An unplanned initial inspection of the heat shield was undertaken as a precaution last Monday after sensors detected "very low" impact readings. After initial fears the shuttle had been damaged by space junk or a micrometeoroid, NASA engineers advised Discovery's crew it did not need to conduct an extra inspection. NASA has insisted on additional safety precautions since damage to a fuel tank lead to the death of Columbia's seven astronauts during re-entry in 2003.

NASA has struggled with the space station's stubborn solar panel for three days, comparing the process to folding a roadmap. Space walkers Christer Fuglesang and Robert Curbeam spent seven hours wrestling with it on Wednesday as part of an electrical refit of the station. The astronauts managed to partially retract the 37-metre panel, which had acted as a temporary power source for the station for six years, so a they could rotate a new, permanent pair of solar wings towards the sun.

The primary goal of today's six hour space walk, scheduled to begin around 2:42pm (6.42am Sunday AEST), is to re-wire two of the station's four major electrical circuits. If time allows, astronauts will then undertake an inspection of a partially retracted solar panel to help engineers figure out how to retract it completely.

The solar panel causing the problems is attached to the P6 tress segment. It is part of the station's backbone but has been temporarily attached to one arm of the station since 2000 - awaiting the P5 tress segment which astronauts installed during their first space walk on Tuesday. The P6 segment will be moved to the end of the P5 segment and the solar panel redeployed in 2007 - completing one end of the station.{moscomment}