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Last Updated: Wednesday, 12 July 2006, 18:38 GMT 19:38 UK
Astronauts finish final spacewalk
Astronauts test shuttle repair kit   Image: Nasa TV
Sealant is applied (above) and then worked into the cracks (below)
Astronauts on space shuttle Discovery have completed 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 were 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.

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).

Space shuttle (Image: BBC)
Mission known as STS-121
Discovery's 32nd flight
18th orbiter flight to ISS
Landing: 0907 EDT (1307 GMT), Monday 17 July
Location: Kennedy Space Center
Objective: To test new safety equipment and procedures
Payload: Cargo bay has 12.75t of equipment and supplies
Crew: Lindsey, Kelly, Fossum, Nowak, Wilson, Sellers, Reiter
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 seven hour, 11 minute-long 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 spatula similar to a putty knife to stop bubbles from appearing in the material.

"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.

Mike Fossum and Piers Sellers in Discovery's cargo bay   Image: Nasa
The astronauts were outside the shuttle for six-and-a-half hours
The follow-up studies aboard Discovery will help engineers understand how temperatures affect the material's performance. The patch jobs will be put to the test in labs on the ground once the shuttle returns.

One of the spatulas used by Sellers to pat down the sealant disappeared when he was not looking. "No sign of the spatula; I think it's gone, gone, gone," he said.

Towards the end of the spacewalk, mission controllers gave the astronauts an extra task; transferring a fixed grapple bar, which allows the International Space Station (ISS) robotic arm to attach to objects in space, to a storage space outside the ISS.

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.


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