WASHINGTON - Two astronauts will make a hastily planned space walk Saturday to try to fix an ammonia leak in the power system of the International Space Station.

The leak in a cooling system was discovered Thursday when "snowflakes" of ammonia were seen flying away from the station. Engineers on Earth were up overnight plotting an impromptu space walk.

Space walks are rarely done on such short notice, but the space agency wanted to check out the leak before all the ammonia escaped and also to take advantage of a spacewalking crew member who is about to return home.

Officials emphasized that the six-member crew is not in danger and the outpost has plenty of power, even though the leak forced NASA to shut off the power channel from one of eight solar panels that supply electricity to the station.

It can operate fine with only seven electrical channels, space station program manager Michael Suffredini said Friday. Power from the affected panel was rerouted to the seven other systems.

Suffredini said the chief suspect for the leak is space junk hitting a cooling tube, but he said the area had a slow, small leak for many years that suddenly accelerated Thursday.

"You're talking a very, very, very small hole," Suffredini said at a NASA news conference.

NASA hopes the leak is in a small pump box. During the six-hour space walk Saturday morning, U.S. astronauts Chris Cassidy and Tom Marshburn will replace the 260-pound box with a nearby spare.

While NASA has had to do impromptu space walks before, they haven't been done on the space station since it was completely built and operating as a finished lab, said chief flight director Norm Knight, who called the move "precedent-setting."

Station commander Chris Hadfield of Canada told NASA flight controllers Friday that the crew was ready.