CAPE CANAVERAL, FL - Two astronauts took a hastily planned spacewalk Saturday to find and, possibly, fix a serious leak at the International Space Station.
Flakes of frozen ammonia coolant were spotted Thursday drifting from the long frame that holds the solar panels on the left side. Less than 48 hours later, Thomas Marshburn and Christopher Cassidy emerged from the orbiting lab to hunt for the leak. They were prepared to replace a pump, if necessary.
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 has never staged such a fast, impromptu spacewalk for a station crew. Even during the shuttle days, unplanned spacewalks were rare.
At the beginning of the spacewalk, neither astronaut spotted any flakes of ammonia or anything else amiss as they reached the spot in question and began an inspection.
"I see nothing off-nominal," Cassidy said. He noted some smudges, but nothing more.
The astronauts needed to take pictures of the equipment, but the connection to the camera's flash was not working and Marshburn had to wait until the space station reached the daylight side of Earth.
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.
NASA's space station program manager Mike Suffredini said it's a mystery as to why the leak erupted. One possibility is a micrometeorite strike. If there is nothing to suggest that the old pump is at fault, then the spacewalkers expect to leave it in place and go back inside as flight controllers figure out the next step.
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.