Washington, Apr 12 (DPA): Two US astronauts completed their 7-hour-26-minute spacewalk Sunday after struggling with a troublesome bolt that forced them to leave some of their work undone.

Rick Mastracchio and Clayton Anderson, who are visiting the International Space Station on the space shuttle Discovery mission, removed an empty ammonia tank and installed a new one. The tank is vital to keeping the station's cooling system working.

The astronauts started their spacewalk ahead of schedule, at 1:30 am (1100 IST), and finished at 8:56 am.

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.

Tasks that were deferred from Sunday's spacewalk because of the problem included the fluid connections to the ammonia tank and retrieval of two micrometeoroid debris shields for return to Earth.

The next spacewalk is set for Tuesday.

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 April 19 at 8:54 am (1824 IST).

The US space agency is scheduled to retire the ageing space shuttle fleet later this year after three remaining missions.