MIAMI (REUTERS) - Two astronauts aboard the International Space Station are preparing for a possible spacewalk to repair an ammonia leak in a cooling system on one of the station's solar arrays, Nasa said on Friday.

The crew spotted a steady stream of small, white frozen ammonia flakes floating away from a coolant line outside the orbital outpost on Thursday.

Mission managers were reviewing images and data gathered overnight and said they would decide on Friday night whether to send spacewalkers out on Saturday.

"The crew is not in danger, and the station continues to operate normally otherwise," Nasa said in a news release.

Ammonia is used to cool the power systems that operate the solar arrays, which provide electricity to the station. Each of the eight solar arrays has its own independent cooling system.

The leak is on the far left-side of the station's truss structure, in an ammonia loop that astronauts previously tried to troubleshoot during a spacewalk in November 2012.

"It is not yet known whether this increased ammonia flow is from the same leak, which at the time was not visible," Nasa said.

If a spacewalk is approved, American astronauts Chris Cassidy and Tom Marshburn would don spacesuits and head outside the station to inspect and possibly repair the leak.

Crew commander Chris Hadfield, a Canadian astronaut, would choreograph from inside the station. Russian cosmonauts Pavel Vinogradov, Alexander Misurkin and Roman Romanenko make up the rest of the crew.

Work was under way to reroute the remaining power channels to maintain full operation of the systems normally controlled by the solar array that is cooled by the leaking loop.

The space station, a US$100 billion (S$123 billion) research laboratory that orbits 250 miles (400km) above Earth, is owned by the United States and Russia in partnership with Europe, Japan and Canada.