Two U.S. astronauts have started the first part of an attempt to replace a failed cooling system for the International Space Station. The spacewalk got underway behind schedule on Saturday. 

The U.S. National Aeronautics and Space Administration said there had been communications glitches causing the slight 20-minute delay, before the spacewalk got underway.

Astronauts Douglas Wheelock and Tracy Caldwell Dyson communicated between each other and with another astronaut and NASA officials as they first tried to position themselves for removing and replacing a broken ammonia coolant pump.

NASA television explained what was going on. "Doug Wheelock giving the go to wedge his feet into the articulating portable foot restraint on the end of the Space Station's Canadarm2."

One of the space station's ammonia pumps shut down last week, knocking out half of the space station's cooling system.

The six-person crew which includes another American and three Russians had to turn off all unnecessary equipment and stop scientific experiments.

Earlier this week, Mike Suffredini, NASA's space station program manager, said he had not expected the pump's failure so soon, but that they were prepared.

"This is an anomaly we knew someday would happen," he said. "It is an anomaly that we have trained for. It is an anomaly that we have planned for.  We are in a good position to go solve this problem. It is a significant failure, though, in terms of systems on board ISS [International Space Station], so it is one we need to go get after."

One of the space station's four spare pumps is being used as a replacement.

Last-minute preparations included astronauts in Houston rehearsing every step of the planned repair mission while submerged in a training pool.

A second spacewalk has been scheduled for Wednesday to connect ammonia lines to the replacement pump.

The space station serves as a research laboratory in low Earth orbit, currently about 350 kilometers above our planet.