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Watch The Moment Nasa's $500m Fermi Space Telescope Almost Hit A Cold War Satellite (VIDEO)

01/05/2013 08:44 BST | Updated 30/06/2013 10:12 BST

Nasa has posted an amazing video of the moment its Fermi Gamma-ray Space Telescope almost collided with another satellite in orbit above the Earth.

The space agency said that it learned with just a few days notice that its $500 million telescope was due to occupy almost the exact same point in space as a Cold-War spy satellite.

It said that project scientist Julie McEnery was just working through her inbox when she saw an automatically generated report from a risk analysis team.

It did not make for pleasant reading. The email said that the telescope was due to come with 700 feet - within 30 milliseconds - of the defunct Russian spy satellite Cosmos 1805.

And while a direct collision was not predicted, Nasa knew that it couldn't be complacement. A projected near-miss of around 1,800 feet between the Cosmos 2251 satellite and the Iridium 33 communications satellite was predicted in 2009 - but the two craft actually hit each other, leaving a cloud of twisted wreckage in orbit around Earth.

As with those satellites, Fermi and Cosmos 1805 are both travelling at many thousands of miles per hour, and any strike would have been catastrophic, potentially sending the two craft hurtling back down to Earth.

"My immediate reaction was, 'Whoa, this is different from anything we've seen before!'" McEnery said, according to Nasa.

Eventually Nasa was forced to fire thrusters on the satellite which were intended only for use at the end of its life - thrusters which had never been tested for real.

"You can't help but be nervous thinking about highly flammable fluids heading down pipes they'd never flowed down before," McEnery said.

It took days to prepare, but when the thrusters were fired they were turned on for just one second. And once they did the danger was avoided - the two craft missed each other by six miles.

But with 17,000 objects currently being tracked in space, it might only be a temporary reprieve.

Watch the drama unfold in the video, above.