Having fulfilled all of its assignments for 2008, the Dawn spacecraft has been unusually quiescent recently. While its operators on faraway Earth have no shortage of work, the probe patiently coasts in its orbit around the Sun, awaiting a brief encounter with Mars on February 17, which will steer it into a new orbit.
On October 31, Dawn completed nearly all the ion thrusting that had been planned for 2008. On November 20, mission controllers directed the spacecraft to execute a short maneuver to fine-tune its trajectory. Its only activity since then has been the routine maintenance of the gimbal system used to point ion thruster #1. On December 3, it moved the mechanism through a range of angles to help redistribute lubricant, following the same commands that were used 2 months earlier.
As viewed from Earth, Dawn passed through solar conjunction this month, appearing to be very close to the Sun. To visualize the geometry, suppose the Sun were at the center of a clock, with Earth at the end of the hour hand and the spacecraft at the tip of the minute hand. With the relative distances at the time of conjunction, the minute hand would be almost 1.6 times the length of the hour hand -- an elegant design indeed. (This analogy applies only for the separation as viewed from Earth under limited circumstances. As explained in an earlier log, while Dawn is indeed farther from the Sun than Earth is, the planet travels more quickly around its orbit than the spacecraft does. This would be more akin to a clock on which the hour hand is longer than the minute hand; such timepieces are back-ordered at Dawn souvenir shops.)
When Earth, the Sun, and the spacecraft are on a straight line, such as at 6:00, the Sun and spacecraft would appear to overlap from the perspective of an observer on Earth, near the bottom of the clock. As we noted last month, Dawn would not pass directly behind the Sun, because it does not orbit in the same plane as Earth. Therefore, the precisely linear arrangement of hands at exactly 6:00:00 never occurs. Pushing the clock analogy beyond its limits of usefulness, the minute hand would be bent toward the clock face, so it does not circle in quite the same plane as the hour hand. We shall ignore that enhancement for now but return to this point below. In the meantime, let’s consider the arrangements that have occurred.
On December 12, when the angle between the Sun and the spacecraft was at its minimum, it would be analogous to the alignment of the hands about 10 seconds from the hour, or the arrangement at 6:00:10. (Remember, this clock only has hour and minute hands; your correspondent types too slowly to be able to construct a useful analogy with a clock that includes a second hand.) When most modern interplanetary craft are within about 2 degrees of the Sun, normal communications may be less reliable. This limitation, which lasted about 2 weeks for Dawn, would correspond to half a minute on either side of 6:00, or between about 5:59:30 and 6:00:30.
Despite the powerful interference caused by radio signals passing through the distorting environment of the Sun on their way from the spacecraft to Earth, enough of the transmissions made it through for engineers to confirm that the spacecraft remained healthy throughout the conjunction period. Dawn was programmed to modify its radio transmissions to account for the angle between it and the Sun. Operators chose to accept a reduced return of information from the ship’s systems in exchange for boosting the quality of the signals used for navigation because of the upcoming flight by Mars. Some usable navigation data were obtained every day, but, as expected, most of the data, particularly during the 4 days when the spacecraft was nearest the Sun, were too degraded to be useful in refining the parameters of Dawn’s orbit.
Now, as Earth and the spacecraft have progressed in their separate travels around the Sun (making an angle today equivalent to about 6:01:45 on our Dawn clock), the radio waves traverse a less tortuous path, so the signal quality has improved. After collecting and analyzing more navigational data, engineers will determine what refinement is needed to the trajectory to guarantee Dawn encounters Mars in just the right way to provide the needed gravitational deflection. Following the same procedure applied to the design of Dawn’s first trajectory correction maneuver (TCM), the team will begin designing the second TCM early next month for the spacecraft to perform on January 15. In fact, the creative process has already begun; the maneuver has been given the imaginative appellation TCM2. Using those 4 characters (and perhaps a few others as well), the next log will report on the maneuver and provide some details on the nature of Dawn’s gravitational interaction with Mars and how it affects the trajectory.
The only reason for Dawn to travel to the vicinity of Mars is for the help to reach its targets in the asteroid belt. Nevertheless, as the probe races by, the team will take advantage of the opportunity to accomplish some bonus goals. Some of the plans will be covered in an upcoming log.
In the meantime, as the thrill of conjunction begins to fade, our vast staff has yet to sort through all the data on how many terrestrial readers used this convenient alignment to guide their mental eyes toward the spacecraft. The Dawn project sincerely hopes all observers reaped the maximum possible inspiration and joy from solar conjunction, as the mission will not offer another like it. Our destinations, Vesta and Ceres, do not orbit the Sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the plane of Earth’s orbit, and no spacecraft has ventured as far out of that plane to orbit another body as Dawn will.) While the probe is already in a slightly different plane from Earth’s orbit now, the gravity of Mars and subsequent ion thrusting will propel it to still a greater angle. As a result, when Dawn and Earth find themselves on opposite sides of the Sun in the future, the alignment will not be as close as it was this month. Dawn’s next apparent encounter with the Sun will be in November 2010, but it will appear to pass far enough north of the Sun that communications should not be significantly compromised. Following that, there will be 3 more times before the primary mission ends in 2015 that Earth and the spacecraft will be on opposite sides of the Sun, but in each case Dawn’s path through Earth’s skies will take it farther north or south of the brilliant landmark than in the 2008 conjunction. Nevertheless, each will be close enough that it may provide a visual reference once again to stir meditation upon the magnificence of a journey far away in the depths of space.
Dawn is 11 million kilometers (7 million miles) from Mars. It is 372 million kilometers (231 million miles) from Earth, or 930 times as far as the moon and 2.53 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.
Dr. Marc D. Rayman
6:01:45 pm PST December 30, 2008
The Dawn spacecraft is healthy and on course for its flyby of Mars early next year. The planet’s gravity will help boost the probe on its way to rendezvous with Vesta. While the spacecraft has its sights set on the asteroid belt (via Mars), its path is now bringing it closer to Earth. Meanwhile, from Earth’s perspective, Dawn appears to be approaching a blindingly close encounter with the Sun. With so much happening in the solar system, all readers, whether local or not, are invited to turn their attention here.
In the last log, we saw that Dawn was nearing the end of an extended period of thrusting with is ion propulsion system that began on December 17, 2007. When it left Earth on September 27, 2007, the Delta II rocket deposited the spacecraft into a carefully chosen orbit around the Sun. By October 31, 2008, the spacecraft had completed the thrusting it needed to change that orbit so it would encounter Mars at just the right time, location, and angle to sling it on its way to Vesta. During this interplanetary cruise phase, Dawn thrust for 270 days, or 85% of the time. Expending less than 72 kilograms (158 pounds) of xenon propellant, the spacecraft changed its speed by about 1.81 kilometers per second (4050 miles per hour).
Although controlling an interplanetary probe across hundreds of millions of kilometers (or miles) of deep space and guiding it accurately enough to reach its remote destination seems as if it should be a very simple task, readers may be surprised to know that it is not. Let’s consider just one aspect of the problem.
Suppose you want to shoot an arrow at a target. Unlike typical archers, you are so far from the target that you can only barely see it. In that case, aiming for the bull’s-eye is essentially out of the question. Adding to the problem may be a variable breeze that could nudge the arrow off course. Shooting sufficiently accurately to get the arrow even to the vicinity of the target would be challenging enough; hitting the precise point you want on the target is just too difficult.
For readers who are principally interested in archery, this concludes our in-depth analysis of the sport.
