[UPDATE (May 11 at 15:45 UTC): Two astronauts performed an EVA this morning to swap out the ammonia pump suspected of causing the leak. After a successful swap with a spare unit, no more leaking of ammonia snowflakes has been seen, so for now it looks like the problem has been fixed! As I write this they are cleaning up and preparing to go back inside the space station, where they will keep a close eye on the pump and the coolant system to make sure eveything is working as it should.]
Right now, NASA is planning at least one EVA—extra-vehicular activity, or spacewalk—for an astronaut on the International Space Station to investigate and hopefully repair a coolant leak. NASA says the crew is not in danger, and the ISS is otherwise operating normally. Pete Harding at NASASpaceflight.com has a detailed article on this situation.
Basically, turning solar energy into power for the station generates heat, which must be removed. A cooling system is used that is essentially a series of ammonia-filled pipes; they run over the heated area, warming up the ammonia, which is then pumped away from the hot spot so it can radiate that heat away into space.
A small leak in one of the pipes has been known for years. It was so tiny it wasn’t a huge concern; a mission a few years back brought more ammonia to the station to refill the pipes. But on May 9, ground controllers noticed the leak had increased suddenly, and astronauts on board ISS saw flakes of frozen ammonia leaking out into space. Because of this, a spacewalk is being scheduled most likely for Saturday, May 11, to take a look and identify the culprit. A second, later EVA will probably then be needed to swap out the faulty part.
UPDATE (May 10, 2013): Video taken on the ISS shows some of these ammonia ice flakes moving away from the affected area:
This situation is serious, but being handled. Everything on the ISS has redundant backups, so the pipe is being shut down and the backup unit getting prepped for use.
This does throw a monkey in the wrench of the schedule; Commander Hadfield, Tom Marshburn (a likely candidate for at least one of the spacewalks), and Roman Romanenko were already preparing for their return to Earth on Monday. That’s now on hold for a while, probably until the situation is under control. It’s not clear how long the delay will be, or what this will mean for the launch of the next crew of three astronauts to the station, due for late May.
Spaceflight enthusiasts are discussing this on the NASASpaceflightNow forum, and it’s fascinating to read. It’s clear NASA is taking this very seriously, as you’d expect, but it doesn’t sound like this is a life-or-death situation. Still, any time a contingency (extra) spacewalk is added in, there are inherent dangers. Hopefully the engineers on the ground and the astronauts on-board the ISS will take care of this quickly and efficiently.