would be in the heat of the sun.
The system that controls the flow of propellant from the gale force that exits the high-pressure main tank to the zephyr that enters the thruster is very complex. (The critical reader may stop to consider that that descriptor could apply to all of the systems on this sophisticated interplanetary probe. Other readers may simply move on.) Each throttle level requires a specific flow rate of xenon, and it takes a while for the system to adjust to changes in the throttle level. In normal operations, this time is not significant, because the throttle changes are infrequent, occurring only about once a week or so. For the calibration, with so many different levels for each thruster, the schedule allowed time for each adjustment and then another 90 minutes of Doppler measurements after the pressures were correctly balanced.
The entire calibration involved nearly two and a half days of thrusting, during which velocity changes as small as about 0.5 millimeters per second (less than 6 feet per hour) were detectable. (Snails, take note!) The accumulated effect of all that thrusting would be to change the spacecraft speed by about 15 meters per second (33 mph). Because the two thrusters were aligned in opposite directions for their respective tests, some of the thrusting canceled, but differences in details of the calibrations left a residual of about 4 meters per second (9 mph). That would be enough to perturb the probe’s carefully designed flight profile to Vesta. As the calibration had been planned well in advance, however, the design of the trajectory had already accounted for that additional velocity during what would otherwise have been a period of coasting.
After a successful week full of instrument and ion propulsion tests, Dawn resumed its familiar routine, propelling itself to Vesta. If you were onboard now, you might be hungry, cold, and hypoxic, but you also would recognize the destination as the brightest starlike jewel in the beautiful display of celestial gems. Soon, the patience that Dawn has demonstrated in its remarkable journey will be rewarded, as the glowing pinpoint of light will grow to reveal a world full of exciting secrets ready to be unveiled.
Dawn is 2.5 million kilometers (1.5 million miles) from Vesta, or 6.4 times the average distance between Earth and the moon. It is also 2.25 AU (337 million kilometers or 209 million miles) from Earth, or 830 times as far as the moon and 2.25 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
11:00 pm PDT March 31, 2011
Deep in the asteroid belt, Dawn continues thrusting with its ion propulsion system. The spacecraft is making excellent progress in reshaping its orbit around the sun to match that of its destination, the unexplored world Vesta, with arrival now less than five months away.
We have considered before the extraordinary differences between Dawn’s method of entering orbit and that of planetary missions employing conventional propulsion. This explorer will creep up on Vesta, gradually spiraling closer and closer. Because the probe and its target already are following such similar routes around the sun, Dawn is now approaching Vesta relatively slowly compared to most solar system velocities. The benefit of the more than two years of gentle ion thrusting the spacecraft has completed so far is that now it is closing in at only 0.7 kilometers per second (1600 mph). Each day of powered flight causes that speed to decrease by about 7 meters per second (16 mph) as their orbital paths become still more similar. Of course, both are hurtling around the sun much faster, traveling at more than 21 kilometers per second (47,000 mph), but for Dawn to achieve orbit around Vesta, what matters is their relative velocity.
It may be tempting to think of that difference from other missions as somehow being a result of the destination being different, but that is not the case. The spiral course Dawn will take is a direct consequence of its method of propelling itself. If this spacecraft were entering orbit around any other planetary body, it would follow the same type of flight plan. This unfamiliar kind of trajectory ensues from the long periods of thrusting (enabled by the uniquely high fuel efficiency of the ion propulsion system) with an extremely gentle force.
Designing the spiral trajectories is a complex and sophisticated process. It is not sufficient simply to turn the thrust on and expect to arrive at the desired destination, any more than it is sufficient to press the accelerator pedal on your car and expect to reach your goal. You have to steer carefully (and if you don’t, please don’t drive near me), and so does Dawn. As the ship revolves around Vesta in the giant asteroid’s gravitational grip, it has to change the pointing of the xenon beam constantly to stay on precisely the desired winding route to the intended science orbits.
Dawn will scrutinize Vesta from three different orbits, known somewhat inconveniently as survey orbit, high altitude mapping orbit (HAMO), and low altitude mapping orbit (LAMO). Upon concluding its measurements in each phase, it will resume operating its ion propulsion system, using the mission control team’s instructions for pointing its thruster to fly along the planned spiral to the next orbit.Those who have navigated around the solar system, as well as others who have contemplated the nature of orbits without having practical experience, recognize that the lower the orbital altitude, the faster the orbital motion. This important principle is a consequence of gravity’s strength increasing as the distance between the massive body and the orbiting object decreases. The speed has to increase to balance the stronger gravitational pull. (For a reminder of some of the details, be sure to go here before you go out for your next orbital expedition.)
Dawn’s winding orbital path obeys the same rules. The lower the orbit, the faster and tighter the spirals, because the orbital velocity is greater. The first few coiled routes around Vesta this summer will be long and slow, taking days to complete. When it is at the lowest altitude, where each orbit takes only four hours, the spirals will be that much faster, so the craft will have to steer with greater agility to synchronize its ion thrusting with its rapidly changing location.
In its interplanetary travels, spiraling outward from the sun to reach Vesta, each loop takes years to complete, so Dawn has not yet had to steer through any tight turns. The direction it aims the operating thruster hardly moves at all during a full week of thrusting.
Pointing a thruster in the direction needed to spiral around Vesta requires turning the entire spacecraft. Each thruster is mounted on its own gimbal with only a very limited range of motion. In normal operation, the gimbal is aimed so that the line of thrust goes through the center of the ship. When the gimbal is swiveled to a different direction, the gentle force of the thruster causes the ship to rotate slowly. This is similar to the use of an outboard motor on a boat. When it is aligned with the centerline of the boat, the craft travels straight ahead. When the motor is turned, it continues to propel the boat but it also turns it. (Most jets, in contrast, do not alter the direction of their thrust to turn but rather use other means.) In essence, Dawn’s steering of its thrust is accomplished in large part by pivoting the thruster itself.
A crucial difference between the boat and our interplanetary ship is that with the former, the farther the engine is turned, the tighter the curving course. For our craft, the gimballing of the thruster needs to be carefully coordinated with the orbital motion, as if the motorboat operator needed to compensate for a constantly curving current. This has important implications at Vesta. Sophisticated as it is, Dawn knows where it is in orbit only by virtue of information mission controllers install onboard to predict where it will be at any time. That is based on their best computations of Vesta’s gravity, the planned operation of the ion propulsion system, and many other considerations, but it will never be perfectly accurate. Let’s take a look at some of the reasons.
Vesta is a member of the elite family of rocky, terrestrial planets that live in the inner solar system. Just as its kin, Mercury, Venus, Earth, the moon, and Mars, have complex gravity fields, it is likely Vesta does as well. The distribution of materials of different densities within the interior creates variations in the strength of the gravitational force, so Dawn will feel a slightly changing tug from Vesta as it travels in orbit. Our ship will be traversing unknown, choppy waters.
In December, we saw that by sensing the irregularities in the gravity field, Dawn will reveal the nature of Vesta’s internal structure. Until those detailed measurements have been made and accounted for in the design of the flight plan, however, the subtle effects of the gravity field will cause deviations from the planned trajectory. Therefore, as the spacecraft travels from one science orbit to another, it will thrust for a few days and then stop to allow navigators to get a new fix on its position. As it points its main antenna to Earth, the Doppler shift of its radio signal will reveal its speed, and the time for radio signals (traveling, as all readers know so well, at the universal limit of the speed of light) to make the round trip will yield its distance. Combining those results with other data, mission controllers will update the plan for where to point the thruster at each instant during the next phase of the spiral travel, check it, double check it, and transmit it to the distant explorer which will put it into action. This intensive process will be repeated every few days as Dawn maneuvers between science orbits.
The as-yet uncharacterized details of the gravity field are not the only reason the flight plan will require regular adjustments. As the ion propulsion system will be changing the orbit, even tiny deviations from the calculated thrust eventually will build up to have a significant effect. This is no different from any realistic electrical or mechanical system, which is sure to have imperfections. If you planned a trip in which you will drive 100 kilometers (62 miles) at 100 kilometers per hour (62 mph), you could expect you would arrive in exactly 60 minutes. But even if you maintained the speedometer as close to 100 as possible, it would not be accurate enough to indicate the true speed. If the actual speed averaged 101 kilometers per hour (63 mph), you would arrive about 36 seconds early. Perhaps that difference wouldn’t matter to you (and if it did, you might consider replacing your car with a spaceship), but such tiny errors, when compounded by Dawn’s repeated spirals around Vesta, will make a difference in achieving its carefully chosen orbit.
Still other phenomena contribute to minor differences between the flight plan controllers send to the spacecraft and what actually occurs. Two of these, the slight force of sunlight on the probe and the larger perturbation from the occasional firing of the small jets to reduce the spin rate of the reaction wheels, were explained in some detail in a previous log, and both will play a role at Vesta.
