Aug. 3 (Bloomberg) -- Two astronauts left Discovery for a seven-hour spacewalk to remove cloth protruding from the shuttle's heat shield to allay concern that the material may cause the craft to overheat when it returns to Earth next week.

Mission specialist Stephen Robinson will ride on the International Space Station's robotic arm to two spots on the underside of the shuttle, where pieces of fabric are sticking out from between the tiles that protect the spacecraft from the heat of the Earth's atmosphere.

``The big concern is that this cloth is sticking up and when the hot air hits it, it's going to change the way the hot air flows over the tile, which could cause some damage because it would create a little more heat behind the cloth that's sticking up,'' former shuttle commander Scott Horowitz said in an interview.

Robinson and Soichi Noguchi opened the hatch at about 3:48 a.m. New York time to begin the spacewalk, as they passed over the east coast of Australia, NASA said on its online television channel They started about half an hour later than scheduled because they were being ``very deliberate'' about preparations, NASA spokesman Pat Ryan said in a phone interview from the Johnson Space Center in Houston.

The ``gap-filler'' cloth protects the tiles from damage as they expand in higher temperatures during takeoff. Mission managers said they are afraid the strips of cloth, which are 0.6 inches to 1.1 inches long, are too large and may cause overheating as the ship reenters Earth's atmosphere. Shuttles have flown back with 1/4-inch pieces protruding from the tiles.

During today's spacewalk, Robinson, and Soichi Noguchi, of the Japan Aerospace Exploration Agency, have been assigned other tasks, including installing an external platform that serves to store spare parts and setting up an experiment that will test the effect on different materials of exposure to ``the harsh environment of space,'' NASA said.

To contact the reporter on this story: Chris Dolmetsch in Princeton at cdolmetsch@bloomberg.net. Alex Morales in London at amorales2@bloomberg.net

Last Updated: August 3, 2005 05:06 EDT