Anomalous Force on the Wilkinson Microwave Anisotropy Probe

摘要

The Wilkinson Microwave Anisotropy Probe orbits the outer Earth-sun libration point, about 1.5 million kilometers outside Earth's orbit, mapping cosmic microwave background radiation. To achieve its orbit on a small fuel budget, the spacecraft needed to perform a lunar gravity assist maneuver. Timing the lunar swing-by required the spacecraft to travel in three high-eccentricity phasing loops with critical maneuvers at a minimum of two, but nominally all three, of the perigee passes. On the approach to the first perigee maneuver, spacecraft telemetry showed a considerable change in system angular momentum that threatened to cause the onboard failure detection and correction software to abort the critical maneuver. Fortunately, the system momentum did not reach the preset failure detection limit; however, the mission operations team did develop a contingency strategy should a stronger anomaly occur before or during subsequent perigee maneuvers. Simultaneously, the team developed and tested various hypotheses for the cause of the anomalous force. The final hypothesis was that water was outgassing from the thermal blanketing and freezing to the cold side of the solar shield. As radiation from Earth warmed the cold side of the spacecraft, the uneven sublimation of ice created a torque on the spacecraft.