These strange objects could explain how water arrived on Earth, but may also be a previously unrecognised threat to our planet. Now, a spacecraft is headed towards one to investigate.

They are some of the strangest rocks in our Solar System. They aren't quite asteroids and not quite comets, but a bizarre mixture of the two. These are "dark comets" – and no-one knows quite what to make of them.

Yet, these mysterious, recently discovered space rocks might be an entirely new class of object in the Solar System that could help to answer questions about how water originated on Earth, according to the scientists trying to study them. They may also pose a previously unrecognised threat to our planet.

And now we have a chance of finding out more about these strange objects thanks to a Japanese spacecraft racing towards one – by complete coincidence – right at this moment. When it gets there in 2031, we might find out for certain what exactly these objects are and how they behave.

The first hint of dark comets emerged in 2016, when astronomers found what they thought was an asteroid that behaved like a comet. While asteroids are rocky, inactive objects commonly found in a wide belt between the planets Mars and Jupiter, comets are rock and ice that have huge tails stretching for millions of miles and tend to originate from the outer solar system.

The object spotted in 2016 was particularly odd. It appeared to move like a comet, but had none of the typical characteristics of one. When researchers studied its orbit around the Sun, the object appeared to receive an occasional, sudden push from something other than gravity that changed its motion ever so slightly. While the movements were small, just fractions of a metre per second, it was enough to be noticeable when looking through telescopes on Earth.

This sort of "non-gravitational acceleration" is normal for comets, where ice heats up rapidly as they near the Sun, triggering a release of gas and dust, which acts a little like a thruster. In the case of the object spotted in 2016, however, there was no visible dust or ice trail and the object appeared relatively inert.

A year later, astronomers spotted another object behaving similarly – a cigar-shaped lump of rock, metal and ice between 115-400m (377-1,312ft) long that has been named 'Oumuamua. It was later found to be one of the first known interstellar objects – an interloper from another star system – to visit our Solar System. The rock sling-shotted around our Sun before heading back out into interstellar space. (Read more about why our Solar System's interstellar visitors are baffling scientists.)

Then in 2023, a team of astronomers led by astronomer Darryl Seligman at Michigan State University in the US announced they had found six similar objects orbiting our Sun in asteroid-like orbits that lacked comet-like tails, but were undergoing unusual bursts of speed. These dark comets, all between 4m (13ft) and 32m (104ft) wide, underwent bursts of acceleration of up one nanometre per second, a tiny amount – but enough to move off their orbits by hundreds of kilometres every few years. At almost the same time, Seligman and his colleagues published research that showed a 300m (984ft) wide near-earth asteroid called 2003 RM also behaved like a dark comet.

In December 2024, they published a new paper with information about yet more of these objects, bringing the total number known in the Solar System to 14. Yet, what lies behind their........

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