The mission control team has devised strategies to accommodate all these tiny contributors (and others) to deviations from the plan. An additional component of preparing for the intricacies of Vesta operations is establishing how accurately Dawn can perform the team’s masterful choreography. It has repeatedly proved that it can execute the slowly changing profile of interplanetary cruise. For the more challenging case of orbiting its protoplanetary destinations, engineers have developed mathematical models and conducted studies with the spacecraft simulator at JPL, but to verify that the results are valid, a test this month of the ship’s ability to steer through some maneuvers was deemed worthwhile. Just as with the activities it practiced in January, the robotic explorer performed very well indeed on its latest demonstration.
Although it is not in orbit around Vesta now, operators commanded Dawn to aim its thruster as it will near the end of the transfer from HAMO to LAMO. For the equivalent of one and a half spiral revolutions (the duration being adequate to assess all the pertinent aspects of the maneuvering), the spacecraft rotated using its thruster, changing the direction of its ion beam in much the same way it will when it is lowering its orbit. Because Dawn will be farther from the sun at that time in the mission than it is now, this trial run used a lower ion throttle level (and hence lower thrust), reflecting the reduced solar power that will be available.
The results confirmed that the spacecraft’s operation matches the mathematical predictions and that the ongoing preparations for these elaborate flight profiles are sound. Although more work remains, the success of this test is a valuable step in becoming ready for reaching the intended orbits around Vesta.
When Dawn has completed its work in LAMO, it will reverse its spirals and begin climbing away from the world it has been studying. It has an appointment with dwarf planet Ceres, so it cannot linger at Vesta indefinitely. Nevertheless, the itinerary allows for the traveler to stop for three weeks at an altitude of about 660 kilometers (410 miles). At the same height as HAMO, this orbit is innovatively named HAMO2. Although there are some differences in the orbital geometry, the principal distinction between HAMO and HAMO2 is that they are separated by about eight months, during which Vesta (with Dawn in tow) will have progressed in its orbit around the sun. As we noted during Earth’s most recent northern hemisphere autumn, Vesta has seasons, and the changing angle of the sunlight on the surface of that alien world during Dawn’s residence there affects its appearance and how much of it is visible to some of the science instruments. Because more of the northern hemisphere will be illuminated, HAMO2 affords the opportunity to see previously hidden landscapes and to gain a new perspective on some terrain observed earlier.
From Dawn’s perspective now, its destination already glows bright. By the middle of March, Vesta will easily outshine all the objects in the adventurer’s sky save the sun. If you take a moment to enjoy the view of Jupiter low in your western evening sky, it will appear about as bright these days as Vesta would for Dawn. But even as Jupiter sinks toward the sun and becomes more difficult for terrestrial observers to see, the point of light in our remote probe’s sky grows ever more luminous as their separation shrinks.
For those on Earth who want their own perspective on the locations of Dawn and Vesta, the changing solar system alignments will help. This summer, just as our planet’s robotic emissary is getting settled in orbit and beginning its survey, Vesta will be easily visible with binoculars and may even be detected by keen, unaided eyes under dark skies, although its visitor from Earth will be quite imperceptible.
In a few weeks, when the moon will be visible at dawn and for much of the first half of the day (regardless of your time zone), you can use it as a guide to the approximate location of Dawn and Vesta. Between about 2:00 PM PDT on March 27 and 10:00 AM PDT on March 28 (the day before the 204th anniversary of Vesta’s discovery), both distant inhabitants of the asteroid belt will be less than 5 degrees from the moon. (For reference, 5 degrees is 10 times the diameter of the moon or about the width of three fingers held together at arm’s length.) You won’t need binoculars or a telescope to see them; you need only your imagination to reveal a distant ship, far from the port from which it set sail more than three years ago. In the silent depths of space, with a faint blue-green trail of xenon ions behind it, the craft will be closing steadily on a mysterious, ancient world that soon will reveal exciting and fascinating new vistas as it bears witness to the very dawn of the solar system.
Dawn is 4.2 million kilometers (2.6 million miles) from Vesta, or 11 times the average distance between Earth and the moon. It is also 2.56 AU (383 million kilometers or 238 million miles) from Earth, or 970 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
9:00 pm PST February 27, 2011
Dawn continues its flight through the asteroid belt, steadily heading toward its July rendezvous with Vesta, where it will take up residence for a year.
Dawn continues its flight through the asteroid belt, steadily heading toward its July rendezvous with Vesta, where it will take up residence for a year. The spacecraft has devoted most of the time since the previous log to its familiar routine of thrusting patiently with its ion propulsion. But both the distant craft and the operations team each took a little time away from this month, spending it instead in the future. Although the project has not yet perfected time travel, it has achieved enough capability to conduct two successful ventures into the near future.
On Jan. 10, Dawn performed some of the activities that it will execute in its low altitude mapping orbit (LAMO) at Vesta. From the probe’s vantage point in LAMO, much of the sky will be filled by the ancient protoplanet, only about 180 kilometers (110 miles) away. Vesta will appear as large as a soccer ball from a mere 7.5 centimeters (3 inches). Completing one revolution every four hours in LAMO, the spacecraft will devote most of its time to training its scientific instruments on the rocky world beneath it, teasing out the secrets Vesta holds about the dawn of the solar system. Other times, the robotic explorer will loop around the body while aiming its main antenna at distant Earth, transmitting its findings and receiving new instructions.
As we have seen in previous logs, to control its orientation in the zero-gravity of spaceflight, Dawn usually relies on one or both of its star trackers. Despite the utterly perplexing etymology, these devices track stars in order to help the attitude control system establish the spacecraft’s orientation (or “attitude”). The tracker’s camera takes pictures of stars five times per second. Its internal computer recognizes patterns of stars, much as you might recognize some of the beautiful constellations visible from your planet and use them to orient yourself at night.
The spacecraft’s two star trackers are mounted so each will have a different but clear view of the star-studded sky when the scientific instruments are pointed at Vesta. But during some turns and while the main antenna is pointed to Earth, there will be times when the nearby world will obstruct their views. You too might have difficulty seeing stars if a soccer ball were positioned immediately in front of your face.
For those times when both star trackers end up pointing at Vesta, and thus are unable to provide the attitude control system with the orientation, the probe relies on gyroscopes. Using these spinning masses, attitude control can sense turns and keep track of how the attitude changes even when the star trackers are unable to yield useful information.
>While Dawn is in the great void of interplanetary space, there is nothing to interfere with the star trackers. (They are aligned so that neither one points near the sun in any of the spacecraft’s normal orientations.) When the ship sailed by Mars for a gravitational boost on its way to the asteroid belt, it used the gyros when the planet blocked the view of the stars. That worked well enough for that brief event, but engineers wanted to confirm that their strategy of swapping from star trackers to gyros would be effective during the much longer events that will occur in orbit around Vesta.
To verify their plans, the spacecraft was configured to operate as it will in LAMO. It pointed its instruments as if Vesta were nearby and rotated to keep them aimed at the surface, just as it will when it circles the colossal body. (The instruments remained unpowered, because this was a test of the attitude control system and, of course, there was nothing for them to observe.) After a time following that pointing profile, Dawn turned to direct its antenna to Earth and held the planet in its radio sights. Following those activities, it conducted a nearly identical pattern. The second time, however, the onboard sequence of commands included a crucial difference. Engineers had incorporated extra instructions to prevent data from the star tracker from being used by the attitude control system when it stopped pointing at the surface of Vesta and started its next turn. This trick in software perfectly mimicked the effect of about 530 kilometers (330 miles) of giant protoplanet blocking starlight. Dawn continued through the sequence without missing a beat. It pirouetted toward Earth and held its orientation there with its gyros, moving with grace and accuracy just as a blindfolded dancer might display on a stage, relying on her vestibular system to accomplish her masterful performance. After two and a half hours, the sequence restored star tracker data.
For the entirety of the test, the spacecraft recorded detailed data on its attitude and on other parameters so engineers subsequently could assess how their plans worked. The analysis showed that the robot’s performance was even better than anticipated.
After the successful test, the spacecraft reconfigured for normal interplanetary thrusting and set course again for the real Vesta. Meanwhile, mission controllers were preparing for their own simulation of life in the vicinity of Vesta.
Dawn’s exploration of that rocky protoplanet will require much more than a sophisticated probe carrying out its assignments in the forbidding depths of the asteroid belt. The operations team will need to keep it healthy and furnish it with up-to-date plans. In many cases, team members will need to analyze or process data quickly and deliver their results to the next person in line, and each step has to proceed on schedule to keep the mission advancing smoothly and productively. In an undertaking as complex as orbiting a remote, massive, and previously unexplored world, surprises are sure to occur, and some of them likely will be unwelcome. As part of formulating intricate plans for the year Dawn will spend in orbit, the team has developed strategies to account for the unexpected. Some of these plans were exercised this month in an “operational readiness test,” or ORT, conducted not in the harsh, alien setting of the asteroid belt but rather in the mysterious, unique environment of JPL.
