On the way to investigate the scene of a historic asteroid collision, a European spacecraft swung by Mars and captured rare images of the red planet's mysterious small moon Deimos, the European Space Agency (ESA) said Thursday.

Europe's HERA mission is aiming to find out how much of an impact a NASA spacecraft made when it deliberately smashed into an asteroid in 2022 in the first-ever test of our planetary defences.

But HERA will not reach the asteroid – which is 11 million kilometers (seven million miles) from Earth in the asteroid belt between Mars and Jupiter – until late 2026.

On the long voyage there, the spacecraft slingshotted around Mars on Wednesday.

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The spacecraft used the planet's gravity to get a "kick" that also changed its direction and saved fuel, mission analyst Pablo Munoz told a press conference.

For an hour, HERA flew as close as 5,600 kilometers from the Martian surface, at a speed of 33,480 kilometers an hour.

It used the opportunity to test some of its scientific instruments, snapping around 600 pictures, including rare ones of Deimos.

The lumpy, 12.5 kilometer-wide moon is the smaller and less well-known of the two moons of Mars.

Exactly how Deimos and the bigger Phobos were formed remains a matter of debate, however.

Some scientists believe they were once asteroids that were captured in the gravity of Mars, while others think they could have been shot from a massive impact on the surface.

The new images add "another piece of the puzzle" to efforts to determine their origin, Marcel Popescu of the Astronomical Institute of the Romanian Academy said.

There are hopes that data from HERA's "HyperScout" and thermal infrared imagers – which observe colours beyond the limits of the human eye – will shed light on this mystery by discovering more about the moon's composition.

Those infrared imagers are why the red planet appears blue in some of the photos.