Now let’s consider how to change the situation to make it more similar to an interplanetary mission. If the arrow had a tiny radio locator mounted on it, you would be able monitor its progress as it flew closer to the target. This would be like watching it on a radar screen. You might see your arrow miss the target entirely or, if you had made a particularly good shot, hit somewhere on it. Now if you could occasionally send a signal to the arrow, perhaps to change the angles of the feathers, you might not be able to alter its course drastically, but you could change it a little. So if your initial shot had been good enough, you could guide the arrow to the desired destination. (To buy your radio controlled archery set, visit the Dawn gift shop on your planet. The set may be found between the display case with xenon ion beam jewelry and the shelves and shelves and shelves and shelves of really cool new Dawn Journal reader action figures -- be sure to buy the one that looks just like you!)
Shooting the arrow is akin to launching a spacecraft, and its flight to the target represents the interplanetary journey, although operating a spacecraft involves far greater precision (and fun!). Our knowledge of where the spacecraft is and where it is heading is amazingly, fantastically, incredibly accurate, but it is not perfect. This point is essential. Keeping most spacecraft on course is a matter of frequently recalculating the position, speed, and direction of travel and then occasionally fine-tuning the trajectory through burns of the propulsion system.
Dawn’s near-constant use of its advanced ion propulsion system for most of 2008 changes the story, but only a little. The thrust plan was calculated before launch and then updated once our arrow was free of the bow. Throughout the interplanetary cruise phase, a new thrust plan was transmitted to the spacecraft about every 5 weeks, each time with slight updates to account for the latest calculations of Dawn’s orbit around the Sun. With this method, the small adjustments to the trajectory have been incorporated into the large, preplanned changes.
The mission control team requires about 5 weeks to design, develop, check, double-check, transmit, and activate a 5-week set of commands. By the time the spacecraft is executing the final part of those instructions, it is following a flight plan that is based on information from 10 weeks earlier. During most of the mission, when there are months or even years of thrusting ahead of it, subsequent opportunities to adjust the trajectory are plentiful. In contrast, for the last period of preplanned thrusting before Mars, controllers modified their normal process for formulating the commands, making a fast update for the final few days of thrusting. By including the latest navigational data in the computations for the direction and duration of the concluding segment of powered flight, the mission control team put Dawn on a more accurate course for Mars than it otherwise would have been.
Even with this strategy, navigators recognized long ago that subsequent adjustments would be required. The plan for approaching Mars has always included windows for trajectory correction maneuvers (which engineers are physiologically incapable of calling anything other than TCMs). Dawn’s first TCM occurred on November 20.
As navigators refined their trajectory calculations after thrusting finished on October 31, they determined that the spacecraft was quite close to the aim point they wanted, but still not exactly on target. In fact, rather than being on a course to sail a few hundred kilometers above Mars, the probe’s path would have taken it to the surface of the planet. Despite the power of the ion propulsion system, Dawn does not have the capability to bore through the rocky planet and continue on its way to Vesta.
Such a situation is not surprising. Suppose in the archery, the bull’s-eye were 30 centimeters (1 foot) in diameter, but we preferred to hit a point 2.2 centimeters (7/8 inch) outside the bull’s-eye, near the 11:00 position (corresponding to where we want Dawn to fly past Mars). As our arrow approached the target, it might turn out that it was going to miss the target entirely, it might be headed for some other point on the target, and it just might be that it was headed for the bull’s-eye itself. Dawn’s case was this last one, so TCM1 put it on track for the destination we desired.
Amazing sports analogies for the fantastic accuracy of interplanetary navigation usually fail to account for TCMs, as most arrows, balls, and other projectiles do not include active control after they are on their way. Your correspondent has presented his own simile for the astonishing accuracy with which a spacecraft can reach a faraway destination, but most such analogies neglect TCMs, without which deep-space missions could not be accomplished. (Note that the accuracy is impressive with or without TCMs. We shall extend our archery example in a future log, making it more quantitative. It will be important, however, to keep in mind that the ion propulsion system provides so much maneuvering flexibility that Dawn does not need to achieve the degree of accuracy in its gravity assist that a mission using conventional chemical propulsion might.)
For reasons we will not divulge, Dawn’s first TCM has been designated TCM1. On November 20, just as it had for all of its previous thrusting, the spacecraft pointed a thruster (TCM1 used thruster #1) in the required direction and resumed emitting the familiar blue-green beam of xenon ions to alter course. While typical thrusting during the mission has lasted for almost 7 days at a time (followed by a hiatus of 7 to 8 hours), in this case only a short burn was necessary. Propelling itself from about 4:31 pm to 6:42 pm PST was just enough to fine-tune its course and change its speed by a bit more than 60 centimeters per second (1.3 miles per hour). This adjustment was modest indeed, as at that time Dawn was traveling around the Sun at more than 22.5 kilometers per second (50,400 miles per hour). Dawn and Mars, following their separate orbits that will (almost!) intersect on February 17, 2009, were moving relative to each other at 3.17 kilometers per second (7100 miles per hour).
Dawn’s second TCM window (inexplicably named TCM2) is in January. Traveling two-thirds of the way from here to Mars, the navigational accuracy then will be still better, with smaller deviations from the planned target point being detectable, so another refinement in the trajectory then is likely. In the meantime, Dawn will follow its orbital path with its ion thrusters idle.
As Dawn travels through space on its own, its path has been essentially independent of Earth’s. We saw in a previous log that the weaker grasp exerted by the Sun at Dawn’s greater distance means that it travels more slowly around the solar system. While Earth has completed more than 1 full revolution (each revolution requiring 1 year) since launch, Dawn has not yet rounded the Sun once. After receding from the Sun until early August, the spacecraft began falling back, albeit only temporarily.
The probe attained its maximum distance from Earth on November 10. (For anyone who was on Earth on that date and plans to use this information in an alibi, it may be helpful to know that the greatest range was reached at about 3:07 am PST.) The spacecraft was more than 384 million kilometers (239 million miles) from its one-time home. Although it will make substantial progress on its journey in the meantime, Dawn’s distance to Earth will continue to decrease until January 2010, when it will be less than one-third of what it is today. In the summer of that year, however, as Earth maintains its repetitive annual orbital motion and the explorer climbs away from the Sun, it will surpass this month’s distance to Earth. (Readers are encouraged to memorize the contents of this log for reference in 2010 in case we fail to include a link to this paragraph.)
The complex choreography of the solar system’s grand orbital dance rarely calls for a circular orbit; rather, the dancers follow ellipses (ovals in which the ends are of equal size) around the Sun. Thanks to the details of the shapes of their orbits, the greatest separation between Earth and Dawn did not occur when they were precisely on opposite sides of the Sun, although the alignment was close to that.
On December 12, their dance steps will take them to points almost exactly on opposite sides of the Sun. For observers on Earth, this is known as solar conjunction, because the spacecraft and the Sun will appear to be in the same location. (Similarly, from Dawn’s point of view, Earth and the Sun will be almost coincident.) In reality, of course, Dawn will be much farther away than Earth’s star. It will be 147 million kilometers (91.5 million miles) from Earth to the Sun but 379 million kilometers (236 million miles) from the planet to its cosmic envoy.
Its apparent proximity to the Sun presents a helpful opportunity for terrestrial readers to locate Dawn in the sky. On December 9 - 15, the spacecraft will be less than 1 degree from the Sun, progressing from east to west and passing just 1/3 degree south of that brilliant celestial landmark on December 12. (As Dawn does not orbit in the same plane as Earth, it will not pass directly behind the Sun.) The Sun itself is 1/2 degree across, so this is close indeed; the spacecraft will sneak in to less than 1 solar diameter from the disk. To demonstrate how small the separation is, if you blocked the Sun with your thumb at arm’s length during this week around conjunction (and you are exhorted to do so), you also would cover Dawn.