The Dawn project performed ORTs in 2007 (as described here, there, and elsewhere) as launch grew near and twice last year (one of which was described in July) to prepare for Vesta. Some ORTs focus on the team’s ability to conduct the mission as planned, and in others, including the one this month, the team faces problems. Organized and overseen by test conductors (also known as simulation supervisors, sim sups, or more inspired names when they dream up more creative challenges for the operations team), the ORTs are nearly as elaborate as real operations, both in their planning and their execution, and they are very valuable experiences for the participants.
In this ORT, the Dawn team spent a week in the summer, when the spacecraft will be in the approach phase, only a few weeks from its first science orbit, known as “survey orbit.” Over the course of the first few days of the exercise, they received more and more bad news. A record-breaking outpouring of radiation from the sun had damaged some of the memory components in Dawn’s central computer and degraded its solar arrays. Dawn’s unique mission to orbit two solar system targets is enabled by its ion propulsion system, and the near-constant thrusting depends on the uniquely powerful solar arrays. Just as the boost in predicted power in 2009 provided an increase in the time Dawn could spend at Vesta, a decrease now would translate to a shorter residence there before having to depart for Ceres. In addition, the simulated damage to the computer meant that many of the images already acquired of Vesta for navigation could not be recovered and transmitted, and the plans for subsequent storage of engineering and science data would have to be curtailed.
It also turned out that analysis of the earlier images in this rehearsal revealed that the tilt of Vesta’s rotational axis was different from what astronomers had calculated from telescopic observations. Vesta, just as all large bodies in the solar system, rotates in a regular fashion around an axis. As we have seen, Dawn will take up a polar orbit around Vesta, the perfect choice for observing all of the illuminated surface. To do so, the orientation of the pole in space needs to be known. In other words, navigators need to know exactly where Vesta’s axis points. To understand this, consider the globe of Earth. It is obvious it spins on its axis, but that axis just happens to point near the familiar star Polaris; it equally well could point elsewhere in space (as it has in the past and will again). Measurements from observatories, including the Hubble Space Telescope in 2007 and 2010, have been used to estimate the direction of Vesta’s axis, and Dawn will improve upon those during the approach phase so the probe can be targeted to an orbit that takes it over the poles. In the manufactured future of the ORT, it was revealed that the actual orientation of the axis was farther from the prediction than many scientists had expected it might be.
That was not enough for the ever-thoughtful, endlessly creative sim sups. They also informed the team that the faux solar radiation inflicted even greater damage on other interplanetary spacecraft, so Dawn’s scheduled use of the Deep Space Network would have to be cut back to allow those other missions to engage in recovery operations. To add to the difficulties, we were astonished to be informed that one member of the team had participated in some quite interesting activities that landed him in prison. Another, after involvement in an incident that merited headlines in some famous tabloids (although, curiously, not the intergalactically read Dawn Journal), also was unavailable to solve technical problems. Other engineers on the team had to fill in to make sure all the work was accomplished.
Still more misfortunes beset the beleaguered operators (who also managed some good laughs over the details provided by the test conductors), but they worked through all the problems, using a combination of the plans they had already developed and creative solutions devised during the course of the ORT. The challenges of these simulated operations in the future were compounded by their being faced when the team also was continuing with real operations in the present.
The actual surprises at Vesta surely will be different from those in the rehearsal (still, your correspondent is going to keep his eyes on those two adventurous team members), but the ORT gave the team an excellent sense of operations in difficult conditions. Although this was the last of the ORTs, more preparatory work remains before Dawn reaches its first intriguing destination.
As the probe continues its journey, less and less of what departed Earth atop a powerful Delta rocket more than three years ago is still onboard. With its famously frugal use of xenon propellant, it was only this month that the fuel gauge dipped to half. Dawn’s tank carried 425 kilograms (937 pounds) at launch. It took more than 2.2 years of powered flight to exhaust half of that supply, during which the ion propulsion system imparted the equivalent of 5.7 kilometers per second (nearly 13,000 mph), far more than any spacecraft has been able to change its own velocity.
And yet the adventurer continues to propel itself, gradually maneuvering so its orbit around the sun will match that of its target, an uncharted world that beckons. Paying no attention to the blue-green wake of xenon ions behind it, the explorer’s sights remain set ahead, on a destination growing ever closer, on the opportunity to uncloak the mysteries of Vesta, on the new knowledge that it will gain, and on the new questions that it will raise. It seeks not to satisfy but rather to sustain the powerful drive for exploration that compelled curious creatures, humble yet bold, confined to the vicinity of almost incomprehensibly distant Earth, to reach within themselves that they might then reach out into the cosmos.
Dawn is 6.1 million kilometers (3.8 million miles) from Vesta, or 16 times the average distance between Earth and the moon. It is also 2.78 AU (416 million kilometers or 259 million miles) from Earth, or 1,065 times as far as the moon and 2.83 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 46 minutes to make the round trip.
Dawn finishes 2010 much as it began the year, thrusting with its ion propulsion system in steady pursuit of a distant world. During the next year, the probe will arrive there and begin its scrutiny. In the meantime, it continues thrusting patiently, but now with a difference.
Dawn is outfitted with three ion thrusters, although it is designed to use only one at a time. (The locations and whimsical names of the thrusters were divulged once the spacecraft was too far from Earth for anyone to see it.) Thruster #3 was the first to see action in the mission, and it propelled the spacecraft until June 16, 2008, after which thruster #1 took over. On Jan. 4, 2010, the ship switched to thruster #2. Prior to this year, #2 had accumulated little more than a day of operation for some tests. But in 2010, it operated flawlessly for 304 days, and Dawn accomplished nearly all of its thrusting this year with only that thruster. While #2 is ready for much more, on Dec. 6 mission controllers assigned thrusting to #3 as part of the strategy of balancing the work for this long journey among the three units.
During its extensive service this year, thruster #2 expended less than 79 kilograms (174 pounds) of xenon propellant. With its uniquely efficient but exquisitely gentle touch, the thruster accelerated the ship by more than 2.2 kilometers per second (4,900 mph). Some of our more dawnscriminating readers will recall that the actual velocity is complicated by the effects of the spacecraft's orbital motion around the sun. This has been discussed in several, if not 602.2 sextillion, logs (this one being a good example), Nevertheless, the change in speed is an effective and common measure of the effect of a spacecraft's maneuvering.
Thruster #3 had remained idle for 2.5 years, waiting with the same patience it displays when thrusting. Earlier this month, the long hiatus ended; it came to life again and operated as smoothly as ever. It is once more powering the ship ahead, impelling the probe toward its July rendezvous with the mysterious and alien world Vesta.
In addition to ensuring Dawn's journey to Vesta continues to go so well, the operations team has been making good progress formulating the detailed instructions the robotic explorer will follow when it is there. Earlier this year, we examined what the ship will do as it approaches the distant port and how it will sail into orbit under ion thrust as well as what it will do in the first two principal science phases, survey orbit and the high altitude mapping orbit (HAMO). Engineers are working now on the timeline of commands, known as sequences, for the third major science phase. To learn more about that, all you have to do is wait a year and read about the activities the spacecraft is conducting. For the more impatient readers, we present an overview here.
From HAMO, at an altitude of about 660 kilometers (410 miles), Dawn will have an excellent view of Vesta, close enough to see plenty of fascinating details and yet far enough to allow its science camera to cover most of the surface of this uncharted world during the month of mapping. In addition to using the camera to develop the global maps, the visible and infrared mapping spectrometer (VIR) will be trained on many regions, providing even better resolution on the minerals that compose the surface than it could achieve from the higher survey orbit. When its work in HAMO is complete, the craft will fly in for an even closer look.
We saw in April that just as the interplanetary traveler has spiraled around the sun from Earth on its way to Vesta (and will do so en route from Vesta to Ceres), it will spiral around Vesta as its ion propulsion system takes it from one orbital altitude to another. Although operation of the ion propulsion system itself is independent of whether it is in orbit around the sun or around Vesta, there is much more to thrusting than that one system. There will be several important differences in how the mission control team plans the flight profile and operates the craft when transiting from one orbit to another at Vesta, and we will consider those in a future log. We also will see that as-yet uncharacterized physical properties of Vesta itself will affect the nature of the trip. The operations team has been working hard to prepare for the many possibilities that might be found at this unexplored world.
Dawn's target after HAMO will be an altitude of around 180 kilometers (110 miles), closer to the surface beneath it than most satellites are that orbit Earth. It may take six to eight weeks to follow the winding path from HAMO to this low altitude mapping orbit (LAMO) under the delicate push of the ion thrust. While that may seem like a long time, a mission to Vesta that relied on conventional chemical propulsion would be quite unaffordable within NASA's Discovery Program, and a mission to both Vesta and Ceres would essentially be impossible. In a real sense then, the time to travel from one orbit to another is about as fast as possible given humankind's present selection of tools for probing the cosmos.
