Astronauts made a rare, hastily planned spacewalk to fix a serious ammonia leak at the International Space Station, and the US space agency said it appeared the repair was a success.
But the following weeks will be the test. Authorities have said the space station's six-member crew was not in danger from the leak.
"I will tell you that we're happy. We're very happy," Joel Montalbano, Nasa's deputy space station program manager, said. "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."
Mr Montalbano expects it will take "a good four weeks, five weeks, maybe even a few weeks longer."
The space agency never before staged such a fast, impromptu spacewalk for a station crew. Even during the shuttle programme, unplanned spacewalks were uncommon.
Christopher Cassidy and Thomas Marshburn installed a 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 kept a sharp lookout for any icy flecks that might appear from the massive frame that holds the solar panels on the left side.
Nasa officials remain mystified as to why the leak erupted. Managers wanted to deal with the leak while it was fresh and before Mr Marshburn returns to Earth on Monday.
The ammonia pump was the chief suspect going into Saturday's spacewalk. 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 increasing confidence as the five-and-a-half-hour spacewalk drew to a close with no flecks of ammonia popping up.
"Gloved fingers crossed," space station commander Chris Hadfield of Canada said in a tweet from inside. "No leaks!" he wrote a half-hour later.
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.