NASA engineers expect to know by Thursday afternoon whether they'll need another spacewalk for another shuttle fix.

The concern is that part of a torn thermal blanket could damage Discovery if a piece tears away during re-entry.

If a spacewalk is needed, it would be the fourth one of this mission.

During a Wednesday morning spacewalk, Astronaut Steve Robinson pulled two pieces of dangling filler from between tiles on the belly of the shuttle.

Mission control announced at 7:55 a.m. CDT that the repairs were complete as pictures transmitted from a camera on Robinson's helmet showed him pulling away from the craft with a piece of material in his gloved hands.

Ten minutes earlier, Robinson had removed material from another site while standing at the end of the international space station's robot arm.

When the work was done, Robinson and Soichi Noguchi were more than four hours into a spacewalk in which they had completed other tasks, including installing a tool platform on the station.

At about 9:16 a.m. CDT, the astronauts re-entered the airlock and ended their spacewalk, which lasted less than six hours.

Before the delicate operation, Robinson said he was "pretty comfortable with using tools very carefully," He said it was a very simple task, but very delicate.

As he was lowered into position, controllers reminded him that the insulation could be sharp and that he should try to avoid touching the tiles that protect the shuttle from the heat of re-entry.

The work was the first time that an astronaut had repaired the outside of the shuttle while in flight and the first time one had moved under the orbiter.

Crew members were also using a remote camera to examine a possible problem where thermal blankets meet protective tiles near the front of the craft.

Discovery is set return to Earth early Monday. It will remain docked to the space station until Saturday.