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An October 6, 2026 arXiv preprint reports that asteroid 491384 is about 2.5 kilometers across and completes one rotation in roughly five minutes. If confirmed, that would make it the largest known ultra-fast rotator—a record based on size, not the shortest rotation period. The result comes from analysis of the Vera C. Rubin Observatory’s Data Preview 2 (DP2) solar-system-object catalog and has not yet been established by peer review.
What the new Rubin result says
Dmitrii E. Vavilov, Siegfried Eggl and Sarah Greenstreet analyzed Rubin DP2 light-curve data and identified 66 “super-fast” rotators, using the classical 2.2-hour spin barrier as their threshold. Two catalog objects, 491384 and 566130, are reported as larger than 2 kilometers and rotating in about five minutes.
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The paper’s abstract states: “At a diameter of about 2.5 km, asteroid 491384 is the largest ultra-fast rotator known so far.” That wording is a preprint finding, not a peer-reviewed discovery announcement. The diameter and period are estimates derived from observations and modeling, so they may be refined as additional data become available.
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Why a five-minute spin is unusual
The 2.2-hour spin barrier
For a strengthless, loosely assembled “rubble-pile” asteroid, gravity generally cannot hold the surface together once rotation becomes too rapid. The classical reference point is about 2.2 hours: objects rotating faster than that are often called super-fast rotators because gravity alone may not prevent material from drifting away.
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Five minutes is far beyond that comparison threshold. At an estimated 2.5-kilometer diameter, 491384 is also much larger than the small, monolithic bodies that can sometimes withstand extreme rotation through material strength. The authors therefore infer that it must possess significant internal cohesion—some combination of tensile strength, cementation or a more coherent structure. Rubin did not directly measure that interior strength; it is a physical interpretation of the estimated size and spin.
How the asteroid’s period was determined
Asteroid rotation is inferred from repeating changes in brightness. As an irregular body turns, different faces reflect different amounts of sunlight, producing a light curve that can reveal a periodic signal.
The DP2 analysis fits each asteroid’s light curve with a Fourier series and chooses an appropriate order for that object. Rubin observations are not continuous movies: measurements can be sparse and irregularly timed, and a light curve can produce misleading periodogram peaks. When the data do not support one unique period, the method retains alternative periods and gives corresponding probability estimates instead of presenting a single uncertain value as definitive.
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What “largest” means—and what it does not mean
491384’s reported record concerns the estimated diameter among ultra-fast rotators. It is not a claim that the asteroid spins faster than every other large asteroid.
| Record or measurement | Asteroid 491384 | Asteroid 2025 MN45 |
|---|---|---|
| Estimated diameter | About 2.5 km, from the October 2026 Vavilov, Eggl and Greenstreet preprint | 710 m, reported in a peer-reviewed Rubin First Look study |
| Rotation period | About five minutes | 1.88 minutes |
| Record wording | Largest known ultra-fast rotator in the October preprint | Fastest-spinning asteroid known above 500 m at the earlier study’s publication |
| Dataset and publication status | Rubin DP2 catalog analysis; arXiv preprint dated October 6, 2026 | Rubin First Look observations; peer-reviewed study summarized in 2026 |
Thus, 2025 MN45 has the shorter reported period, while 491384 is substantially larger. Those are different comparison axes, so describing 491384 as “faster” than MN45 would be incorrect.
How this fits Rubin’s early solar-system science
The earlier First Look sample used observations collected from April 21 to May 5, 2025. It reported 76 asteroids with reliable periods, including 16 super-fast rotators and three ultra-fast rotators. The October result comes from a separate DP2 catalog analysis and should not be treated as a result from a completed Rubin survey.
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Rubin Observatory began its planned 10-year Legacy Survey of Space and Time in July 2026. As the survey accumulates longer and more regularly sampled time series, researchers should be able to test candidate periods, improve diameter estimates and determine how common cohesive, rapidly rotating asteroids are.
What remains uncertain
- Peer-review status: The 491384 claim is currently in an arXiv preprint, so its methods and conclusions have not been independently validated through journal peer review.
- Size: The roughly 2.5-kilometer diameter is an estimate; asteroid dimensions can change with improved brightness, albedo and shape information.
- Period: The paper explicitly models alternative periods when sampling is ambiguous, meaning a five-minute solution should be understood with its reported probability and uncertainty rather than as an unqualified exact value.
- Interior structure: The inferred cohesion is not a direct measurement of 491384’s material strength or composition.
Why the finding matters
A large body rotating once every few minutes tests the boundary between gravity-dominated rubble piles and objects whose material strength helps hold them together. Confirming more such asteroids would improve models of how collisions, fragmentation and reaccumulation create coherent or partially coherent bodies. It would also show why dense, time-series surveys matter: a single image cannot reveal a rotation period, but repeated brightness measurements can expose extreme behavior in otherwise ordinary-looking catalog objects.
The Bottom Line
Rubin DP2 data analyzed in an October 2026 arXiv preprint identify asteroid 491384 as an estimated 2.5-kilometer body rotating in about five minutes—the largest ultra-fast rotator reported so far. The result implies substantial internal cohesion, but both the record and the inferred structure remain subject to confirmation.
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