After a final inspection of Endeavour's thermal shield, astronauts aboard the linked shuttle-station complex began preparing for a Saturday spacewalk to store on the station the laser-tipped boom they used to search for damage.
Every shuttle crew that's flown since the Columbia disaster has used the boom to check for problems with the shield that protects the ship from the searing heat of re-entry.
The boom is being left at the space station because there won't be room for it in Discovery's payload bay on its next mission because the enormous Japanese Kibo lab will take up almost every square inch.
Astronauts Robert Behnken and Michael Foreman plan to attach the 50-foot inspection boom to the outside of the space station on Saturday during the fifth and final spacewalk of Endeavour's mission. Discovery will carry the boom back to Earth after its mission ends.
Shortly after reaching orbit last week, the astronauts attached the boom to Endeavour's 50-foot robot arm to check the wings and nose for any launch damage. None was found. They repeated the inspection Friday in the remote chance the wings or nose were struck by a micrometeorite or space junk during the past week and a half.
The inspection went smoothly and nothing stood out in early examinations of the images, shuttle flight director Mike Moses said. He expected the analysis to be complete by Sunday.
"The crew was well trained. They executed well. Everything went perfect," Moses said.
After the boom is secure, Behnken will try again to attach a couple of suitcase-sized science experiments to the outside of the European Columbus lab. He had trouble getting the first experiment to latch down in an earlier spacewalk.
Meanwhile, Foreman will conduct yet another inspection of a solar rotary joint that's been broken since last fall. NASA still doesn't know what might be causing the metal parts to grind, clogging the joint with shavings.
Foreman will check out a pockmark spotted in images gathered by other spacewalkers. If it's a divot, Moses said, it could be "a hint towards our smoking gun." Then again, he said, it could just be a large clump of debris.
Foreman also will remove some thermal covers to check whether it's possible a micrometeorite hit showered debris throughout the joint, which is supposed to continuously rotate 360 degrees to keep the solar wings pointing toward the sun.
Saturday night's spacewalk will wrap up Endeavour's space station work and clear the way for undocking on Monday night. It will be the most spacewalks ever performed during a joint shuttle-station flight.
NASA, meanwhile, may be forced to delay some of the year's later shuttle flights _ including the Hubble Space Telescope repair mission at the end of August _ because of a slowdown in building new external fuel tanks with post-Columbia design changes. Shuttle officials are evaluating the schedule and what can be done, if anything, to keep the launches on track.