MOSCOW, Aug. 3 (Xinhuanet) -- Two astronauts of space shuttle Discovery ventured out into space Wednesday for a third time, with an unprecedented mission of fixing worrisome pieces of filler dangling from the ship's belly, the NASA group at the Russian Mission Control said.

    The two spacewalkers, American Stephen Robinson and Japanese Soichi Noguchi, who started off at 0848 GMT, will first install a stowage platform outside the Quest air lock of the space station and replace some experiment equipment before Robinson tackles the repair mission underneath Discovery.

    The spacewalk will last about six and a half hours and the repair is expected to take a little more than an hour.

    Filler material at the ship's belly sticks out about one inch from the gap between heat protective tiles in two spots near the nose. The upper limit should be about 0.5 centimeter. NASA fears the protruding material could cause dangerous overheating during re-entry, leading to a repeat of the Columbia disaster.

    The ceramic coated-fabric gap fillers are used to prevent hot gas from seeping into gaps between the shuttle's thermal tiles. The intruding hot gas caused shuttle Columbia to disintegrate during re-entry into Earth's atmosphere on Feb.1, 2003.

    Robinson is the first astronaut in the 24-year shuttle program to venture beneath the ship in space.

    Robinson, who will reach Discovery's belly with the help of theshuttle's robotic arm and remain attached to a foot restraint at the end of the robotic arm, is expected to tug out the ceramic-fabric filler with gloved hands.

    Should the fibers resist the tugging, Robinson will use a makeshift hacksaw to cut away the material. The saw was fashioned out of a blade, plastic ties, duct tape, Velcro and other items aboard the space station.

    Astronauts have never attempted to fix their ship's thermal shielding in flight. The repair mission is highly risky because during the unrehearsed maneuver the astronaut might accidentally cause more damage to the shuttle's fragile heat shield.

    "I am pretty comfortable with using tools very carefully," Robinson said from space to a news conference Tuesday at the NASA space center in Houston, Texas. He said the repair would be "very delicate" and "simple."

    Discovery's other astronauts and Mission Control would see him the whole time via cameras, but he would be out of sight of Noguchi during the space walk.

    Discovery will remain docked at the space station until Saturday and is expected to land on Earth early Monday. Enditem