Two astronauts stepped out of the International Space Station late Thursday afternoon, leaving the orbiting outpost empty for the first time when all of its habitants are working outside.

The tasks in front of US astronaut Michael Foale and his crewmate, Alexander Kaleri, who is a Russian, include retrieving and deploying Japanese and Russian science experiment devices on the exterior of the station's Zvezda service module, which is made by Russia.

Among the devices is a torso-like one called Matroshka, which is designed to measure the effects of radiation on simulated human tissue.

Usually, when two astronauts are conducting space walks outside the International Space Station, a third person will remain inside to monitor systems or assist in a crisis.

After the Columbia tragedy in Feb. 1, 2003, the three remaining US space shuttles have been grounded and the space station has depended on Russian spaceships to transport passengers, cargoes and supplies. The normal space station crew of three was also reduced to two.

Russia has pushed for the first International Space Station two-person space walk. After months of safety reviews, the US space agency NASA gave the finally go-ahead on Tuesday.

Thursday's space walks are scheduled to last about five hours and 45 minutes.

(Xinhua News Agency February 27, 2004)