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Mars probe fuel 'not a threat' (2)

By Xin Dingding  (China Daily)

08:19, November 18, 2011

The Russian probe could be the third uncontrolled large object to fall back to Earth in recent months following the crash of a 5.6-ton US climate satellite into the Pacific Ocean on Sept 24 and the plunge into the Bay of Bengal of Germany's 2.4-ton Rosat space telescope satellite on Oct 23.

"As more spacecraft are sent into space, more objects will re-enter Earth's atmosphere. In fact, trackable space debris with a diameter of more than 10 cm returns to Earth's atmosphere every day but only a few of these objects actually hit the Earth," he said.

Orbital debris includes derelict spacecraft. NASA said more than 20,000 objects larger than 10 cm are known to exist.

The debris poses a threat to functioning spacecraft. Even debris just 1 millimeter in size could travel at an average speed of 10 km per second, 10 times faster than a bullet and easily penetrate a spacecraft without protection, he said.

Some debris will take centuries to decay.

Usually, the higher the altitude the longer the debris will remain in Earth orbit. Debris left in orbits below 600 km normally falls back to Earth within several years.

Retired in 2005, the US climate satellite, in a 578-km orbit, took six years to re-enter the Earth's atmosphere, while Germany's Rosat satellite, circling in a 585-km orbit, retired in 1999 and took 12 years to fall back to Earth.

As for space junk at altitudes of 800 km, where the greatest concentrations of debris are found, the time for orbital decay is often measured in decades. Above 1,000 km, orbital debris will normally continue circling the Earth for a century or more.

Experts warned that once a "critical density" of space debris is reached, a process called collisional cascading (or chain reaction) - collision fragments will trigger further collisions - would start, around the year 2050. Consequently, the Earth could be covered by a cloud of debris too dense to allow any satellites.

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helen at 2011-11-18141.0.9.*
China will have to rely on its own research and efforts to launch its Mars space craft. Nothing like self reliance to smooth out contradictions.
  

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