CAPE CANAVERAL, Florida (AP) - Two astronauts will venture outside the International Space Station on Saturday to tackle an ammonia leak in the power cooling system.

The leak was discovered on Thursday and the repair job was quickly planned for the weekend.

Spacewalks usually aren't done on such short notice. But NASA wanted to try to fix the leak before all the ammonia escaped.

The space agency also wanted to take advantage of a veteran spacewalker who is due to return home on Monday with two other astronauts.

The two spacewalkers - Americans Chris Cassidy and Tom Marshburn - will swap out a pump box on one of the eight solar panels that supply electricity to the station.

NASA says the station has plenty of power, and the six-man crew is not in danger.