For those interested observers who lack the requisite superhuman visual acuity to discern the remote spacecraft amidst the dazzling light of the Sun, conjunction still may provide a convenient occasion to reflect upon this most recent of humankind’s missions far into the solar system. This small probe is the product of creatures fortunate enough to be able to combine their powerful curiosity about the workings of the cosmos with their impressive abilities to explore, investigate, and ultimately understand. While its builders remain in the vicinity of the planet upon which they evolved, their robotic ambassador now is passing on the far side of the extraordinarily distant Sun. This is the same Sun that has been the unchallenged master of our solar system for 4.5 billion years. This is the same Sun that has shone down on Earth throughout that time and has been the ultimate source of so much of the heat, light, and other energy upon which the planet’s inhabitants have been so dependent. This is the same Sun that has so influenced human expression in art, literature, and religion for uncounted millennia. This is the same Sun that has motivated scientific studies for centuries. This is the same Sun that acts as our signpost in the Milky Way galaxy. This is the same Sun that is more than 100 times the diameter of Earth and a third of a million times the planet’s mass. And humans have a spacecraft on the far side of it. We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty.
Solar conjunction means even more to Dawn mission controllers than the opportunity to meditate upon what magnificent feats our species can achieve. As Earth, the Sun, and the spacecraft come closer into alignment, radio signals that go back and forth must pass near the Sun. The solar environment is fierce indeed, and it causes interference in those radio waves. While some signals will get through, communications will be less reliable. Therefore, controllers plan to send no messages to the spacecraft from December 5 through December 18; all instructions needed during that time will be stored onboard beforehand. Deep Space Network antennas, pointing near the Sun, will listen through the roaring noise for the faint whisper of the spacecraft, but the team will consider any signals to be a bonus.
There is plenty of other work to do while waiting to resume communications after conjunction. In addition to preparing for the visit to Mars, engineers will continue to interpret the results of election day. On November 4, the Dawn team voted unanimously for more power. They commanded the spacecraft to execute a set of steps to yield data that will reveal the full potential of the enormous solar arrays to generate electrical power. The method was tested first on July 21, and then refined for a test on September 22. For this month’s measurement, the commands were identical to those used for the second test with one exception that had been planned from the beginning: the solar arrays were rotated to point 60 degrees away from the Sun instead of 45 degrees. The solar arrays are so powerful that when they are pointed directly at the Sun, the spacecraft could not draw enough power to measure their full capability.
The data collected show the electrical behavior of the arrays as the ion propulsion system was commanded through its start-up, drawing more and more power. Unlike the two tests, this calibration was designed so that with the arrays pointed so far from the Sun, they would not be able to provide as much power as was requested. Engineers wanted to find the point at which the arrays would no longer be able to satisfy the demands. They were not disappointed; power climbed up and up until no more was available. The prospect of having a spacecraft not be able to meet its own power demands may seem risky, but the procedure was carefully designed, analyzed, and simulated, and it executed perfectly. When the ion propulsion system asked for more power than the arrays could deliver, in the language of the trade, the solar arrays “collapsed.” Now to some (including even some engineers unfamiliar with the terminology), this suggests something not entirely desirable, such as 2 bent and twisted wings, each with 5 warped panels, and 11,480 shattered solar cells, the fragments sparkling in the sunlight as they tumbled and floated away from the powerless probe. In this case though, “collapse” is an electrical, not a mechanical, phenomenon and hence would be somewhat less visually spectacular and quite reversible -- a key attribute for a mission with well over 6 years of space exploration ahead of it. Once all the data are analyzed, controllers will have a better prediction for how much power the arrays will be able to generate for the rest of the voyage.
Dawn is 20 million kilometers (12 million miles) from Mars. It is 383 million kilometers (238 million miles) from Earth, or 950 times as far as the moon and 2.59 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 43 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 am PST November 26, 2008
Asteroids. The word conjures images of pitted rocks zooming through space, the cratered surfaces of planets and moons, and for some, memories of a primitive video game. Just how hazardous are these nearest neighbors of ours? We think that one contributed to the extinction of the dinosaurs, giving rise to the age of mammals. How likely is this to happen again?
The Wide-field Infrared Explorer (WISE) mission, an infrared telescope launching in about a year, will observe hundreds of near-Earth asteroids, offering unique insights into this question. The risk posed by hazardous asteroids is critically dependent on how many there are of different sizes. We know that there are more small asteroids than large ones, but how many more, and what are they made of?
Asteroids reflect sunlight (about half of which is the visible light that humans see), but the sun also warms them up, making them glow brightly in infrared light. The problem with observing asteroids in visible light alone is that it is difficult to distinguish between asteroids that are small and highly reflective, or large and dark. Both types of objects, when seen as distant points of light, can appear equally bright in visible light. However, by using infrared light to observe asteroids, we obtain a much more accurate measurement of their size. This is because the infrared light given off by most asteroids doesn’t depend strongly on reflectivity.
WISE will give us a much more accurate understanding of how many near-Earth asteroids there are of different sizes, allowing astronomers to better assess the hazard posed by asteroids. The danger posed by a near-Earth asteroid depends not only on its size, but also on its composition. An asteroid made of dense metals is more dangerous than one of the same size made mostly of less dense silicates. By combining infrared and visible measurements, we can determine how reflective the asteroids are, which gives us some indication of their composition.
Dear Presidawntial Candidawnts,
The Dawn spacecraft continues on course and on schedule for its bold campaign to unexplored worlds. The probe is thrusting gently with its ion propulsion system, as it has been for most of its time in space, gradually modifying its path around the Sun.
New research in the well-named Department of Recent Earthling Communications and Knowledge at the increasingly popular Galactic University of Fatuity and Frivolity (GUFF) has revealed that the significant majority of these logs written since Dawn’s interplanetary cruise phase commenced on December 17, 2007, have begun with something similar to that introductory paragraph. That may not be very surprising, as humankind would not be able to accomplish this ambitious and exciting mission without a reliable, ion-propelled spacecraft. (Note to other readers: for bureaucratic reasons, earthlings have chosen not to collaborate with more technologically advanced species on this mission. Rest assured, though, that it’s nothing personal!) Nevertheless, as you will see in a second (assuming you can read about 800 words per second), this familiar story will change quite soon, as the typical content of our opening remarks will no longer be fully applicable. First, let’s review what Dawn has accomplished since the last log besides 28 days of thrusting.
On September 29, as its own silent but joyous celebration of its first anniversary of being in space was winding down, the spacecraft stopped thrusting so mission controllers could conduct routine maintenance on components in 2 of its subsystems: attitude control and ion propulsion. (Thrusting is suspended during these activities principally because the orientation in which the main antenna is aimed at Earth is different from the orientation required to point an ion thruster in the direction needed for changing the craft’s course through space.) Attitude control is responsible for the orientation (known to engineers as “attitude”) of the probe in the zero-gravity of spaceflight. Despite its name, this subsystem is as pleasant a member of the onboard crew as any other. Ion propulsion, of course, reshapes the spacecraft’s orbit so it will rendezvous with distant Vesta and Ceres and maneuver at each to obtain the precious scientific secrets they hold.
Some of the work during this week was to verify that the contents of the computer memory in certain components remained intact. On September 30, engineers confirmed that the memory in each of the 2 ion propulsion computer control units was in good condition. On October 2, the backup star tracker was tested, and it also remains healthy and ready for use whenever needed. A star tracker helps the attitude control system determine the orientation of the spacecraft by imaging groups of stars and recognizing patterns, much as you might orient yourself on a dark, cloudless night if you were familiar with the constellations. (Readers who travel frequently, and hence must keep track of where they are in their galaxy in order to know what the arrangement of stars should be, have a more difficult problem than Dawn’s star trackers face. The solar system is so tiny compared to interstellar distances that the views of the stars remain essentially unaffected by where the spacecraft is, just as the shapes of constellations are the same for observers anywhere on Earth.)
