I just received a MESSENGER mission news update stating that the MESSENGER spacecraft, en route to Mercury via two Venus flybys, has passed another milestone on its long journey: it has, for the last time, passed from Earth's environs toward the inner solar system. This event is marked on the MESSENGER mission by a 180-degree rotation of the spacecraft that points its protective sunshade toward the Sun. Since its launch, MESSENGER has spent a considerable amount of time in Earth's neighborhood, beyond 0.95 Astronomical Units (AU) from the Sun, where the sunshade wasn't necessary (and where, in fact, it was necessary to have the spacecraft body facing the Sun, in order to keep warm). From now on, MESSENGER will be within 0.95 AU, and will need to bounce that solar energy away to keep cool. The MESSENGER team's terminology for these maneuvers is cute: a 180-degree rotation that faces the sunshade toward the Sun is a "flip," and a 180-degree rotation that does the opposite is called a "flop". The final flop was on March 8, and the final flip occurred today at 13:34 UTC.
I always find myself drawing comparisons between New Horizons and MESSENGER because they're visiting the extreme "ends" of the solar system; they both have unusually long cruises toward their final destinations (6.5 years for MESSENGER, 9.5 years for New Horizons); but New Horizons is taking the straightest, fastest possible path, while MESSENGER has to perform loop after loop after loop in order to settle into orbit at Mercury. MESSENGER will finally arrive at Mercury right about the time that New Horizons passes the distance of Uranus' orbit from the Sun, in March 2011.
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