Flakes of frozen ammonia coolant were spotted Thursday drifting from the long frame that holds the solar panels on the left side. Barely 48 hours later, ThomasMarshburn and Christopher Cassidy hunted in vain for the leak as they inspected and removed an old pump.
No new, major flaking occurred, to everyone's disappointment. NASA had ordered up the fast, impromptu spacewalk in hopes that ammonia flakes might lead the astronauts to the bedeviling hole or crack, which is too small to notice without a trail of icy evidence.
"All the pipes look shiny clean, no crud," Cassidy reported 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."
Despite the lack of visible damage, Mission Control ordered the spacewalkers to install a spare pump. The spacewalk was going to last no longer than the allotted six hours, the astronauts were advised.
The ammonia pump was the chief suspect going into Saturday's spacewalk, and the fact that nothing amiss was found meant that the problem, in all likelihood, was going to continue to vex NASA in the weeks ahead.
"There's nothing to lose" by putting in a fresh pump and pressing ahead with "additional detective work," Mission Control explained. "The mystery mounts."
NASA said the leak, while significant, poses no safety threat. 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 rare.
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 1 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."