Picture a rock more than 2.5 kilometres wide hurtling through space and completing a full rotation in the time it takes to make a cup of tea. Then consider that, by every established rule of planetary science, it should have torn itself apart.
Asteroid 491384 completes a turn every 4.18 minutes and measures an estimated 2.53 kilometres across, making it the largest ultra-fast rotating asteroid ever reported, according to a study posted to the arXiv preprint server. Its colours suggest it is a C-type asteroid, a class of generally dark, carbon-rich objects, with a classification confidence of 97 per cent.
The study was led by Dmitrii Vavilov at the University of Washington, which combed through the Vera C. Rubin Observatory’s second data preview alongside additional asteroid observations. The find has confronted astronomers with a puzzle they cannot yet fully explain: what is holding this thing together?
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What Is The Spin Barrier And Why Does It Matter?
Many asteroids are not solid boulders at all. They are thought to be “rubble piles” loose heaps of rock held together mostly by their own feeble gravity. Spin one too fast and centrifugal forces fling the pieces apart, like mud flying off a bicycle wheel. That is why asteroids bigger than 150 metres almost never complete a turn in less than 2.2 hours. Astronomers call this ceiling the “spin barrier.”
Asteroid 491384 spins more than 30 times faster than that limit.
The spin barrier is one of the most robust empirical rules in asteroid science. It emerges from basic physics: the faster a rubble pile rotates, the greater the centrifugal acceleration pulling material away from the surface. Below a certain size, small cohesive forces between dust and grain particles can resist that pull. Above a certain size, gravity alone should govern and the spin barrier is where theory predicts that gravity loses its grip on a rotating body.
Asteroid 491384 sits well above that size. Its rotation rate is not a marginal breach of the limit. It is a shattering of it.
(This artist’s concept depicts a near-Earth asteroid with an elongated orbit. Credit: NASA)
How Is The Asteroid Staying In One Piece?
When a large asteroid blows past the spin barrier, something else must be keeping it in one piece. One possibility is that the material inside the asteroid has enough internal strength to resist breaking apart. Tiny forces between grains of rock and dust could help hold it together, although astronomers have not yet determined its internal structure.
The authors compare the required cohesion to that of lightly cemented regolith. In other words, these asteroids may be more like cemented rubble than loose gravel heaps. Regolith is the term used for the loose surface material, composed of dust, soil and fragmented rock, that covers the surface of many rocky solar system bodies including the Moon and Mars.
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