China's second manned spaceship changes its elliptical orbit to a circular at 3:50 p.m. Wednesday afternoon.
It is a very important step in telemetric control work, Dr. Liu Yingchun, a spacecraft orbit expert, said. The Shenzhou series from unpiloted 2 to 4 versions and manned Shenzhou-5 did the same in past voyages.
The spaceship was 250 km above Earth when it entered the preset orbit. Moving at high speed, the spaceship was not running on a circular orbit, but elliptical with its altitude varying from 250 km at perigee to 350 km at apogee.
"The orbit change is made for self-prompted emergency landing purposes," Liu said. After the change, the orbiting trajectory will be the same on the first, third and fifth days, which will be advantageous for the spacecraft returning to the priority landing site, according to Liu. It is also easier to make emergency return plan for a spaceship on a circular orbit than on an elliptical one.
"The earliest Shenzhou-1 ran on the elliptical orbit, but it orbited Earth only one day." It is relatively difficult to predict a proper landing as an elliptically running spaceship's altitude and speed are changing and its work data vary at every position, Liu said.
To make the change, the ground command center sends the order to the spacecraft, adjusting work of engines onboard to tune its orbit close to a circular one, according to Liu.