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Manned space docking's make-or-break precision revealed


16:24, June 19, 2012

BEIJING, June 18 (Xinhua) -- Chinese scientists and engineers have been working hard over the past three days, calculating data from its manned spaceship to ensure the precision needed for the country's first manned space docking mission, the program's control officials said on Monday.

"The frequency of guidance control from a long distance and its requirement on precision is unprecedented," said Ma Yongping, deputy head of the Beijing Aerospace Control Center (BACC), where every single command for the space docking mission is issued.

Shenzhou-9, with its crew of three astronauts, successfully docked with the Tiangong-1 lab module on Monday afternoon after repeated orbit corrections and a long journey catching up with its target.

The space lab received its first visitors after the three Chinese astronauts entered it to conduct experiments, exercise and rest.

"Only with precision can we avoid risks, ensure safety and achieve success," explained Qian Yongping, chief designer of the tracking and communication system of the manned space program. "Our mission is to be as precise as possible."

For the first step of the docking, Shenzhou-9 had to be guided to the rendezvous point about 52 km behind Tiangong-1.

The space rendezvous and docking requires a precise match of the orbital velocities of Shenzhou-9 and Tiangong-1, allowing them to remain at a constant distance through orbital station-keeping.

Both the spacecraft and the lab module are flying at a speed of 7.8 km per second, making it very difficult to maintain their relative position and height under the command of the BACC.

According to the original plan, the control center had to conduct guidance control and changed the orbit for the spaceship five times before the docking, using the tracking and control system on the ground.

"But we did it after only four orbit corrections," said Tang Geshi, a researcher at the BACC.

Tang noted the center canceled one orbit modification for Shenzhou-9 when it was orbiting Earth for the 13th time.

The revision of the original plan came as a result of calculations, in which researchers found they could use the fixed deviations from the orbit changes when the spacecraft was orbiting Earth for the fifth and 19th time to substitute a correction at the 13th orbiting.

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