FILE - U.S. astronaut waves during spacewalk outside International Space Station.
Two astronauts from the U.S. space agency have completed the first stage of an effort to repair a faulty pump that shut down part of the International Space Station's critical cooling system several days ago.
NASA says astronauts Rick Mastracchio and Mike Hopkins disconnected the faulty ammonia pump while working outside the space station for more than five hours on Saturday.
The agency says the two men will try to install a spare pump module during their next spacewalk on Monday, with a final session outside the spacecraft on Wednesday.
The station has since been using a backup system for cooling internal and external equipment. The problem has forced the astronauts to turn off all non-essential equipment, including some science experiments.
Astronauts Fix Pump at the International Space Station
1/11Astronaut Mike Hopkins and Astronaut Rick Mastracchio replace a faulty pump on the International Space Station during a spacewalk, Dec. 24, 2013.
2/11Astronauts Rick Mastracchio, top, and Michael Hopkins work to repair an external cooling line on the International Space Station, Dec. 24, 2013.
3/11Astronaut Mike Hopkins is seen at the end of the robotic arm on the International Space Station during a spacewalk, Dec. 24, 2013.
4/11Astronaut Mike Hopkins works outside the International Space Station during a spacewalk, Dec. 24, 2013.
5/11Astronauts Rick Mastracchio, top, and Michael Hopkins work to repair an external cooling line on the International Space Station, Dec. 24, 2013.
6/11NASA astronaut Mike Hopkins is seen during the spacewalk, received December 22, 2013.
7/11In this image made from video provided by NASA, astronaut Rick Mastracchio performs a space walk outside the International Space Station, Dec. 21, 2013.
8/11Astronaut Rick Mastracchio (L) checks out the spacesuit that he will wear during a spacewalk with crew member Mike Hopkins at the International Space Station in this undated image taken from video from NASA TV.
9/11Astronaut Mike Hopkins checks out the spacesuit he will be wearing on a spacewalk at the International Space Station, in this undated handout photo from NASA.
10/11Flight engineers Michael Hopkins and Rick Mastracchio perform a series of spacewalks outside the International Space Station in this Dec. 21, 2013 still image taken from a NASA handout video.
11/11Astronauts Michael Hopkins and Rick Mastracchio perform a series of spacewalks outside the International Space Station in this Dec. 21, 2013 still image taken from a NASA handout video.
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Until Saturday, the station's crew members had not ventured out into space since July, when a still-unexplained problem nearly drowned an Italian astronaut whose helmet filled with water during a spacewalk.
NASA said Mastracchio and Hopkins attached snorkels to their spacesuits to use for breathing in case of a helmet water leak.
Some information for this report was provided by Reuters.
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