The MESSENGER team announced today that they accomplished the penultimate trajectory correction maneuver necessary to line the spacecraft up for its second gravity-assist flyby of Venus, due to take place about a month from now, on June 6. According to the mission update, there was a slight hiccup with this maneuver, but it wasn't serious enough to require any contingency actions to be taken:
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.
I'm really looking forward to this flyby. Unlike the first Venus flyby, this time around MESSENGER is going to be able to do science, taking pictures and other data. And judging from the beautiful animation they captured as they receded from the first Earth flyby on August 2, 2005, the MESSENGER images of Venus should be quite spectacular.