BBC_Audio_Limelight_The_System_Series_3.txt

The mysterious 'dark comets' prowling our Solar System

These strange objects could poker minasexplain 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 erratic movements remains a mystery.

"We still have no clue what's actually causing it," says Seligman.

Astronomers believe most asteroids and comets are remnants from the early period of planet formation that occurred within the Solar System 4.6 billion years ago. Some asteroids are fragments of material that never coalesced into planets, while comets are the result of ice and dust further away from the Sun that clumped together. Experts typically divide these objects into two groups, with inactive asteroids on one hand and the turbulent comets whose ice sublimates, or turns from a solid into a gas, ejecting material into space to create their dramatic tails, on the other.

The existence of dark comets suggests there might not always be a strong dividing line between these two kinds of object. "This is part of the emerging picture of asteroids and comets as a continuum," says Michele Bannister, an astronomer at the University of Canterbury in New Zealand. "A historical viewpoint is a comet is something that has a very prominent tail, and an asteroid is something that's very rocky and dry. That picture has completely gone."

The simplicity of the asteroid-comet binary was already being questioned due to the 1996 discovery of active asteroids – a type of asteroid where material is ejected from its surface, producing a comet-like tail. This "activity" could be the result of ice beneath the asteroid's surface being exposed through ?a collision with another asteroid or due to it spinning quickly enough to break it apart. The ice, when exposed, gets heated by the Sun, causing material to burst off its surface, making the asteroid "active".

"Some of them behave like comets," says astronomer Jessica Agarwal at the Max Planck Institute for Solar System Research in Germany, who has studied active asteroids for years.

Dark comets, however, are different. Unlike active asteroids, they do not show any signs of material being ejected from their surface, yet they are clearly undergoing some sort of comet-like acceleration.

Their accelerations are also deemed too large to have resulted from heat radiating off them causing them to rotate faster, a process called the Yarkovsky effect. "We're seeing them drift faster than the Yarkovsky effect," says Teddy Kareta, an astronomer at the Lowell Observatory in Arizona.

In order to better understand dark comets, we're going to have to get closer to them. But a complete coincidence will provide just such an opportunity within the next six years.

In December 2020, Japan's Hayabusa2 spacecraft flew past Earth carrying samples it collected from an asteroid called Ryugu in 2019. It dropped off a capsule containing the samples, which provided invaluable information on the origin of water and life on Earth. But the main spacecraft still had fuel left on board, so the Japanese space agency (Jaxa) decided to send it on an extended mission to an asteroid called 1998 KY26.

Amazingly, it turns out 1998 KY26 was one of the six dark comets identified by Seligman and his colleagues. "We had not known about dark comets at the time of the extended mission's planning," says Yuichi Tsuda at Jaxa, the project manager for Hayabusa2. "It's really exciting."

Jaxa selected 1998 KY26 because of its peculiar properties – properties it happens to share with certain dark comets. Namely, its rapid rotation once every ten minutes and its small size of 30m (98ft) across. Should Hayabusa2 arrive successfully, 1998 KY26 will actually be the smallest cosmic object ever visited by a spacecraft.

Hayabusa2 is due to arrive at 1998 KY26 in 2031. Initially, the spacecraft will remain several kilometres above its target, says Tsuda. From here, it will take images of the asteroid and any ice melting away, giving 1998 KY26 a boost, should be visible. "If it is comet-like outgassing which is causing the accelerations, Hayabusa2 will certainly detect that," says Seligman.

Later in the mission, Hayabusa2 might descend towards the asteroid and even land on it, says Tsuda. The spacecraft still has some of the equipment it used to collect a sample from Ryugu, including a mechanism that shoots a bullet-sized projectile. This could break through 1998 KY26's surface to create a small crater and reveal at whatever lies underneath. "By tracking the particles ejected by the impact, we may be able to estimate the internal structure of the asteroid," says Tsuda.

In the nearer term, astronomers hope that powerful telescopes like JWST could reveal more detail about dark comets. "JWST would be fabulous," says Kareta, though Seligman says he has previously submitted proposals to use the telescope for dark comet research without success.

Other telescopes might still be useful. Kareta has access to the Lowell Discovery Telescope during the next three years, which will allow him to study known dark comets. "We're going to continue to follow them and understand the strength of their non-gravitational acceleration," he says. Some of the objects are also expected to brighten in the next few years as the Sun heats them, which could allow Kareta and his colleagues to measure what they're made of.

These investigations promise to unravel some of the secrets of dark comets. But, in the meantime, the mystery surrounding them goes on. In July last year, a group of researchers led by Aster Taylor, an astrophysicist at the University of Michigan and works with Seligman's team, published a study proposing that dark comets might have begun life as asteroids in the asteroid belt with ice trapped beneath their surface, but were then knocked towards the Sun, perhaps by the gravitational tug of Jupiter.

"Then, at some point, they split apart," says Taylor. "That can be from spinning up fast, or getting hit. That exposes ices and makes them become dark comets." The process would also spin such objects to high speeds, which happens to be a characteristic shared by many dark comets – they spin as fast as once every six minutes, compared to once an hour, like other asteroids of a similar size.

The discovery of more dark comets in December 2024 has also hinted that these objects might come in two forms: outer dark comets, which are around 100m-1km (330-3,300ft) in size and originate near Jupiter, and smaller, inner dark comets 10-20m (30-60ft) in size that have a circular orbit, similar to Earth's. That suggests that the inner dark comets may be asteroids that have broken apart, exposing their ice, while the existence of more distant dark comets might be explained by another process – perhaps they are dying comets in the twilight of their lives.

"We're seeing them while they're about to run out of gas – the last absolute gasps of cometary activity," says Kareta. "We're seeing what is a very brief phase in some of these objects' lifetimes."

If dark comets are asteroids that contain ice, that might help solve the mystery of how water arrived on Earth. A popular theory is that water got here via asteroids or comets that crashed into our planet early during the history of the Solar System. Dark comets are a promising candidate for this scenario. "If there are lots of objects near Earth that we didn't know were hydrated, then it's possible they contributed," says Seligman.

Besides their potential role in the Earth's past, dark comets could also have implications for planet's future too. We might spy an object that seems relatively safe, but suddenly then darts in the direction of our planet. "You can have an Earth impactor that you didn't expect," says Taylor.

Several dark comets – including the relatively large 2003 RM – have been found in the near-Earth neighbourhood.

Thankfully for the time being, dark comets seem relatively rare, with only small changes in direction from the bursts they experience. But they are still something to be wary of. "If we can't detect these objects correctly, we will not be able to know if they're going to hit us," says Taylor.

--

If you liked this story,?sign up for The Essential List newsletter?– a handpicked selection of features, videos and can't-miss news, delivered to your inbox twice a week.

For more science, technology, environment and health stories from the BBC, follow us on?Facebook and?Instagram.

Copyright 2025 BBC. All rights reserved. The BBC is not responsible for the content of external sites.?Read about our approach to external linking.