China's test on January 11 2007, in which it used a ballistic missile to destroy an ageing Chinese weather satellite at an altitude of some 800 kilometres (500 miles), sparked an international outcry.

At a stroke, it created hundreds of pieces of additional debris, posing a potential threat to satellites in low orbit.

Fernand Alby, in charge of space debris issues at France's National Centre for Space Studies (Cnes), said a European conference in Toulouse, southwestern France, last week painted a mixed picture about the junk problem.

There are now more than 12,000 objects that are monitored in orbit, around 1,500 of them debris from the Chinese test, and the tally is increasing by 200 to 250 objects per year, he said.

"It already amounts to a genuine collision problem, although it is not preventing space exploration," he said.

Of the total, 11,500 pieces are in low Earth orbit, which is at an altitude of between 800 and 1,500 kilometres (500 and 950 miles), where there are many commercial, military, scientific and navigational satellites. The maximum altitude of the International Space Station (ISS) is around 450 kilometres (280 miles).

Another 1,147 pieces are in geostationary orbit, about satellite orbits in the direction of the Earth's rotation, at an altitude of approximately 35,786 km (22,240 miles), where telecommunications satellites are typically deployed.

At this location, the satellite remains stationary relative to its position on Earth.

Alby, in his briefing on the Toulouse conference, said that satellite operators were now doing better on obeying recommendations for disposing of disused satellites in geostationary orbit.

Old satellites or boosters left in geostationary orbit not only take up valuable "slots" for replacements. With time, in the harsh environment of space, their batteries or residual fuel can spontaneously explode, creating a perilous debris field.

Two such events have already occurred in geostationary orbit -- one was the 1978 explosion of a Russian Ekran-2 television satellite, and the other, in 1992, was the explosion of a US Titan 3 rocket stage, according to the Inter-Agency Space Debris Coordination Committee (IADC), whose 11 members include all the major spacefaring nations.

The IADC recommends that dying satellites be parked in a graveyard 300 kms (187 miles) beyond geostationary orbit, and be emptied of any remaining fuel and their batteries run down before being permanently shut down.

In 2007, out of 12 satellites that reached the end of their operational life, 11 were correctly re-orbited, but one -- a Russian satellite -- was re-orbited too close to the geostationary zone, said Alby.

In 2006, nine satellites were correctly reorbited, seven were reorbited too close and three were abandoned. In previous years, the average was one third correctly reorbited, one third too close and one third abandoned, said Alby.

Space debris is typically only small in size, but because these pieces travel at huge speeds, they impact with high energy.

In 1996, a French spy satellite, Cerise, was wacked at about 50,000 kms (30,000 miles) per hour by a fragment left from an exploded Ariane rocket.

In June 1983, the windscreen of the US space shuttle Challenger had to be replaced after it was chipped by a fleck of paint measuring 0.3mm (0.01 of an inch), that impacted at four kms (2.5 miles) per second.

In low orbit, debris can stay adrift for decades before they eventually burn up in Earth's atmosphere.

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