In an unprecedented spacewalk and repair operation, Discovery astronaut Stephen Robinson successfully removed two fibre fillers protruding from the heat shield on the belly of the space shuttle.

The delicate operation happened 225 miles above the earth on Wednesday. Because a mistake could have been disastrous, for both the crew and their ship, it took Robinson six hours just to reach the area.

He used the Canadian-designed robotic arm to bring him closer to the gap fillers, while NASA officials watched apprehensively from earth.

Robinson was delighted to find that he could remove the ceramic-covered cloth pieces easily with his gloved hand and didn't need to tug the pieces out with forceps or slice them off with a hacksaw.

"I'm pulling. It's coming out very easily," Robinson said as he quickly removed the first piece.

"I don't see any more gap filler," he said as he removed the second piece. "I'm doing my own inspection here. It is a very nice orbital belly. It looks like this big patient is cured."

"You guys did a great job," Mission Control radioed back. "Congratulations."

The two pieces of filler material, just a few millimetres thick, cushion the fragile heat-shield tiles from each other.

Although the pieces were protruding by only an inch in one case, NASA was worried they could overheat during re-entry and change the craft's aerodynamics during landing on Aug. 8.

Until now, no astronaut had ever attempted to perform an unrehearsed emergency repair job to the exterior of a shuttle. It was also the first time an astronaut has ventured beneath the shuttle to work on its belly.

NASA admits it didn't know if the protruding strips were a danger to the shuttle, but with the Columbia disaster still fresh in their minds two and half years later, it wanted to take no chances.

Ahead of the repair, Robinson and fellow astronaut Soichi Noguchi spent a couple of hours on the key task of the mission's third spacewalk: installing an external tool and parts platform on the International Space Station, where Discovery has been docked since Thursday.

But this may not be the end of the repairs. NASA officials said on Tuesday they are considering another spacewalk on Friday to fix a protrusion in an insulating blanket outside the shuttle commander's window.

There is concern that the blanket might come off during re-entry and smack into the shuttle.

Deputy shuttle program manager Wayne Hale said engineers were still in the process of assessing whether something needed to be done to the blanket.

Discovery is scheduled to return to earth on Monday, at about 4:30 a.m. EDT.

With a report from CTV's Murray Oliver in Washington