Dawn Journal: Conjunction junction

Here's our monthly checkup with the Dawn mission, contributed by Marc Rayman, the mission's Project System Engineer. Thanks Marc! --ESL

by Dr. Marc D. Rayman

Dear Dawncember30ths,

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 two 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.

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