CAPE CANAVERAL, Florida - Astronauts making a rare, hastily planned spacewalk replaced a pump outside the International Space Station in hopes of plugging a serious ammonia leak.
Spacewalkers Christopher Cassidy and Thomas Marshburn found "no smoking guns" as they removed a pump suspected of spewing flakes of frozen ammonia coolant two days earlier. After they installed the new pump, no new leakage was detected.
That's good news -- at least for now. NASA will continue to watch for any seepage and it could be days before victory can be declared.
"So far so good," Mission Control radioed.
Cassidy and Marshburn hung around for a while, on the lookout for any icy flecks that might appear along 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.
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 had 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.
"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. As the minutes passed and no flakes were spotted, the entire team-- weary and stressed by the frantic pace of the past two days -- gained more and more confidence.
Flight controllers in Houston had 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 (410 kilometers) 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 1 1/2 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," space station commander Chris Hadfield said in a tweet Friday. "A happy, busy crew, working hard, loving life in space."