CAPE CANAVERAL, Fla. - Astronauts making a rare, hastily planned space walk replaced a pump outside the International Space Station on Saturday in hopes of plugging a serious ammonia leak.

The prospects of success grew as the minutes, then hours, passed and no frozen flecks of ammonia appeared. Mission Control said it appeared the leak may have been plugged, although additional monitoring over the coming weeks will be needed before declaring a victory.

"I will tell you that we're happy. We're very happy," said Joel Montalbano, NASA's deputy space station program manager. "We didn't see any obvious signs of a leak, but it's going to take some time . . . for us to look at the system, evaluate the system and make sure we did, indeed, stop the leak."

Montalbano expects it will take "a good four weeks, five weeks, maybe even a few weeks longer."

Christopher Cassidy and Thomas Marshburn installed the new pump after removing the old one suspected of spewing flakes of frozen ammonia coolant two days earlier. They uncovered "no smoking guns" responsible for the leak and consequently kept a sharp lookout for any icy flecks that might appear from the massive frame that holds the solar panels on the left side.

"Let us know if you see anything," Mission Control urged as the fresh pump was cranked up. Thirty minutes later, all was still well. "No snow," the astronauts radioed.

"We have our eyes on it and haven't seen a thing," Marshburn said.

NASA said the leak, while significant, never jeopardized crew safety. But managers wanted to deal with the trouble now, while it's fresh and before Marshburn returns to Earth in a few days.

The space agency never before staged such a fast, impromptu space walk for a station crew. Even during the shuttle program, unplanned walks were uncommon.

The ammonia pump was the chief suspect going into Saturday's space walk. Engineers determined there was nothing to lose by installing a new pump, despite the lack of visible damage to the old one.

Flight controllers in Houston worked furiously to get ready for the operation, completing all the required preparation in under 48 hours. The astronauts trained for such an emergency scenario before they rocketed into orbit.

That area on the space station is prone to leaks. The ammonia coursing through the plumbing is used to cool the space station's electronic equipment. There are eight of these power channels, and all seven others are operating normally.