In addition to performing maintenance on software, the mission control team needs to keep Dawn’s hardware in peak condition. The 3 ion thrusters are mounted on separate mechanical apparatuses that allow each 8.9-kilogram (19.5-pound) thruster to be pointed accurately. These thruster gimbal assemblies, known as TGAs to team members who find themselves too busy to use entire words (such people are themselves known as being TBTUEW), need to have lubricant in their bearings redistributed occasionally. Even when a TGA is in use for an operating thruster (thruster #1 has been the active one since June), the usual motion is not enough to accomplish the needed spreading of lubricant. Therefore, all 3 TGAs were moved through a prescribed pattern, ensuring that they will be able to continue to operate smoothly and point correctly.
Dawn is outfitted with 4 reaction wheels, devices whose spin is controlled electrically. Changing a wheel’s spin rate allows the attitude control system to rotate the spacecraft. The wheels are mounted in different orientations, but any 3 are sufficient for normal operations. Wheel #3 has been off since May. On October 2, it was powered on again and wheel #2 was deactivated, beginning its turn as the backup.
Gyroscopes, which will help attitude control perform the accurate pointing of science instruments at the 2 protoplanetary destinations, normally are turned off, as they are not needed for most of Dawn’s assignments along the way. A few times each year they do need to be operated to ensure they remain in good condition. The last such time was in May. On September 29, the units were activated again, and they remained powered on until October 3.
With all maintenance completed successfully, normal interplanetary thrusting resumed on October 3. Soon however, interplanetary thrusting will no longer be the norm. Some of the unusual principles of an interplanetary journey driven with ion propulsion were considered in a log written while Dawn was still gravitationally anchored to Earth. One essential characteristic of such missions is the long periods of thrusting, familiar now to those fortunate enough to have followed Dawn’s progress since the beginning of the interplanetary cruise phase. But, thrusting is not required for the entire voyage; indeed, at some times thrusting is helpful to the mission and at other times it would be detrimental. Extensive analysis is devoted to computing the thrusting schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the masses and orbits of Earth, Mars, Vesta, Ceres, and myriad other bodies) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when thrusting is not permitted (e.g., during spacecraft maintenance periods).
As hinted obscurely only a second ago, the period in which thrusting is beneficial for reaching Vesta on schedule is drawing to a temporary close. For nearly all of the next 7 months, Dawn will coast in its orbit around the Sun (just as do most objects in the solar system, including other spacecraft and planets), no longer mounted atop a bluish-green pillar of xenon ions. Still, its orbit will change dramatically during this interval, as its flight by Mars in February will deflect its path through the solar system. As we shall see in the next log, to achieve exactly the gravitational bending needed, the spacecraft will execute some special thrusting in November and again in January, but very little indeed.
The interplanetary cruise phase has gone so smoothly that the completion of thrusting is being reached somewhat sooner than had been expected earlier in the mission. Commands already stored in Dawn’s central computer will terminate the thrust on October 31 at 3:22 pm PDT. In the next log, we will discuss a bit about the process the team used to determine that time, as it bears on another activity planned for November; contrary to what you might conclude however, leaving enough time for team members to don their costumes in preparation for going door to door to collect Halloween treats was not a factor. (Your correspondent, who disguises himself in costumes at JPL most days, won’t need any extra time at all tomorrow to outfit himself for perfectly frightening appearances on Halloween.)
Although thrusting will be uncommon over the coming months, there will be plenty of other news to look forward to in these logs, including the reversal of Dawn’s departure from Earth, the first attempt to measure the total power generating capability of the solar arrays, passage of the spacecraft nearly behind the Sun, plans for and results of the brief visit to Mars, a dramatic increase in the quality of writing [Note from writer to sponsor: Now that I’ve made such a promise to our readers, I hope you’ll come through with that generous raise I’ve been requesting. Note from sponsor to writer: OK, you win. We agree to a 2% raise from the current $0.00 per log, and we will pay 1% of your tuition if you can buckle down, gain readmission to GUFF, and finally receive your degree.], and much more.
Dawn is 384 million kilometers (238 million miles) from Earth, or 950 times as far as the moon and 2.58 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 43 minutes to make the round trip.
Dr. Marc D. Rayman
9:00 pm PDT October 30, 2008
On the first anniversary of its departure from Earth, Dawn continues with what it has been doing for most of its time in space: with the greatest patience it is gently reshaping its orbit around the Sun with its ion propulsion system.
In its first year of travels, the spacecraft has thrust for a total of about 253 days, or 69% of the time. Dawn has been in powered flight for 85% of the time since the beginning of its interplanetary cruise phase in December 2007 and about 0.000000005% 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 67 kilograms (148 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) 1 year ago.
The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 1.68 kilometers per second (3760 miles per hour). As the preceding log described, 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-eighth of the thrust time planned for its entire mission, Dawn has already exceeded the velocity change required by many spacecraft. (For a comparison with probes that enter orbit around Mars, visit the red planet yourself or refer to a previous log.)
Since launch, our readers who have remained on or near Earth have completed exactly 1 revolution around the Sun. (This log, including the date it is filed, disregards that 2008 is a leap year and that Earth actually takes almost 365.25 days to complete one orbit. Oops -- it isn’t being disregarded; in fact, it’s right there in the previous sentence, and the longer this parenthetical text goes on, the more attention is being drawn to it. As it makes no significant difference, we request readers do a better job of ignoring it than the writer is doing. Please return to the flow of the log.) Orbiting farther from the Sun than Earth, and moving at a more leisurely pace, Dawn has not traveled even two-thirds of the way around the Sun. Of course, unlike Earth, when it has completed 1 full circuit (in 2009), it will not be at the same place it started. Earth’s orbit is quite repetitive, but the combined effects of the powerful rocket launch, the extensive ion thrusting, and the gravitational deflection from Mars next February will cause the spacecraft to be farther from the Sun at the end of its first revolution than it was at the beginning.
As readers who have followed the Dawn mission during 2008 know, the spacecraft occasionally engages in activities other than routine thrusting as its adventure progresses. On August 26, mission controllers commanded the primary and backup cameras to execute their calibration routines. This not only served to confirm that both units remain healthy, but it also let engineers verify one of the new features in the software radioed to each camera in April that was not tested at that time.
On September 22, an updated version of a method to establish how much power Dawn’s extraordinary solar arrays can generate was tested successfully. The first test was conducted in July, and it yielded only some of the desired information. The revised procedure was very similar to the earlier one, principally differing in the timing of some instructions and values of parameters based on the analysis of that initial run. Because the entire activity, even including the 41-minute round-trip travel time for radio signals, required less than 3 hours in the middle of the afternoon, among the most significant changes that ever-observant mission controllers detected was that no meals were incorporated into the carefully engineered plan.
As in July, the test included rotating both solar array wings 45 degrees, so they did not point directly at the Sun, thus reducing how much light they received and converted to electrical power. The test was carried out during the spacecraft’s routine weekly interruption in thrusting to point its main antenna to Earth, but the ion propulsion system was commanded into service when it otherwise would have been idle. Its role then was not to provide propulsion (although it did so); rather, it participated because it is the greatest consumer of power onboard. Dawn’s enormous solar arrays, even turned partially away from the Sun and more than 1.66 times farther from the radiant orb than Earth is, were able to provide the 2.5 kilowatts requested by the ion drive at full power. Later in the mission, after all the data have been analyzed thoroughly, the next step in the solar array calibration will be to command the arrays to rotate farther, where they are not expected to be able to deliver all the power requested.
