BEIJING, Oct. 15 (Xinhuanet) -- 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-6, the nation's second manned spacecraft, is unlikely 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-6 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.