Fast rotator
A fast rotator is an asteroid that spins once in less than about two hours, too fast for a loose rubble pile to hold together.
Most asteroids larger than a few hundred metres spin no faster than about once every 2.2 hours. Faster than that, loose rubble held together only by weak gravity would fly apart at the equator. This limit, sometimes called the spin barrier, is strong evidence that many asteroids are rubble piles.
Smaller asteroids, however, often spin much faster. A fast rotator may turn in minutes or even seconds. Such bodies must be held together by something more than gravity: either they are solid monoliths, or tiny cohesive forces between grains, similar to the forces that make flour stick, are strong enough at that size.
Spin rates are changed over millions of years by the YORP effect, a gentle torque from sunlight absorbed and re-emitted unevenly. Understanding fast rotators matters for planetary defence, since many small near-Earth asteroids belong to this group, including Hayabusa2's final target, 1998 KY26.
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