Washington: US space shuttle Endeavour’s crew on Friday prepared for their fourth and final
spacewalk outside the International Space Station with a key milestone in view, NASA has said.

Mission specialists Mike Fincke and Greg Chamitoff are to start the walkabout at 12:46 am local time, and they plan to stay out for about six and a half hours.

"If the spacewalk begins on time, the 1,000th hour spent spacewalking for assembly and maintenance of the International Space Station will be reached at 5:33 am ," NASA said in a statement.

The astronauts' mission is to stow the boom on the station truss, replace an electrical fixture, and work on some new installations for the Canadian robot Dextre and the new
Canadarm2 base on the Russian side of the station.

The spacewalk will be the last by a US shuttle crew docked at the orbiting lab. A spacewalk is planned during the shuttle Atlantis's mission in July but that will be done by space station crew.

Endeavour blasted off on its final mission, STS-134, on May 16 with six astronauts on board -- five Americans and one Italian -- and docked at the ISS on Wednesday.

The shuttle will remain at the space station until May 30, returning to the United States on June 1.

The 30-year US space shuttle program formally ends later this year with the flight of Atlantis set for July 8, leaving Russia's space capsules as the sole option for world
astronauts heading to and from the orbiting research lab.