Asteroid that broke up over Berlin was fastest-spinning one ever seen

Long-exposure photo showing the trail of asteroid 2024 BX1 shortly before impact. The changes in brightness are caused by the asteroid’s spin

L. Buzzi, Schiaparelli Astronomical Observatory, Italy (MPC 204)

An asteroid that hit Earth’s atmosphere earlier this year was spinning once every 2.6 seconds, faster than any we knew of.

Called 2024 BX1, the object – probably no more than 1 metre wide – entered Earth’s atmosphere on 21 January, breaking apart over Berlin, Germany. Some pieces survived the fireball and were recovered. It was a rare example of a tracked asteroid fall, in which the incoming rock is spotted before it encounters Earth, in this case just 3 hours ahead of the event.

Maxime Devogele at the European Space Agency’s Near-Earth Object Coordination Centre in Italy and his colleagues took images of the asteroid prior to its impact. Despite it moving at some 50,000 kilometres per hour, its elongated shape meant changes in its brightness caused by rotation were particularly prominent in these images.

Those changes in brightness corresponded to a rotation time of 2.588 seconds – roughly 30,000 rotations per day. “It’s the fastest [spin] we’ve ever observed,” says Devogele.

Asteroids spin for a number of reasons, such as collisions earlier in their life. In general, space rocks larger than a kilometre can’t rotate more than once every 2.2 hours because they would break apart. But smaller asteroids like 2024 BX1 can withstand much faster spins because they are more compact. “They have internal strength, so they can rotate faster,” says Devogele.

Gauging the spin of objects like this could be useful for planetary defence, letting us know how strong a small asteroid is and how likely it might be to survive its passage through Earth’s atmosphere. “If it’s hard, it will react differently than if it’s a piece of snow that has no internal strength,” says Devogele.

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