Spacewalking astronauts have replaced an ammonia pump outside the International Space Station in hopes of plugging a serious leak.

Christopher Cassidy and Thomas Marshburn took a hastily planned spacewalk Saturday to find and, possibly, fix the leak.

Flakes of frozen ammonia coolant were spotted Thursday drifting from the long frame that holds the solar panels on the left side. The astronauts found "no smoking guns" as they removed a suspect pump. After the men installed the new pump, however, no new leakage was detected.

That's good news _ at least for now. NASA will continue to watch for any seepage. Mission Control says it will take time before it's able to declare victory.

NASA says the leak, while significant, never jeopardized crew safety.