The Vera C. Rubin Observatory identified the largest ultra-fast spinning asteroid yet. A team led by Dmitrii Vavilov used new software to analyze brightness measurements from 9,417 asteroids. They found an asteroid over 2.5 kilometers across that completes a full rotation in just over four minutes, breaking the spin barrier.
The speed of an asteroid's spin can tell astronomers a surprising amount about what it is made of. However, existing approaches to measuring spin based on telescope observations can often produce misleading estimates.
Now, a team led by Dmitrii Vavilov at the University of Washington has developed new software that automatically weeds out unreliable measurements. Their updated approach revealed an asteroid more than 2.5 kilometers (1.6 miles) across that completes a full rotation in just over four minutes—making it the largest known ultra-fast-spinning asteroid. Their research has been posted to the arXiv preprint server.
Super-fast rotators
Many asteroids are thought to be "rubble piles": loose collections of rock held together only by their own weak gravity, which would rapidly tear apart if they rotated too quickly. As a result, asteroids larger than about 150 meters (490 feet) rarely complete a full rotation in less than 2.2 hours: a limit known as the "spin barrier." If any larger asteroid is spinning faster than this, it must be held together by some force stronger than gravity alone.
Since most asteroids are irregularly shaped, they reflect more or less sunlight as they turn. By tracking how an asteroid's brightness changes over time, astronomers can harness this effect to estimate the speed of its spin.
In early observations by the Vera C. Rubin Observatory in Chile, astronomers have already used this approach to identify 19 "super-fast rotators" that appear to spin beyond this barrier.
However, since telescopes like Rubin can only catch asteroids at scattered and irregular moments, these gaps can easily lead to the wrong spin rate. Although astronomers can check for these errors by hand, it becomes time-consuming for the vast numbers of asteroids that Rubin observes.
Flagging likely errors
To tackle this problem, Vavilov's team developed new software that analyzes asteroid brightness measurements automatically. Their program tailors its approach to each asteroid individually, identifying gaps in coverage and flagging suspicious measurements. Where a single spin rate can't be pinned down, it instead offers a list of possible values, each with an estimate of how likely it is to be correct.
The researchers applied their software to repeated brightness measurements taken by Rubin. Among 9,417 asteroids, they found 66 super-fast rotator candidates, two of which were larger than 2 kilometers (1.2 miles) across but spin around roughly once every five minutes.
One of these objects stood out in particular: Named 491384, it appears to rotate once every 4.18 minutes and has an estimated diameter of 2.53 kilometers (1.57 miles), making it the largest ultra-fast rotator reported so far. To avoid tearing itself apart, it must have substantial internal strength, making it far more like a single solid rock than a loose rubble pile.
Better understanding asteroids
With further improvements, Vavilov and colleagues hope their software could automate the study of asteroids across large astronomical surveys. In particular, if many different asteroids show the same odd brightness readings in a single telescope exposure, this could reveal that the exposure itself is unreliable, allowing it to be flagged.
Ultimately, this could give astronomers a far clearer picture of how asteroids are put together and how fast they can spin, potentially challenging existing theories of how these bodies have formed and evolved over the history of the solar system.