The Ultra-Fast Spinning Asteroid That Breaks Every Rule

A space rock more than two and a half kilometers across is spinning so fast it completes a full turn in barely four minutes — and physics says it should have ripped itself apart. Asteroid 491384, estimated at 2.53 kilometers (1.6 miles) wide, rotates once every 4.18 minutes, according to research posted to the arXiv preprint server. The team behind the find calls it the largest Ultra-Fast Spinning Asteroid known so far.

The discovery matters because most big asteroids are believed to be rubble piles — loose heaps of rock and dust held together only by their own weak gravity. Bodies wider than about 150 meters (490 feet) rarely rotate faster than roughly 2.2 hours, a limit astronomers call the spin barrier. Spin much quicker and centrifugal force at the equator would fling material into space. This Ultra-Fast Spinning Asteroid turns more than thirty times faster than that limit, so it must be held together by something far stronger than gravity: genuine internal cohesion, closer to a single solid boulder than a gravel heap.

Software that catches its own mistakes

The find came from early data collected by the Vera C. Rubin Observatory in Chile, whose giant camera sweeps the southern sky night after night. Astronomers work out an asteroid's spin by watching its brightness change as its irregular shape turns — a flickering light curve that reveals the rotation period. But the telescope only catches each asteroid at scattered moments, and those gaps can produce convincingly wrong answers.

A team led by Dmitrii Vavilov of the University of Washington built new software that checks the reliability of every measurement automatically, as phys.org reported. Instead of forcing one answer out of patchy data, the program tunes its mathematical fit to each asteroid individually and flags periods it cannot trust. When the observations leave the spin ambiguous, it lists the possible answers along with the odds for each one.

Applied to repeated brightness measurements for 9,417 asteroids in the Rubin Data Preview 2 catalog, the method surfaced 66 candidate super-fast rotators spinning past the classical barrier. Even earlier, plain eyeballing of the observatory's first observations had turned up 19 "super-fast rotators" spinning beyond the limit, according to phys.org — suggesting the new software is finding a real population, not a glitch.

A record-holder with company

Asteroid 491384 stood out from the pack because it combines remarkable size with remarkable speed. Two candidates in the sample exceeded two kilometers in diameter while spinning roughly once every five minutes. The runner-up, listed as 566130, is just under two kilometers across and turns about every five and a third minutes — a hair too slow for the study's under-five-minute cutoff for the ultra-fast label.

Diameter estimates do carry uncertainty: the headline figure for the record-holder assumes a typical surface brightness, with the true width depending on how reflective the rock really is. Even at the low end of the plausible range, it would remain the biggest object known to spin this fast. And the researchers are careful not to overclaim: they recommend dedicated follow-up observations to confirm the rotation periods independently before the record is carved in stone.

If the period holds up, the implications are straightforward. Collisions over billions of years grind big asteroids down and gravity re-gathers the fragments, which is why the rubble-pile model became standard. An object this large surviving this much spin means some asteroids are far more solid than that picture allows — a fact worth knowing as astronomers map what's actually out there. Surveys like the MMX Phobos mission and recent near-Earth asteroid detections keep adding to that map.

What Rubin finds next

The 3,200-megapixel camera at the heart of the Rubin Observatory has barely begun its decade-long survey, which will catalog billions of objects and revisit each patch of sky hundreds of times. Every one of those revisits adds another dot to thousands of asteroid light curves — and another chance for careful software to tell a genuine fast spinner from a sampling artifact.

Vavilov's team hopes to push their reliability-checking approach further, so it can run automatically across whole surveys. As other giant observatories come online, the same trick could flag unreliable telescope exposures themselves whenever many unrelated asteroids show the same odd flicker in a single image. The full paper is available as a preprint at arXiv:2610.07584, where the authors, Vavilov, Siegfried Eggl, and Sarah Greenstreet, lay out the method in detail.

For now, 491384 keeps spinning — a two-and-a-half-kilometer boulder doing in four minutes what most rocks its size need more than two hours to manage, and daring astronomers to explain how it holds together.