Astronauts on space shuttle Discovery are set to carry out the mission's first spacewalk, which will include a test of repair kits for the orbiter.

Soichi Noguchi and Steve Robinson will step outside Discovery in their spacesuits at 0845 GMT.

This will be the first time either astronaut has carried out a spacewalk.

Nasa is continuing its analysis of images taken to inspect for damage sustained during the climb to orbit, when foam debris came off the shuttle.

"Mission management is ready for the first American spacewalk in some time. We're all excited about the tasks that are going to be performed tomorrow," said Wayne Hale, the shuttle's deputy programme manager.

Preparations

Discovery is also expected to extend its 12-day mission by one day, in response to a request by the International Space Station (ISS) to help its crew carry out additional work.

On the first of three spacewalks scheduled for the STS-114 mission, Noguchi and Robinson will test repair methods for the shuttle's heat shield tiles, restore power to a failed control moment gyroscope, which, together with others, helps the station maintain its correct position in space.

Noguchi will demonstrate a repair technique called the Emittance Wash Applicator, which will apply a gritty hardening paste to tiles with simulated damage held in a box. He will have an hour to complete his task.

Meanwhile, Robinson will work with a repair kit the Reinforced Carbon-Carbon panels which line the wing leading edge, nose cap and other areas of the shuttle that are exposed to the highest heating.

Robinson will use a gun which dispenses a sealant called Noax (Non-oxide Adhesive Experimental) on to a sample panel with artificial damage. He will then use scrapers a bit like putty knives to work the material into the sample.

On Friday, the astronauts began preparing for the walk by breathing pure oxygen for one hour to purge nitrogen from their bloodstreams. This will help protect them against the bends.

Discovery's crew have also dropped the cabin pressure in the shuttle to help the spacewalkers prepare for the lowered pressure they will experience in their spacesuits.

The spacewalk is due to end at 1515 GMT.

Analysis of images and data gathered through inspections of Discovery with a 15m-long boom equipped with a camera and laser scanner are ongoing. But Discovery crew member Andy Thomas said on Friday that the orbiter was "remarkably clean".

Risk assessment

Laser scanning of a damaged tile in a sensitive area near the nose landing gear doors shows the chip is three quarters of an inch long, three inches wide and a third of an inch deep.

Shuttle deputy programme manager Wayne Hale said the depth of the damage was such that it is not likely to have breached the shuttle's outermost thermal barrier, which sits below the tiles.

Mr Hale said experts had now found two "gap fillers" protruding from the underside of Discovery.

Made of a ceramic fabric, these plug spaces between heat-shield tiles as they follow the curvature of the shuttle's underside.

These protrusions can disturb the aerodynamic flow under the shuttle as it re-enters Earth's atmosphere, increasing air turbulence and raising temperatures heat shield tiles downstream of the protrusion.

He said he thought temperatures could be raised in the region of 100 degrees or so, but experts were assessing the possible impact of this on re-entry. Mr Hale added that part of a thermal protection blanket around a window had come loose.

"All these things are not serious in the sense that they do not cause us immediate alarm.

"We are going to go through and understand with engineering rigour and analysis how these things are going to perform on entry," said Mr Hale.

The mission is the US space agency's first in the two-and-a-half years since the Columbia accident. But shuttle flights have been grounded once more after multiple pieces of foam debris were shed during launch.

A suitcase-sized piece of foam punched through Columbia's wing during launch causing super-heated gases to enter the wing and tear the orbiter apart when it attempted re-entry 16 days later.

Earlier on Friday, mission commander Eileen Collins said she was "surprised" and "disappointed" to hear about the foam debris that had peeled off during ascent.