Media playback is unsupported on your device Media caption The collision happened within a jet of material blasted out by the black hole inside galaxy NGC 3862

Astronomers have witnessed two big blobs of plasma, shot into space by a black hole, cannoning into each other in a dramatic cosmic billiard-shot.

The supermassive black hole sits at the heart of an elliptical galaxy called NGC 3862, blasting out a jet of plasma at 98% of the speed of light.

A fast-moving knot within that jet bore down on another, in a dazzling impact.

It was captured in a series of images snapped by the Hubble Space Telescope, spanning 20 years.

The discovery is reported in the journal Nature.

"Something like this has never been seen before in an extragalactic jet," said Dr Eileen Meyer of the Space Telescope Science Instititue (STScI) in Baltimore in the US. "This will allow us a very rare opportunity to see how the kinetic energy of the collision is dissipated into radiation."

The collision will unfold further in the coming decades, allowing scientists to continue watching it and learn more about these mysterious, spectacular jets - which can only rarely be seen in visible light.

This particular jet was spotted in Hubble images from 1992; the time-lapse movie that revealed the collision comprises four images taken from 1994 to 2014.

Astronomers believe the jet is made up of highly energised plasma, propelled into space by the supermassive black hole that forms the "active nucleus" of NGC 3862 - an elliptical galaxy 260 million light-years from Earth.

The beam of material is actually being blasted roughly in our direction. This compresses our view of the "string of pearls" of blobs within the plasma jet, and makes the images more difficult to interpret.

The observations were only possibly because of the sensitivity of the Hubble telescope - and the fact that it has been peering into deep space, from its orbit 500km above the Earth, for more than 20 years.

Image copyright NASA/ESA/E. Meyer (STScI)

Martin Hardcastle, a professor of astrophysics at the University of Hertfordshire in the UK, said it was "very nice to see" the evidence of such a collision.

"These chunks of stuff follow each other up the jet and you would expect fast ones to run into slow ones," Prof Hardcastle told the BBC.

"When that happens what you get is a hydrodynamic shock, which means that energy is transferred from blob number two, to material that used to make up blob number one. As a result, you increase the energy of the particles in the fluid… so you see a brightening effect.

"There's a lot of interpretation going into this, because even Hubble can't see this as sharply as you would like. But it's perfectly plausible that's what they're seeing."

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