Astronauts on board the Shuttle Discovery are preparing for a third space walk, during which they will try to make unprecedented repairs to the underside of the craft.

Two so-called "gap filler" strips are sticking out of the space-plane's belly, in between heat-shield tiles. NASA is worried that this could cause significant overheating - increasing the load on nearby tiles by a couple of hundred degrees - to the Shuttle as the crew tries to return to Earth.

On a flight in 1995, the Shuttle Columbia had a similar problem with gap-fillers sticking out by nearly one-and-a-half inches. According to NASA's reports, the Shuttle did experience additional heating when it made its return journey through the atmosphere.

In this case, the strips are sticking out about an inch each. Astronaut Stephen Robinson will try to pull the gap-fillers all the way out. If that doesn't work, he'll cut them off closer to the Shuttle's belly, NASA says.

Astronauts have never tried to make repairs to, or venture underneath the Shuttle while it is in orbit. But NASA says the known dangers of a spacewalk are preferable to the uncertainties of letting the craft return to Earth without the repair being completed. ®

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