A large quantity of space debris were left in space as a result of human activities over the past 48 years, posing a great threat to the safety of space vessels.

Under the current technological development, space debris larger than 10 centimeters, totaling 9,600 pieces, can be avoided in space flights, while those smaller than one millimeter pose no disastrous threat to space vehicles, said Gong. "But we cannot monitor debris pieces between one millimeter and 10 centimeters, and call them threatening debris since they pose great threat to spaceship," he said.

To avoid collision, the United States requires its space vessels to have an orbit maneuver when a piece of space debris is monitored 25 kilometers ahead. "The safety distance is set at 100 kilometers for Shenzhou-6," said Gong, adding that in fact, it takes a very long time for some of the space debris pieces to cross the Shenzhou-6 running orbit, and some will never meet Shenzhou-6 spacecraft.

According to him, Shenzhou-6 is able to resist hit by debris less than one millimeter in size. China's self-developed space debris warning system can provide fairly accurate forecast for the next 24 hours and preliminary forecast for the next three days, he said.

The Shenzhou-6 spacecraft, blasted off Wednesday morning with two taikonauts, or astronauts, aboard, is flying in a circular orbit 343 kilometers above the Earth after the orbit shift from an elliptical orbit in its fifth circle.

Besides space debris, high-radioactive high energy particles also pose threats to space vessels, which radiate the low orbit through the shielding of the Earth's magnetic fields, according to Gong. Astronauts at the International Space Station hide in special zones when there is a burst-out of high energy particles caused by abnormal solar activities. China's Shenzhou spacecraft series avoid the burst-out by variation of launching time, Gong said.

Another major threat in space mentioned by Gong is meteors and slight meteor showers. Some small meteors fly at a maximum speed of 70 kilometers per second, while the space debris run only at 7.9 kilometers per second. Therefore greater damage would be caused if the spaceship collided with meteors.

However, meteors and meteor showers can be forecast in advance. The launch time of China's Shenzhou-1 in November 2001 was postponed because a Leo meteor shower was forecast, said Gong, noting that no meteor shower is forecast during the flight of Shenzhou-6.

(Source: Xinhua)