Theory 013/Earth & Space /Small bodies
Asteroid Shape Records Spin-Up
Shapes are read as collision debris. They may be what happens when a rubble pile is spun.
The shape of a small asteroid is set by its spin history rather than by its collisional history, so elongated bodies indicate past spin-up and oblate bodies indicate spin-down, independent of family membership.
01The evidence
Small asteroids show a wide range of shapes, including elongated and oblate forms, and many are near the spin limit at which loose material lifts off.
02The tension
Shape is attributed to the collision that formed the body, which predicts family members should share shapes. They often do not.
03The mechanism
A rubble pile reshapes as it spins. Spin-up drives material toward the equator and produces elongation; spin-down lets it relax toward a more spherical or oblate form.
That makes shape a record of spin history, which can differ within a family because spin evolves after formation through YORP.
04What this is not
Collision models predict family-wide shape similarity. This predicts shape from spin history, which varies within families.
05Predictions
- P1Elongation should correlate with spin-up indicators.
- P2Shapes should vary within families.
- P3Near-limit rotators should be most elongated.
06Refutation
Shape tracking family membership regardless of spin would restore collision inheritance.
07What it changes
It would make shape a spin-history proxy in surveys.