Hope fades for missing Mars Global Surveyor craft
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 21, 2006
NASA's $377 million Mars Global Surveyor, the oldest of four spacecraft currently in orbit around the red planet, apparently fell victim to what amounts to severe arthritis Nov. 2 when one of its two solar panels jammed and stopped tracking the sun. While the 10-year-old spacecraft may still be alive, hunkered down in electronic hibernation awaiting instructions from Earth, flight controllers have not been able to regain contact and fear the aging satellite may be lost, officials said today.
"However, MGS has been a good friend, it's had an illustrious career, the data it's collected has taught us a lot about Mars and it will continue to teach us a lot about Mars," Li told reporters in a teleconference. "We're certainly feeling that we might be losing a good friend from our family here. We're still holding out some hope, but we are fully prepared in our hearts that we may never be able to talk to the spacecraft again."
The Mars Global Surveyor was launched Nov. 7, 1996. After a one-year cruise to Mars, the spacecraft braked into an elliptical orbit Sept. 11, 1997.
To save money, MSG was not designed to carry enough fuel to brake directly into a circular mapping orbit. That would have required a more powerful launch vehicle, a larger spacecraft and a much higher price tag. Instead, the flight plan called for repeated dips into Mars' atmosphere to lower the high point of the initial orbit.
That process should have taken four months or so to complete. But because of concern about the strength of one of the craft's two big solar panels, the so-called aerobraking maneuver was stretched out to a full year. MGS began studying Mars in earnest in April 1999.
The original mission requirement was to map the surface of the planet for two years. NASA recently approved the mission's fourth two-year extension.
But on Nov. 2 at 6:35 p.m. EST, when MGS emerged from behind Mars as viewed from Earth, telemetry indicated major problems with one of its solar arrays - the same array that caused concern when the spacecraft reached Mars in 1997.
"In fact, the spacecraft had decided on its own to switch over to the backup electronics that drives the motor that moves the solar array and also to move to a redundant power bus on the spacecraft," Li said. "The spacecraft then regained its functions and performed nominally through the rest of the orbit.
"Then it went behind Mars one more time and at about (8:27 p.m.) when we were expecting it to come back out from behind Mars to talk to us again, we were not able to re-establish nominal communications."
Three days later, flight controllers detected what may have been an extremely weak carrier signal from MGS during portions of four orbits. But nothing has been heard since then, despite more than 800 commands sent "in the blind" to restore communications.
Last Friday and again on Monday, cameras on NASA's recently arrived Mars Reconnaissance Orbiter searched the presumed orbital track of the Global Surveyor in hopes of visually spotting the spacecraft to determine its orientation and the orientation of its solar panels.
"Our preliminary analysis so far has not yielded any definitive images of MGS," Li said.
Tom Thorpe, MGS project manager at JPL, said that even in safe mode, flight controllers should have been able to re-establish communications.
"The solar array minus panel is the panel we believe cracked shortly after launch," he said. "It's deployment was very hard, we believe the damper arm broke and we noticed during cruise (to Mars) that the flexibility of the panel had increased. This ultimately resulted in a delay of our aerobraking by about a year's time as we were forced to aerobrake very gently through the atmosphere with the panel turned in the opposite direction. Now, that may be totally unrelated to this event, but the same panel seems to be the one that caused this problem."
On Nov. 2, the solar arrays had been commanded to a slight offset from the sun. Both panels moved as expected but subsequently, "this minus panel sent back errors in its tracking performance. So we went into eclipse. When we came out, there was no signal."
"During eclipse, the panels perform what's called an 'unwind,' so they're ready to see the sun when we come out of eclipse," Thorpe said. "That's about a 200-degree travel for these panels. We believe somewhere during that eclipse, the panel failed to move, got stuck. The problem is, we don't know at what attitude it got stuck.
"Now, when the fault protection software determined that the panel was stuck, it tried the backup gimbal, the backup electronics, then declares that panel stuck and moves the spacecraft so the stuck panel is face on to the sun and the other panel tracks the sun, giving us optimum power. That's not good for communications, however, and we believe that we lost communications due to that pitch."
But the details are not yet clear. In such a "safe mode," the spacecraft is programmed to change its orientation periodically to help a low-gain antenna receive instructions from Earth.
"That is not optimum for power and we don't know the attitude this stuck panel is in and that could provide a drain on the power available to the spacecraft," Thorpe said. "From there on, it's a question of are we losing power with time? We were unable to raise the transmitter on the spacecraft, that is one of the puzzles that still exists as to why we can't get a signal from the low-gain transmitter."
Later this week, NASA's Opportunity rover, one of two robots currently working on the surface of Mars, will attempt to pick up UHF signals from MGS as it passes overhead. If the spacecraft is still alive, its UHF transmitter may be functioning, providing clues about what went wrong and what might be needed to restore the craft to operation.
But engineers are not optimistic.
"We are now into the 10th year of operation," Li said. "If you look at a typical human life, I don't know what the consensus is, what a normal human life is, but it's probably around 70 years. It's almost like having a friend who's 350 years old."