Dawn will spend about two months in LAMO, revolving around the rocky body once every four hours. The science camera will acquire many exciting pictures from this new vantage point, and while they certainly will afford a better view of some regions than could be obtained in HAMO, that is not the primary objective of LAMO. Images and observations with VIR will be a valuable bonus, but the primary science data will be in two other areas.
The gamma ray and neutron detector (GRaND) will have been operating continuously since the approach phase, but it will be in LAMO that it is most effective. (Despite its name, even in LAMO when GRaND is the star of the show, it retains its endearing humility.) The instrument is designed to detect the byproducts of cosmic rays hitting Vesta. Cosmic rays are energetic radiation, composed of a variety of particles, that pervades space. As Vesta's surface is exposed directly to space, cosmic rays strike the nuclei of atoms in the uppermost meter (yard). (The radiation would reach Earth's surface as well if not for our protective atmosphere and magnetic field.) Some of the gamma rays and neutrons produced by these impacts make their way back into space, carrying the signatures of the atoms. When GRaND is in LAMO, it will sense enough of the emitted particles to reveal the identities of many kinds of atoms in the surface. It also will record some radioactive decays of atoms there.
GRaND can detect some of the cosmic rays directly, and it has done so whenever it has been tested in flight, far from a planetary body (see, for example, here). It also observed gamma rays and neutrons from Mars during the spacecraft's brief visit there last year.
Unlike the relatively bright light reflected from Vesta's surface that the camera and VIR detect, the subatomic particles GRaND measures provide a very faint signal. Just as taking a picture of a dim object requires a longer exposure than for a bright one, to make GRaND's “picture” of Vesta will require a very long exposure. So, much of Dawn's time in LAMO will be devoted to pointing GRaND at Vesta and letting it measure the energy and other properties of the particles that come its way.
Dawn was designed with all instruments pointing in the same direction. Even when GRaND is the principal instrument, simultaneous bonus observations with the others (when not precluded by other constraints) will greatly enhance the data returned from LAMO.
In addition to providing GRaND's measurements of the elemental composition, LAMO is designed to enable another important method of characterizing Vesta. As Dawn travels in its orbits, its motion is dictated by the combined gravitational attraction (which depends on the mass and distance) of all of the matter within the giant protoplanet. By making ultrasensitive measurements of the probe's orbit (more accurate even than for the normal needs of its deep-space navigation), scientists can calculate the arrangement of Vesta's constituent masses. If, for example, there is a volume far below the surface filled with rock of greater density than the surrounding regions, even though it is hidden from the instruments, its stronger gravitational pull will reveal it. Dawn will accelerate just a little as its orbit brings it closer to this feature and decelerate just a little when it has passed by. These effects are miniscule and the measurements very challenging, but the view of the interior of Vesta, from crust to core, will be rewarding.
There is good reason to believe Vesta has a complex internal structure, as do the other large rocky residents of the inner solar system, one of which is immediately beneath your correspondent as he writes this and some of you as you read it. In addition to Earth and Vesta, Mercury, Venus, the moon, and Mars all are thought to have grown very hot as they were forming, and that caused the minerals within them to separate into layers of different composition. In this process, known to planetary geologists as differentiation, the denser materials tend to sink while the lighter materials rise to the top, and when the body cools, the layers are frozen in place. Other processes during the history of the planet may create pockets of higher or lower density rock as well.
Vesta may be the smallest relict from the solar system's formation to have experienced planetary differentiation, and the information scientists glean from studying the interior structure (in concert with all of Dawn's other measurements) will contribute to understanding the process by which planets formed. Even though it is Lilliputian compared to the planets, Vesta is Brobdingnagian compared to most asteroids. In the context of planetary formation mechanisms, its closer brethren are the rocky worlds named above.
To make a good map of the interior structure, measurements of Dawn's orbital motion need to occur above all parts of Vesta. In essence, scientists use the spacecraft to map variations in Vesta's gravitational field. Several different kinds of data are required to do this, but the principal type is the Doppler shift of a radio signal transmitted from one of the giant antennas of NASA's Deep Space Network to Dawn, which then sends a signal back to the same antenna. This technique was applied to calibrate the gentle thrust of Dawn's ion thrusters early in the mission, and it will be performed so exquisitely at Vesta that changes in the distant craft's speed of about 0.1 millimeters per second (1 foot per hour, or a five-thousandth of a mile per hour) will be evident. So with the ship sailing over the alien world, mapping the gravity field is accomplished not with an instrument pointed at the surface but rather with the main antenna aimed at distant Earth.
It is likely that the irregularities in the gravity field not only will permit insights into the interior of Vesta, but they also will perturb Dawn's path enough that the probe will have to maneuver to maintain the orbit within the parameters needed for operations. Therefore, the ion propulsion system will be used about once a week for a few hours to adjust the orbit. The specifics of these maneuvers will depend on the details of the gravity field, which won't be known until Dawn measures them. Nevertheless, engineers plan windows for the orbital corrections now and will work out those specifics when they measure the craft's orbit.
In order to obtain the data needed for the map of the atomic constituents and the map of the interior mass of Vesta, Dawn will spend more time in LAMO than any of the other parts of the orbital mission. In both of the preceding science phases, survey orbit and HAMO, as well as the fourth phase, which will be the subject of a future log, Dawn will devote most of its time over the illuminated side of Vesta to acquiring data with the science camera and VIR and most of its time over the unilluminated side to radioing that precious information to Earth. Neither the GRaND nor gravity measurements depend on the sun shining on Vesta, however, so that regular schedule will not be followed in LAMO. Completing one revolution every four hours, it would be inconvenient and inefficient anyway to switch back and forth so often between directing the antenna to Earth or the instruments to the surface. Rather, the timing is determined by strategies to ensure good global coverage for the two primary scientific investigations, the need to transmit data when memory is full, and other considerations.
Even though LAMO is the lowest altitude from which Dawn will observe Vesta, it is not the final phase of the Vesta mission. Nevertheless, by the time the spacecraft is ready to climb back above LAMO, it will have returned a wealth of breathtaking information that will allow scientists to begin transforming this unfamiliar world into one we know. Dawn's removal of the veil that shrouds Vesta in secrets will nourish everyone who hungers for the exhilaration of new knowledge and new understanding, the excitement of adventure and exploration, and the thrill of discovery.
Dawn is 0.060 AU (8.9 million kilometers or 5.5 million miles) from Vesta, its next destination. It is also 2.96 AU (442 million kilometers or 275 million miles) from Earth, or 1,175 times as far as the moon and 3.01 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.
Dr. Marc D. Rayman
9:30 p.m. PST December 30, 2010
Dawn is maintaining its smooth and steady course through the solar system as it gradually closes in on Vesta. With the utmost patience and persistence, it continues thrusting with its ion propulsion system, heading toward its July rendezvous with the second most massive member of the main asteroid belt. Even as the spacecraft climbs farther from the sun, Earth's orbit is beginning to bring the planet closer to the probe.
Having thrust for two-thirds of its time in space, Dawn has now achieved the velocity equivalent of about 5.5 kilometers per second (more than 12,000 miles per hour). We have seen before that this does not represent the actual change in speed, but it is still a very useful measure of the effect of the thrusting. Although it has long since surpassed the record for propulsive change in velocity, Dawn is only now at the halfway point in the planned profile of thrusting for its ambitious eight-year expedition. By the time it completes its mission at dwarf planet Ceres (the asteroid belt's most massive resident) in 2015, it will have accomplished twice the effective velocity change it has achieved so far.
At the beginning of this month, mission controllers installed new parameters in the software used to control the spacecraft’s orientation (which engineers refer to as “attitude”) in the zero-gravity conditions of spaceflight. The attitude control system has four methods of keeping Dawn stable or turning to point in a new direction. When the spacecraft is not thrusting with the ion propulsion system, it has two techniques: it can rely on its reaction wheels or on its reaction control system. The wheels are gyroscope-like devices which, when electrically spun faster or slower, rotate (or stop the rotation of) the spacecraft. As we saw early last year, the reaction wheels alone are not sufficient, so when they are in operation, the reaction control jets also are used, although only occasionally. (The jets also are known as “thrusters,” nomenclature used in previous logs, but to avoid confusion with the ion thrusters in this discussion, we will refer to them only as “jets.”) The conventional rocket propellant hydrazine is fired through the jets to impart a small force to the ship, causing it to turn or to stop turning.
