December 30, 2008
Dear Dawncember30ths, Dr. Marc D. Rayman
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.
6:01:45 pm PST December 30, 2008
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Dr. Marc D. Rayman