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Last Updated: Sunday, 31 July 2005, 00:44 GMT 01:44 UK
Nasa to give Discovery all-clear
By Paul Rincon
BBC News science reporter, Houston

Mission specialist Charles Camarda waves from the mid-deck of the shuttle Discovery
The Discovery has not yet been given the all-clear
Space shuttle Discovery is expected to be given the all-clear for its return to Earth.

Orbiter managers have said the heatshield tiles and insulation blankets are in good shape for the fiery re-entry to Earth's atmosphere.

A six-and-a-half-hour spacewalk to test repair techniques for heatshield tiles was completed successfully by two of Discovery's astronauts on Saturday.

Nasa has decided to extend the mission by one day.

Heatshield panels that line the shuttle's wing leading edge are expected to be cleared on Monday.

But some outstanding issues are preventing managers from giving Discovery 100% clearance to return.

Rarely does an image and a feeling grip one than your first time stepping out of the vehicle on a spacewalk and looking down 250 miles and travelling at 18,000mph
Dave Wolf
Nasa astronaut

These include protruding gap fillers on Discovery's underside and black spots on reinforced carbon-carbon panels that line the wing leading edge.

Mission managers have also formally approved extension of Discovery's 12-day mission by a single day, at the request of International Space Station officials.

Discovery's damage assessment is based on data from inspections of the shuttle in space using a 15m-long boom.

These showed some pockmarks on the shuttle's underside and one gouge near the nose landing gear doors.

They also found that a thermal insulation blanket had billowed out just under the commander's window.

Space walk trim?

"All of those items have been formally assessed through rigorous engineering models in great detail and found acceptable to fly home as is," shuttle deputy programme manager Wayne Hale told reporters in Houston, Texas.

But two gap fillers are known to be sticking out from the shuttle's underside just aft (rear) of the nose landing-gear doors.

Astronaut Steve Robinson takes picture of fellow astronaut Soichi Noguchi during spacewalk
The two astronauts spent nearly seven hours on the outside
Made of a ceramic fabric, these plug spaces between heat-shield tiles.

When gap fillers protrude, they can disturb the aerodynamic flow under the shuttle as it re-enters the atmosphere. This can raise temperatures on heatshield tiles downstream of the protrusion by as much as 200 degrees.

It can also have implications for flight control.

Shuttle managers have been mulling whether to send an astronaut out on a spacewalk to physically trim the fabric fillers.

"That is not completely off the table," said Mr Hale.

Black spots

The deputy programme manager said he believed experts assessing images of the reinforced carbon-carbon panels lining the wing leading edge would formally clear those for re-entry on Monday.

"Once we get that clearance we will be able to tell you the heatshield of Discovery is completely ready for re-entry," he said.

If commercial flights carried the same risks as spaceflight, there would be about 500 airliner crashes per day
Bob, Cheshire, England

The only issue that has provoked any concern is the appearance of small black spots on these panels.

But Mr Hale said he thought the team now understood this phenomenon, which was probably down to a form of surface blistering and did not have important implications for the return to Earth.

Shuttle experts think the protruding gap fillers may have be shaken out by vibrations during the climb to orbit.

All the other minor damage to the shuttle is thought to be caused by the impact of foam debris from the external tank.

Extra supplies

Early on Saturday, astronauts Steven Robinson and Soichi Noguchi carried out the first of three spacewalks scheduled for the STS-114 return-to-flight mission.

They tested repair techniques for the heatshield tiles and carried out maintenance tasks for the International Space Station (ISS), including the restoration of power to a failed control moment gyroscope, replacing a GPS antenna and recovering materials experiments mounted outside the station.

Long lens view of Discovery (Nasa)
Experts are still confident the shuttle will return safely to Earth
"It was overall a great day for getting things done on the station in a timely manner," said Bill Gerstenmaier, ISS programme manager.

Discovery will transfer more hardware and supplies during the day-long extension to its period docked to the ISS in case another shuttle does not return for some time.

Despite Discovery sustaining far less damage than the average for previous shuttle flights, Nasa has grounded its fleet until the problem of foam debris from the external tank has been solved, placing a scheduled September launch in doubt.

Multiple chunks of debris were shed during Discovery's climb to orbit, the largest of which weighed about one pound - slightly smaller than the debris that hit space shuttle Columbia in 2003, tearing a hole in its wing that allowed superheated gases to enter the orbiter on re-entry, ripping it apart and killing seven astronauts.

Despite the foam problem, Nasa administrator Mike Griffin has said he is optimistic the agency can solve it and fly another shuttle mission this year.

See the astronauts embarking on the spacewalk

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