Two astronauts from the US shuttle Discovery undertook a second spacewalk yesterday as part of efforts to replace a depleted ammonia coolant tank outside the International Space Station.

Mission specialists Rick Mastracchio and Clay Anderson started their work in open space at 1.30 a.m. EDT (0530 GMT), or around three quarters of an hour earlier than had been initially planned.

They have since installed two radiator grapple fixture stowage beams on a space station truss, Nasa officials said.

The procedure has allowed station robotic arm operators Jim Dutton and Stephanie Wilson to attach a new ammonia tank to the truss, replacing a depleted one.

The spacewalkers were to complete the installation of the tank by engaging four bolts and connecting electrical cables as well as nitrogen and ammonia plumbing lines.

Then they prepare the depleted tank for its return to Earth.

The spacewalk, scheduled to last six and a half hours, was being managed by Discovery crewmember Dottie Metcalf-Lindenburger.

During the first spacewalk on Friday, Anderson and Mastracchio retrieved an experiment from the Japanese Kibo Laboratory on the station's porch and replaced a Rate Gyro Assembly, part of the station's navigation system.

They began work to withdraw batteries from the four solar antennae on the ISS. The batteries will be replaced on a future mission.

A third and final spacewalk of the mission is scheduled for tomorrow.

The Discovery, which blasted off from Kennedy Space Centre at Cape Canaveral, Florida last Monday, docked on Thursday with the International Space Station.