Not content simply to coast through the solar system, as most interplanetary probes do, Dawn devotes the majority of the time to using one of its ion thrusters to change its course, constantly applying a light pressure to its orbit to bring it closer and closer to that of its destination. In addition to accelerating the ship, an ion thruster can be used to rotate it by slightly changing the angle of the thrust. This provides the attitude control system two other means of control when the spacecraft is in powered flight: it can use either the thruster plus the wheels (again with the occasional help of the reaction control system) or the thruster plus the more frequent use of the jets.
Over the summer, engineers powered the reaction wheels off, preserving them for use in orbit around Vesta and Ceres. The reaction control system took over quite smoothly and has been keeping the craft stable ever since, most of the time in concert with the ion propulsion system.
Even before deciding to deactivate the wheels for the rest of the interplanetary phases of the mission, engineers began working on a technique to use the hydrazine more efficiently, ensuring that the supply would last to the end of the long journey. When it left Earth more than three years ago, Dawn carried 45.6 kilograms (101 pounds) of hydrazine. It still has more than 38 kilograms (nearly 84 pounds) onboard, and frugal operators want to continue to use the precious resource sparingly. They devised a means to reduce the rate of propellant consumption when the reaction control system is the primary control method. (The expenditure of hydrazine during ion thrusting was already so low that there was no need for an improvement in that control mode.)
The new control parameters were finalized after extensive analysis and simulation at Orbital Sciences Corporation and JPL. Without needing to change the software, operators radioed the values to Dawn in October and timed them to go into effect on Nov. 1 during the normal weekly hiatus in thrusting (and thus when attitude control is reliant exclusively on the jets) to point the main antenna to Earth. Thanks to this successful upgrade, the system now uses only about one-eighth as much hydrazine during those periods that it is holding steady and not applying ion thrust.
To rotate from one attitude to another requires firing some jets to start the huge ship (the largest NASA has ever sent on an interplanetary voyage) turning and then others to stop it. To achieve a further savings in the hydrazine, controllers reduced the rate at which Dawn executes its turns. The standard speed had been a whiplash-inducing 0.1 degrees per second; that’s the same pace at which the minute hand of a clock moves (except for some of the clocks sold in Dawn gift shops, and we’re still processing your refund requests on those). On Nov. 1, the speed was lowered to half that, meaning less propellant is needed to initiate a rotation and less is needed to terminate it. The only turns in a typical week are those required to shift between the attitude required for ion thrusting and the attitude required for pointing the main antenna to Earth, and a little extra time spent turning is easily affordable.
On Nov. 8, all four reaction wheels were powered on for a short time. For wheels 1, 2, and 3, this served as routine maintenance, keeping them in top condition so they will be ready to return to duty as the Vesta phase of the mission begins next summer. Running wheel 4 provided additional data on its condition so that engineers could assess its long-term prospects.
Dawn is 0.083 AU (12 million kilometers or 7.7 million miles) from Vesta, its next destination. It is also 3.05 AU (456 million kilometers or 283 million miles) from Earth, or 1235 times as far as the moon and 3.09 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 p.m. PST November 30, 2010
Continuing its journey to collect treats in the main asteroid belt, Dawn is making excellent progress toward its July 2011 rendezvous with protoplanet Vesta. The gentle thrust of its ion propulsion system is gradually changing its trajectory around the sun, smoothly helping the spacecraft move onto a path that will perfectly match Vesta's own motion. This pas de deux is just one part of the grand solar system dance, and the choreography is about to provide a particularly pleasing tableau.
While Dawn and Vesta move ever closer into synchronized orbits in the depths of space, nearer the sun and hence in a more forceful gravitational grip, Earth is traveling more quickly in its own orbit. We have seen before that tighter orbits require greater velocity in order to counterbalance the stronger pull of gravity. Mercury and Venus orbit the sun faster than Earth. Mars moves around the sun more slowly than Earth, and all residents of the more distant asteroid belt (including Dawn) revolve at an even more leisurely pace. Last month, Earth completed its third lap around the sun since Dawn was launched, but the craft is still only about two-thirds of the way through its second loop.
In January of this year, our planet and its cosmic ambassador were on the same side of the sun. Since then, as Dawn has continued its climb up the solar system hill, the relative positions have shifted, and now they are on opposite sides. So from the vantage point of our readers who happen to be on or close to Earth in November, Dawn will appear to be near the sun. Because warranties are expiring on all those clocks sold in Dawn gift shops located throughout the universe, let's take a look at some of the new models that are in stock to help visualize the upcoming arrangement.
With the sun at the center of the clock face, Earth is on the end of the hour hand and Dawn is situated at the tip of the minute hand, which is a bit more than twice as long. The hands on this clock will not reveal how fast the orbits progress (remember, unlike the way clock hands move, Earth travels faster than Vesta), but we offer an attractive discount on the price of these souvenirs, so they are still a good choice for picturing the relative positions. Nine months ago, the star, planet, and spacecraft were aligned as they would be at 6:32:16 and Dawn was considerably closer to Earth (and nearer to the sun). Although their orbits advance in the same direction (counterclockwise, as viewed from the north), because of their differing speeds, now they are reaching their greatest separation.
On Nov. 10, the alignment will correspond to 6:00:42. Because Dawn and Earth orbit in different planes, they will not achieve the perfect collinearity of the hands pointing in exactly opposite directions at 6:00:00. Dawn's route through space takes it north of the sun from Earth's perspective. The distant spacecraft will be located less than 4 degrees north of our brilliant star from Nov. 8 to 12. (For scale, the sun itself is half a degree in diameter.) During those days, when you go outside to gaze just above the sun and contemplate the grandeur of a craft from Earth traveling out there, far on other side of that star, if you block the blinding light with two fingers, they can cover not only the sun, but Dawn as well.
The last time such a close alignment occurred for terrestrial observers was December 2008. On that occasion, Dawn moved even closer to the sun, corresponding to 6:00:10. (Its orbit had not yet undergone the dramatic changes from the subsequent gravitational bending by Mars plus another 465 days of ion thrusting.) Radio signals traveling between Earth and the spacecraft had to traverse the fiery plasma surrounding the star, creating powerful interference, but the radio silence presented opportunities for reflections upon the nature of such interplanetary voyages. In the arrangement this November, Dawn will skirt just far enough from the sun that communications should not be compromised.
The faint radio waves that provide the explorer's only bond with home will have a long way to travel. During this time, Dawn will be about 3.06 astronomical units (AU) from Earth. Because the orbits are not quite as simple as the motions of hands on a clock, the instant of greatest separation does not occur until Nov. 13 (at 6:10 p.m. PST, for those who want to be with friends and family at the appropriate moment). Then as Earth continues its more rapid progression around the sun, the range will begin declining and will continue to do so until next summer. It will not be until January 2012 -- while Dawn is hard at work in orbit around Vesta, taking pictures and acquiring other precious scientific data -- that it will be this far again. By then, it will have climbed still farther from the solar system's center, so it will attain even greater distances from Earth that year.
Even this time, however, the distance is staggering. Dawn will be nearly 458 million kilometers (284 million miles) from its erstwhile home. (Quite by coincidence, given that the probe's orbit has changed significantly, the last time it reached a temporary maximum in the range to Earth was almost exactly two years ago. At that time, the distance was 384 million kilometers, or 239 million miles.) Well over one million times farther from Earth than the International Space Station, farther than Mercury, Venus, or Mars can ever be, farther than all but a handful of other trailblazing craft have ever reached, the ship sails ahead patiently and calmly on the cosmic seas, propelled by a zephyr of xenon.Dutifully following the course set for it by operators on remote Earth, Dawn ventures toward the horizon and keeps going. When it was
last descried by a telescope, the probe had been in space only three days and was more than 470 times closer to the planet than it is now. While much too far now for our eyes and even our telescopes to detect, the greater power of our passionate imaginations still can bring the faraway explorer into sharp focus. On the far side of the sun, the indefatigable spaceship continues to strive toward the unexplored world Vesta, the first stop on an exceedingly ambitious undertaking assigned to it by beings confined to the vicinity of a distant planet. At the same time we are humbled by how limited our place and our role are in the cosmos, we are emboldened by our own astonishing successes in essaying to grasp the nature of the universe. On the far side of the sun, Dawn stands atop a pillar of blue-green xenon ions, a monument to the vigor of human ingenuity, determination, curiosity and adventure.
Dawn is 0.11 AU (16 million kilometers or 10 million miles) from Vesta, its next destination. It is also 3.05 AU (456 million kilometers or 284 million miles) from Earth, or 1240 times as far as the moon and 3.07 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.
Dr. Marc D. Rayman
6:00:42 p.m. PDT October 31, 2010
P.S. For Halloween this year, your correspondent found that he could not fit into his costume from a year ago, so instead he is disguised as someone who can refrain from wearing a costume. Oops.
Recently, one of our fans on the NASAJPL Facebook page asked a good question about the efficiency of solar arrays on the Dawn and Rosetta spacecraft.
