CAPE CANAVERAL, Fla. – Instead of enjoying a relaxing day at the international space station, the space shuttle Discovery's seven astronauts prepared Sunday for an unplanned, fourth spacewalk to get a stubborn, half-retracted solar array to fold up.
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 1:47 p.m. EST Monday. NASA managers hope fixing the array problem will not take more than 4 1/2 hours, but the spacewalkers will be equipped to stay longer than that.
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.
Unlike the mission's three previous spacewalks, for which astronauts had several months of practice in a large swimming pool at Johnson Space Center, Monday's trip outside has been put together on the fly.
"Of course it's a little more challenging because we haven't done the training on the ground for it," Curbeam said in an interview Sunday evening. "But we did have a lot of generic training for solar array wing deploy and retract... I have a pretty good feeling that we got a good chance for success."
Curbeam said he'll treat the spacewalk like any other, but added, "Probably the most important thing is for us to keep in mind that since it's my fourth — and you tend to get more and more comfortable as you go along — just to make sure we don't make any mistakes."
At a briefing Sunday, lead spacewalk officer Tricia Mack outlined a vague spacewalk plan, noting that adjustments may be made as the spacewalk is happening, based on how it progresses.
The space station's crane-like robotic arm will carry Curbeam to areas of the array that need troubleshooting. Fuglesang will be floating near the base of the array and have with him insulated tools to pass to Curbeam, such as a hook, pliers and scraper. Based on what he sees firsthand, Curbeam can use the artillery to flip or fan out some stuck grommets, the source of the array-folding problem.
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.