(CBS News) Two U.S. astronauts had just two days to plan a spacewalk to fix a leak, but it could be a month before we know if the mission was a success.
With a view of the Earth below, astronauts Thomas Marshburn and Christopher Cassidy carefully searched for the ammonia leak.
NASA was already aware of a very small leak that has been releasing about 5 pounds of ammonia per year, but last Thursday something changed, and the leak rate jumped up to 5 pounds per day.
The ammonia could be seen as small white flakes moving away from the space station. It crystallizes in the deep cold of space.
"All spacewalks are inherently dangerous," CBS News space consultant Bill Harwood said. "The astronauts are flying around the Earth at 5 miles per second, and obviously in that environment in a total vacuum that's not something to take lightly."
Ammonia is the coolant of choice aboard the space station to carry away the heat generated by the lab's electrical gear.
The leak was not considered an immediate threat to the crew, but NASA hopes to operate the space station through at least 2020.
"Losing a coolant loop in and of itself is not a terribly critical failure, but it's a lot like taking a long trip in your car and you lose your space tire," said Harwood. "You can still drive the car, but you sure don't want another flat."
After not being able to locate the leak, Marshburn and Cassidy removed and replaced the pump assembly in case that's where the leak originated.
All told, the exercise took over five hours. NASA was encouraged by the results.
The astronauts will now keep an eye on that part of the space station for the next few days to see if they fixed the problem.
Although they didn't see any ammonia during their space walk, Marshburn and Cassidy did what's called a "bake out" before returning to the space station. That's where they let the sun's intense rays burn off ammonia that may have gotten onto the suits. They didn't want to bring any of the toxic chemicals back inside.