Astronauts aboard the International Space Station are preparing for a fourth spacewalk today to unstick a jammed solar array.
Astronauts Robert Curbeam and Christer Fuglesang will try to clear snarled guidewires that are preventing the 33-metre panel from retracting into a 50cm-high storage box at its base.
The array is supposed to fold like a Venetian blind. It has already retracted enough to allow the crew to successfully complete its primary mission of rewiring the space station.
The fourth spacewalk starts at 7.12 pm and is expected to last four to six hours.
Construction on the station is due to be completed in 2010 but was halted for over two years by the 2003 shuttle Columbiadisaster.
To meet that deadline before the ageing shuttle fleet is retired, Nasa expects to fly 13 more shuttle missions.
The fourth spacewalk adds an extra day at the station for the Discoveryastronauts, and the shuttle's landing is now targeted for Friday afternoon at Kennedy Space Center in Florida.