Two astronauts from the space shuttle Atlantis are making the first spacewalk of their mission, BBC reported.
Michael Foreman and Robert Satcher will hook up a spare antenna and cables on the International Space Station (ISS).
Three spacewalks are planned during the shuttle's 11-day visit, which began with a smooth docking on Wednesday.
Atlantis blasted off from Florida's Kennedy Space Center on Monday; it will deliver spare parts to the station and bring back a crew member.
The spacewalk is due to last for about six hours.
The astronauts have already unloaded several tonnes of large spares, including pumps and gas tanks.
The hardware should keep the space station operating well past next year's retirement of the space shuttle fleet.
None of the other spacecraft which visit the station is big enough to carry such large pieces of equipment.
The six all-male crew of Atlantis will spend the US Thanksgiving holiday in orbit.
They will return to Earth with a seventh crew member, Nicole Stott, who has been living at the space station for nearly three months.
But this will be the last such shuttle crew rotation before the fleet's retirement.
