The US space agency NASA said astronauts Heide Stefanyshyn-Piper and Shane Kimbrough left the ISS decompression chamber at 1658 GMT, some 45 minutes earlier than planned, for a spacewalk to last about 6.5 hours.

The spacewalkers plan to continue work begun on a seven-hour spacewalk Tuesday to unstick a jammed joint on a solar array, NASA said.

But a glitch marred Tuesday's spacewalk when Stefanyshyn-Piper lost her grip on a tool bag and watched helplessly as it floated off into the void of space.

The spacewalks are part of an ambitious "home improvement" project designed to double the station's crew capacity from three to six. On Wednesday, the team already began installing a freezer and an oven for scientific experiments by NASA's Destiny Laboratory Module.

The addition also is to include two new sleeping quarters, exercise equipment, a second toilet, two new ovens and a refrigerator.

The astronauts will also set up a machine for recycling urine into drinking water. The 250-million-dollar device is essential for doubling the accommodation capacity, as it would be able to recycle the station's 6.8 tonnes of waste water produced each year.

Once the water recovery system is in place, it will no longer be necessary to regularly ferry vast quantities of water to the space station, officials said.

Thursday's spacewalk comes on the 10th anniversary of the ISS, one of the most ambitious space projects ever and a key launching board for exploration of the solar system, including Mars and beyond.

The project commenced on November 20, 1998, with the launch into orbit of the first station element, a Russian-built module.

In orbit some 190 miles (350 kilometers) above the earth's surface, the ISS now has a permanent crew of three astronauts that remain aboard for stays lasting several months. The crew will increase to six in 2009 after the addition installed by the Endeavour crew.

The United States has financed the bulk of the project, estimated to cost some 100 billion dollars. Fifteen other countries have made financial contribution to the project, including Russia, Japan, Canada, Brazil and eleven EU nations.

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