These views of MESSENGER show the orientation at the start of trajectory correction maneuver 23 (TCM-23). Because TCM-23 was the third large course-correction maneuver using the large bi-propellant thruster, TCM-23 is also called deep space maneuver 3 (DSM-3). The blue (top view) or gold (bottom view) squares represent the front (sunlit) or back (shadowed) sides of the solar arrays, respectively. The large white (top view) or light gray (bottom view) feature is the spacecraft's sunshade, which points toward the Sun when the spacecraft is near or closer than Earth's distance from the Sun. Colored arrows show the directions of Earth, the Sun, the spacecraft's velocity with respect to the Sun, and the course-correction velocity change (delta-V or ΔV). The "spacecraft +x axis" label identifies an axis direction in the local spacecraft body-fixed coordinate system.
The first TCM after the first Mercury flyby, TCM-23, targeted the spacecraft for its second close encounter with Mercury and is the first TCM to test the rotation of the spacecraft required for the 18 March 2011 Mercury orbit insertion (MOI) maneuver. The rotation of the spacecraft, which occurred during the final 110 seconds of the 150-second TCM, was less than four degrees – about 11% of the turn required for the mission-critical MOI. The “net” ΔV of 72.24 meters per second (161.6 miles per hour), achieved using the large bi-propellant thruster, will increase the spacecraft’s speed relative to the Sun. During TCM-23 the sunshade protects heat-sensitive parts from direct sunlight that warms the spacecraft (which was about the same distance from both the Earth and the Sun during TCM-23).
TCM-23 targeted the spacecraft for its next major mission event: the second Mercury flyby on October 6, 2008. Mission controllers at The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, verified the start of the maneuver about 5.7 minutes after the start of TCM-23, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network (DSN) tracking station outside Goldstone, California. The 150-second maneuver began at 3:30 pm Eastern Daylight Time.
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