Two defunct satellites racing across the sky at a combined speed of 33,000 miles an hour narrowly missed smashing into each other on Wednesday night.

The satellites - a pioneering international space telescope and an experimental US craft travelling in opposing orbits - crossed paths 550 miles above Pittsburgh 'without incident' according to US Space Command, which is part of the US Department of Defence.

Satellite tracking company LeoLabs had predicted there was a one in 1,000 chance of a collision.

If the satellites had collided, they would have created a cloud of debris, endangering other spacecraft orbiting the Earth, NASA said.

But the US space agency added the collision would have posed no danger to anyone on the ground.

The Infrared Astronomical Satellites (IRAS) space telescope was launched in 1983 as a joint project of NASA, Great Britain, and the Netherlands, and its mission lasted only 10 months

Space junk has become a growing concern as the debris in Earth's orbit has the potential to cause damage to other active communication and research satellites.

It is estimated there are 4,500 satellites in orbit, of which 3,000 are inactive.

'Events like this highlight the need for responsible, timely deorbiting of satellites for space sustainability moving forward,' LeoLabs said in a Tweet.

The last time a huge collision occurred was in 2009 when an inactive Russian communications satellite known as Kosmos-2251 smashed into an active US commercial satellite called Iridium 33.

The event produced more than 1,000 pieces of large debris in addition to thousands of smaller ones.

While the debris didn't directly hit any other satellite, it did force the International Space Station to perform an avoidance manoeuvre two years later in 2011.

In 2009 commercial communications satellite Iridium 33 collided with the derelict Russian military satellite Kosmos-2251 at a speed of 26,000 miles per hour - the blue lines in this graphic show the orbit of the thousands of pieces of debris that resulted in the crash

There are around 20,000 catalogued pieces of debris bigger than a softball orbiting the planet, travelling at speeds up to 17,500 miles per hour.

Satellite operators have to frequently adjust their trajectory accordingly, which isn't possible once a satellite dies.

'Imagine how dangerous sailing the high seas would be if all the ships ever lost in history were still drifting on top of the water,' says ESA Director General Jan Wörner.

'That is the current situation in orbit, and it cannot be allowed to continue.'

Last month, the European Space Agency (ESA) announced plans to remove debris from orbit as part of its ClearSpace-1 mission, which is planned for launch in 2025.

ESA will commission the technology and operations of the mission from commercial companies in a hope of 'triggering a new industry in deorbit services'.

In the same way private rocket launch companies take payment for putting satellites in orbit, in future private organisations could be paid to bring them back down.