"A question about Dr. Marc D. Rayman's comment in his Dawn journal, saying that 'its tremendous solar arrays [are] the most powerful ever used on an interplanetary mission.' Is that really true? According to JPL's Dawn website, the solar arrays have a total span of 19.7 meters. By comparison, each of Rosetta's two arrays is 14 meters in length (28m total). Are Dawn's arrays so much more efficient? Thanks."
Here's an answer from Dawn Chief Engineer Marc Rayman:
Yes, this is really true. Dawn's solar arrays, although smaller than Rosetta's, could indeed produce more power because they are more efficient. Fortunately, it is not a competition! Both missions seek fascinating new insights into the complex history and character of the solar system as they take all of us on adventures to exotic destinations. To overcome some of the daunting challenges of traveling moderately far from the sun, engineers on each mission have turned to powerful solar arrays.
Rosetta is a fabulous mission, promising exciting results from comet Churyumov-Gerasimenko. It is with the greatest enthusiasm that I look forward to the astonishing discoveries that await its rendezvous with this solar system relict.
The spacecraft carries the largest solar arrays ever flown on an interplanetary mission. The two 14-meter (47-foot) arrays project in opposite directions from the main spacecraft itself, creating a structure about 32 meters (105 feet) tip-to-tip, and the total area of solar cells is 53 square meters (573 square feet). Composed of silicon, these cells could have produced somewhat in excess of seven kilowatts when at Earth's distance from the sun. Of course, Rosetta did not need that much power, but as it travels into the depths of space, every watt will be precious. When the spacecraft is more than five times Earth's distance from the sun and the light from our star is much weaker, the giant arrays still will generate 400 watts, just enough to keep the probe operating. (Rosetta will arc out to that distance on its way to the comet, but it will be closer to the sun, and hence able to produce more power, when it arrives and conducts its investigations of this mysterious body).
Dawn's solar arrays, while the largest used on a NASA interplanetary mission, are smaller than Rosetta's. This bird's wingspan is about 20 meters (65 feet), and the solar arrays, each more than eight meters (27 feet) in length, have a total of about 32 square meters (341 square feet) to capture sunlight. The panels are populated with advanced cells composed of three different materials that work together to convert a larger percentage of the incident light into electrical power. The combination of indium gallium phosphide, indium gallium arsenide, and germanium makes these cells so much more efficient that despite the smaller collecting area, together they produce higher power under the same conditions. These arrays could have generated more than 10 kilowatts at Earth's distance from the sun. Dawn not only did not need such tremendous power, but like Rosetta, it was not even capable of using it all. But it too ventures far from home to remote locations where sunlight is less abundant.
Dawn's ambitious mission to orbit the two most massive residents of the asteroid belt, Vesta and Ceres, would be quite impossible without its use of ion propulsion. The key to ion propulsion's extraordinary capability is its conversion of electrical power into thrust, so Dawn carries such powerful arrays to ensure that even when exploring dwarf planet Ceres at three times Earth's distance from the sun, it can produce sufficient power to thrust and operate all other systems. I describe more about the importance of power to the mission in my Dawn Journal of July 27, 2008.
I appreciate your interest in Dawn, and I hope you will continue to join us as we travel to two of the last unexplored worlds in the inner solar system. In only 10 months, Dawn will become the first spacecraft ever to orbit a resident of the main asteroid belt as it begins its exploration of protoplanet Vesta, and to put it quite simply, this is going to be really cool!
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On the third anniversary of traveling through the solar system on its own since dispatching Dawn on a separate journey, Earth continues to orbit the sun in much the same way it has been. Meanwhile, the spacecraft is thrusting with its ion propulsion system, making steady progress in reshaping its orbit to rendezvous with Vesta in July 2011.
In its three years of interplanetary travels, the spacecraft has thrust for a total of about 715 days, or 65% of the time (and about 0.000000014% of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn's wont. All this thrusting has cost the craft only 189 kilograms (417 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.
The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 5.01 kilometers per second (11,200 miles per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft's propulsive work. Having accomplished only one-third of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)
Since launch, our readers who have remained on or near Earth have completed three revolutions around the sun, covering about 18.9 AU (2.82 billion kilometers or 1.75 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 15.1 AU (2.26 billion kilometers or 1.40 billion miles). As it climbs away from the sun to match its orbit to that of Vesta, it will continue to slow down to Vesta's speed. Since Dawn's launch, Vesta has traveled only 12.0 AU (1.80 billion kilometers or 1.12 billion miles).
Readers with eidetic memory have already noticed that much of the text in the three preceding paragraphs is taken nearly verbatim from the logs that commemorated Dawn's first and second anniversaries of being in space, with the principal changes being that the numbers are updated here and we have generously expunged more (but not all!) humor each time. For those who wish to cogitate about the extraordinary nature of this interplanetary journey, comparing the first half of this log with those others may be helpful. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we reuse some more of the text here.
Another way to investigate the progress of the mission is to chart how Dawn's orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores.
Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family follow their paths around the sun, they sometimes move closer and sometimes move farther from it.
In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth's orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body's orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and no spacecraft has had to venture as far out of that plane in order to achieve orbit around another body as Dawn will.)
Now we can see how Dawn has been doing by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts' websites when their readership extends to one more elliptical galaxy than ours does.)
The table below shows what the orbit would have been if the spacecraft had terminated thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on September 27, 2007, its orbit around the sun was exactly Earth's orbit. After launch, it was in its own solar orbit.
|Minimum distance from the Sun (AU)||Maximum distance from the Sun (AU)||Inclination|
|Dawn's orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn's orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn's orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn's orbit on Sept. 27, 2009||1.42||1.87||6.2°|
|Dawn's orbit on Sept. 27, 2010||1.89||2.13||6.8°|
Readers may disregard the table or gaze into it for insight or inspiration for as long as they like. The point of it, however, is to illustrate that Dawn has come a long way since the launch pad, and while considerably more work remains to climb to Vesta, the ascent ahead is not as daunting as what has already been accomplished. On its next anniversary, the probe will be in the same orbit around the sun that Vesta is in; Dawn will be orbiting that distant world, where much of the mission's scientific destiny lies.
During the intervening year, there is a great deal more to look forward to than further travels through interplanetary space. In recent logs, we have had an overview of the activities during the "approach phase," which begins in less than eight months, and how the ship will slip into orbit around this mysterious protoplanet in July, becoming the first spacecraft to circle a body in the main asteroid belt. We also summarized the plans for the first intensive science phase, known as "survey orbit," which will be conducted in August.
Dawn is a mapping mission. In survey orbit it will use the science camera and the visible and infrared mapping spectrometer (VIR) to map most of the surface. (The gamma-ray and neutron detector, GRaND, will record radiation, but it will not be until a much lower altitude that the full power of its measurements will be achieved.)
Dawn will not be able to observe all of the surface from survey orbit, because some of it will not be illuminated. The reason is simple: Vesta has seasons. This is no different from Earth and most of the other planets. Of course, the seasons don't refer to weather on this airless world but rather to the sun's apparent motion relative to Vesta's equator, a consequence of the tilt of Vesta's pole. Each of the seasons lasts about 11 months. When Dawn begins surveying Vesta, it will be summer in the southern hemisphere; the high northern latitudes will be in the darkness of winter and hence concealed from the camera and VIR. By the latter phases of our mission at the protoplanet, in 2012, the seasons will have progressed, the sun will illume more of the northern hemisphere, and the instruments will see previously invisible terrain. We will consider the effect of the seasons and the implications for observing Vesta in a future log. For now, we will focus on the science phase that follows survey orbit.
While survey orbit affords the robotic explorer a broad overview of the uncharted world, subsequent observations from a lower orbit will reveal more details. This second mapping orbit is known to insiders on the Dawn project, now including you, as the high altitude mapping orbit (HAMO). (Most team members have avoided the disquietude of trying to understand why that name is not applied to survey orbit, and readers are encouraged to do the same.)
With its survey from an altitude of about 2700 kilometers (1700 miles) complete, the ship will set sail again with the gentle touch of its ion propulsion system, gradually spiraling down for a month until it arrives at the HAMO altitude of about 660 kilometers (410 miles). As with survey orbit, the final selection of the parameters of the orbit will be possible only when Dawn is in the vicinity of Vesta and the massive asteroid's gravity field has been measured accurately.
Dawn will follow a polar orbit again, affording it the opportunity to pass over all latitudes, from the north pole to the equator to the south pole and then back, as Vesta spins on its axis. Unlike the nearly three days to complete one revolution in survey orbit, in HAMO the spacecraft will loop around the world beneath it twice a day. Between Dawn's orbital motion and Vesta's rotation, ten orbits will be required to present most of the illuminated surface to the camera's eye. Although they will take a brief break today to celebrate with some cake decorated with an image of Dawn's launch (and with real xenon in the frosting), mission controllers are now developing the detailed sequences of commands for the spacecraft in six groups, each corresponding to one of these 10-orbit/five-day cycles in HAMO.
