ISS crew conduct spacewalk to address ammonia leak

(CNN) -- Two International Space Station crew members are heading out Saturday morning for a spacewalk to address an ammonia leak in the orbiter's cooling system, an emergency that forced NASA officials to work round-the-clock to hatch a plan.

First detected early Thursday morning, the leak was causing ammonia to enter space -- which could be seen in the form of "flakes of snow" -- at a rate of 5 pounds per day, said Mike Suffredini, NASA's space station program manager. Ammonia is used to cool each of the solar arrays that provide electricity to station systems.

The discovery spurred teams at NASA, over a busy 24-hour stretch, to go into "a full-court press to understand what the failure is" and how to address it, NASA flight director Norm Knight said at a Friday news conference.

The space station's six-man crew is in "good spirits" in anticipation of the spacewalk, added Knight, a sentiment echoed by a tweet Friday afternoon by its commander.

"This type of event is what the years of training were for," wrote Cmdr. Chris Hadfield of Canada. "A happy, busy crew, working hard, loving life in space."

Hours earlier, Hadfield wrote on Twitter that his crew was planning for a Saturday spacewalk to be conducted by NASA astronauts Chris Cassidy and Tom Marshburn.

Hadfield -- who is with the Canadian Space Agency -- spent Friday preparing for his role as choreographer of the spacewalk, while Cassidy and Marshburn worked in an airlock to check out the spacesuits they will wear in space, among other tasks.

Cassidy and Marshburn have done two spacewalks apiece, working together both times in 2009 while on a space shuttle Endeavour mission to the space station.

"Since the crew is prepared and our ops team is ready to go, we're going to try to get them outside," said Suffredini.

Scheduled to wake up at 2 a.m. ET, the astronauts will begin their formal preparations about 1 hour and 15 minutes later, Knight said. A hatch will open around 8:15 a.m., sometime after which Cassidy and Marshburn will start moving along a truss the 150 feet out to the site of the leak on the space station's U.S. segment.

Once there, Knight explained, the astronauts will do a "visual inspection" of the leak -- the location of which space agency officials have "narrowed down" thanks to imagery, though Suffredini notes any cracks may be "very, very small." They will also check and possibly replace a pump controller box on the truss, which NASA explains is "the oldest component of the station's backbone."

The entire walk is expected to take about 6½ hours.

The space station's crew, which also includes three Russian cosmonauts, is not in danger from the leak, NASA has said. Moreover, the agency has said the rest of the orbiter is otherwise operating normally.

The leak is in a cooling loop in a solar array that has leaked before. NASA said crew members tried to fix a leak in November. It's unclear whether this is the same leak or a new one.

The ammonia coolant for the power channel, one of eight used to supply electricity to the station, is likely to run out by late Friday morning and it will be shut down, NASA said.

"It is a serious situation, but between crew and experts on the ground, it appears to have been stabilized," Hadfield tweeted Thursday.

In the Friday news conference, NASA's Suffredini said the spacewalk and ammonia leak won't affect the departure -- scheduled for Monday at 7:08 p.m. ET -- of Hadfield, Marshburn and Russian cosmonaut Roman Romanenko.

Three crew members, Cassidy and Russian cosmonauts Alexander Misurkin and Pavel Vinogradov, will remain on the space station when the others leave.

They will be joined at the end of the month by three new crew members: NASA astronaut Karen Nyberg, Russian cosmonaut Fiyodor Yurchikhin and European Space Agency astronaut Luca Parmitano, who are due to launch aboard a Russian Soyuz spacecraft on May 28.