China has joined two space vehicles together in orbit for the first time.
The unmanned Shenzhou 8 craft, launched earlier this week, made contact with the Tiangong-1 space lab at 1729 GMT. The union occurred over China itself.
Being able to dock two space vehicles together is a necessary capability for China if it wants to start building a space station towards the decade's end.
Although no astronauts were in the Shenzhou craft this time, future missions will carry people.
Tuesday's procedure (Beijing time 0029, Thursday) took place at an altitude of about 340km. It was automated but overseen on the ground at the Beijing Aerospace Flight Control Centre.
The vehicles used radar and optical sensors to compute their proximity to each other and guide their final approach and contact. A video feed from orbit showed the final moments of the vehicles coming together.
Shenzhou 8 was the active craft in the docking. It fired its thrusters to push its front end towards the docking port of Tiangong-1. Once the vehicles' docking rings had made a good capture, 12 hooks were deployed to fix the craft in place. Protruding pins made electrical connections.
From first contact to confirmation of a seal took about 10 minutes.
Shenzhou 8 and Tiangong-1 will spend two weeks circling the globe together before Shenzhou 8 heads back to Earth.
"After a joint flight of about 12 days, they will separate," explained Prof Yang Yuguang from the China Aerospace, Science and Industry Corporation.
"Then the Shenzhou 8 will draw back to about 140m. After that they will perform a second docking. Then they will have a flight of two days. Then they will be separated to about 5km; this will be a safe distance for both vehicles. Then the re-entry procedure will be performed by the Shenzhou 8 spaceship," he told state broadcaster China Central Television (CCTV).
The return capsule will land by parachute to allow experiments carried into orbit to be recovered for analysis. The German space agency (DLR) has supplied an experimental box containing fish, plants, worms, bacteria and even human cancer cells for a series of biological studies.
"This is a great moment for Chinese space activities," observed DLR chairman Prof Jan Woerner.
"This co-operation between China and Germany is called the Simbox; and it's a box in which we have small apparatus to investigate the cells of plants and animals, to see their behaviour under zero gravity conditions.
"It's a really sophisticated experiment and I hope that the results will help also on Earth to learn more about the behaviour of cells in cancer and the immune system."
- Tiangong-1 was launched in September on a Long March 2F rocket
- The unmanned laboratory unit was first put in a 350km-high orbit
- Shenzhou 8 was sent up to rendezvous and dock with Tiangong-1
- The project is testing key technologies such as life-support systems
- China aims to start building a 60-tonne space station by about 2020
Assuming the current flight of Shenzhou 8 goes according to plan, two manned missions (Shenzhou 9 and 10) are likely to try to make similar dockings in 2012.
Chinese astronauts - yuhangyuans - are expected to live aboard the conjoined vehicles for up to two weeks. There is speculation in the Chinese media that one of these missions could also include the country's first female yuhangyuan.
Beijing sees the Tiangong and Shenzhou dockings as the next phase in its step-by-step approach to acquiring the skills of human spaceflight operations.
It is a learning curve China hopes will eventually lead to the construction of a space station. This could start taking shape before 2020.
At about 60 tonnes in mass, this future station would be considerably smaller than the 400-tonne international platform operated by the US, Russia, Europe, Canada and Japan, but its mere presence in the sky would nonetheless represent a remarkable achievement.
Concept drawings describe a core module weighing some 20-22 tonnes, flanked by two slightly smaller laboratory vessels.
Officials say it would be supplied by freighters in exactly the same way that robotic cargo ships keep the International Space Station (ISS) today stocked with fuel, food, water, air, and spare parts.
China is investing billions of dollars in its space programme. It has a strong space science effort under way, with two orbiting satellites having already been launched to the Moon and a third mission expected to put a rover on the lunar surface.
Next week should see its first Mars orbiter - Yinghuo-1 - begin its journey to the Red Planet.
There has been talk - from the Europeans, at least - of asking China to join the ISS project, but this is unlikely to happen given current relations between Beijing and Washington. The US has concerns over China's intentions in space, particularly in the military realm.
Space analyst Prof John Logsdon from George Washington University said the two nations needed to learn to trust each other first, perhaps in the field of robotic space science, before there could be wider co-operation.
"Human spaceflight, both in the United States and in China, is a very visible, very politically charged activity," he told CCTV. "[We need] to learn to work with one another, get comfortable, develop mutual trust, hopefully solve our political problems, and then maybe at the end of this decade or in the next one start working together in human spaceflight."