"This is China's first attempt (at a spacewalk) so there are a lot of uncertainties," Wang told reporters at a news conference in Beijing.
The 20-minute spacewalk is scheduled for Saturday afternoon; the exact timing depends on the readiness of equipment and personnel, Wang said. The event will be broadcast live on television, he said.
Performing a successful spacewalk is a key step in mastering techniques for linking two orbiters, required to create China's first space station, expected within the next few years.
The two astronauts donning suits for the spacewalk will be supported by Russian experts throughout the mission. Only one astronaut will actually leave the orbiter module to retrieve scientific experiments placed outside, described by the official Xinhua news agency as solid lubricant samples. The ship will then release an 88-pound (40-kilogram) satellite which will circle the orbiter and send back images to mission command.
Shenzhou 7 commander Zhai Zhigang is expected to carry out the spacewalk. Like his fellow astronauts, he is a 42-year-old fighter pilot; the three have each logged more than 1,000 hours of flying time.
Earlier Friday, the three-module capsule shifted from an oval orbit to a more stable circular orbit 213 miles (343 kilometers) above Earth, meaning it is circling Earth at a constant distance.
The change in orbit ensures Earth's gravitational pull will not vary during the spacewalk attempt, and will allow for smooth operation of the ship's instruments, the agency said.
A round orbit will also help Shenzhou make a precise landing on the Inner Mongolian Steppe on Sunday after its re-entry vehicle bursts through Earth's atmosphere, Xinhua said.
Thursday's launch of China's third manned mission in five years dominated front pages of the entirely state-controlled media, largely supplanting coverage of China's continuing scandal involving contaminated milk, which has killed four babies and sickened more than 54,000.
The Communist Party's flagship newspaper People's Daily showed President Hu Jintao waving to the astronauts before the launch and congratulating staff at mission control after liftoff.
Such coverage underscores the weighty role of politics and patriotism in the space program, portrayed by the party as an illustration of China's rising technological might and global influence.
As the 16-year-old manned spaceflight program gathers speed, technicians will begin to step up production of Shenzhou spacecraft to ensure a constant supply, its chief designer said.
Zhang Bainan offered no numbers, but said the spacecraft would be used to transport astronauts and cargo to a future Chinese space station. Zhang said other countries might be offered room on the craft.
Ramped-up production of the three-module Shenzhou, based loosely on Russia's Soyuz capsule, also points to a need for more astronauts beyond the 14-member corps now trained.
To meet the demand, a new round of astronaut selection will be held using revised standards following completion of this year's mission, Chen Shanguang, director of the China Astronaut Research and Training Center, was quoted as saying by the Communist Party's China Youth Daily newspaper.
Officials are also drafting criteria for female astronauts, although it has no specific plans to begin recruiting them, China's first astronaut, Yang Liwei, now Chen's deputy, was quoted as saying.
"Many standards have already been set and there has been strong support from society as a whole," Yang said.
