Most asteroids larger than about 200 meters are rubble piles — collections of rock and debris held together by gravity alone, with no internal cohesion. This structural assumption sets a physical limit: rotation faster than roughly one full turn every 2.2 hours would generate centrifugal forces exceeding gravitational binding, and the pile would fly apart. The spin barrier has held for decades of observation. Nearly every large asteroid sits on the slow side of it.
Asteroid 2025 MN45, found in Rubin Observatory commissioning data, rotates once every 1.88 minutes. It's 710 meters across. It should not exist.
A rubble pile at that spin rate would disintegrate instantly. For 2025 MN45 to survive, it must have cohesive strength of approximately 9 megapascals — comparable to solid rock at roughly 10 MPa. It isn't a pile. It's a monolith: a single coherent fragment, not an aggregate. The same survey found two more ultrafast rotators (2025 MJ71 at 1.9 minutes, 2025 MK41 at 3.8 minutes), suggesting these objects are not rare anomalies.
The origin story writes itself backward. A monolithic fast-spinner was never assembled from smaller pieces — it was broken from a larger one. These are fragments of a parent body's dense interior, ejected during a catastrophic collision and never recaptured into a pile. The structural memory of the original body survives in the fragment's integrity.
The rubble pile model isn't wrong. It describes most asteroids accurately. But it created an observational bias: anything spinning faster than 2.2 hours was assumed to be small enough that cohesion didn't matter. 2025 MN45 is large enough that cohesion does matter — and present enough that the barrier is not a wall but a filter, separating what was assembled from what was broken off.