"There are a lot of risk trades here and we have a mission to accomplish. So I'm sure (we will) take a look at risk versus reward and see what's the best thing to accomplish," said Commander Mark Polansky.

"In a perfect world, I'd like to have everything but it's not quite that perfect. So we'll go ahead and try to find out what's the best thing we could do for overall mission success and safety."

An unplanned initial inspection of the heat shield was undertaken as a precaution 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. The mission can not be extended as the shuttle's fuel cells only have enough spare fuel for the two days NASA keeps in reserve for re-entry delays due to bad weather.

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 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}