With data collected months before its main survey begins, the Vera C. Rubin Observatory is already changing what we thought we knew about asteroids.
In the Main Asteroid Belt between the orbits of Mars and Jupiter, the telescope spotted a large asteroid spinning shockingly fast. Its name is 2025 MN45it measures 710 meters (2,330 feet) in diameter – and has a rotation period of just 1.88 minutes.
That’s far, far above the 2.2-hour rotation barrier beyond which asteroids larger than 150 meters should fly into pebbles, as centrifugal forces override the asteroid’s supposed structural integrity.
Furthermore, the observations have identified 18 additional asteroids spinning at “impossible” high speeds. These results suggest that asteroids may be much more powerful than scientists previously thought.
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“The unexpected prevalence of football-field-sized asteroids (greater than 500 meters in diameter) completing 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 the National Science Foundation’s National Optical-Infrared Astronomy Infrared Laboratory.
The Solar System has more minor planets – that is, bits of stuff that are smaller than full-fledged planets and not comets – than anything else. These objects often preserve clean records of the composition of the Solar System from the time of its formation.
However, they are not easy to study. They are quite small, dark and distant and move around a lot. This means that detailed catalogs of their features, 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 before, dramatically expanding our understanding of these ancient and mysterious objects.
The telescope hit the ground during its observing period before the inspection. For decades, astronomers thought they understood how fast asteroids could safely spin without breaking up. That’s because most asteroids are thought to be “debris heaps” – aggregates of pebbles, dust and boulders loosely bound by gravity.
If one of these piles of debris spins too fast, that weak link is overcome by centrifugal force. Think of a Gravitron and how the people riding it are thrown outwards against the wall as it spins.
If you put a single large, coherent mass at the center of the Gravitron, that mass would stay in place. If the mass were made up of smaller components held together only loosely, it would fall apart.
For large Main Belt asteroids, that breakpoint was set at a rotation 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 were really piles of debris, and while there might be more solid bodies, they were thought to be few.
Rubin’s observing campaign took place over nine nights between April 21 and May 5, 2025, during which it collected information on about 340,000 asteroids. From this wealth of data, Greenstreet and her colleagues measured the spins of 76 asteroids—75 in the Main Belt and one orbiting near-Earth space.
Nineteen of these asteroids had rotation periods shorter than the rotation barrier: 16 were super-rapid rotators with periods between 2.2 hours and 13 minutes, and the remaining three were ultra-rapid rotators with periods of less than five minutes.
This is a huge surprise: most fast rotators discovered so far 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 fast spinners was a near-Earth object.
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2025 MN45 is obviously the records, but the other asteroids cannot 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 has to be made of a material that has a very high strength to keep it in one piece because it’s spinning so fast,” Greenstreet says. “We calculate that it would need a cohesive force similar to that of solid rock.”
This is extremely interesting. Chunks of solid rock like this could be survivors of the unusually violent collisions that took place 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 for missions like 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 collision histories, and to a greater extent our understanding of the formation and evolution of the Solar System.”
The findings were published in Letters from the Astrophysical Journal.