The space Shuttle Discovery is fit to return to Earth, NASA says, after the inspection of the heat shield found no problems with the insulation.

The crew completed the inspection of the craft on day four of the mission. NASA engineers on the ground have been reviewing the footage of the Shuttle since then, both from the launch and images taken by the crew, and have now declared it 100 per cent cleared for re-entry.

Today, mission specialists Piers Seller and Mike Fossum will go outside to continue their work on the International Space Station's mobile transporter. They will also install a spare pump for the thermal control system on the outside of the Quest Airlock, NASA says, in a space walk that should last for around six and a half hours.

Another space walk is planned for Wednesday, during which the crew will try out repair techniques that could be vital if a future mission did sustain damage on its way into orbit.

Discovery, the only Shuttle to fly since the loss of Columbia, is due to return to Earth on 17 July. ®