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Two astronauts on the US space shuttle Discovery have begun a unique spacewalk to carry out heatshield repairs.

Armed with forceps and a saw Stephen Robinson will go beneath the orbiter's underside to remove two ceramic strips sticking out between heatshield tiles. Nasa engineers are concerned the strips may cause part of Discovery to overheat as it re-enters the Earth's atmosphere.

The crew has said they had some initial concerns about the job but say they are confident they can repair the shuttle.

'Simple task'

The US space agency has identified two gap fillers that are sticking out between heatshield tiles near Discovery's nose section, each by less than 3cm (1.2in).


Made of ceramic-coated cloth, the gap fillers act as seals between Discovery's heat-resistant tiles
Astronaut Stephen Robinson will attempt to remove a section of protruding filler either by hand or using a hacksaw

See tile areas of concern

The pieces of ceramic fabric, which act like a grouting for the tiles, were probably shaken out during the vibrations of launch, Nasa said.

Robinson, who is accompanied by crewmate Soichi Noguchi, has received instructions in a 12-page email to prepare him for the operation.

Now outside of the orbiter, he will be manoeuvred into position by the shuttle's Canadarm2 18-metre (58-foot) robotic arm, which for this operation has been attached to the ISS.

Once in place, Robinson will attempt to pull away the two pieces of ceramic-coated cloth with his fingers.

If they don't come away easily, he will use a pair of forceps to try to tweeze them out and if that fails use a hacksaw to cut them off.

"I'll have to be careful but the task is very simple," Robinson told reporters before the spacewalk.

'Easy decision'

Wayne Hale, the shuttle's deputy programme manager, said Nasa could not be certain what effect the protruding gap fillers would have as the shuttle heats up during re-entry.


No doubt about it, this is going to be a very delicate task, but as I say, a simple one
Stephen Robinson

Protrusions on the otherwise smooth belly of the shuttle could disturb the air flow during re-entry, causing turbulence that raises temperatures on heatshield tiles downstream.

"When we looked at the unknown versus what we do know about [spacewalks], it was a very easy decision," said Mr Hale.

"The bottom line is there is large uncertainty because no one has a very good handle on aerodynamics at those altitudes and those speeds. Given that large degree of uncertainty, life could be normal during entry or some bad things could happen."

Helmet watch

A team of managers, engineers and aerodynamicists had been working to address the issue for three days.

Nasa will want to be sure the spacewalk does not make matters worse - by damaging tiles in the process of pulling out the fillers.


Astronauts have cobbled together a make-shift hacksaw

Enlarge Image

"The thing I'll be watching most closely is the top of my helmet... because I'll be leaning in toward the orbiter... so that's what I'll be most careful with," Robinson said before embarking on the task.

Commander Eileen Collins said she had few worries about the re-entry set for Monday, 8 August.

This is the first orbiter flight since Columbia was lost with its crew in February 2003.

A suitcase-sized piece of foam fell from Columbia's external tank during launch, punching a hole through heatshield panels in the shuttle's left wing.

The damage did not present problems in orbit, but as the vehicle tried to re-enter the Earth's atmosphere, super-heated gases entered the wing and tore the ship apart.

Discovery has suffered minor tile damage to its protective tiles at launch because of flying foam debris, but Nasa says laser scans together with still-image and video surveys have shown up no major problems.