The mission by Fei and Nie, both former fighter pilots, is expected to be longer and riskier than the 2003 flight, which carried one astronaut and lasted just 21 1/2 hours.
The government has not said how long the flight will last, but news reports say it could be up to five days. The official China News Service said Wednesday the capsule was expected to orbit the Earth 80 times before landing in China's northern grasslands.
Wen, the premier, visited Fei and Nie before dawn in their quarters at the Jiuquan base and called their flight a "glorious and sacred mission," the official Xinhua News Agency reported.
"You will once again show that the Chinese people have the will, confidence and capability to mount scientific peaks ceaselessly," Wen was quoted as saying. He said he spoke for Hu and the party leadership.
Xinhua said that during their flight, the astronauts will take off their 22-pound (10-kilogram) space suits to travel back and forth between the two halves of their vessel —— a re-entry capsule and an orbiter that is to stay aloft after they land.
Fei's and Nie's identities were not announced until early Wednesday, hours before their flight. Xinhua said the astronauts —— known in Chinese as yuhangyuan, or "travelers of the universe" —— were picked from a field of six finalists.
Nie was one of three finalists for China's first space mission in 2003 but lost out to Yang Liwei, also a former fighter pilot, who became a national hero.