Discovery's astronauts spent months practicing the first three spacewalks of their mission.
But they've put together the fourth walk on the fly.
Monday afternoon, American Robert Curbeam and Christer Fuglesang of Sweden will try to get a stubborn, half-retracted solar array to fold up.
NASA hopes the job will take about four-and-a-half hours but the spacewalkers will be equipped to stay outside longer than that.
Though the walk wasn't planned ahead of time, Curbeam says they've had a lot of "generic training" and he's expecting a "good chance for success."
Discovery's mission has been extended a day because of the extra spacewalk. The shuttle is now scheduled to return to Earth on Friday.