Station Commander Peggy Whitson and rookie flier Jack Fischer floated outside on the station's 200th spacewalk for four hours, marking a milestone moment for the international space agency.

The pair began what was originally expected to be a six hour spacewalk almost two hours late, after a cable supplying power and cooling water to Fischer's spacesuit developed a leak.

Fischer was forced to share Whitson's spacesuit servicing system, which caused both to burn through their spacesuits' battery power faster than usual, prompting the space agency to shorten the outing.

A NASA veteran, Dr Whitson has a history of shattering records in space. Last month, the 57-year-old surpassed the 534-day record for most time spent in space by an American. Whitson is also the first woman to complete nine successful spacewalks, logging an extraordinary 53 hours outside of airlock.

As the duo prepared to return to the station, Whitson said it was "amazing" to have taken part in the landmark spacewalk. Fischer agreed, calling the experience "awesome".

"Oh my gosh, this is beautiful", Fischer then says, his voice cracking over the intercom.

Whitson and Fischer installed a cable to troubleshoot a cooling system problem with the station's $2 billion dark matter detector, mission commentator Rob Navias said.

They also attached a debris shield onto an exposed docking port that is being prepared for commercial space taxis under development by Space Exploration Technologies, or SpaceX, and Boeing Co.

"All of the major work has been accomplished," Navias said. "It's been a highly successful spacewalk in spite of a somewhat uncertain beginning".

A collective of international astronauts are responsible for the maintenance of the $100 billion laboratory, which flies about 400 km above Earth.

There are currently five astronauts living on the space station. The crew has spent a total of 6,035 cumulative days in orbit.

With Reuters, AP