Beijing - When Zhai Zhigang floats out of a space capsule wearing his 120-kilogram, 30-million-dollar pressurized suit, he is to set another landmark for China's ambitious space programme as it moves toward assembling a permanent space station by 2020.

Nearly three years after China's last manned space mission, Shenzhou VII was to blast off Thursday from the Jiuquan space centre in the north-western province of Gansu.

If all goes well, Zhai should make his spacewalk Friday.

"The spacewalk is risky," Australian space analyst Morris Jones told Deutsche Presse-Agentur dpa. "The Chinese spacesuit has not been tested in space before."

"The Shenzhou's orbital module has never been depressurized before, either," he said.

The spacewalk was expected to last about 40 minutes and was to be televised live via cameras mounted outside the space capsule and on a mini-satellite to be released from Shenzhou VII in orbit, state media reported.

Engineers have changed much of the equipment at the Jiuquan space centre since the launch of Shenzhou VI in 2005, launch commander Cui Jijun told the official Xinhua news agency this week.

About 6,000 ground staff at Jiuquan carried out meticulous preparation of the Long March-2F carrier rocket and the Shenzhou spacecraft, the agency reported.

The engineers devised plans to cope with 239 different emergencies, Cui said.

China is only the third country to launch an astronaut into space after Russia and the United States.

"It's at a 1967 level, roughly," Jones said of the Chinese space programme, compared with the US and Russian ones.

The Shenzhou human space flight programme includes future plans for docking manoeuvres and space laboratories.

Shenzhou VIII, expected to be launched in 2010, was designed to form the first stage of a crude space station made from two linked orbital modules.

The space programme is run by military units, and the previous two Shenzhou missions were believed to have carried out military surveillance.

All the astronauts who have served on Shenzhou missions are former military fighter pilots.

But Jones said he believes the three-day Shenzhou VII mission is unlikely to perform military tasks.

Cui denied that the Shenzhou programme had any military purposes, saying it was for "serving China's economic development."

"So far, China's manned space programme hasn't carried out a single military task," Cui said.

Sheng Jie, one of the chief designers of the launch systems, said the mini-satellite to be released from Shenzhou VII was for "civil scientific research to improve China's communication technology" and admitted controlling the satellite would be a "challenge."

"The key part of this research is to make sure the small satellite keeps a safe distance from the Shenzhou spacecraft," state media quoted Sheng as saying.

Astronaut Liu Boming is charged with monitoring Zhai's tethered spacewalk and helping him to leave and re-enter the orbital module while Jing Haiping is to remain inside the capsule.

A second trio of astronauts was on standby in case of last-minute problems with the first crew before the launch.

Liu would reportedly wear a Russian spacesuit while Zhai's suit was made in China but improved with Russian help. The suits are capable of protecting astronauts for up to seven hours in space, the China Daily newspaper said.

The mission was delayed earlier this year because of technical problems with the air lock and spacesuit, state media quoted Huang Chunping, an adviser to the manned space programme, as saying in July.

The spacewalk is to be a cautious exercise "mainly focussed on demonstrating the dexterity and functionality of the spacesuit," Jones said in a recent article published on the Dragon Space website.

The Shenzhou VII spacecraft consists of the orbital, propulsion and re-entry modules. It is similar to the Russian Soyuz spacecraft but larger.

The re-entry module is to be monitored by four ships and was scheduled to land in the remote Wulanchabo area of China's Inner Mongolia region, from where it was expected to be transported to the Beijing Space City, which also houses mission control.

China released plans in 2000 for an ambitious 20-year programme to build an "integrated ground-space network" for space exploration and manned space research, including space stations.

It said it hopes to send exploration vehicles to the moon and Mars as part of its long-term plan for developing its commercial space industry.

But most analysts said they believe China is unlikely to be influenced by the plans of other nations or be drawn into a "space race."

"China will simply carry out its plans at its own pace and protect its overall space flight ambitions with a well-woven veil of secrecy," Jones said.