At 4:30 a.m. PDT on Thursday, the flight software onboard Mars Global Surveyor commanded the spacecraft into safe mode. Entry into this operational mode placed the spacecraft in a safe power, thermal, and communications configuration. This precautionary measure is taken if the spacecraft detects an unexpected event in one or more of its subsystems.
The chain of events that resulted in safe mode began Wednesday night. At that time, the flight team was finishing the second of two calibrations of Surveyor's gyroscopes. These calibrations involved commanding the spacecraft to rotate in various directions in order to ascertain the performance of the gyroscopes. Surveyor had just completed the calibration that involved a +Z-axis rotation when the flight software commanded the spacecraft into contingency mode. This mode is similar to safe mode, but involves fewer precautionary measures taken to safe the spacecraft.
Entry into contingency mode was triggered when the direction to the Sun as measured by Surveyor's Sun sensors disagreed with the predicted direction to the Sun as calculated by the onboard flight software. This discrepancy in Sun position was approximately 5 degrees. Entry into safe mode occurred about five hours later when a flight software task timed out and failed to report back Surveyor's central processor. At this time, the flight team is identifying the software task that timed out.
The entry into contingency and safe mode resulted in the flight software terminating the execution of the current command sequence, powering off the science payload and non-essential components, and turning the spacecraft toward the Sun to guarantee adequate power. Analysis of telemetry transmitted from Surveyor over the last 24 hours indicates that all systems are healthy. After the exact cause of safe- mode entry is identified and resolved, the flight team will command the spacecraft back into its normal operational mode. This process will consume at least the next few days.
Late Thursday night, the flight team transmitted a series of commands to Surveyor for thermal maintenance purposes. One set of commands shut off the secondary set of heaters to avoid overheating the spacecraft's 12 attitude-control thruster rockets. The other set of commands changed Surveyor's pointing orientation from high-gain antenna pointed directly toward the Sun to antenna pointed 10 degrees away from the Sun. This orientation change allowed for more sunlight to maintain warm temperatures on the science instruments.
After a mission elapsed time of 183 days from launch, Surveyor is 101.43 million kilometers from the Earth, 33.90 million kilometers from Mars, and is moving in an orbit around the Sun with a velocity of 23.58 kilometers per second. This orbit will intercept Mars 125 days from now, slightly after 6:00 p.m. PDT on September 11th (01:00 UTC, September 12th). Although the spacecraft is currently operating in safe mode, all systems are functioning properly, there are no spacecraft hardware problems, and there is no threat to the mission.
Status report prepared by:
Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109