Chasing an orbiting space laboratory module at speeds of nearly 5 miles per second, China's Shenzhou 8 capsule is on final approach for an automated docking to test key technologies for a future Chinese space station.

Artist's concept of the Tiangong 1 spacecraft (left) and the Shenzhou 8 vehicle near docking. Credit: China National Space Administration
 
China hasn't released the exact time of the scheduled docking, but it's expected to occur around 1730 GMT (1:30 p.m. EDT) Wednesday, or at about 1:30 a.m. Beijing time Thursday. Chinese engineers want the first docking to occur during the night phase of its orbit to reduce sun glare.

It will be the first automatic docking in Chinese space history, catapulting the country into another elite space club with Russia, the United States, Japan and the European Space Agency. The docking demo is a crucial step for China's ambition to build a 100-ton space complex in orbit by 2020.

China became the third nation to launch a person into orbit in 2003. Two more spaceflights have occurred since then, most recently a mission in 2008 which included China's first spacewalk.

Since launching Monday from the Jiuquan space base in northwest China, the unmanned 17,800-pound Shenzhou capsule has completed five engine maneuvers to fine-tune its approach to Tiangong 1, a slightly larger bus-sized module fitted with a docking port.

Tiangong 1 launched from Jiuquan on Sept. 29.

The five orbital maneuvers were completed with guidance from tracking sites and controllers on the ground, but Shenzhou 8's automated navigation and control system was expected to take over flying the spacecraft at about 1500 GMT (11 a.m. EDT).

Shenzhou 8 established a two-way communications link with Tiangong 1 earlier Wednesday when the craft were about 30 miles apart.

The final rendezvous sequence includes targeting, approach and horizontal closing phases lasting about two-and-a-half hours, according to Wu Ping, a spokeswoman for the China Manned Space Engineering Office, a division of the Chinese military.

Shenzhou 8 will be the active vehicle in Wednesday's docking. Its computers will automatically process navigation data from a high-tech suite of radar, laser and optical sensors designed to collect information on the range and closing rate between Shenzhou 8 and Tiangong 1.

Artist's concept of the Tiangong 1 spacecraft (left) and the Shenzhou 8 vehicle docked together.
 
Moments before contact, Shenzhou 8 will deploy a circular docking ring one foot into front of the spacecraft to link up with a receptor on the Tiangong target module. Then a robotic system will bring the spacecraft together for a tight connection with 12 locking pins.

The Shenzhou docking collar is similar to the Russian-designed APAS system, which was used in the joint U.S.-Soviet Apollo-Soyuz Test Project, the assembly of the Russian space station Mir and by space shuttles visiting the International Space Station.

The androgynous docking system is designed so either spacecraft can be active or passive.

China says the docking system would permit Shenzhou capsules to attach to the ISS if the country was ever invited to join the multinational research laboratory.

The Shenzhou docking system is designed to automated or manual dockings on missions with a crew on-board.

Zhang Bonan, chief engineer of the China Aerospace Science and Technology Corp., said controllers in Beijing will check the alignment and pressure seal between the two spacecraft in the moments after docking.

"The rendezvous and docking phase will begin from the contact of the docking mechanisms," Wu said. "Within 15 minutes, the processes of capturing, buffering, approaching and locking will be completed. Finally, the two vehicles will be physically connected to form an assembly."

Shenzhou 8 and Tiangong 1 will form a mini-space station 60 feet long with more than 700 cubic feet of habitable volume inside.

"There are a lot of plugs for gas and electrical circuits that must be perfectly aligned," Zhang said. "The two vehicles must also be precisely positioned to be connected. If not, the docking cannot be achieved."

Once the spacecraft are firmly bolted together, Tiangong 1 will assume control of the flight of the dual vehicles.

The craft will stay together for 12 days before Shenzhou 8 undocks, backs away to 460 feet and returns for a second docking.

Shenzhou 8 will depart Tiangong 1 for good and its re-entry module will parachute back to Earth on Nov. 17, according to Chinese state media.

One or two manned Shenzhou flights are scheduled for next year to dock with Tiangong 1 for short-duration research missions.