A scare triggered by orbital debris that on Tuesday came within a couple of hundred metres of the International Space Station sheds light on an acutely worsening problem.
Millions of chunks of metal, plastic and glass are whirling round earth, the garbage left from 4,600 launches in 54 years of space exploration.
The collision risk is low, but the junk travels at such high speed that even a tiny shard can cripple a satellite costing tens of millions of dollars.
Around 16,000 objects bigger than 10 centimetres across are tracked by the US Space Surveillance Network, according to Nasa’s specialist newsletter.
There are around 500,000 pieces between one and 10 centimetres, while the total of particles smaller than one centimetre “probably exceeds tens of millions”, Nasa says elsewhere on its website.
The rubbish comes mainly from old satellites and upper stages of rockets whose residual fuel or other fluids explode while they turn in orbit. As the junk bumps and grinds, more debris results.
Another big source, though, is a Chinese weather satellite, Fengyun-1C, which China destroyed in a test of an anti-satellite weapon in 2007.
Debris specialists and satellite operators were incensed. At a stroke, it helped increase the tally of large debris by more than a third.
In May 2009, a 10-centimetre chunk from Fengyun-1C passed within three kilometres of the US space shuttle Atlantis, prompting plans for evasive manoeuvres that proved to be unneeded.
Four known collisions have occurred between tracked objects, France’s National Centre for Space Studies says.
In 1991, a Russian navigation satellite, Cosmos 1991, collided with debris from a defunct Russian satellite, Cosmos 926, although this event only came to light in 2005.
In 1996, a fragment from an exploded Ariane rocket launched in 1986 damaged a French spy micro-satellite, Cerise.
In 2005, the upper stage of a US Thor launcher hit debris from a Chinese CZ-4 rocket.
And in 2009, a disused Russian military satellite, Cosmos 2251, smacked into a US Iridium communications satellite, generating a debris cloud in its own right.
In low earth orbit, which is where the ISS is deployed, debris impacts at around 10 kilometres per second, says the CNES.
An aluminium pellet just one millimetre carries roughly the same kinetic energy as a cricket ball or baseball fired at 280 mph.
In June 1983, the windscreen of the shuttle Challenger had to be replaced after it was chipped by a paint fleck just 0.3 mm across that impacted at four kms per second.
To cope with such threats, the ISS has some shielding but depends mainly on manoeuvring to get out of the way, an operation it has done several times.
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