As Dawn begins its second year (as measured back on Earth) of interplanetary flight, the probe steadfastly continues its long journey in the quiet solitude of space, quite isolated from events on or near the distant planet that used to be its home. While no spacecraft has left the vicinity of the Earth-moon system in the year since Dawn’s departure, much has happened there, even as the explorer has remained focused on accomplishing its voyage in deep space. From the first circulation of protons at the Large Hadron Collider 100 meters (330 feet) underground, to the beginning of the Fermi Gamma-ray Space Telescope mission 550 kilometers (340 miles) overhead, to the arrival of SELENE (Kaguya) and Chang’e 1 at the moon, humankind’s thrilling work to understand nature has continued. Apparently there have been some other kinds of news as well, from shocking revelations about celebrities, to competitions among athletes and among politicians, to still more shocking revelations about celebrities, but such information is harder to find, given the news media’s nearly exclusive focus on myriad science topics. (News coverage may be different on your planet.)
Dawn is 374 million kilometers (232 million miles) from Earth, or 980 times as far as the moon and 2.49 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 42 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 am PDT September 27, 2008
The Dawn spacecraft continues to make good progress on its adventure to unlock scientific secrets hidden deep in the main asteroid belt, between Mars and Jupiter. Its path to that distant realm of the solar system is now bringing it closer to the Sun, and thanks in part to all the thrusting it has accomplished with its remarkable ion propulsion system, it has recently achieved its lowest speed so far in the mission. To understand this enigmatic behavior, read on!
As most of you who have read about or visited the solar system know, the asteroid belt is much farther from the Sun than Earth is. Dawn passed outside the orbit of Mars in June, but it has not yet traveled far enough from the Sun to reach asteroid Vesta, its first destination. Dwarf planet Ceres, Dawn's second target, resides still farther in the depths of space. So readers with memories that extend as far back as the previous paragraph may wonder why Dawn apparently is backtracking, now approaching the Sun.
Despite the many innovations that make this project so fascinating, the Dawn team has not yet discovered how to travel backwards in time. (If it had, while writing this log, we would be able now to prevent the misspelling that occurred while writing the last log.) To see why Dawn seems to be reversing course, both heading toward the Sun and traveling more slowly now than at the beginning of its mission, we need to consider some of the principles that govern space travel.
Your correspondent offered some comments on these concepts in a log for a different interplanetary mission, Deep Space 1. If not for some unexpected legal issues with certain species in spiral galaxies capable of abstract thought, we would simply reprint that material here. Instead, we shall consider some of the same ideas but with different words.
The goal of this text is to provide only the gist of some of the fundamentals. In an act of selfless charity to help our hungry friends the Numerivores of Q2237+0305, this log will include more numbers than usual. It is not necessary to study them in detail; some readers may find them helpful and others may feel free to gloss over them. In any case, we can provide an absolute guarantee that by the end, with even a casual comprehension of this material, the interested reader would not find even the doctorate level examinations from the prestigious Galactic Institute of Space Travel (known to many as "the prestigious Galactic Institute of Space Travel") to be difficult.
The overwhelming majority of craft humans have sent into space have remained in the vicinity of Earth, accompanying that planet on its annual revolutions around the Sun. The satellites of Earth (including the moon) remain bound to it by its gravity. As fast as they seem to travel compared to residents of the planet, from a solar system perspective, their incessant circling of Earth means their paths through space are not very different from Earth's itself. Everything on the surface and in Earth orbit travels around the Sun at an average of around 30 kilometers/second (67,000 miles/hour), completing one full solar orbit every year. To undertake its interplanetary exploration and travel elsewhere in the solar system, Dawn needed to break free of Earth's grasp, and that was accomplished by the rocket that carried it to space last year. Dawn and its erstwhile home went their separate ways, and the Sun became the natural reference for the spacecraft's position and speed on its travels in deep space.
Despite the enormous push the Delta II rocket delivered (with affection!) to Dawn, the spacecraft still did not have nearly enough energy to escape from the powerful Sun. So, being a responsible resident of the solar system, Dawn remains faithfully in orbit around the Sun, just as do Earth and the rest of the planets, asteroids, comets, and other members of the Sun's entourage.
Whether it is for a spacecraft or moon orbiting a planet, a planet or Dawn orbiting the Sun, the Sun orbiting the Milky Way galaxy, or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to a sizeable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight path. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the Sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its path; if you let go of the string, the weight's natural motion would carry it away in a straight line (ignoring the effect of Earth's gravity).
The force of gravity diminishes with distance, so the Sun's pull on a nearby body is greater than on a more distant one. Therefore, to remain in orbit, to balance the relentless tug of gravity, the closer object must travel faster, fighting the stronger pull. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, less than 400 kilometers from the surface) must streak around the planet at about 7.7 kilometers/second (more than 17,000 miles/hour) to keep from being pulled down. The moon, orbiting 1000 times farther above, needs only to travel at about 1.0 kilometers/second (less than 2300 miles/hour) to balance Earth's weaker grip at that distance.
Notice that this means that for an astronaut to travel from the surface of Earth to the International Space Station, it would be necessary to accelerate to quite a high speed to rendezvous with the orbital facility. But then once in orbit, to journey to the much more remote moon, the astronaut's speed eventually would have to decline dramatically. Perhaps speed tells an incomplete story in describing the travels of a spacecraft, just as it does with another example of countering gravity.
A person throwing a ball is not that different from a rocket launching a satellite (although the former is usually somewhat less expensive and often involves fewer toxic chemicals). Both represent struggles against Earth's gravitational pull. To throw a ball higher, you have to give it a harder push, imparting more energy to make it climb away from Earth, but as soon as it leaves your hand, it begins slowing. For a harder (faster) throw, it will take longer for Earth's gravity to stop the ball and bring it back, so it will travel higher. But from the moment it leaves your hand until it reaches the top of its arc, its speed constantly dwindles as it gradually yields to Earth's tug. The astronaut's trip from the space station to the moon would be accomplished by starting with a high speed "throw" from the low starting orbit, and then slowing down until reaching the moon.
The rocket that launched Dawn threw it hard enough to escape from Earth, sending it well beyond the International Space Station and the moon. Indeed, the spacecraft is now more than 1 million times farther away than the station. Dawn's maximum speed relative to Earth on launch day was so high that Earth could not pull it back. As mentioned in an earlier log, Dawn was propelled to 11.46 kilometers per second (25,600 miles per hour), well in excess of the space station's orbital speed given 3 paragraphs above. But it remains under the Sun's control.
If the spacecraft had never operated its ion propulsion system, it would have coasted away from the Sun (even while Earth continued circling the Sun on its own), slowing down the entire time, reaching the top of its interplanetary arc on July 2, 2008. Then, at almost 242 million kilometers (150 million miles) from the Sun, as it succumbed to the massive star's pull, it would have begun its inward fall.
Many solar system residents find measuring distances in millions of kilometers (or miles) to be inconvenient, so it is common to use the "astronomical unit" (AU). The average distance between Earth and the Sun, nearly 150 million kilometers (93 million miles) is defined to be 1 AU. So Dawn would have reached almost 1.62 AU from the Sun without thrusting.
After that, the probe would not have fallen all the way back to the Sun, ending in a useless blaze. Because it departed from Earth, which was already orbiting the Sun, and not from a stationary platform, it began with energy adequate to keep it at Earth's orbit. Upon allowing the Sun to pull it back, it would come only to about that same orbit, reaching a minimum distance of 1.00 AU from the Sun on April 1, 2009. So Dawn would have been in an elliptical solar orbit, ranging from 1.00 AU to 1.62 AU. It would have traveled faster and faster as it swooped to its smallest distance and then slowed down again as it sailed back out to the largest distance, much like a person on a swing, slowing near the top, speeding through the lowest point, and then repeating that pattern. The change in speed with distance is an essential characteristic of all orbits.
