A 2km-size asteroid challenging the rubble-pile spin barrier – A case for cohesion

Abstract

The rubble pile spin barrier is an upper limit on the rotation rate of asteroids larger than ~200-300. m. Among thousands of asteroids with diameters larger than ~300. m, only a handful of asteroids are known to rotate faster than 2.0. h, all are in the sub-km range (≤0.6. km). Here we present photometric measurements suggesting that (60716) 2000 GD65, an S-complex, inner-main belt asteroid with a relatively large diameter of 2.3-0.7+0.6km, completes one rotation in 1.9529. ±. 0.0002. h. Its unique diameter and rotation period allow us to examine scenarios about asteroid internal structure and evolution: a rubble pile bound only by gravity; a rubble-pile with strong cohesion; a monolithic structure; an asteroid experiencing mass shedding; an asteroid experiencing YORP spin-up/down; and an asteroid with a unique octahedron shape results with a four-peak lightcurve and a 3.9. h period. We find that the most likely scenario includes a lunar-like cohesion that can prevent (60716) 2000 GD65 from disrupting without requiring a monolithic structure or a unique shape. Due to the uniqueness of (60716) 2000 GD65, we suggest that most asteroids typically have smaller cohesion than that of lunar regolith. Keywords: Asteroids; Asteroids, rotation; Rotational dynamics; Photometry