“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.
Two kinds of dark comet
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.
Alamy/ Nasa“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.
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