NASA HST Image: Galactic liaisons

April 24th marks the Hubble Space Telescope's 21st anniversary in space. To celebrate, NASA has released this new image of two interacting galaxies.

It contains clues about a past rendez-vous, and hints of a third, hidden object.

The disc of the upper, spiral galaxy, called UGC1810, has been distorted by the gravitational pull of the smaller galaxy below.

The position of its long outer arm is likely to have come about when the smaller galaxy dived deep, and slightly off-centre, through UGC1810.

Meanwhile, the nucleus of the smaller galaxy, UGC1813 shows intense star formation, also a result of its encounter with the larger galaxy.

Now look closer and a possible third galaxy can be spotted in the top right of the image, nestled between the ends of the two outermost arms of UGC1810. The give-aways are a mini-spiral and several clusters of young, blue stars.

The image is just one of the thousands released by Hubble over its working life. "After all this time, new Hubble images still inspire awe and are a testament to the extraordinary work of the many people behind the world's most famous observatory," said Charles Bolden, who piloted the Discovery space shuttle as it deployed Hubble in 1990, and is now NASA administrator.