LIVE: NASA's Emergency Spacewalk To Repair Leak On Space Station [VIDEO}

Two astronauts aboard the International Space Station made a hastily prepared spacewalk Saturday to investigate and repair a coolant leak in the station's power system.

The walk was devised and executed just 48 hours after a ammonia leak was detected on a pump controller box on the space station's far port truss (P6).

A new pump was installed on the P6 truss by the astronauts, and the latest reports indicate no ammonia leakage. The pump will soon be tested by the astronauts before then complete the spacewalk.

Mission controllers planned 6 ½ hours for the spacewalk. A post-spacewalk briefing will be broadcast on NASA TV no earlier than 4:30 p.m. EDT.

The two-man crew is working about an hour ahead of schedule, according to a report by NASA TV, which is broadcasting a live stream of the spacewalk and repair process here.

Expedition 35 Flight Engineers Chris Cassidy and Tom Marshburn began a spacewalk at 8:44 a.m. EDT Saturday. This is the third spacewalk by Cassidy and Marshburn, who conducted two spacewalks together during the STS-127 mission of the shuttle Endeavour to the station in July 2009.

NASA has never staged such a hasty, impromptu spacewalk. Even during the space 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.

On the ground at mission control in Houston, flight controllers worked furiously to get ready for Saturday's operation. 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, which all astronauts have advance training for.

While the leak has been described as a "serious situation," NASA reports there problem was not critical from a safety standpoint.