Two astronauts stepped outside the International Space Station for a spacewalk on December 21 as they began trying to fix a faulty cooling system.
NASA said U.S. astronauts Rick Mastracchio and Michael Hopkins are planning three spacewalks in total -- the next on December 23 and another on Christmas Day.
The spacewalk on December 21 was expected to take about six hours. It is Mastraccio's seventh but the first for Hopkins.
It is also the first spacewalk by NASA since July when there was a problem with a space helmet worn by Italian astronaut Luca Parmitano.
The fault with the cooling system was detected last week.
The crew on board are said to be in no danger.
The Russian side of the space station has a separate cooling system.
The station, a project involving 15 countries, orbits at more than 400 kilometers above the Earth.
Based on reporting by dpa and AFP