As before, the probe will devote most its time over the day side of Vesta to acquiring a wealth of information with its sensors and most of the time over the night side beaming those precious data back to eager scientists on Earth. There is a whole new world to explore, and the instruments will be commanded to gather more data during passage over the lit side than the communications system can transmit to the Deep Space Network during the half of the orbit over the dark side. As a result, Dawn's computer memory will fill faster than it can be emptied, and the spacecraft will leave HAMO with some of its treasure trove still onboard. In the course of its flight to the next science orbit (suggestively and even appropriately named the low altitude mapping orbit) it will radio the stored data during the regular hiatuses in thrusting to point its main antenna to Earth.
In two of the HAMO cycles, Dawn will peer straight down at the exotic landscape below it, taking pictures with the camera and recording spectra with VIR. By performing these mapping observations twice, there will be a ready opportunity to see most of the visible surface even if some observations are not completed because of the occasional glitches that are inevitable in such a complex undertaking in an unknown environment. The process of designing, testing, verifying, and executing the intricate sequences for mapping Vesta is far too complicated for the mission control team to wait until any such minor problem occurs and then formulate plans to acquire the missed data.
During the other four HAMO cycles, instead of pointing its instruments at the nadir, the spacecraft will aim them off at an angle, providing a different view in each cycle, effectively acquiring stereo imagery. Scientists will combine the pictures taken in all the directions to create topographic maps, revealing the heights of mountains, the depths of craters, the slopes of plains, etc. This will be of great value in inferring the nature of the geological processes that shaped this protoplanet. In addition, it will yield exciting perspectives for everyone who wants to visualize this alien world.
As this journey of exploration enters its fourth year, we continue to be exceedingly grateful for the many readers who travel with us on this voyage. An adventure of humankind, Dawn raises our collective sights and our spirits as it strives on behalf of everyone who longs to see far past mere terrestrial horizons. While physically we remain confined to the vicinity of our planet, we do not allow that to limit our reach. Powered by our ambition, our imagination, our curiosity, and our determination, missions like Dawn take all of us along to real places that lie beyond what our imaginations could construct. Although readers throughout the cosmos have participated in this experience, it is ironic that some very nearby have not been able to. Thanks to the generous efforts of Pablo Gutiérrez-Marqués, the operations manager for Dawn's science camera at the Max-Planck-Institut für Sonnensystemforschung (Max Planck Institute for Solar System Research) in Katlenburg-Lindau, Germany, it is with great delight that we welcome our hispanophone friends onboard. Small as the spacecraft is, there is plenty of room for everyone who shares in the wonderment of the cosmos, the fulfillment of gaining new knowledge and new insights, the exhilaration of discovery, and the thrill of exploration.
Dawn is 0.15 AU (23 million kilometers or 14 million miles) from Vesta, its next destination. It is also 2.96 AU (443 million kilometers or 275 million miles) from Earth, or 1120 times as far as the moon and 2.95 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2010
Dawn's journey ever-deeper into the asteroid belt continues to go well, as the spacecraft carries out its familiar routine of thrusting gently with its ion propulsion system. But the interplanetary traveler has changed some of its habits, performing certain activities a little differently now from what its many followers have been accustomed to.
Dawn is now so far from the sun, that even with its tremendous solar arrays, the most powerful ever used on an interplanetary mission, it does not receive enough sunlight to generate sufficient electrical power to operate all systems and still achieve maximum thrust.
The largest consumer of power onboard the ship, the ion propulsion system is power hungry. Indeed, the key to its remarkable effectiveness is that, in concert with the solar arrays, it converts the renewable energy from the omnipresent sunlight into thrust with a high velocity beam of xenon ions, in contrast to conventional propulsion systems, which only work with the more limited energy stored within the chemical propellants. The importance of high power to thrusting has been discussed in detail in several previous logs, including in July 2008 and September 2009. In preparation for the spacecraft's being as far from the sun as it is now, the mission control team has conducted a great deal of work (including the solar array calibration that was explained in the first of those two logs and performed in November 2008). Now we can see how some of the detailed planning has been put into effect in the operation of the ship.
The electrical power generated by a solar cell depends on its temperature. When a cell is warmer, it is a little less efficient at transforming light into electricity, so it yields a little less power, even if the intensity of light impinging on it is unchanged.
When Dawn is not thrusting, the onboard power demand is much lower, so less power is drawn from the two huge wings of cells. That means that less of the light captured by the solar arrays is converted to electrical power. So where does the energy of that light go? When the solar panels do not need to generate as much electricity, the excess energy of the absorbed light simply turns to heat, warming the wings. Therefore, the arrays are warmer when Dawn is not thrusting; and when thrusting commences, the arrays need to cool down before they can achieve their best performance. Earlier in the mission, when Dawn was closer to the sun and could produce far more power than it needed (even when the arrays were warm), this small difference was entirely unimportant. Now it is very important.
Each week (usually on a Monday), the probe stops thrusting for around eight hours so that instead of pointing an ion thruster in the direction needed to climb out to Vesta, it can aim the main antenna to Earth in order to exchange radio signals with an antenna in NASA's Deep Space Network. At the end of the communication session, the spacecraft returns to the thrust orientation and resumes its powered flight.
For the last few months, the solar arrays have been able to meet the onboard demand when they were cool, but when they were warm, the available light was too weak to yield the power required by all systems. There were several solutions to this (some of which probably would have lent themselves to simpler and more jocose descriptions), and the one engineers chose was to initiate thrusting at a somewhat reduced throttle level, demanding less power than at full thrust. That drew enough power from the arrays to bring their temperatures down, allowing them to approach their highest efficiency. Then the sequence running in the main computer commanded the ion propulsion system to throttle up, and the arrays were able to provide the additional power. This strategy has been in use every week since April 19 and has worked flawlessly.
As Dawn moved farther from the sun, the power diminished still more. The team knew well in advance that by May, even when the array temperatures were low, there would not be enough power for all systems while at the maximum throttle level. So, beginning May 17, when the spacecraft completes its weekly communication session with its main antenna, it powers off its radio transmitter. Prior to that date, the transmitter had always been left on, even when Dawn was not in contact with Earth. The radio signal had been directed through an auxiliary antenna that broadcasts over a very wide angle in exchange for making the signal much weaker at the receiving antenna on Earth. (This is no different from the control on a flashlight. A narrowly focused beam can easily illuminate a small area, just as Dawn's main antenna allows it to transmit a strong signal toward Earth. For the same power, a wide cone of light from the flashlight provides fainter illumination over a far broader area, but it does not require precise pointing; similarly, Dawn can spread out a weak signal without choosing an orientation specifically designed for communications.) With the transmitter off, Dawn reduced its power needs, thereby ensuring it had enough to allocate to ion thrusting.
Each week (usually on a Thursday or Friday), the Deep Space Network listens in to Dawn's radio whisper while the spacecraft is thrusting. Faint though it is, the reception is adequate to confirm that the probe is generally healthy. Although the transmitter has been powered off for thrusting since May, the sequence turns it back on for two hours to permit this verification of the craft's status midway between the main communications sessions on Mondays. The onboard battery was available to help cover the increased power demand for that short time. (So if you were wondering why you haven't been hearing from the spacecraft at other times during the week recently, now you know. Now the probe travels in radio silence except when communication is scheduled.)
By July 26, that strategy was no longer sufficient to accommodate the ever-decreasing power. Since then the ion propulsion system has been throttled down during the mid-week health check. Reducing power for the ion thrust allows power to be devoted to the transmitter.
Based on extensive analyses performed in 2009, engineers had estimated that Dawn would no longer be able to sustain the highest throttle level by the last week of July 2010, even with the radio off. As it turned out, however, the craft exceeded their expectations and persisted through August 23. Finally, at a distance of 2.02 AU from the sun, it was time to reduce the power to the ion drive. From now on, Dawn will gradually decrease thrust as it travels still farther from the brilliant star. Even at lower throttle levels, however, the ion propulsion system's efficiency is far beyond what is achievable with chemical propulsion.
In February we took a detailed look at Dawn's daily change in velocity and recognized that it would continue to increase (thanks to the decrease in the total mass) until throttled operation would be necessary. As predicted then, the space traveler has now reached its peak acceleration of 7.6 meters per second per day (17 miles per hour for a day of thrusting). As this had been accounted for long ago in the design of the trajectory, and motivated some of the tests performed shortly after launch, the future gradual reductions in thrust have already been incorporated into the plan for keeping the ship on a steady course to Vesta and then to Ceres.
