SIRTF: "Update: 27 August 2003
This is Day 3 of the 90-day In-Orbit Checkout and Science Verification period. Initial checkout of the pointing and control system has been completed. Pointing performance meets expectations, and pointing stability or 'jitter' over a 10-minute period is excellent. During a test of the reaction wheel system, which controls the orientation of SIRTF, one of four reaction wheels was unable to supply the requested torque. (Only three reaction wheels are required to operate SIRTF; any one of the four can be regarded as redundant.) The fault protection system responded correctly by turning off that wheel and putting SIRTF in safe mode. Preliminary analysis suggests that this is a temperature effect. Friction internal to reaction wheels, called 'drag torque,' increases at low temperature, affecting some wheels more than others, and the temperature of the wheel in question is now at its minimum for the mission. When the science instruments are turned on, its temperature will rise. Additional analysis is in progress. All other SIRTF systems are operating very well, both on the spacecraft and with the teams at NASA's Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena, and Lockheed Martin Astronautics in Denver. "