Astronauts using two Canadian-built robotic arms installed a new stowage platform on the exterior of the International Space Station on Tuesday, a day before NASA is expected to decidewhether the Endeavour crew will need to make a spacewalk to repair a deep gash on the space shuttle's belly.

Mission specialists aboard Endeavour used the shuttle's Canadarm to lift the stowage platform from the payload bay and hand it off to Canadarm 2, the space station's larger robotic arm. The station arm then attached the platform to the station's Port 3 truss.

The platform installation was the only activity outside the station as the Endeavour crew prepares for a third spacewalk on Wednesday.

NASA is also expected to make a decision on whether to repair a small gouge in the shuttle's heat shield tiling during a fourth spacewalk.

Mission managers have already determined the damage to the heat shield poses no threat to crew safety or the mission, but are concerned the gouge might grow larger upon re-entry and require more extensive repairs.

The unevenly shaped gouge, discovered on Sunday using the Canadian-made robotic arm, showed the damage went through the 2.5-centimetre-thick tiles, exposing the felt material sandwiched between the tiles and the shuttle's aluminum frame.

Assembling key components

Canadian astronaut Dave Williams and fellow astronaut Rick Mastracchio have already completed two spacewalks since the space shuttle arrived at the space station on a mission to assemble key components of the orbiting platform.

On Saturday the pair successfully installed a roughly two-tonne beam to the backbone of the space station during a spacewalk of more than six hours, and on Monday they replaced a faulty gyroscope to maintain the station's orientation in space.

A third spacewalk is planned for Wednesday, with Mastracchio and space station crew member Clayton Anderson assigned to prepare a segment of the station for relocation during the next mission.

A fourth spacewalk has been scheduled for Friday, with Saskatoon-born Williams the likely candidate.

To patch the gouge, astronauts would have to perch on the end of the Canadarm and apply a protective paint and caulk-like filling.

Canadian astronaut Steve MacLean told CBC News the task would be a difficult one, both because of the inherent risks of spacewalks and the precision required to fix the tiles.

"Any time you put an astronaut on a boom, you have the risk that you might hit the tile and cause more damage," MacLean told CBC News.

NASA has already extended the mission from its original 11 days to 14 days in order to give them time to assess whether a repair is necessary.