Now think again of the ball you throw. If it had a tiny propulsion system to help it along its way, that extra boost would propel it higher, supplementing the energy you imparted to it when it left your hand. Unlike a powerful rocket that accelerates as it ascends, if the additional thrust were low, it might not be able to completely counter the slowing from Earth's gravity, but it might help reach a higher altitude before beginning its fall.
As Dawn's famously efficient ion propulsion system has given it a delicate but steady push on its flight away from the Sun, the spacecraft has been able to resist the Sun's incessant pull longer. Instead of turning back in early July, Dawn flew outward until August 8. Even with all the thrusting, it was constantly slowing down, and when the Sun's gravity overwhelmed it, it began its inward flight. But by then the ion thrusting had changed the shape of its elliptical orbit so it would not fall back as far as Earth's orbital distance. If it undertook no more thrusting after August 8, it would come back only to 1.16 AU from the Sun, reaching that distance on June 14, 2009. As we will see in future logs, it will not have the opportunity to drop even that close to the Sun however, because ion propulsion continues modifying its orbit. In addition, on approximately February 18, 2009 (the exact date and time depend on the progress of future thrusting), the probe will pass by Mars, and the gravitational deflection will cause still more changes to its orbit around the Sun, which eventually will take it to the asteroid belt.
On September 27, 2007, some 2 minutes after it had separated from its rocket, Dawn reached its highest speed relative to the Sun for the entire mission. At that time, it was traveling at about 38.95 kilometers/second (87,130 miles/hour). Earth (and its residents, including this writer) were moving around the Sun at the more leisurely pace of 29.70 kilometers/second (66,440 miles/hour).
So what has been the effect of Dawn's thrusting since then? By August 8 it had expended about 55.4 kilograms (122 pounds) of xenon propellant, some for tests during the initial checkout phase of the mission and most with the specific intent of altering its orbit around the Sun. If this were solely for the purpose of accelerating (as it is usually described, in these logs and elsewhere) without the complex patterns involved in orbital dances, the effect would have been to increase Dawn's speed by 1.38 kilometers/second (3090 miles/hour). But because of the way forces and velocities work in space travel, in pushing Dawn away from the Sun, allowing it to travel "higher" before the Sun pulled it back, the ion propulsion system helped Dawn continue away from the Sun until, on August 8, it was more than 1.68 AU from the master of the solar system. By then, its speed had fallen to 20.77 kilometers/second (46,460 miles/hour). At the same moment Dawn was orbiting the Sun at that rate, distant Earth was racing in its orbit at 29.38 kilometers/second (65,710 miles/hour).
When Dawn began coming back in toward the Sun, it was in a different part of the solar system from where it would have been had it never applied its ion propulsion system to so patiently yet persistently change the orbit the Delta rocket left it in. In the absence of any ion thrusting, the spacecraft would have been 0.45 AU (68 million kilometers or 42 million miles) away from its actual location on August 8.
Dawn will rendezvous with Vesta in about 3 years. To match that asteroid's orbit around the Sun, our robotic explorer will have to continue tuning its orbital parameters so that it will be almost 2.3 AU from the Sun while traveling at about 20 kilometers/second (45,000 miles/hour), farther and slower than its current orbit or that of its quondam planetary domicile.
Achieving the required speed and distance alone is not enough to ensure Dawn can slip into orbit around Vesta, but we will consider other aspects of this problem in a future log. In the meantime, we can think of the general problem of flying elsewhere in space as similar to climbing a hill. For terrestrial hikers, the rewards of ascent come only after doing the work of pushing against Earth's gravity to reach a higher elevation. Similarly, Dawn is climbing a solar system hill with the Sun at the bottom. It started from Earth, at 1 AU in elevation; and its first rewards await it higher up that hill at 2.3 AU, where Vesta, traveling at only about two thirds of Earth's speed, keeps its records of the dawn of the solar system. Ceres is still higher up the hill, moving even more slowly to balance the still-weaker pull of the Sun.
If this were only a climb, it would be easy to stop at the correct spot on the solar system hill. This simple analogy fails us here though, because everything is in orbital motion. With a big enough rocket, or gravitational boosts, it would not be difficult to throw Dawn hard enough that it would fly out to Vesta or beyond, and some other spacecraft have coasted past that distance from the Sun. But to enter orbit, Dawn must precisely match Vesta's path around the Sun, joining it for a portion of the asteroid's regular 3.6-year circuit around the Sun, just as Earth's natural and human-made satellites stay with it throughout its 1-year orbit. That is part of the reason the spacecraft needs ion propulsion. The ion propulsion system allows Dawn not only to carry its scientific instruments up that hill but also to "stop" on the slope, neither falling back toward the Sun nor coasting by the asteroid. When a subsequent log addresses what more is required than speed and distance, we will see why this is more difficult than it may appear. (And as we surely will have a link from that log to this one, on behalf of all present readers, we send greetings from the past to you future readers.) We are confident that in meeting this great challenge, should Dawn remain healthy, it will be a shoo-in in the next solar system Olympics, aiming not for a bronze medal, nor for one of silver or gold but rather for the most highly coveted: the xenon medal.
We promised near the beginning that for those who completed this arduous log (perhaps a challenge even greater than Dawn's interplanetary journey), the examinations at the prestigious Galactic Institute of Space Travel would not prove difficult. The reason is simple: there is no such organization; we made it up. Nevertheless, following Dawn's long and ambitious journey does not require mastery of the concepts touched upon here. All that really is needed is the desire to learn more about the cosmos, to share in one of humankind's bold adventures to explore the unknown as we set our sights on extraordinarily distant goals and aspire to something well beyond the confines of our humble home in the universe.
Dawn is 352 million kilometers (219 million miles) from Earth, or 955 times as far as the moon and 2.33 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 39 minutes to make the round trip.
Dr. Marc D. Rayman
9:30 pm PDT August 24, 2008
Dawn continues its flight through the solar system with all systems functioning well. It is vitally important that the spacecraft is reliably staying on course and on schedule, gently and steadily thrusting with the bluish glow of its ion propulsion system; yet that doesn't lend itself to the sorts of spine-tingling, heart-pounding, hair-raising, planet-shattering logs for which Dawn is famous (at least among immigrants from brown dwarf systems reading these reports in the vicinities of active galactic nuclei). So let's turn out attention to consider a particular aspect of flying a mission with ion propulsion.
Perhaps that requires a bit more detailed consideration...
Engineers are developing a method to determine how much power the solar arrays can produce. It might seem odd that with the spacecraft having been in interplanetary flight for 10 months, engineers don't already know the answer. (Other facts might seem odd as well, such as the phrase "nihil ad rem" being in this sentence. This log will address only one oddity however.)
When the spacecraft was at Earth's distance from the Sun, shortly after launch, the solar arrays would have been able to supply more than 10 kilowatts, enough to operate about 10 average homes in the US (and nearly as much as your correspondent's cat Regulus generates when Mr. Vacuum Cleaner emerges from his closet). Dawn cannot use that much electrical power, but as it pushes deeper into space, the weaker illumination by the Sun will yield less power. The craft's two solar array wings, each about 2.3 by 8.3 meters (more than 7 by 27 feet), were designed to be large enough to meet the needs of the power-hungry ion propulsion system plus all other spacecraft systems even in orbit about dwarf planet Ceres. To thrust at nearly twice Mars' average distance from the Sun, Dawn carries the most powerful solar arrays ever used on an interplanetary mission.
