image from the internet- wiki
image from the internet- wiki
Aug. 21 proved a remarkably bad day for the nation’s space program. NASA engineers lost all communication with the Mars Observer shortly before the $980 million spacecraft was to begin orbiting the Red Planet. And NOAA-13, a new weather satellite, fell silent as well.
The reason for the sudden silence from the Mars Observer remains a mystery. Glenn Cunningham, project director for Mars Observer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., says he remains “cautiously optimistic” that the craft has retained a set of critical commands — designed to send it into an elliptical orbit around Mars on Aug. 24 — beamed to the craft the day before communications ceased.
But he and other scientists worry that the commands may have become distorted or erased from the craft’s on-board computer. If so, Mars Observer could not have fired its engines as planned and thus will not go into a low orbit around the planet. The craft simply may have sailed past Mars, ending a two-year mission before it ever began.
If communications should resume by the end of August, engineers could still instruct a wayward Mars Observer to orbit the planet. But calculations show the craft would take 40 days in a highly elliptical path to orbit Mars instead of the 118-minute circular path desired for a detailed study of the planet.
Trouble began about 9 p.m. Eastern Daylight Time on Aug. 21. Technicians had commanded the craft to pressurize its fuel tanks, a procedure that requires the craft to detonate a valve so that helium gas can flow into the tanks. Because the explosion might shock the craft’s transmitters, technicians had deliberately switched them off minutes before, silencing the craft’s only link with Earth. Five minutes later, with pressurization completed, engineers commanded the transmitters to turn on again. The craft remained mute.
Cunningham says there’s “less than a 0.1 percent chance” that the valve detonation –a routine procedure with no more power than a firecracker–might have destroyed the craft. But a NASA contractor who wished to remain anonymous was less confident: “They [JPL officials] are calling it a ‘loss of downlink.’ But you might just as well call it a ‘loss of spacecraft.'” He added that if a leak occurred in the tanks as pressure increased, it could have spun the craft wildly, hurling its solar arrays and other vital parts into space.
Scientists familiar with the Venus-orbiting Magellan spacecraft said that its transmitters — built by a different company than those on the Mars Observer–were not turned off before valve detonations. “That’s just the time [during detonation] when you’d like to have as much communication with the craft as possible,” notes Frank McKinney, project manager for spacecraft mission operations at Martin Marietta Astronautics in Denver.
Cunningham says that if the transmitters had remained on during pressurization, engineers might have gathered more hints about the cause of the communications failure. But he adds that the manufacturer had recommended that NASA turn off the transmitters during such procedures, advice followed previously without problems.
NASA expressed even greater pessimism about the fate of NOAA-13, a new weather satellite intended to replace an aging, but identical model that monitors Earth from a polar orbit. Researchers attribute the loss to a bad battery and don’t expect to recover a signal.