Washington - Two astronauts from the space shuttle Discovery were working outside the International Space Station Thursday to install a new ammonia tank that is crucial for keeping the orbiting laboratory cool. US astronaut John "Danny" Olivas and Swede Christer Fuglesang left the ISS at 2212 GMT on a planned six-and-a-half hour spacewalk. They got started nearly an hour later than planned because of problems with a communications device in Olivas' spacesuit.
The pair will remove an 816-kilogramme ammonia tank from a cargo carrier attached to the station and move it with a robotic arm to its home outside the station. The tank will be the largest item ever moved by astronauts during a spacewalk.
Much of the work involves connecting bolts and cables to hook the tank into the station's cooling system.
Kevin Ford and Nicole Stott will operate the robotic arm.
Earlier this week, Olivas and Stott removed an empty ammonia tank from the station. The old tank will be put into the carrier to be brought back to Earth at the end of Thursday's spacewalk.
The planned 13-day mission is designed to transport new equipment and experiments to the space station to boost its capacity as an orbiting laboratory.
The crew of six US and a Swedish astronaut brought two "racks" for scientific experiments and a freezer to store experiments set to be sent back to Earth. One of the racks will be used to conduct research on metals, semiconductors, crystal, glass and other materials in microgravity. The other will be used to study liquids and gels in space.
The racks were transferred to the station Wednesday.
