HOUSTON — As the crew of the space shuttle Challenger worked Tuesday to repair the most important experiment on board, space agency officials said that two faulty thermometers may have caused a premature engine shut-off during the spacecraft's Monday launching, forcing it into a lower orbit that could affect the success of the scientific studies on the seven-day mission.
If an analysis of the engines after Challenger returns to Earth Monday proves that the two heat sensors were at fault, that will be good news for the National Aeronautics and Space Administration, which hopes to launch the shuttle Discovery later this month. A full-blown engine failure would probably require a lengthy review before another shuttle is launched.
The Challenger, carrying 13 experiments ranging from astrophysics to a study of how plants grow in zero gravity, had pulled itself into orbit despite the dramatic engine shutdown less than six minutes after liftoff from the Kennedy Space Center.
Besides the lower orbit--195 miles from Earth, about 40 miles lower than planned--the emergency caused the shuttle crew to jettison precious fuel that was needed for some of the experiments.
Officials at the Johnson Space Center here said they hoped for some degree of success with most of the experiments. But the astronauts hit a major problem Tuesday with the malfunction of a $60-million European-built telescopic guidance system that will be used on a shuttle mission next year to study Halley's comet.
The device, known as the Instrument Pointing System, carries four experiments and is designed for detailed tracking of the sun. The machine is powerful enough to track a dime from two miles away, but Tuesday it was unable to home in on the sun with the degree of accuracy needed to complete the experiment.
Software May Be Faulty
Flight Director Al Pennington said that the trouble could be in the software of the instrument and that it might be repaired by deprogramming it and inserting new data.
"Basically, the orbiter is in absolutely fine shape," he said.
But he noted that, because of the fuel that had to be jettisoned and the lower orbit, experiments that called for the firing of Challenger's maneuvering rockets would have to be scaled down to make sure there was enough fuel for a return to Earth. The major experiment in which the maneuvering rockets are needed involves using them to burn small temporary "holes" in the Earth's upper atmosphere for a radio astronomy study. NASA officials said that only two of the eight burns were expected to be carried out.
The seven-member crew, commanded by Gordon Fullerton, worked through the day on the experiments, contained in the shuttle's cargo bay within a spacelab built by the European Space Agency.
Late Tuesday afternoon, NASA issued a statement saying that the tiny wire sensors, and not engine failure, were almost certainly the cause of the liftoff emergency. On-board computers, fed readings that indicated a fuel pump had overheated, shut down one of the main engines. The crew was then instructed to override the sensors on a second engine to keep the computers from shutting it down.
Previous Sensor Failures
The statement said that there had been sensor failures in the past, but not two in the same engine. New sensors will be used on future shuttle missions, the statement said.
"Trouble-shooting to verify the sensor failures will begin as soon as Challenger lands at the conclusion of this Spacelab 2 mission," the statement said.
"It's been an adventuresome day," said Fullerton, referring to the liftoff Monday.
But the problems of the first two days did not diminish the enthusiasm of mission scientist Loren W. Acton.
"You just cannot believe how beautiful this payload is when the sun comes up," he said. "There is more pretty stuff out there than you can shake a stick at against the black sky."
Other members of the crew are pilot Roy D. Bridges Jr., flight engineer F. Story Musgrave and scientists Karl Henize, Anthony England and John-David Bartoe.