Loose Rubble Should Fly Apart. These Two Asteroids Are Holding On

Two asteroids, each about two kilometres wide, are turning once every five minutes, far faster than a loose heap of rubble should survive, according to a new study of Vera C. Rubin Observatory data. ^[1][2][3]

The finding is a preprint, posted to the arXiv server on October 6, 2026 and not yet peer reviewed. Its authors, Dmitrii Vavilov, Siegfried Eggl and Sarah Greenstreet, say the two bodies, numbered 491384 and 566130, must have real internal strength to stay in one piece. ^[2][3]

That cuts against a tidy and long-standing picture of what big asteroids are made of.

The spin barrier

Collisions over billions of years break them up, and gravity gathers the pieces back into loose heaps held together by little more than their own weak pull. Astronomers call these rubble piles. ^[3]

A heap like that has a speed limit. Put gravel on a turntable and spin it up, and the stones at the edge fly off first. An asteroid does the same thing, only slowly: spin it fast enough and material at the equator is flung away. For asteroids above about 150 metres, very few known objects rotate faster than about 2.2 hours per turn. That figure is called the spin barrier. ^[3]

These two cross it by a wide margin. Two hours and twelve minutes is 132 minutes. Five minutes is about 26 times quicker. ^[2][3]

What the team found

The researchers applied an improved method for estimating rotation periods to the Rubin Observatory's Data Preview 2 catalogue. Rubin is a new survey telescope in Chile that photographs the sky again and again. ^[2][3]

The asteroids are never resolved as shapes. They are dots.

What the telescope records is how bright each dot is on each visit. An irregular rock reflects more sunlight when its broad side faces us and less when it turns edge-on, so the brightness rises and falls in a repeating pattern. The length of that pattern gives the spin period. ^[3]

The trouble is that sparse, unevenly spaced observations can produce false candidate periods. The team's method fits a mathematical curve to the brightness data, a Fourier series, and chooses how complicated that curve should be separately for each asteroid. Where no single period is reliable, it reports alternatives instead of forcing an answer. ^[2][3]

New Scientist reports that the team analysed a sample of more than 9,000 asteroids and measured rotation periods for 260 of them. ^[1] The study counts 66 asteroids spinning faster than the 2.2-hour barrier. Two of them are the big ones. ^[2][3]

The study puts 491384 at about 2.5 kilometres across and calls it the largest ultra-fast rotator known to the authors. ^[2][3]

Rubble or rock

The logic of the inference is simple. If an asteroid of that size, turning that fast, were a gravel heap, it would not be there. The authors conclude that both need significant internal cohesion, meaning some real strength binding the material together. ^[2][3]

Vavilov told New Scientist that asteroids of this size should, over billions of years, have been smashed and reassembled into rubble piles. The fast spinners may have escaped that fate and could be monolithic, single solid bodies. He said they could be pristine objects from the early solar system, but presented that as a possibility, not a result. ^[1]

If the idea held up, it would matter well beyond two rocks. Vavilov suggests the discovery may challenge models of how asteroids collide and reassemble across the solar system's 4.6 billion years. ^[1]

That is his proposed implication, and nobody has yet shown that those models need rewriting. Two odd objects can also just be two odd objects.

The alternative is that something spun them up. Vavilov raised the YORP effect, in which sunlight can gradually change an asteroid's spin. But he said he was uncertain, because these objects are far from the Sun. ^[1] The authors have not established how the asteroids got their spins. ^[1]

What it means for the threat question

Both asteroids are in the main belt between Mars and Jupiter, and the sources do not identify either as a near-Earth object. Their spins do not by themselves signal an impact threat. ^[1][3]

The link to planetary defense is indirect. When scientists judge how a body might respond to a nudge or to other forces, they lean on a picture of what asteroids are like inside. A population of large bodies tougher than expected would complicate that picture. How much it would, no one can say yet. ^[1][3]

There is a plain limit on the evidence. The spins are inferred from changes in the asteroids' brightness, not from images of their surfaces, and the sizes are estimates. ^[1][2][3]

The preprint has not gone through peer review, and no independent confirmation of either period has been reported so far. ^[2][3] The densities, shapes and interiors of the two rocks are unknown.

What the result does give is a pair of specific targets. Two large asteroids that ought to have shed themselves into space are still here, turning every five minutes, which suggests the main belt has more variety in it than the rubble-pile story allows. That is a good problem to have been handed by a telescope that has only just begun to look.

Vavilov told New Scientist he hopes to study the two objects in more detail. ^[1]