Scientists in the US have warned Nasa that the amount of so-called space junk orbiting Earth is at tipping point.

美国科学家向宇航局警告道，地球外围太空垃圾的数量已经达到了一个临界值。

debris¹ in low Earth orbit" src="http://www.enread.com/upimg/allimg/110902/1_110902061446_1.jpg" width="304" height="171" />
Artist's impression from the European Space Agency of debris in low Earth orbit
A report by the National Research Council says the debris could cause fatal leaks in spaceships or destroy valuable satellites.

It calls for international regulations to limit the junk and more research into the possible use of launching large magnetic nets or giant umbrellas.

The debris includes clouds of minuscule²（极小的） fragments, old boosters and satellites.

Some computer models show the amount of orbital rubbish "has reached a tipping point, with enough currently in orbit to continually collide and create even more debris, raising the risk of spacecraft failures," the research council said in a statement on Thursday.

Situation 'critical'

Hopes of limiting the amount of space junk in orbit suffered two major setbacks in recent years.

There are 22,000 pieces of debris large enough to track from the ground, but smaller objects could still cause serious damage.

The International Space Station must occasionally dodge³（躲避） some of the junk, which flies around the Earth at speeds of up to 17,500 mph (28,164 km/h).

In June, some debris narrowly missed the space station, forcing its six crew to go to their escape capsules and prepare for an emergency evacuation back to Earth.

The situation is critical, said Mr Kessler, a retired⁴ Nasa scientist, because colliding debris creates even more of the junk.

"We've lost control of the environment," he said.

The report makes no recommendations about how to clean up the field of debris.

But it refers to an earlier study for the Pentagon's science think-tank, the Defence Advanced Research Projects Agency (Darpa).

The Darpa report, dubbed⁵ "Catcher's Mitt⁶", suggested a range of technologies, including harpoons⁷（鱼叉） , nets and an umbrella-shaped device that would sweep up the debris.

The aim would be to push the debris further towards the earth where it would burn up, or into a higher but safer orbit.