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.

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The prospects of success grew as the minutes passed and no frozen flecks of ammonia appeared. Mission Control said it appeared as though the leak may have been plugged, although more monitoring was needed before declaring a victory.

"No evidence of any ammonia leakage whatsoever. We have an airtight system — at the moment," Mission Control reported.

This image from a NASA space shuttle mission shows the International Space Station in orbit. The space station is the size of a football field and home to six astronauts.

(NASA)

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.

Just in case, the spacewalkers hung around for a while, on the lookout for any icy flecks that might appear from the long frame that holds the solar panels on the left side.

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

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"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 days, unplanned spacewalks were uncommon.

The ammonia pump was the chief suspect going into Saturday's spacewalk.

It was disheartening for NASA, at first, as Cassidy and Marshburn reported seeing nothing amiss on or around the old pump.

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"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."

Mission Control noted, "The mystery mounts."

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 spacewalk drew to a close and still no flecks of ammonia had popped 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; the repair job is among NASA's so-called Big 12.

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 were operating normally. As a result, life for the six space station residents was pretty much unaffected, aside from the drama unfolding Saturday 255 miles above the planet.

The loss of another power channel, however, could threaten science experiments and backup equipment.

NASA's space station program manager Mike Suffredini said it's a mystery as to why the leak erupted. Possibilities include a micrometeorite strike or a leaky seal. Ammonia already had been seeping ever so slightly from the location, but it increased dramatically Thursday.

Marshburn has been on the space station since December and is set to return to Earth late Monday. Cassidy is a new arrival, on board for just 1½ months.

"Suddenly very busy," Marshburn said via Twitter on Friday.

By coincidence, the two 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."

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New York City, posted on April 23, 2013. Cmdr. Chris Hadfield tweets: Incredibly clear, before the trees have filled with leaves. (Chris Hadfield/NASA)