Shortly after 4 a.m. Wednesday morning, MESSENGER reached its greatest speed relative to the Sun. The spacecraft, nearly 70 percent closer to the Sun than Earth, was traveling nearly 140,880 miles per hour (62.979 kilometers per second) relative to the Sun. At this speed, MESSENGER would traverse the distance from Earth to Earth’s Moon in only 1.7 hours! Even at this great speed, MESSENGER is slightly slower than the fastest spacecraft: Helios 2. That spacecraft, launched into a solar orbit on January 15, 1976, reached a top speed of 157,078 miles per hour (70.220 kilometers per second) relative to the Sun in April of 1976. Due to MESSENGER’s near-perfect Mercury flyby trajectory on October 6, the mission design and navigation team decided that a trajectory-correction maneuver (TCM) scheduled for October 28 will not be needed. The next maneuver for the mission, scheduled to be carried out in two parts on December 4 and December 8, will re-target the spacecraft for the third and final encounter with Mercury in just under a year on September 29, 2009.
The NAC image, taken about 85 minutes after MESSENGER’s closest approach during the mission’s second Mercury flyby, shows a view of Astrolabe Rupes, named for the ship of the French explorer Jules Dumont d’Urville. Rupes is the Latin word for cliff. Mercury’s day/night transition (the terminator) is located on the left side of the image, and the Sun is striking the cliff face of Astrolabe Rupes in the upper right of the image. Also visible in the image are additional unnamed rupes, whose cliff faces are casting dark shadows. One of these rupes intersects the crater Ghiberti, named for the Italian Renaissance sculptor. Rupes on Mercury are thought to have formed as the interior of Mercury cooled and the planet consequently contracted slightly. Determining the number and extent of rupes on Mercury can thus be used to understand the thermal history of the planet. Photo Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
