Astronauts finish Hubble repairs

Two US astronauts on Monday put the final vital maintenance touches on the Hubble Space Telescope, replacing old insulation and a guidance sensor on the ageing satellite before its scheduled release back into orbit Tuesday.

The seven-hour-plus space walk completed a marathon five straight days of work in a risky mission by the Atlantis shuttle that entailed a stand-by shuttle on the launch pad in Florida for a rescue mission if needed.

NASA had originally decided against the maintenance mission because of the risk involved and the pressures to complete International Space Station construction by 2010. But US politicians and world astronomers fought successfully to keep alive the instrument that has expanded knowledge of space history by billions of years back into time.

Astronauts John Grunsfeld and Drew Feustel Monday replaced a second battery pack on the telescope plus one of its three fine guidance sensors, and covered sensitive equipment with thermal blankets on three bays to protect them from the extremes of space.

The fine guidance sensors provide pointing information for the spacecraft and help determine the precise locations of stars. They are so precise it is akin to focusing a laser beam on a coin 320 km away, NASA said.

It was not clear if Grunsfeld and Feustel had also carried out the intended replacement of a second 209-kg battery module to help power Hubble's telescope during the night portion of its orbit.

The first battery module was installed in an earlier spacewalk.

After 5 hours and 26 minutes, the two men were cleaning up debris before "closing up the telescope for good," NASA said on its website. It was to be the shortest of the mission's five spacewalks.

The thermal blanket replacements were to have been completed Sunday, but the task was dropped after problems with a stripped bolt put spacewalkers Mike Massimino and Mike Good two hours behind schedule.

In the mission, the space shuttle Atlantis crew has also installed a new camera and spectrograph on the telescope, replaced a computer and the gyroscopes that keep it aligned, and repaired its Space Telescope Imaging Spectrograph.

The Atlantis crew is to release Hubble back into orbit starting at 1253 GMT Tuesday and head back to Earth Friday to land at Kennedy Space Centre in Florida.

Scientists say the repairs and upgrades, which US space agency NASA hopes will extend Hubble's functioning life span until at least 2014, will continue to provide clues about the origin and nature of the universe.

Since its launch in 1990, Hubble has helped scientists to place the age of the universe at 13.7 billion years, learn that black holes are at the centre of most galaxies, monitor planetary formation and discover that the universe is expanding at an ever-faster pace.