Dawn Journal: Aiming away from a bull's eye at Mars
Posted by Marc Rayman
2008/11/30 10:12 CST
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 five 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 five weeks to design, develop, check, double-check, transmit, and activate a five-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 seven days at a time (followed by a hiatus of seven to eight hours), in this case only a short burn was necessary. Propelling itself from about 4:31 pm to 6:42 pm Pacific time 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 (7,100 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 Pacific time.) 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 a degree from the Sun, progressing from east to west and passing just a third of a 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 half a degree across, so this is close indeed; the spacecraft will sneak in to less than a 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 two bent and twisted wings, each with five 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 six 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