One of the trio, most likely mission leader Zhai Zhigang, was scheduled to leave the Shenzhou VII spacecraft for the historic 20-minute walk in orbit at 4:30 pm (0830 GMT), state media reported.

An "intensive psychological shock" would be unavoidable once the astronaut left the capsule 343 kilometres (215 miles) over the Earth, Xinhua news agency said, citing Yang Liwei, who piloted China's maiden space flight in 2003.

China's first space walker will be tethered to the spaceship with two safety wires and is scheduled to walk along the exterior of part of the Shenzhou craft.

He will take a test sample of solid lubricant from the surface of the spacecraft, in a bid to replicate the type of task that future space walkers will have to do.

"Actually, it's not like walking, but floating," Wu Bin, an expert at the China Astronaut Research and Training Centre, told Xinhua.

The astronauts, who took off from the Jiuquan Satellite Launch Centre in the desert of northwest China late Thursday, passed most of their first day in space preparing for the walk.

On Friday, 41-year-old Zhai and his colleague Liu Boming spent 12 hours unpacking and assembling the special China-made space suit that must be used during the walk outside the Shenzhou craft.

"A job that people can finish in one hour on Earth usually needs two to three hours in space," said Li Xiqiang, a scientist attached to the space programme, according to the China Daily newspaper.

It was the first time the two tried to assemble the suit in conditions of perfect weightlessness as it is impossible to create a real zero-gravity environment on Earth, the paper said.

Being able to spacewalk is seen as a precondition for accomplishing China's next extraterrestrial ambition: establishing a space station.

By 2010 two more unmanned craft will have been sent up, as well as another manned spaceship with a crew of three to start work on the lab, according to the China Daily.

The astronauts also had time Friday to enjoy the view, witnessing 16 sunrises during their first 24 hours in orbit, and to sample the 80-dish menu they brought with them on their 68-hour mission.

Astronaut Jing Haipeng, from north China's Shanxi province where vinegar is often added to food, was grateful to find the seasoning on board, according to Xinhua.

Sleep was necessarily limited, but the spacecraft did provide facilities in the form of sleeping bags hooked to the wall.

However, the astronauts were told to keep their hands inside the bags in order to avoid them accidentally pushing a button or flipping a contact while asleep, Xinhua said.

After China sent its first man into space in 2003, it followed up with a two-man mission in 2005.

The Shenzhou VII is scheduled to land in the northern Inner Mongolia region after the mission is completed.

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