Astronauts on space shuttle Discovery have started the third and final spacewalk of the current mission.Piers Sellers and Mike Fossum headed outside on Wednesday to practise repair techniques for the ship’s heat shield.

They are testing a sealant for fixing cracks in wing panels which must be able to survive searing temperatures when the shuttle returns to Earth.

The US space agency, Nasa, has given Discovery a clean bill of health and cleared it for a return to Florida.

The repair techniques were developed as part of Nasa’s attempts to improve flight safety in the wake of 2003’s Columbia disaster.

“Our number one goal is to never have to use them,” said Nasa lead flight director Tony Ceccacci.

Polymer filler

At 0720 EDT (1120 GMT), Sellers and Fossum began making their way to the back of Discovery’s cargo bay to open a box that contains a pallet of 12 pre-damaged panels made of the composite material Reinforced Carbon-Carbon (RCC).

These panels are identical to those which line the shuttle’s wing leading edge protecting the orbiter’s aluminium shell from re-entry temperatures that get as high as 1,700C (3,000F). During the six-and-a-half-hour spacewalk, the astronauts filled cracks in the panels using a caulk gun loaded with a gloopy sealant called Noax.

Wearing bulky spacesuit gloves, Sellers and Fossum massaged the sealant with a putty knife to keep it from bubbling in zero gravity.

“Those bubbles behave differently in [low gravity] - they don’t rise to the surface,” Mr Fossum said on Tuesday. “If you have too many bubbles trapped beneath the surface, then you’re not going to get the repair you want.”

The Noax substance - which is a polymer with the consistency of peanut butter - was tested during the first post-Columbia mission last year.

The follow-up studies aboard Discovery will help engineers understand how temperatures affect the material’s performance.

It was RCC panels on Columbia that were pierced by flyaway foam from the vehicle’s external tank during launch - damage that led directly to the ship’s destruction on re-entry.

Discovery’s own surface tiles were subjected to a thorough examination following the vehicle’s launch from the Kennedy Space Center (KSC) in Florida last week; and although the investigation found some defects in the heat shield they were not considered serious enough to prevent the orbiter’s safe return to Earth.

A landing at KSC has been scheduled for 0914 (EDT) on 17 July.