China's two unmanned spacecraft separated and then re-docked smoothly on Monday, following the first successful docking on Nov 3, the Beijing Aerospace Control Center said.

Experts said this second attempt was considered more difficult as it was mostly carried out in sunlight, which could interfere with the optical system's performance.

After orbiting together since Nov 3, the Shenzhou VIII spacecraft and Tiangong-1 space lab module received an order to undock at 7:24 pm and separated from each other in four minutes. 

Shenzhou VIII withdrew to a position 140 meters behind Tiangong-1. Then it drew close to Tiangong-1 under the control of the ground station, and the two docked with each other at 8 pm. 

"Most of the rendezvous process was done in sunlit areas to test the optical telemetry devices' performance in such areas," the control center said in a release. "With the data gained, our goals have been achieved."

The final docking was still completed in shadow, it said.

In comparison, the first rendezvous and docking practice was entirely carried out in the shadow of the Earth. 

"You think you can see objects clearer with sunlight on the Earth. But the spacecraft's telemetry system actually produces more accurate results in darkness without interference from sunlight," said Bai Mingsheng, deputy chief designer of the space lab system.

The two rendezvous and docking attempts in different environments are expected to give China a breakthrough in the rendezvous and docking technology, and also master the technology better, said Pang Zhihao, deputy editor-in-chief of the monthly publication, Space International.

The second exercise is also expected to prove the docking port is up to standard for repeated docking, Pang said.

Ma Yongping, vice-director of the Beijing Aerospace Control Center, said that Shenzhou VIII and Tiangong-1 will fly together for two more days.

Then Shenzhou VIII will separate from Tiangong-1 and maneuver into an orbit 200 km above earth before its return on Thursday evening, he said. 

The spacecraft is expected to fall in the Inner Mongolia autonomous region.

At the same time, Tiangong-1 will elevate its orbit to prepare for the next rendezvous and docking mission in 2012 with Shenzhou IX, which could be manned, he said.

The docking system, developed by Shanghai Academy Spaceflight Technology, is compatible with the docking mechanism and structure used on the International Space Station, the Soyuz spaceship and space shuttles, Xinhua News Agency cited Tao Jianzhong, a research fellow with the academy, as saying.

"We've simulated 1,101 dockings and 647 separations on the ground and the first docking precisely fit into our simulation," said Tao, who is an expert on docking simulation.

He said that future international cooperation in space would be convenient in light of the compatibility. 

The country's manned space program spokeswoman, Wu Ping, told an earlier news conference that China's spacecraft would conduct two more space docking missions in 2012, at least one of which will be manned.

China aims to build a space lab by 2016 and a space station around 2020, she said.