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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.
Two astronauts aboard the International Space Station conducted a hastily arranged spacewalk on Saturday to look for the source of a worrisome leak that had forced the shutdown of one of the eight power systems that provide electricity to the orbiting laboratory.
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 might have been plugged, although additional monitoring will be needed.
The two astronauts, Thomas H. Marshburn and Christopher Cassidy, both American, spent 5 ½ hours outside the space station. They were unable to locate the source of the leak, but did install a new pump, which appeared to solve the problem.
“No evidence of any ammonia leakage whatsoever. We have an airtight system — at the moment,” Mission Control reported.
The new pump started up normally, and with the two astronauts looking for escaping crystals of ammonia coolant, there were no signs of the leak.
Dr. Thomas H. Marshburn and Christopher J. Cassidy installed the 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.
“We’re not seeing anything,” Dr. Marshburn reported.
“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.
Pressure in the coolant loop was also holding, another indicator that the problem had been resolved.
“We have our eyes on it and haven’t seen a thing,” Dr. Marshburn said.
On Thursday, astronauts spotted a stream of frozen flakes emanating from one of the station’s trusses. From videos, engineers concluded that the source of the leaks was a pump that pushes ammonia coolant to regulate temperatures on one of the eight expansive solar arrays, which provide electricity to the station.
NASA said the leak, while significant, never jeopardized crew safety. But managers wanted to deal with the trouble now, while it was fresh and before Dr. Marshburn returns to Earth in just a few days.
Each array has an independent cooling loop, and the leak did not have an immediate impact on the day-to-day operations.
The space agency never before staged such a fast, impromptu spacewalk for a station crew. Even during the shuttle days, unplanned spacewalks were uncommon.
NASA officials said there was no danger to the six-member crew. But they worried that if a problem developed with a second cooling loop, the station might have had to cut back on its power consumption, limiting science experiments.
The ammonia pump was the chief suspect going into Saturday’s spacewalk. So it was disheartening for NASA, at first, as Mr. Cassidy and Dr. Marshburn reported nothing amiss on or around the old pump.
NASA has tussled with coolant leaks previously and last November, two other astronauts took a spacewalk to fix a leak in the same general area.
“All the pipes look shiny clean, no crud,” Mr. Cassidy said as he used a long, dentist-like mirror to peer into tight, deep openings.
“This has been something that has been troubling us for some time,” Joel Montalbano, the deputy space station program manager, said at a news conference after the spacewalk.
“I can’t give you any good data other than nominal, unfortunately. No smoking guns.”
The balky loop had been losing coolant at a rate of five pounds per year, but on Thursday, that rate jumped to five pounds per day.
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 five-and-a-half hour spacewalk drew to a close with no flecks of ammonia popping up.
With three of the crew members, including Dr. Marshburn, scheduled to head back to Earth on Monday, NASA decided it would be prudent to repair the leak sooner rather than later.
“Gloved fingers crossed,” the space station commander Chris Hadfield said in a tweet from inside. “No leaks!” he wrote a half-hour later.
In less than 48 hours, plans for the spacewalk and the repairs were put together. At 8:44 a.m. Eastern time on Saturday, Dr. Marshburn and Mr. Cassidy switched on the power to their spacesuits, marking the start of the spacewalk. When they reached the site of the suspected leak, at the far left end of the space station, they were not able to spot evidence of it. Even after removing the pump, they did not see much unusual.
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.
“It looks really, really clean,” Dr. Marshburn said. “Surprisingly so.”
This area on the space station is prone to leaks.
NASA engineers thought the leak could be inside the pump and decided that a new one should be installed.
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 two power channels, however, could threaten science experiments and backup equipment.
Dr. Marshburn and Mr. Cassidy did take a moment to admire the view as they passed over the southern end of Africa.
“We may not have found exactly the smoking gun,” Mr. 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.”
“Did you see the moon?” Dr. Marshburn asked.
NASA’s space station program manager Mike Suffredini said it was a mystery as to why the leak erupted. Possibilities include a micrometeorite strike or a flawed seal. Ammonia already had been seeping ever so slightly from the location, but the flow increased dramatically Thursday.
“I did, oh my God!” Mr. Cassidy replied. “Burn that in your memory!”
Dr. 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 a month and a half.
It was the 168th spacewalk related to construction or maintenance of the space station since the first piece of the space complex was launched in 1998.
By coincidence, the two performed a spacewalk at this troublesome spot before, during a shuttle visit in 2009.
“We’re happy, we’re very happy,” Mr. Montalbano said after the repair.
“This type of event is what the years of training were for,” Mr. Hadfield said in a tweet Friday. “A happy, busy crew, working hard, loving life in space.”
He warned that it would be weeks or months before engineers were confident that the problem would not recur.
“Obviously, the longer you go, the more confidence you get,” Mr. Montalbano said.