As the spacecraft continues its ambitious expedition through the asteroid belt, engineers have recently changed another aspect of its operation as well. On August 23, following instructions that had been stored in the main computer the previous week, Dawn powered off all four of its reaction wheels. (It's only coincidental that that is also both the date the ion drive was throttled down to save power and the date there was a power failure in the ice cream shop at the Tribute to Coincidence.) This was the first time since the day it was launched that all the wheels were off. For most of its mission so far, Dawn has used three of these units at a time to help hold its orientation or to turn to a different orientation in the zero-gravity, frictionless environment of spaceflight by electrically changing the speed at which they spin. Wheel no. 4 developed increased friction on June 17, so it was turned off, and wheels 1 - 3 have been in use since then. Mission controllers subsequently elected to turn all the wheels off in order to help preserve them for use at Vesta and Ceres.
With the wheels being given a rest, the reaction control system takes over their function. This system fires conventional rocket propellant (perhaps even the same hydrazine formulation you use in your rocket) through small thrusters aimed in different directions to provide the required control of the craft's orientation.
Since smoothly accomplishing the transition, Dawn has maintained its usual schedule of devoting 95% of the time to thrusting, gradually changing its orbit so that it will match Vesta's orbit in about 11 months.
The occasional brief pulses from the hydrazine thrusters are very effective at keeping the spacecraft stable, but they are too weak to contribute much to reshaping the trajectory around the sun. Even if the entire 45.6-kilogram (101-pound) supply of hydrazine Dawn carried into space had been devoted to changing the probe's velocity, the effect would be less than 0.1 kilometers per second (220 miles per hour), quite insignificant compared to the 4.81 kilometers per second (10,800 miles per hour) the xenon ion thrusting has already achieved (and even that is less than half of what is planned for the entire mission). The ion propulsion system is so much more efficient that it remains the only system capable of propelling the ship to its distant ports, Vesta and then Ceres.
Dawn continues to make excellent progress in its voyage to those ancient and unexplored worlds. In the next log, we will consider how far it has come and look ahead to more of what is store for it at the first of these enticing targets.
Dawn is 0.19 AU (29 million kilometers or 18 million miles) from Vesta, its next destination. It is also 2.82 AU (421 million kilometers or 262 million miles) from Earth, or 1070 times as far as the moon and 2.79 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 47 minutes to make the round trip.
Dr. Marc D. Rayman
9:30 p.m. PDT August 30, 2010
Dawn is flying smoothly through the asteroid belt, now less than a year from entering orbit around Vesta, the first of its two cosmic destinations.
Earlier in July, while the spacecraft was devoting its time to gentle thrusting with its ion propulsion system, members of the mission control team spent some of their time in August 2011. Impressively capable as NASA is, time travel is not within its powers. (If it were, your correspondent could travel back in time after this is posted to remove his controversial and inappropriate comments above, thereby preventing anyone from ever having seen them and avoiding the regrettable consequences of his poor judgment. Alas, that awaits a future capability.) Instead, the team simulated being in the future, when Dawn will be finishing its approach to survey orbit around Vesta, where it will begin its intensive scrutiny of the alien world. That will be a very busy period not only for the spacecraft but also for the human members of the team.
To account for details of the normal variations in the trajectory (as illustrated in a log last year) as well as the properties of the protoplanet that will be determined as Dawn closes in on it, controllers may adjust the sequences of commands that are being developed this year for execution at Vesta . For example, once the brightness of different regions of the surface is known, the instructions for the science instruments to perform their observations may need to be changed accordingly. The procedures to make these modifications reliably are complex, and the time available between receiving the pertinent data from the spacecraft and radioing the refined sequences back often will be only a few days. The team has formulated detailed schedules for all the necessary work, including the checks needed at every step. Engineers have established the criteria for making revisions, determined exactly what data must be presented and in what format for each meeting at which a decision will need to be made, and developed the computer programs to be used for verifying that no unintentional changes are made along with the intentional ones.
For these complex operations involving many participants, the team cannot wait until arrival at Vesta to verify that the plans are sound, so they rehearse major elements of it. Such operational readiness tests, or ORTs, also were conducted before launch. This time, with great creativity and care, some engineers had concocted data from the science instruments and navigational data, all representing results from the approach phase. The rest of the operations team treated the data as if they were real and went through all the steps to be followed when Dawn is nearly ready to begin surveying Vesta.
The ORT was successful, concluding with your correspondent providing the final approval to transmit the fine-tuned sequences to the spacecraft. The ORT allowed team members to identify opportunities for improvements in their software tools and procedures, such as parts of the schedule that allowed more time than needed for some steps and not enough time for others. While such details may seem prosaic, they are essential for the accomplishment of a grand and challenging endeavor. All the improvements will be incorporated into the final plans for how operations will be conducted at Vesta.
Even as the team was simulating activities in the future, Dawn remained committed to its present task of thrusting with its ion propulsion system. Last month, it exceeded the greatest propulsive change in speed by any spacecraft. On July 8, it passed another milestone when its ion thrusters had yielded 10,000 mph over the course of the mission. Such an achievement may seem somewhat less noteworthy when expressed in metric units, as the project does, but 4.47 kilometers per second is just as great a velocity!
Although most of Dawn's interplanetary travel is dedicated to thrusting, the design of the flight profile included coasting during most of the week of July 19 to accomplish some other work. Each of the science instruments was activated and tested, confirming that all remain healthy and ready to reveal Vesta's secrets to eager earthlings. A small software update was transmitted to both the primary and the backup science cameras, correcting a minor bug that would have added some complexity to the acquisition of images at Vesta. Subsequent tests showed the software “patch” works perfectly. The spacecraft also pointed the primary science camera to selected stars as part of its regular calibration, as it has done before. In addition, the camera imaged Ceres, the dwarf planet it will study at close range in 2015. At a distance of 3.3 AU (almost 500 million kilometers, or nearly 310 million miles), the giant of the asteroid belt appears only as a faint dot, no different from many background stars, but the data are helpful in monitoring the camera's performance during its years of spaceflight before reaching Ceres. (At the time of these observations, as each object followed its own orbit around the sun, Ceres happened to be closer to Earth, less than 2.0 AU, than to the spacecraft.)
Measurements were taken of the alignment between the primary science camera and the visible and infrared mapping spectrometer (VIR) to augment those made in April. All these will be of value in combining data from the two complementary instruments at Vesta and at Ceres.
The gamma ray and neutron detector (GRaND) sensed space radiation, as it has during previous tests, including the first time it was operated in space. Perhaps ironically, while the device measures some kinds of radiation, one of the gamma-ray sensors in the unit includes a crystal that deteriorates slowly under the constant exposure to other kinds of radiation that originate far outside the solar system. The changes can be reversed by carefully controlled heating of the crystal, and GRaND includes the electronics necessary to perform this function. While the instrument remains in excellent condition, scientists expect to implement this annealing prior to beginning measurements of Vesta in order to ensure the device is at peak performance. By conducting this test, engineers were able to verify that the crystal attains the correct temperature.
As all readers who have flight software 9.0 commemorative tattoos can readily attest, in June, engineers installed this new version of software on the primary spacecraft computer. As we have seen before, Dawn carries four copies of the software. In case the primary copy is corrupted by radiation or any other problem, the computer has a backup available. And if the primary computer suffers a problem from which it cannot recover, a backup computer, with its own primary and backup copies of the software, is ready to take over operation of the ship. The activity in June was to load the primary copy on the primary computer, and it has been running smoothly ever since. On July 22, controllers stored the two copies of the software on the backup computer. Dawn's computer systems do not allow copying programs from one location to another, so the software was transmitted from Earth twice, once to each of the required locations in computer memory. (The backup copy for the primary computer will be radioed to the spacecraft during a future opportunity.) Unlike the work last month, this did not require rebooting the computer (with the consequent entry into safe mode), because the software is not active.
As Dawn gradually closes in on Vesta, it continues to climb away from the sun. On July 26, it was exactly 2 AU from the master of the solar system, or twice Earth's average distance from the sun. In the northern hemisphere summer, Earth is at its farthest from the sun, so it will not be until August 24 that Dawn will be precisely twice as far from the sun as Earth is. Even then, as most days, while our home world continues following its repetitive loops around the sun, the spacecraft will persist in enlarging its orbit. The region of the solar system occupied by our planet, its inhabitants, and the companions it brings along under the grip of its gravity is now alien to Dawn. The sun that burns bright and large in our sky, although brilliant throughout the solar system, grows dimmer and smaller as the ship sails on. Dawn is a permanent denizen of the asteroid belt, the only spacecraft ever to take up residence there. Far from the sun, far from Earth, the probe carries with it the spirit of exploration and the quest for knowledge that thrive in the home it left behind as it looks ahead in its search for new insights into the dawn of the solar system.
Dawn is 0.26 AU (39 million kilometers or 24 million miles) from Vesta, its next destination. It is also 2.55 AU (381 million kilometers or 237 million miles) from Earth, or 940 times as far as the moon and 2.51 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
10:30 p.m. PDT July 26, 2010