After entering its fifth day of flight in space, China's second
manned spacecraft Shenzhou VI?is prepared for a journey back
home.
According to Sui Qisheng, commander-in-chief of the space
program's landing system, said that in the return process the
spaceship undergoes four stages of decelerating flight, free
descending, aerosphere flight and landing.
With a return order from the ground control center, the
spacecraft's position is adjusted and the re-entry capsule is
separated from the orbital module. The orbiter remains running in
orbit to carry out more experiments.
After the separation, the re-entry capsule's position is
readjusted for a return trajectory. Retroengines are ignited to
make the capsule deviate from its circular orbit and switch to the
return trajectory.
Ignition timing must be extremely accurate, for one second means
nine km of difference of the capsule's landing place.
Along the return trajectory, the spaceship maintains unpowered
free descending. About 140 km above the Earth, the propulsion
module is separated from the re-entry capsule, with the former to
be burnt in the aerosphere.
The re-entry capsule keeps descending in the aerosphere. About
100 km above ground, strong friction between the capsule and
aerosphere produces a large amount of heat on the surface of the
capsule.
Accordingly, a high-temperature plasma gas layer over the
capsule surface shields electromagnetic signals, making the capsule
lose contact temporarily with the ground control center.
The blackout phenomenon disappears when the re-entry capsule
drops to 40 km above the Earth, and its communication with the
ground resumes.
When the capsule is only 10 km from the ground, the final
landing phase is activated. The guiding parachute, deceleration
parachute and main parachute are unfurled, and the capsule drops
slowly.
When the capsule is one meter above the ground, four
reverse-thrust rocket engines are ignited to achieve the capsule’s
soft landing at a speed of one to two meters per second.
(Xinhua News Agency October 16, 2005)