The Hubble Space Telescope went into "zero gyro safemode," a form of electronic hibernation, early Tuesday when one of the three stabilizing gyroscopes then in operation failed. The telescope is programmed to stop science observations, spin down its remaining gyros and go into safe mode whenever fewer than three gyros are running.

Credit: NASA

Engineers knew gyro No. 3 was having problems but in two previous instances when the device stalled, they were able to get it restarted. This time around, repeated efforts proved fruitless and it's now considered down for the count.

Engineers hope to resume science operations by 8 p.m. this evening after restarting gyros 1, 2 and 4. Gyro No. 6 will be held in reserve for use as needed. Gyro No. 5 failed quite a while back and is no longer operational.

"Gyro 3 has been ill for well over a year," said Hubble project scientist David Leckrone. "We did some tests on it during the last (shuttle) servicing mission. It made it through those tests with flying colors and since we were tight on EVA (spacewalk) time, we elected not to replace it. We've managed to nurse it along now for over a year."

Hubble is equipped with six gyroscopes but only requires three to do science. In its current configuration, one more gyro failure can be tolerated with no impact to science. Playing it safe, Leckrone said engineers are working on computer programming changes that would enable Hubble to carry out at least some observations with just two operational gyros. But at present, that scenario is two more failures away.

Before the shuttle Columbia's destruction in February, the next Hubble servicing mission was targeted for November 2004. When that mission might get off the ground under NASA's still-evolving return-to-flight scenarios is not yet known. But engineers are hopeful Hubble's gyros will remain healthy enough for normal science operations until the next shuttle visit, whenever that might be.