NASA extends shuttle mission for repair

The starboard S3/S4 truss segment is attached to the International Space Stations' arm over the Earth in this NASA TV June 11, 2007 video grab. A team of spacewalkers will install the (S3/S4) truss segments during todays planned six-hour spacewalk.

HOUSTON (Reuters) - NASA said it will fix a worrying rip in space shuttle Atlantis' heat shield system and spacewalking astronauts hooked a giant metal truss to the International Space Station on Monday.

The U.S. space agency's decision to be safe rather than sorry meant Atlantis will stay in a space two days more than planned and a fourth spacewalk will be added to the flight.

NASA does not think the inches-long tear in a thermal blanket near the tail poses a great danger to the shuttle, but deputy shuttle program manager John Shannon said mission managers were being cautious.

"It was 100 percent consensus that the unknowns in the engineering analysis and the potential damage ... was not acceptable and we wanted to go and fix it," he said in a briefing at Johnson Space Center.

NASA engineers have been eyeing the torn blanket since it was spotted shortly after Atlantis took off on Friday.

If not fixed, the tear could cause heat damage to the shuttle when it returns to Earth on its new landing day, June 21, Shannon said.

NASA has treated heat shield issues with great caution since Columbia disintegrated during its return home in 2003 after its wing heat shield was damaged during its launch by loose insulating foam from the fuel tank.

The seven astronauts on board were killed.

FIRST SPACEWALK

On Monday, Atlantis astronauts James Reilly and John "Danny" Olivas floated out of the space station's airlock at 4:08 p.m. EDT (2008 GMT) to begin the first of four spacewalks during the 13-day mission.

The shuttle reached the space station, a project involving 16 nations that is slightly more than halfway built, on Sunday after a two-day chase in Earth's orbit.

They spent much of the day installing on the station a 45-foot-(14-metre) long, 35,678-pound (16,183-kg) metal structure flown up by Atlantis. The $367 million unit includes a pair of wing-like, electricity-generating solar panels scheduled to be unfurled on Tuesday.

A second spacewalk is scheduled for Wednesday, when astronauts Pat Forrester and Steve Swanson will go out to retract an old solar panel.

Whether the blanket repair will occur on the third spacewalk on Friday or the additional fourth spacewalk had not been decided, said mission operations representative Joe Montalbano.

If the repair is performed during the third, the fourth will include a variety of tasks put off on previous missions, said lead spacewalk officer Kieth Johnson.

NASA plans another 12 trips to the station to haul major components and laboratories before retiring the shuttles in 2010. The U.S. space agency also would like to make two additional missions to stash spare parts at the station and make a final servicing call to the Hubble Space Telescope.

Eventually the station's backbone will span 365 feet (111 metres) and support four massive U.S.-built solar wings that stretch out 240 feet (73 metres) from end to end.

The last set of panels, needed to power laboratories built by Europe and Japan, are scheduled to be flown to the station in late 2008 or early 2009.