The only way to measure the power of the arrays is for the spacecraft actually to pull the power from them, and its ability to do that is limited. When thrusting at full throttle and using all systems normally, Dawn consumes 3.2 kilowatts. Even now, traveling farther from the Sun than Mars ever ventures, the solar arrays can provide about 4 kilowatts. If the spacecraft activated all of its nonessential components, it still could not draw this much power. That leaves engineers without an accurate determination of the full potential of the arrays.
Of course, engineers thoroughly tested the electrical power system before launch, including each of the 11,480 solar cells and all other components, and from that they constructed a mathematical simulation of the arrays. But laboratory measurements do not perfectly reproduce conditions in space, so the computational model has some uncertainty. In-flight measurements are needed to improve their simulation of how much power the solar arrays can furnish at different distances from the Sun.
Who cares how much power is available? Well, first and foremost, our readers do! After all, you've gotten this far (and even farther right now) in this log, so you must have some reason for spending otherwise good time reading about the solar arrays. The Dawn project appreciates your interest, and we want to provide the information you apparently seek, even though we have no idea why you suddenly are eager to understand the solar array performance.
As it turns out though, there is another reason for establishing the true capability of the solar arrays. As explained in many (but fewer than 10,001) previous logs, Dawn's unique mission is possible only through the persistent use of its ion propulsion system. Rather than thrusting for minutes, as most spacecraft do, Dawn will thrust for years. As power diminishes in the dim depths of space, Dawn must throttle its ion thruster to lower power (and lower thrust) levels.
Because the throttle level depends on how much power is available, to formulate the details of the craft's trajectory and other plans for the mission, engineers require knowledge of how much power the arrays will provide at any distance from the Sun. After all, it is misleading to think of ion thrusting as an ion propulsion subsystem function; rather, it is a spacecraft system function, requiring most subsystems to operate together. Apart from the inevitable (and quite unpredictable) glitches and anomalies on the spacecraft and appearances of cake in mission control, and contrary to many people's preconceived notions, since well before launch the greatest technical uncertainty in the planning of Dawn's flight has been what the solar array power will be. So far, mission engineers have incorporated a reasonable, but conservative, estimate into the solar array simulation, but to refine the plans, they need to verify or correct the numbers.
Although the arrays produce more power than can be measured now, they would produce less power if they were not pointed directly at the Sun. That could reduce their output low enough to allow the spacecraft to draw as much power as the arrays could generate in that orientation, providing the calibration measurement that is needed. (Engineers would extrapolate to reveal how powerful the arrays would be when Sun-pointed at different distances.) As is usually the case in controlling interplanetary spacecraft, the details make such a test much less simple than it might appear at first blush.
With the normal switching of heaters on and off throughout the spacecraft, the total power consumption fluctuates, and that could add "noise" to the data, making the results harder to interpret and less accurate. If the spacecraft tried to draw more power than the solar arrays could produce, the battery would temporarily make up the difference but, depending upon the circumstances, protective software onboard would intervene to turn some systems off and place the spacecraft in safe mode. While that would not threaten the health of the spacecraft, it would threaten the solar array calibration. (By the way, the battery can store only enough energy to operate the spacecraft for about an hour. The solar arrays keep it charged for its occasional use.)
The solar array calibration working group (a runner up in the highly competitive Least Cool Dawn Team Name Contest) devised a method to calibrate the solar arrays that accounted for all these and many other considerations, including the solar panel thermal equilibration time and the dependence on temperature of the power vs. voltage curve, high voltage down converter phase margin, the solar array voltage set point, power processor unit undervoltage trips, the voltage-temperature control loop for the battery on the low voltage bus, and spacecraft safety even in the event of an unrelated anomaly during the test.
While conceptually simple (rotate the solar arrays by a certain angle and measure how much power the spacecraft can draw), the calibration proved complex enough that a somewhat simplified test was deemed appropriate. The objective was to verify how the spacecraft would operate in the test conditions before committing to the full calibration. The plan was to execute the test on July 21, and if everything went perfectly, the final version would be attempted the next day. Last year, when the planning for this began, it was decided to schedule a backup opportunity late in 2008 in case the first time did not yield the desired data. (In addition, the calibration will be repeated occasionally over the course of the mission to monitor changes in the solar array characteristics, ensuring the power predictions remain accurate.)
Because electrical power is essential to the operation of all subsystems, a test of this nature calls for all subsystem personnel to scrutinize spacecraft telemetry for symptoms of unpredicted and infelicitous behavior. All commands were contained in a single file transmitted to the spacecraft, and immediate intervention would not be physically possible, as radio signals revealing the condition of the spacecraft would take nearly 18 minutes to reach Earth, and commands sent in response would require the same time to travel back to the spacecraft. Nevertheless, the team needed to be prepared to take action in the very unlikely case a problem developed, so two key measures were put in place: all stations in mission control were at the ready, and pizza was provided to help fill the gaps in this early-evening test while radio signals raced across the solar system.
The result: overall the test went well, although there was unexpected spacecraft behavior and unexpected toppings on the pizza. For the former, no response was required by the flight team, as the spacecraft executed all the commands correctly and returned to its normal configuration at the end. The test yielded only a partial set of calibration data however, apparently because some of the reconfigurations of the electrical power system and the ion propulsion system for the purposes of the test led to a few responses that were not anticipated. The spacecraft transmitted a large volume of supporting data, which will take longer to digest than the pizza, and when the satiated engineers have finished, they will determine what modifications to make for a new test. A future log will describe the next test and any corresponding changes in the food delivered to mission control.
Turning their attention on July 22 to a different topic, the team modified software in one of the many computers onboard. In January, with neither permission nor warning, a subatomic particle traveling through the solar system hit a sensitive electronic component on the spacecraft, triggering a quick sequence of events that culminated in the spacecraft entering its safe mode. Since then, programmers have developed a way to prevent space radiation that reaches that particular circuit from having the same effect. With the updated software, now the only consequence would be a notice to controllers that the device was hit, and the spacecraft would not need to enter safe mode or interrupt its activities.
The solar array test and the software change were conducted during a planned 2-day pause in thrusting. On schedule on July 23, Dawn resumed propelling itself with xenon ions. Once again the special lights adorning a wall in mission control were turned on, emitting a blue glow to remind everyone who visits or works there of the probe's patient pursuit of intriguing and unexplored worlds in the asteroid belt.
As Dawn travels through space, Earth and the Sun grow more remote. Although the journey will never bring it near the part of the solar system it used to consider home, we will see in the next log that its path to Vesta and then to Ceres is not as direct as some might expect. As part of the explanation, the log also may reveal something about this mispelling.
Dawn is 324 million kilometers (202 million miles) from Earth, or more than 885 times as far as the moon and 2.14 times as far as the Sun. Radio signals, traveling at the universal limit of the speed of light, take 36 minutes to make the round trip.
Dr. Marc D. Rayman
8:30 pm PDT July 27, 2008
Almost everyone has had the frustrating experience of getting lost. To avoid this problem, the savvy traveler carries a map. Similarly, astronomers need maps of the sky to know where to look, allowing us to make the best use of precious time on large telescopes. A map of the entire sky also helps scientists find the most rare and unusual types of objects, such as the nearest star to our sun and the most luminous galaxies in the universe. Our team (lead by our principal investigator, Dr. Ned Wright of UCLA) is building a new space telescope called the Wide-field Infrared Survey Explorer that will make a map of the entire sky at four infrared wavelengths. Infrared is a type of electromagnetic radiation with a wavelength about ten or more times longer than that of visible light; humans perceive it as heat.
