The three Chinese astronauts aboard the Shenzhou VII spacecraft were in fine health on Friday, ready for the extra-vehicular activities (EVA) scheduled for Saturday, a spokesman for the space mission said.

Zhai Zhigang and Liu Boming started assembling the 120-kg EVA spacesuits since 10:20 am, Wang Zhaoyao, a spokesman for China's manned space program, told a press conference in Beijing on Friday. Xinhua reported that they finished the work in about 12 hours.

The two space suits were fixed to the module wall in parts when the spaceship was launched, to be assembled by the astronauts later.

Unpacking the suits comprises 21 steps and each step is divided into sub-steps and detailed moves.

Once the suits are assembled, the astronauts started testing and wearing them, in addition to a 100-minute rehearsal in the orbital module before the spacewalk.

"A job that people can finish in one hour on Earth usually needs two to three hours in space," Li Xiqiang, a researcher for the Shenzhou VII mission, said at a separate press briefing. But he promised the astronauts would not feel exhausted. "There's enough time for them to take a rest."

It was the first time for the two to assemble the suits in zero-gravity as it is impossible to create a real zero-gravity condition on Earth.

One of the astronauts will walk out of the orbital module at 4:30 pm on Saturday, Wang said. "Since it is the first spacewalk for China ... there are many uncertainties. We will very likely adjust the time for EVA according to the astronauts' conditions," he said.

The astronauts will make a series of difficult moves in space, Wu Bin, the expert in charge of astronaut training with the China Astronaut Research and Training Center, told Xinhua.

Zhai, the No 1 astronaut, is most likely to walk into space. Assisted by Li, who will stay on in the orbital module during the process, Zhai will take a test sample of solid lubricant from the surface of the spacecraft, which was placed there before the launch.

The astronaut, tied to the spaceship with two safety wires, is scheduled to walk along the orbital module exterior.

"Actually, it is not like walking, but floating," Wu said.

A set of handrails on the spaceship surface will help the astronaut move, he said. He must make sure that both wires are tied to the handrails before taking any step forward.

During the mission, the astronauts will depressurize and re-pressurize the cabin, as well as exit and re-enter the orbital module.

To ensure the safety of the astronaut who will do the spacewalk, the space suit is equipped with three oxygen tanks. An 8-m-long electricity wire, linking the astronaut with the spaceship, will send his physiologic data back and act as a spare safety wire.

"The EVA is expected to last 20 minutes. If the time for opening and closing the module door is factored in, it will be 30 to 40 minutes," Wang said.

The historic moment will be broadcast live.

The spaceship will aslo release a 40-kg satellite which will circle the orbiter and send back images to mission command.

Earlier on Friday, Shenzhou VII shifted from an oval orbit to a more stable circular orbit 343 km above Earth.

The spaceship atop a Long March II-F rocket lifted off at 9:10 pm on Thursday from Jiuquan Satellite Launch Center in Gansu province, in China's third manned space mission.