CAPE CANAVERAL, Fla. - Astronauts making a rare, hastily planned spacewalk 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 as though 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."

"Obviously, the longer you go, the more confidence you get," he told reporters.

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 just a few days.

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

The ammonia pump was the chief suspect going into Saturday's spacewalk. So it was disheartening for NASA, at first, as Cassidy and Marshburn reported nothing amiss on or around the old pump.

"All the pipes look shiny clean, no crud," Cassidy said as he used a long, dentist-like mirror to peer into tight, deep openings.

"I can't give you any good data other than nominal, unfortunately. No smoking guns."

Engineers determined there was nothing to lose by installing a new pump, despite the lack of visible damage to the old one. The entire team - weary and stressed by the frantic pace of the past two days - gained more and more confidence as the 5 1/2-hour spacewalk drew to a close with no flecks of ammonia popping up.

"Gloved fingers crossed," space station commander Chris Hadfield said in a tweet from inside. "No leaks!" he wrote a half-hour later.

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

This 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.

Life for the six space station residents has been pretty much unaffected since Thursday's ammonia shower. The loss of an additional power channel, however, could threaten science experiments and backup equipment.

"We may not have found exactly the smoking gun," Cassidy said, "but to pull off what this team did yesterday and today, working practically 48 straight hours, it was a remarkable effort on everybody's behalf."

NASA officials remain mystified as to why the leak erupted. Ammonia already had been seeping ever so slightly from the location, but the flow increased dramatically Thursday.

Montalbano did not know, as of Saturday evening, how much ammonia was lost. Another spacewalk will be needed to replenish the supply.

With the repair work behind them, the astronauts and ground controllers turned their attention to the impending departure of three of the six crewmen.

Marshburn has been on the space station since December and is set to return to Earth late Monday, along with Hadfield, a Canadian, and Russian Roman Romanenko. Cassidy is a new arrival, on board for just 1? months.

By coincidence, the two Americans performed a spacewalk at this troublesome spot before, during a shuttle visit in 2009.

"This type of event is what the years of training were for," Hadfield said in a tweet Friday. "A happy, busy crew, working hard, loving life in space."