Hubble telescope shuts itself down

"Fluke" not regarded as serious

WASHINGTON -- The Hubble Space Telescope has shut itself down temporarily because of a computer programming error that rotated its communications antenna into a technical no-parking zone.

The Hubble went into a mode that shut down some but not all its systems and halted scientific work Monday morning. It is programmed to take this kind of action whenever necessary to protect itself from harm; in this case, it was at the hands of what officials called a "fluke" buried undiscovered in its millions of lines of computer code.

The antenna normally "parks" in a certain position between the times when it is required to move to transmit data to a relay satellite. But the Hubble computer sent the antenna a command that went 20 degrees off the scale, according to Edward Weiler, NASA's chief Hubble scientist.

This caused the antenna to "get lost," so the computer ordered it to swivel at a high speed to get back to its correct position. That action "exceeded its torque limit and shut it down," Weiler said.

On the relative scale of Hubble's troubles, this one is deemed not serious, Weiler said. The loss of precious time for scientific observations is expected to total just over two days.

Meanwhile, the $2.1 billion Hubble is churning out scientific data, and a number of papers based on its images will soon be published, despite problems with its solar arrays, gyroscopes and an observing instrument, as well as the infamous flaw built into its main mirror.

However, the shutdown is a reminder of the uncertainties involved in planning for a crucial repair flight, tentatively scheduled for November 1993. How to make the most of the flight is now being debated in NASA.

"If we all think we have a final list of things to do, we're being very naive," said Milt Heflin, a shuttle flight director at the Johnson Space Center.

Hubble managers may be forced to abandon some of the planned repairs if the time required for the crew to perform them all, with a margin of safety for unexpected problems, is more than four six-hour spacewalks by teams of two at a time, Heflin said.

By early spring, space-suited astronauts working in underwater tanks are expected to determine more precisely the required repair time.

Some Hubble officials would like to fly the mission aboard the newest orbiter, Endeavour, Heflin said. That orbiter, which has never flown and is behind schedule because of technical problems, could stay in orbit a day longer than other space planes in the fleet, providing up to five days for spacewalking repairs. This is because it can carry more fuel for the on-board electrical power supply.