Cape Canaveral (Florida), Nov 20 (DPA) Two US astronauts left the safety of the International Space Station Thursday to install spare parts that will help it continue running smoothly in the future.

Space shuttle Atlantis crew members Michael Foreman and Robert Satcher spent six hours and 37 minutes on the spacewalk.

They installed a backup antenna to the outside of the station and completed other maintenance with extra time to spare, NASA said.

After completing all assigned tasks, they used the last two hours of the spacewalk to install hardware to build cargo storage outside the station. The task had originally been planned for Saturday's spacewalk after astronauts on previous missions had not been able to complete the work.

The Atlantis brought thousands of kilogrammes of extra supplies to the station as the shuttle programme enters its expected final year in 2010.

The shuttle delivered two platforms with 12,360 kg of spare parts, which will be installed on the outside of the station. Most of the gear was to be installed using the robotic arm, but some pieces, like the antenna in Thursday's spacewalk, must be physically bolted on by spacewalkers.

As the first of several flights devoted largely to delivering spare parts, this mission is laden with the highest-priority items.

The so-called Express Logistics Carriers contain a variety of crucial parts: Gyroscopes that help keep the ISS at the proper altitude in space; an extra hand for the station's robotic arm; a gas tank for providing oxygen to the airlock during spacewalks; parts for the station's cooling system.

During the spacewalk, Foreman and Satcher also conducted a variety of other construction and maintenance tasks, including lubricating the Japanese robotic arm.

Two more spacewalks are planned for later in the mission.

The retirement deadline of the shuttle is fast approaching with just five more launches scheduled and one year left to go in the programme.

NASA is at work on developing the next generation spacecraft with an eye on returning humans to the moon or travelling to Mars and beyond. But full support for the plans is still pending in the halls of government..

If the programme goes forward, it would carry the crew on top of the rocket in a configuration that recalls the Apollo moon missions and which engineers say is safer than the space shuttle design, following the explosion of the Columbia in 2003.