The Hubble owes its remarkable ability to make discoveries to the fact that astronauts have visited it repeatedly, not just to replace batteries and ailing gyroscopes but to install new instruments that keep expanding the telescope's abilities.

In tonight's spacewalk, astronauts began to replace failed gyroscopes, two electronic control units and some fuses.

National Aeronautics and Space Administration officials said the shuttle Discovery and a team of space-walking astronauts would be dispatched to replace malfunctioning gyroscopes that stabilize the 12.5 ton telescope and allow it to point accurately at targets across the universe.

In a series of spacewalks, astronauts from the shuttle Atlantis will replace vital gyroscopes and batteries and install a new camera and a new spectrograph, extending the telescope's capabilities into new realms of the electromagnetic spectrum, Dr. Griffin announced in October.

A crew of seven, led by Scott D. Altman, who commanded the last Hubble repair mission in 2002, will capture the observatory in its 380-mile-high orbit and conduct five spacewalks to replace its aging batteries and stabilizing gyroscopes.

SED16 sensor has been the first to be used to replace gyroscopes in satellites.

Mr. O'Keefe announced on Jan. 16 that for safety reasons he would not authorize a fourth shuttle mission to the telescope to upgrade its instruments and replace gyroscopes and batteries that would stretch its life past the end of the decade.

For example, Dr. Kirshner said, the last 10-year report by astrophysicists, in 2002, assumed that shuttle astronauts would be sent to the Hubble Space Telescope to replace batteries and gyroscopes and to add new instruments, two of which are being built.

The next trip, scheduled for next year, would have replaced batteries and gyroscopes and installed two new instruments.