Two US astronauts left the International Space Station Sunday on the second of three spacewalks to install a new ammonia tank that is part of the orbiting laboratory's cooling system. Rick Mastracchio and Clayton Anderson, who are visiting the ISS on a space shuttle Discovery mission, will remove an empty ammonia tank and install a new one during the planned six-and-a-half-hour spacewalk, according to a report of the German Press Agency "DPA". The astronauts started their spacewalk ahead of schedule, at 1:30 am (0530 GMT). The multi-step process of installing the new tank with the aid of the station's robotic arm requires three spacewalks, the first of which was Friday. The ammonia tank is part of a cooling system on the station and must be periodically changed. The next spacewalk is set for Tuesday. On Saturday, NASA extended Discovery's mission by one day to allow the crew to inspect the shuttle's heat shield while still docked at the ISS. Discovery is now slated to land at Kennedy Space Centre in Florida on April 19 at 8:54 am (1254 GMT). The US space agency is scheduled to retire the ageing space shuttle fleet later this year after three remaining missions.
