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A new analysis of Vera C. Rubin Observatory data identifies asteroid 491384 as the largest ultra-fast rotator known so far: the October 2026 preprint estimates it is about 2.5 kilometres across and completes a rotation in roughly five minutes. That is a preprint finding, not a peer-reviewed result, and the authors’ conclusion that the asteroid needs substantial internal cohesion is an inference—not a direct measurement of its interior.
What the Rubin analysis reports
In an October 6, 2026 arXiv preprint, Dmitrii E. Vavilov, Siegfried Eggl and Sarah Greenstreet analyzed the Rubin Observatory Data Preview 2 (DP2) solar-system-object catalog. They identify asteroid 491384, estimated at about 2.5 km in diameter, as the largest ultra-fast rotator known so far. The preprint reports a rotation period of about five minutes for 491384; it also reports that asteroid 566130 is larger than 2 km and rotates in about five minutes. Read the authors’ preprint.
The authors identify 66 “super-fast rotators” in their DP2 analysis, using a 2.2-hour spin period as the classical spin-barrier threshold. Their headline claim about 491384 is narrower: it is the largest object they identify in the ultra-fast-rotator category. The paper’s abstract says, “At a diameter of about 2.5 km, asteroid 491384 is the largest ultra-fast rotator known so far.” That wording describes the authors’ preprint finding, not a settled, peer-reviewed record.
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An asteroid’s spin creates outward forces that oppose gravity holding the body together. For a loosely bound “rubble pile,” a sufficiently rapid rotation can cause material to shift or escape. The 2.2-hour spin barrier is a conventional threshold used to describe the point beyond which a typical gravity-bound, cohesionless asteroid would be expected to have difficulty remaining intact. It is a useful comparison benchmark, not a claim that every asteroid slower than that is stable or every faster one must break apart.
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At roughly five minutes, 491384 rotates far faster than that threshold while being estimated at kilometres in size. The authors therefore infer that it must have significant internal cohesion to survive the spin. The catalog analysis does not directly inspect or measure the asteroid’s interior, so the finding does not establish what material holds it together or how strong that cohesion is.
How the study estimated rotation periods
Rotating asteroids can show repeating changes in brightness as their shape and orientation change. Researchers use those light curves—brightness measurements over time—to estimate a rotation period. Rubin’s DP2 catalog provides the observations analyzed in the preprint.
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Period estimates can be ambiguous when observations are sparse or taken at irregular intervals: multiple candidate periods may fit the sampled brightness data, and a periodogram can produce misleading minima. The authors’ method selects a Fourier-series order for each asteroid. When the observations do not support one reliable, unique period, the analysis reports alternative periods with corresponding probability estimates instead of presenting a single candidate as certain. That approach acknowledges uncertainty in the time-series fit; it does not turn an estimated period into a direct measurement of the asteroid’s interior.
How this differs from the 2025 MN45 result
The 491384 claim is not that it spins faster than asteroid 2025 MN45. The two reports concern different records and separate Rubin data analyses:
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| Object and study | Estimated diameter | Reported rotation period | What the record refers to | Publication status and dataset |
|---|---|---|---|---|
| 491384; Vavilov, Eggl and Greenstreet, October 2026 | About 2.5 km | About five minutes | Largest ultra-fast rotator known so far, according to the preprint | October 2026 arXiv preprint analyzing the DP2 catalog |
| 2025 MN45; Rubin First Look study, reported by LSST:UK Science Centre in 2026 | 710 m | 1.88 minutes | Fastest-spinning asteroid known above 500 m at the time of that report | Peer-reviewed First Look study; observations collected April 21–May 5, 2025 |
MN45 has the shorter reported rotation period, while 491384 is estimated to be much larger. “Largest ultra-fast rotator” and “fastest known above 500 metres” are distinct categories, so the two claims do not conflict. The First Look study presented 76 asteroids with reliable periods, including 16 super-fast rotators and three ultra-fast rotators; those figures describe that earlier sample, not the October DP2 analysis. Rubin began its 10-year Legacy Survey of Space and Time in July 2026, so DP2 should not be described as a result from a completed survey. LSST:UK Science Centre’s account of the First Look result.
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What is established—and what remains uncertain
- Reported in the preprint: 491384 is estimated at about 2.5 km across and has a rotation period of about five minutes; the authors identify it as the largest ultra-fast rotator known so far.
- Interpretation, not an interior measurement: surviving such a rapid spin at that estimated size implies significant cohesion under the authors’ analysis.
- Status: the claim comes from an October 2026 arXiv preprint and should not be presented as peer-reviewed.
- Comparison: 2025 MN45’s reported period is shorter, but its record was for spin speed among asteroids above 500 m, not largest size among ultra-fast rotators.
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