(AP Photo/NASA TV)
Discovery's crew prepared spacesuits, relocated the station's robotic arm and mobile platform so they can be used during the spacewalk and moved cargo from the station to the shuttle for the trip home.
The spacewalk by U.S. astronaut Robert Curbeam and Swedish astronaut Christer Fuglesang of the European Space Agency was to start at 12:47 p.m. today.
Discovery's return to Earth was pushed back a day to Friday because of the extra spacewalk. Because of supply limits, the astronauts need to be on the ground no later than Saturday.
It will be the third spacewalk for Fuglesang since Discovery's arrival at the international space station almost a week ago, and the fourth for Curbeam, who will set a record for most spacewalks during a single shuttle mission.
"It will be exciting to see because a lot of times when we're doing operations, we've trained them for many months before they actually execute it," Melanie Miller, a space station robotics officer, said Sunday. "But in this particular case, we had to do a lot of work on the ground, while the crew is on board. So we'll be seeing it for the first time in real time."
The decision to send astronauts outside for a fourth time was made by NASA managers Saturday in the middle of a spacewalk by Curbeam and astronaut Sunita "Suni" Williams, during which they successfully finished rewiring the orbital outpost from a temporary power source to a permanent one.
Because they finished their tasks ahead of schedule, flight controllers asked them to inspect the half-retracted solar array, which had generated power for the station's temporary system. The old array retracted halfway by remote control Wednesday before getting stuck, and since then, the space agency has tried a number of approaches to fix it.
The spacewalking pair Saturday pushed on a box into which the accordion-like, 115-foot-long array folds. That managed to free some stuck grommets, and enabled other astronauts in the space station to retract the array further by remote control, but then more grommets became stuck.
NASA considered having the current station crew or the next shuttle crew make a spacewalk to fix the array after Discovery's departure, but nixed those options. Managers didn't feel the three-member station crew had enough astronauts for the job, and they wanted to gain experience in solving the problem since the next shuttle crew in March will be performing similar work on another array.
The snagged solar array has been the only difficulty in an otherwise smooth mission.
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