WASHINGTON (AP) - Two astronauts will make a hastily planned spacewalk 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 spacewalk. Spacewalks 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 re-routed to the other seven 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 on 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 spacewalk on 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 spacewalks 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 is completely ready for the spacewalk. "It's the right thing to do," he radioed down to Earth. Hadfield tweeted that the crew was working "like clockwork" and said the two spacewalkers were already getting their spacesuits ready. While he also described it as a "serious situation," Suffredini characterized it more as annoyance. If the cooling system can't be fixed in Saturday's spacewalk, it can be fixed in later spacewalks, he said. And if it isn't fixed, NASA can and has operated the station fully on seven power channels, he said. But that would leave the station little margin for error. If there are more problems, some experiments on board may have to be shut down to conserve power. But, he said, "it's not critical from a safety standpoint." NASA also just wants to figure out what happened. If it was space junk, that leaves a tell-tale signature, Suffredini said. "What's causing the leak is unknown because there's a lot of plumbing underneath the box itself," NASA spokesman Rob Navias said. He said the repair is what NASA calls one of the "Big 12" types of emergency repair work that all spacewalking astronauts train for in advance, Navias said. In 2009, Cassidy and Marshburn flew to the space station on the shuttle Endeavour and walked in space together to swap out a battery in the same location, so "they know this worksite inside and out," Navias said. NASA Administrator Charles Bolden told reporters in Virginia on Friday that their return will go ahead as planned, leaving three astronauts on board. Another reason to do the repair quickly: There may be some ammonia left which will help the spacewalkers find the leak because of the white flakes, Suffredini said. Last fall, station instruments revealed a leak that was so small that it wasn't visible. In November, two other spacewalking astronauts tried to reroute coolant lines to bypass the tiny leak but it wasn't successful.