NASA managers are expected to add a fifth spacewalk to the shuttle Discovery's upcoming space station assembly mission, officials say. The additional EVA will be devoted to testing a heat-shield repair tool that could prove useful in the event of damage like the tile gouge experienced during the last shuttle mission.

While the putty-like STA-54 repair material and an applicator gun were on board Endeavour last month, NASA managers decided against making repairs after ground testing and computer analysis showed Endeavour could safely land as is.

But the repair tool, a caulk gun-like device called the TPS repair ablator dispenser, or T-RAD, has never been tested in space. NASA had planned to test it on a mission next year, but after the Endeavour incident, managers started discussions aimed at moving it up to Discovery's mission.

Discovery is scheduled for launch Oct. 23 and adding a spacewalk this late in the flow is unusual. While official approval has not yet been granted, the astronauts are already training for the repair test and barring a major problem, officials say, approval is expected.

Assuming an on-time launch, Discovery will dock with the space station on Oct. 25. The next day, the crew's flight-day four, astronauts Scott Parazynski and Doug Wheelock will stage a six-and-a-half-hour spacewalk to hook up the multi-hatch Harmony module carried up aboard Discovery.

Two days later, on flight day six, Parazynski and newly arrived station astronaut Dan Tani will carry out a second spacewalk to prepare the stowed P6 solar array for relocation on the far left end of the station's main power truss. Parazynski and Wheelock will venture outside on flight day eight to assist with the actual P6 relocation.

The original flight plan called for station commander Peggy Whitson and flight engineer Yuri Malenchenko to stage a fourth spacewalk to make preparations for Harmony's eventual move to the front of the Destiny lab module.

Instead, the mission's fourth spacewalk, carried out by Parazynski and Wheelock, would be devoted to testing the T-RAD repair technique. Deliberately damaged tiles will be carried up inside the shuttle's crew cabin. The spacewalkers will take them to Discovery's cargo bay and use the T-RAD applicator to make simulated repairs. The tiles will be left on a mount in the cargo bay for return to Earth.

The spacewalk is expected to last about three-and-a-half hours.

The next day, flight day 11, Whitson and Malenchenko will stage the fifth spacewalk to make preparations for Harmony's move.

A revised mission flight plan, reflecting the additional spacewalk, will be posted as soon as possible.