A Giant Asteroid Spins Once Every 4 Minutes. Scientists Aren’t Sure How It Holds Together – USFTB

Picture a rock more than 1.6 miles (2.5 kilometers) wide. Now picture it whipping around a full turn every 4.18 minutes. In the time it takes you to brew a cup of tea, this thing has already spun around more than once.

Meet asteroid 491384. According to a study posted to the arXiv preprint server, it’s the largest ultra-fast rotating asteroid ever reported. And by the usual rules, it really shouldn’t hold together.

Loosey-goosey asteroid

Here’s the thing about asteroids. Many of them aren’t solid boulders at all. They’re 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’s why asteroids bigger than 490 feet (150 meters) 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.

So, when a large asteroid blows past that limit, 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.

Until recently, though, the known speed demons were mostly small near-Earth objects. That’s where the Vera C. Rubin Observatory comes in.

An asteroid that shouldn’t hold together

The study, led by Dmitrii Vavilov at the University of Washington, combed through Rubin’s second data preview alongside additional asteroid observations and zeroed in on two remarkable objects.

The first is 491384. It completes a turn every 4.18 minutes and measures an estimated 1.57 miles (2.53 kilometers) across. Its colors suggest it’s a C-type asteroid, a class of generally dark, carbon-rich objects, with a classification confidence of 97%. The diameter is also an estimate, based partly on how much sunlight astronomers assume its surface reflects.

The second, asteroid 566130, is roughly 1.2 miles (two kilometers) wide and spins every 5.34 minutes. That’s just over the five-minute cutoff the authors use for “ultra-fast,” but still astonishing for a body this size. Its required cohesive strength is lower, at less than one megapascal.

To put those numbers in perspective, 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.

Still, the researchers aren’t popping the champagne just yet. They’re calling for follow-up observations to confirm both spin periods.

How do you time a spinning rock?

The trick is to watch the light. Most asteroids are lumpy and irregular, so the amount of sunlight they reflect changes as they turn. Track the rhythmic brightening and dimming, which astronomers call a light curve, and you can work out how fast the rock spins.

The problem is that telescopes like Rubin catch each asteroid only at scattered, irregular moments. Those gaps breed false signals, including echoes of the day-night cycle and multiples of the true period. Checking every candidate by eye is hopeless for the millions of asteroids Rubin will eventually see.

So Vavilov and colleagues built software that does the vetting automatically. It describes each light curve as a stack of waves added together (a Fourier series) and decides, object by object, how many waves it needs. When it can’t settle on a single spin rate, it offers a short list of possibilities, along with estimates of how convincingly the preferred answer beats the alternatives.

The team let their software loose on 9,417 asteroids with at least 80 brightness measurements each, using observations collected between April 2025 and January 2026. Taken at face value, the raw data suggested 1,487 of them spun faster than the spin barrier, and 899 completed a turn in under five minutes.

Most of those were statistical mirages. After strict filtering, 66 super-fast rotator candidates survived, including five ultra-fast ones. Even then, 20 still carry some ambiguity.

That enormous drop, from 1,487 apparent speed demons to just 66 promising candidates, shows how easily astronomers can be fooled by incomplete observations. It’s also why the new software matters almost as much as the record-breaking asteroid itself.

All of this came from a mere data preview. Rubin’s Legacy Survey of Space and Time kicked off on June 29, 2026, and is expected to observe up to five million small Solar System bodies over the coming decade.

With further improvements, the authors hope their software can automate asteroid studies across huge surveys like this one. That could give astronomers a far clearer picture of how asteroids are put together and just how fast they can spin. It might even challenge existing theories of how these bodies formed and evolved over the history of the Solar System.

And if 491384’s extraordinary rotation is confirmed, it will offer astronomers a tantalizing clue about what can hold an asteroid together when gravity alone isn’t nearly enough.