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Postmortem on Loss of Solar Probe
16 July 1998 7:00 pm
Faulty computer code compounded by a disastrous command from ground controllers caused the SOHO spacecraft to spin out of control last month, officials reported today. The Solar and Heliospheric Observatory--operated jointly by NASA and the European Space Agency (ESA)--went radio silent on 25 June (see ScienceNOW, 26 June) and hasn't been heard from since. Researchers are still hopeful they may be able to reestablish contact with the sun-watching satellite in the next few months.
An ESA and NASA investigative board identified three separate causes for the accident. The first was a preprogrammed software sequence that lacked a command to turn on a gyroscope necessary to reorient SOHO toward the sun in case of an emergency. Tragedy ensued when a second faulty command sequence caused one of the spacecraft's three gyroscopes to return an incorrect reading. That reading sent SOHO into Emergency Sun Reacquisition (ESR) mode, which failed because of the first bad bit of code. Finally, controllers on the ground, confused about which gyro was malfunctioning, sent SOHO a command to turn off a functioning gyroscope. That sequence of events apparently sent the craft into a spin that misaligned its communications antenna and turned its solar panels away from the sun.
The hasty decision to shut down the working gyroscope was the direct cause of the loss, says Michael Greenfield, an official in NASA's Office of Safety and Mission Assurance, who co-chaired the review panel. "I think the team had sufficient time in an ESR--over 24 hours the spacecraft would have been stable--to reevaluate what to do," says Greenfield. "You generally stop, call in experts, senior management. That was not done."
But Joseph Gurman of NASA's Goddard Space Flight Center and the U.S. project scientist for SOHO, who was on vacation at the time, also emphasizes the errors in writing and testing the software. "Each of us can share part of the blame," he says.