China's three astronauts have spent their first day in orbit preparing for the mission's spacewalk.
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It will be the first time Chinese yuhangyuan (astronauts) have ventured outside their spacecraft.
Their Shenzhou VII capsule soared into orbit on a Long March II-F rocket from Jiuquan spaceport in north-west China.
The rocket put the Shenzhou capsule in a near-circular orbit more than 300km above the Earth.
Mr Zhai is joined on the mission by two other "yuhangyuan" - Liu Boming and Jing Haipeng.
Zhang Jianqi, one of the chief engineers for China's space programme, said keeping three men in the spacecraft, and then sending one outside, would be a "big test".
"This is a big technological leap," he told state-run news agency Xinhua.
"The risks are quite high. Sending up three astronauts is a jump both in quantity and quality."
When Mr Zhai carries out his extra-vehicular activity (EVA), he is expected to wear a Chinese-made spacesuit thought to have cost between Ј5m and Ј20m ($10m-$40m).
The yuhanguan will be tethered to the capsule with a cable that provides him with life support and a communications link with the spacecraft.
His back-up, Mr Liu, will monitor the activity, presumably to reel the spacewalker back inside if there is an emergency.
Mr Zhai will retrieve an externally mounted experiment and oversee the release of a satellite.
At the end of the mission, the Shenzhou re-entry capsule will target a landing in north China's Inner Mongolia Autonomous Region.
China became only the third nation after the United States and Russia to independently put a man in space when Yang Liwei, another fighter pilot, went into orbit on the Shenzhou V mission in October 2003.
Two years later, Fei Junlong and Nie Haisheng completed a five-day flight on Shenzhou VI.
According to the Associated Press, China's official news agency posted an article on its website prior to the lift-off that was written as if Shenzhou VII had already been launched into space.
The article reportedly carried a date of 27 September and came complete with a dialogue between the astronauts.
Chinese media report that this latest mission is the "most critical step" in the country's "three-step" space programme.
These stages are: sending a human into orbit, docking spacecraft together to form a small laboratory and, ultimately, building a large space station.
The Shenzhou VIII and IX missions are expected to help set up a space laboratory complex in 2010.
China launched an unmanned Moon probe last year about one month after rival Japan blasted its own lunar orbiter into space.
(BBC)
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