New images of Endeavour confirmed last night that the shuttle’s protective thermal shield has been breached, prompting an intensive flurry of tests to establish whether it could be at risk during its return to Earth.

Managers at Nasa said that laser assessments of a small gouge on the spaceship’s underside, close to the right wing, had revealed that the damage has pierced the entire thickness of a heat-resistant tile, raising the possibility that hot gases could seep in and overheat the vehicle during re-entry later this month.

They stressed, however, that based on the small size of the hole and its position on the shuttle, it was still unclear whether such a risk exists - and even if it does, the crew is equipped to carry out simple repairs while in orbit to make it safe.

“This is something that we would rather not deal with, but we are really prepared for exactly this kind of case,” said John Shannon, chairman of Nasa’s mission management team, last night.

“I feel very comfortable that whatever is required we can go and do - and do successfully.”

Technicians at Johnson Space Centre in Houston, Texas, will today run tests simulating how the gouge will react under the scorching conditions of atmospheric re-entry, when that particular area of the shuttle can endure temperatures of up to 2,300F (1,260C).

The damage being investigated involves a small patch measuring just one inch by 0.2 inches.

Endeavour is currently docked at the International Space Station, 220 miles above Earth, where it does not face any heating risk. Two astronauts will also undertake today the second of four scheduled spacewalks to install a new gyroscope on the ISS.

The damage to Endeavour’s heatshield was caused by a piece of solid insulating foam flying off a bracket on the external fuel tank during launch from Cape Canaveral, Florida, last Wednesday, and striking the shuttle’s under-belly.

Mr Shannon admitted last night that similar problems have occurred on previous flights, causing managers to now consider a redesign of the bracket area for future missions.

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