Steve Robinson approaches Discovery's underside. Click here for a video showing removal of the gap filler.© NASA

It has been a troubled mission for Discovery, but today's task could not have gone better for the crew.

In an unprecedented attempt to repair the shuttle during a spacewalk, astronaut Steve Robinson easily plucked two protruding flaps of fabric from the shuttle's heat shield.

The pieces of ceramic-coated fabric, which are meant to provide a cushion between Discovery's protective tiles, were found poking through the shuttle's skin during an inspection of the craft on 31 July. Mission managers feared that turbulence around the material could cause the craft's skin to overheat during re-entry into the Earth's atmosphere.

The fix has given NASA the chance to prove its commitment to safety. This mission was always intended to be a test flight for the safety modifications made since the Columbia accident - but no one anticipated in-flight repairs.

Announcing the decision to make the repair, deputy shuttle programme manager Wayne Hale explained: "This is the new NASA. If we cannot prove that it's safe, then we don't want to go there."

Damage limitation

The fibrous filler was spotted sticking out of Discovery's underside soon after it arrived at the International Space Station.© NASA

This mission was always bound to reveal more dents, dings, and other problems with the shuttle than have been known about in-flight before, thanks to a new battery of cameras and sensors plastering the craft.

International Space Station (ISS) crewmembers spotted the dangling fabric while Discovery performed a slow somersault just before it docked with the station, allowing its belly to be inspected.

Previous missions have flown with protruding pieces of fabric, mainly because no one knew about them until the craft returned to Earth. In 1995, Columbia safely landed with a similar piece of filler sticking out of its tiles.

On this re-entry, Columbia's tiles heated up about 1,400 °C, compared with the usual 1,200 °C. Engineers suspected that the loose filler could have been the culprit.

Take out the trash

Carrying out the first in-orbit repair job on a shuttle sounds dangerous, but the work was actually quite simple.

Robinson was strapped to the end of the station's robotic arm and manoeuvred into place. He had a makeshift hacksaw and forceps in case the filler proved too stubborn to move. But in the end the stray piece came out very easily, in just a few seconds. After removing the second flap of fabric, Robinson said: " It looks like this big patient is cured."

He was assisted by fellow crewmember Soichi Noguchi in the spacewalk, which lasted about six hours. The pair also fitted a specialized tool rack to the outside of the ISS, which will help with future building work, and mounted an experiment on the outside of the station that will track how solar cell materials behave when exposed to cosmic rays.

Now the crew's main job is to transfer supplies from the shuttle to the ISS, before loading Discovery up with trash for the return journey on 8 August.