With a gentle tug of his gloved right hand, Discovery astronaut Stephen Robinson removed two worrisome pieces of filler material from the shuttle's belly Wednesday in an unprecedented space repair job that drew a big sigh of relief from NASA.

　　But he may have to go out again to fix yet another trouble spot. Robinson was barely back inside the shuttle and out of his spacesuit when Mission Control informed the crew there was a chance that a fourth spacewalk may be needed Friday to deal with a torn thermal blanket below a cockpit window.

　　The concern is that a roughly 1-foot section of the blanket could rip away during re-entry, whip backward and slam into the shuttle, perhaps causing grave damage. Engineers expect to know by Thursday afternoon whether the danger is real and whether any blanket trimming is required.

　　Discovery and its crew of seven are scheduled to return to Earth on Monday.