MESSENGER Lines up for Second Pass at Venus
3 May 2007
(Source: Johns Hopkins University Applied Physics Laboratory)
The MESSENGER trajectory correction maneuver (TCM-15) completed on April 25 lasted 140 seconds and adjusted the spacecraft's velocity by 0.568 meters per second (1.86 feet per second). One more course correction will be performed before the probe?s second Venus flyby on June 5 to ensure precise targeting of the gravity assist.
The maneuver started at 1:30 p.m. EDT. Mission controllers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., verified the start of TCM-15 about 10 minutes later, when the first signals indicating thruster activity reached NASA?s Deep Space Network tracking station outside Madrid, Spain.
Although this maneuver was designed to adjust MESSENGER's velocity by 0.767 meters per second (2.52 feet per second), mission controllers estimated that about 26% less than the intended velocity change was achieved. There was almost no error in the direction for the velocity change. "The result is that the current trajectory aim point is about 200 kilometers, or 124.3 miles, higher than the ideal target point for the flyby," explains APL's Eric Finnegan, the MESSENGER mission systems engineer.
According to Finnegan, the spacecraft orientation began to jitter slightly shortly into the maneuver. The spacecraft responded properly by pulsing other thrusters to maintain accurate pointing through the TCM, but this pulsing reduced the efficiency of the maneuver.
"The spacecraft compensated properly for the attitude oscillation, but because of the additional thrusting activity, the system would have needed more time to produce the commanded velocity adjustment," he says. "As a safety precaution in all MESSENGER maneuvers, the team determines the maximum expected maneuver time and instructs the spacecraft to shut the maneuver down if that time is exceeded. That's what happened here, so the maneuver was stopped before it reached 100% of the planned velocity change."
The flight team is analyzing the data from the attitude control system and tracking data to identify what caused the jitter so they can design future maneuvers to avoid it. Although TCM-15 resulted in a shortfall, it was sufficiently successful that a contingency maneuver, held in reserve for May 5, is not needed. The team can accommodate all adjustments in TCM-16, scheduled for May 25, to direct the spacecraft to the intended aim point 337 kilometers (209 miles) above the surface of Venus.
For graphics of MESSENGER's orientation during the maneuver, visit the "Trajectory Correction Maneuvers" section of the mission Web site at http://messenger.jhuapl.edu/the_mission/tcm15.html.