With data collected months before its main survey is due to begin, the Vera C. Rubin Observatory is already upending what we thought we knew about asteroids.
In the Main Belt of asteroids between the orbits of Mars and Jupiter, the telescope spotted a large asteroid spinning shockingly fast. Its name is 2025 MN45, it measures 710 meters (2,330 feet) across – and it has a spin period of just 1.88 minutes.
That's far, far past the 2.2-hour spin barrier, beyond which asteroids larger than 150 meters should fly apart into pebbles as centrifugal forces supersede the asteroid's supposed structural integrity.
Moreover, the observations identified 18 additional asteroids rotating at 'impossible' high speeds. These results suggest that asteroids can be vastly stronger than scientists previously thought.
Related: Two Weird Red Rocks Live in The Asteroid Belt, And They Don't Belong There
"The unexpected prevalence of asteroids the size of several football field lengths (diameter greater than 500 meters) that complete a full rotation in the extremely short period of less than two minutes requires us to refine our understanding of the formation and evolution of asteroid rotations," writes a team led by astronomer Sarah Greenstreet of US National Science Foundation National Optical-Infrared Astronomy Research Laboratory.
The Solar System has more minor planets – that is, chunks of things that are smaller than full-fledged planets and not comets – than anything else. These objects often preserve pristine records of the composition of the Solar System from the time of its formation.
They're not easy to study, though. They're quite small, dark, and distant, and they move around a lot. This means that detailed catalogs of their characteristics, such as size, shape, and rotation, are difficult to obtain.
Part of Rubin's mission will be to take an inventory of asteroids that is more detailed than any to date, dramatically expanding our understanding of these ancient, mysterious objects.
The telescope has hit the ground running during its pre-survey observing period. For decades, astronomers thought they had a good understanding of how fast asteroids can safely spin without breaking apart. That's because most asteroids are thought to be 'rubble piles' – aggregations of pebbles, dust, and boulders loosely bound by gravity.
If one of these rubble piles spins too quickly, that loose binding is overcome by the centrifugal force. Think of a Gravitron, and the way the people riding it are flung outward against the wall while it spins.
If you put one single, large, cohesive mass in the center of the Gravitron, that mass would stay put. If the mass consisted of smaller components only held together weakly, it would break apart.
For large Main Belt asteroids, that break-apart point was set at a spin period of about 2.2 hours – a hard limit suggested by theory in the 1990s and then confirmed in 2000 by observations of the Main Belt that showed very few objects larger than 150 meters with a rotation period shorter than that threshold.
The implication was that most asteroids are indeed rubble piles, and, while more solid bodies may exist, they were thought to be few and far between.
Rubin's observing campaign took place on nine nights between 21 April and 5 May 2025, during which it gathered information on around 340,000 asteroids. From that wealth of data, Greenstreet and her colleagues measured the spins of 76 asteroids – 75 in the Main Belt, and one hanging around in near-Earth space.
Nineteen of those asteroids had rotation periods shorter than the spin barrier: 16 super-fast rotators with periods between 2.2 hours and 13 minutes, and the remaining three were ultra-fast rotators, with periods of less than five minutes.
This is a huge surprise: Most fast rotators discovered to date are near-Earth asteroids closer to the Sun. Main Belt asteroids were thought to be much slower and more stable. Only one of the new speedy spinners was a near-Earth object.
2025 MN45 is obviously the record-smasher, but the other asteroids can't be ignored, either. The fact that such a large percentage of the sample defied the spin barrier implies that we may have dramatically underestimated the number of Main Belt asteroids with high density and structural integrity.
"Clearly, this asteroid must be made of material that has very high strength in order to keep it in one piece as it spins so rapidly," Greenstreet says. "We calculate that it would need a cohesive strength similar to that of solid rock."
This is tremendously exciting. Chunks of solid rock like this may be survivors of unusually violent collisions that occurred in the chaos that reigned during the early Solar System, preserving internal structures that most asteroids lost long ago.
This bodes well for future Rubin observations, as well as missions such as Lucy, a NASA spacecraft currently underway to explore asteroids up close.
"With potentially unusual compositions, internal structures, and/or formation histories," the researchers write, "a much larger sample of these extremely fast rotating asteroids is very likely to transform our understanding of asteroid physical structures and collisional histories and to a greater extent our understanding of the formation and evolution of the Solar System."
The findings have been published in The Astrophysical Journal Letters.