Why do we want to map the sky in the infrared? Three reasons: First, since infrared is heat, we can use it to search for the faint heat generated by some of the coldest objects in the universe, such as dusty planetary debris discs around other stars, asteroids and ultra-cold brown dwarfs, which straddle the boundary between planets and stars. Second, we can use it to look for very distant (and therefore very old) objects, such as galaxies that formed only a billion years after the Big Bang. Since light is redshifted by the expansion of the universe, the most distant quasars and galaxies will have their visible light shifted into infrared wavelengths. And finally, infrared light has the remarkable property of passing through dust. Just as firefighters use infrared goggles to find people through the smoke in burning buildings, astronomers can use infrared to peer through dense, dusty clouds to see things like newborn stars, or the dust-enshrouded cores of galaxies.
So how does one go about building an infrared space telescope? And why does it need to be in space in the first place? Since infrared is heat, you can imagine that trying to observe the faint heat signatures of distant astronomical sources from our nice warm Earth would be very difficult. A colleague of mine compares ground-based infrared astronomy to observing in visible light during the middle of the day, using a telescope made out of fluorescent light bulbs! Putting your infrared telescope in the deep freeze of space, well away from the warmth of Earth, improves its sensitivity by orders of magnitude over a much larger ground-based infrared telescope.
On the Wide-field Infrared Survey Explorer project, our team is in the middle of one of the most exciting phases of building a spacecraft -- we're assembling and testing the payload. Right now, the major pieces of the observatory have been designed and manufactured, and we're in the process of integrating all these pieces together. The payload is elegantly simple. It has only one moving part -- a small scan mirror designed to "freeze-frame" the sky for each approximately 10 second exposure as the spacecraft slowly scans. After six months, we will have imaged the entire sky. The telescope is flying the latest generation of megapixel infrared detector arrays, along with an off-axis telescope that gives us the wide field of view that we need to cover the whole sky so quickly. In the next few months, we'll be setting the focus on our telescope, characterizing our detector arrays, and verifying the thermal performance of our cryostat. The observatory's cryostat is essentially a giant thermos containing the cryogenic solid hydrogen that we use to keep our telescope and detectors at their operating temperatures near absolute zero.
We are also in the midst of making detailed plans for verifying that the spacecraft is working properly once we launch. This is called the "in-orbit checkout" phase. For this mission, checkout is fast -- only 30 days! The checkout commences right after our November 2009 launch, when we wake the spacecraft up and begin switching on its various subsystems: Power generation and distribution, communications, attitude control and momentum management, and the main computer system. We'll also power on the payload electronics and detectors. Next, we will begin the calibration observations that we need to start the survey, such as verifying the telescope's image quality and the way our detector arrays respond to light. Once these steps are completed, we'll be ready to extend our gaze across the universe using the observatory's infrared eyes.
The great thing about the mission's all-sky dataset is that it will be accessible to everyone in the entire world via a Web interface. So you will literally be able to access some of the coldest, most distant and dustiest parts of the universe from the comfort of your couch. Stay tuned to explore the universe with us!
We've been steadily learning about what it takes to run this thing called the Phoenix lander. As expected, not everything has gone exactly as planned. But that in its own way was planned -- we work to maintain flexibility in our schedule and our design, so that we can absorb new things that happen without throwing the whole team into a tizzy! So what have we been doing?
The Robotic Arm Camera on Phoenix captured this image underneath the lander on the fifth Martian day of the mission. The abundance of excavated smooth and level surfaces adds evidence to a hypothesis that the underlying material is an ice table covered by a thin blanket of soil.
The wet chemistry experiment in one of the lander's instruments called the Microscopy, Electrochemistry and Conductivity Analyzer, or MECA, also found salts in the soil samples. Salts are only formed when water has been present! So that is another indicator that there was abundant water in this region of Mars. What are these salts? They appear to be chemicals containing sodium, magnesium, potassium and chlorine. The soils were found to be alkaline, with a pH greater than 7 -- similar to soils in the upper dry valleys of Antarctica.
But, like I said, everything hasn't been totally smooth. The team discovered that the Martian soil is lumpy and sticks together. That made the first sample difficult to deliver! So the team thought about how to make the process easier, and we figured out various ways to break up the lumps. We tried three methods: de-lumping, sprinkling and agitation.
De-lumping refers to shaking the acquired material in the scoop by running a Dremel-like tool that vibrates the entire scoop, breaking up clumps. Then there is sprinkling: By running the rasp while slightly tipping the scoop, the team can command Phoenix to send a small shower and sift particles down into the TEGA (Thermal and Evolved-Gas Analyzer ) and MECA instruments rather than dumping a whole load of clumped-up dirt onto each instrument. As for agitation, the TEGA instrument has a method to shake itself -- it has an agitator which shakes the sample loose if anything has stuck to its entry port. The sprinkle and agitation methods have been routinely adopted for sample delivery.
The neat consequence of this is that it solves what had always been our worry about how to deliver the same sample to each instrument for comparison of science results. The sprinkle delivery method enables us to put a large sample into the scoop and deliver part of it to MECA microscopy, part to MECA wet chemistry and part to the TEGA instrument. Same sample problem: solved!!
When life gives you lemons, make lemonade! Or in this case, Marsade!
Here we are, four years after the Cassini spacecraft entered orbit around Saturn. We're about to begin the extended mission, termed the Cassini Equinox Mission. Cassini has been a scientifically remarkable mission and a fantastic return on the investment. If you are reading this blog, then you might already know about Cassini's discoveries at Enceladus, Titan, the other icy moons, the rings, the magnetosphere and Saturn itself. But if you're new to following this mission, you can catch up on those discoveries by reading about them here: http://saturn.jpl.nasa.gov/news/features/feature20080627.cfm.
This great science is accomplished by an international team of scientists and engineers. I am thrilled to be able to carry the scientific reins for Cassini as its incoming project scientist. The project scientist is essentially the mission's chief scientist, who watches out for the overall scientific integrity of the mission.
My own background is in the geology of icy moons of the outer solar system. Though the planets have always enthralled me, I trace this specific icy interest back to a course I took as an undergraduate at Cornell University in about 1984, taught by Carl Sagan and his post-doctoral research associate Reid Thompson, entitled "Ices and Oceans in the Outer Solar System." The course included discussion of Jupiter's moon Europa, which it was thought might have a globe-girdling ocean beneath its icy surface -- an idea that would be further tested by the Galileo spacecraft when it arrived at Jupiter a decade later. We also learned about Saturn's haze-shrouded moon Titan, which might just have seas of organic rain and liquids on its surface -- but we wouldn't know for certain until the Cassini spacecraft arrived at Saturn two decades later. Who could possibly wait so long? And who would have thought that once we all did, both of these seemingly far-fetched ideas would turn out to be correct? (If only Carl and Reid could be here today to know it.)
Two years ago I came to JPL with the goal of getting the next flagship mission to the outer solar system off the ground. It takes a great deal of time and energy to make such a mission a reality. They are relatively expensive and take a long time from concept to completion. But just as others before me -- such as Galileo Project Scientist Torrence Johnson and Cassini Project Scientist Dennis Matson -- have worked to send those missions into space, I would help create the next mission, potentially to orbit Jupiter's moon Europa. Currently I serve as JPL's study scientist for the Europa Orbiter mission concept. This mission concept is in friendly competition with a mission that would orbit Titan. I hope that somehow, in time, we can make both of these spectacular mission concepts come to fruition.
Entering into the wonderland that is Cassini, my eyes are wide open to the science and engineering behind the curtain, while wary of its history and complexity. My operating philosophy is to always be true to the science. With good planning and good fortune, Cassini will keep going down the road for many years to come, following up on its prime mission discoveries and in making new ones that we can't dream of yet.
Stay tuned for more to come. It'll be a great ride!