posted 11-09-2006 08:09 PM               

The orbiter is the oldest of five NASA spacecraft currently active at the red planet. Its original mission was to examine Mars for a full Martian year, roughly two Earth years. Once that period elapsed, considering the string of discoveries, NASA extended the mission repeatedly, most recently on Oct. 1 of this year.

The orbiter has operated longer than any other spacecraft ever sent to Mars. It has returned more information about Mars than all earlier missions combined and has succeeded far enough beyond its original mission to see two later NASA orbiters arrive: Mars Odyssey and Mars Reconnaissance Orbiter. Among many important accomplishments so far, Mars Global Surveyor has found many young gullies apparently cut by flowing water, discovered water-related mineral deposits that became a destination for NASA's Opportunity rover, mapped the planet topographically and examined many potential landing sites on Mars.

On Nov. 2, one orbit after commands were sent for a routine maneuver to move the solar panels, the spacecraft reported that the motor moving one of the arrays had experienced errors. Onboard software responded as programmed, switching the spacecraft to a backup motor controller, then to a backup circuitry connection.

Following these indications of difficulty, a two-day lapse in contact occurred on Nov. 3 and 4. The signal from the spacecraft was received on Nov. 5 during four different orbits, but it did not carry any data from the spacecraft. The signal's frequency indicated that the spacecraft had entered safe mode, a pre-programmed state of restricted activity in which it awaits instructions from Earth.

No further signal was heard during subsequent orbits on Nov. 5 and Nov. 6. Engineers concluded that the spacecraft had made an additional pre-programmed response, intended to help it survive when it senses that a solar array is stuck. The spacecraft turns that array toward the sun to maintain its power supply and rotates the rest of the spacecraft in the same direction, thereby making communication with Earth less effective.

"The spacecraft has many redundant systems that should help us get it back into a stable operation, but first we need to re-establish communications," said Tom Thorpe, project manager for Mars Global Surveyor at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

posted 11-09-2006 08:13 PM               

If the spacecraft does not receive commands from Earth for seven days in a row, it is programmed to stop whatever it is doing and try to transmit a signal to Earth using its high gain antenna. This could happen at about 0014 GMT on Friday (1614 PST on Thursday), so NASA will be listening for a signal from MGS's high gain antenna at that time.

Even if the spacecraft cannot transmit a high gain antenna signal to Earth, it might still be able to listen to commands. So NASA will send a command to have the spacecraft point its low gain antenna at Earth as a fallback.

If no signal is heard on Thursday, NASA will listen again on Saturday, since it already tried to upload commands to the spacecraft on 4 November.

...if no signal is heard on either day, NASA may call on the Mars Reconnaissance Orbiter (MRO) to take pictures of MGS early next week. The two spacecraft pass within about 100 kilometres of each other several times each week.