Human activities are not only leaving wastes on the Earth, but also polluting space. Space debris has posed a great concern in the countries which pursue space exploration.

However, Chinese experts say the operating Shenzhou VI, the nation's second manned spacecraft, is unlikely to be hit by space debris as scientists are capable of monitoring space debris.

Space no more a clean place

Since the former Soviet Union sent the first satellite into space in 1957, human exploration of space has left numerous debris, or rubbish, in the vast space.

Space debris refers to artificial objects or fragments cast off in space, whether on purpose or not. A total of 29,000 larger pieces of space debris have been tracked in the past 48 years of space activities. About 20,000 of them have fallen to the Earth.

Currently, 9,600 pieces of space debris which are larger than 10 cm are monitored. Only the United States and Russia conduct monitoring all such debris, said Gong Jiancun, a research fellow with the space technology and application center under the Chinese Academy of Sciences.

The pieces bigger than one mm but smaller than 10 cm cannot be monitored, Gong said, and those smaller than one mm pose no disastrous threat to space vehicles.

There are some 500 pieces of space debris passing through the orbit of Shenzhou VI spacecraft, which is flying in a circular orbit 343 km above the Earth. It is able to resist the hit by debris less than one mm in size, said Gong.

About 45 percent of space debris was produced by the United States and 48 percent by Russia or former Soviet Union. China produced only 1.2 percent.

Growing Threat to Space Mission

The average speed of space debris is 10 km per second and the maximum speed can be 16 km per second. Explosion or disassembly could happen if the craft is hit by a large piece of space debris. So far, three disastrous collisions have occurred, according to Gong.

Even a 10-gram piece of debris can generate a collision force in space equaling to the crash of a car running at 100 km per hour, said Du Heng, chief scientist with China's space debris action program.

On the average, a space shuttle has to reinstall 1.41 windows after each flight, according to the National Aeronautics and Space Administration of the United States. In July 1999, 64 collision traces were found on the American space shuttle "Discovery" after it returned to the Earth.

Most debris float in a zone 300 to 2,000 km from the Earth, where most space activities are carried out, and the number of debris larger than 10 cm is increasing by 200 each year.

Therefore, some scientists warn that if not controlled, the quantity of space debris will rise twice in 100 years, and no space vehicles can exist in space in a few centuries.

Monitoring, improving, avoiding

So far no effective way to collect space rubbish or avoid them from collision has been worked out. Satellite and spacecraft launchers can only try producing as less debris as possible while monitoring large debris and improving solidity of space vessels.

In 1993, the US, Japanese and European space authorities initiated the Inter-Agency Space Debris Coordination Committee. China joined the committee in 1995.

Today, less debris is produced by the explosion of abandoned rockets with surplus fuel. The committee is working on the design and production standards of space vehicles in the effort to reduce debris, according to Du Heng.

Improvement in spacecraft has also been made, including strengthening solidity and designing special protection areas. The measures on the International Space Station seem effective in resisting the collision with small debris, said Du.

Space powers are also trying to enhance monitoring abilities, wanting to lower the size of traceable debris to one cm.

China's task in the next five years is to independently monitor large debris, he said.

However, to avoid approaching large debris is the best way. The United States requires its space vessels to have an orbit maneuver when a piece of space debris is monitored 25 km ahead. China sets the safety distance at 100 km for Shenzhou VI, said Gong.

According to figures released by the US National Aeronautics and Space Administration, in the 50-odd flights from 1988 to 1997, its space shuttles changed orbit to avoid space debris at least five times.

On October 26, 1999, the International Space Station changed orbit for first time when a large debris approached. The action cost 30 kg of fuel, jetting itself away from the debris from less than one km to a safety distance of 140 km. Four such changes took place each year ever since.

(Xinhua News Agency October 15, 2005)