Asteroid rotation excitation by subcatastrophic impacts

Abstract

Photometric observations of asteroids show that some of them are in non-principal axis rotation state (free precession), called tumbling. Collisions between asteroids have been proposed as a possible asteroid rotation excitation mechanism. We simulated subcatastrophic collisions between asteroids of various physical and material parameters to find out whether they could be responsible for the excited rotation. For every simulated target body after the collision, we computed its rotational lightcurve and we found that tumbling was photometrically detectable for the rotational axis misalignment angle β greater than about 15 • . We found that subcatastrophic collisions are a plausible cause of non-principal axis rotation for small slowly rotating asteroids. The determining parameter is the ratio of the projectile orbital angular momentum to the target rotational angular momentum and we derived an approximate relation between this ratio and the angle β. We also compared the limiting energy for the onset of tumbling with the shattering energy. Slowly rotating asteroids of diameter 100 m and larger can be rotationally excited by collisions with energies below the shattering limit.

Slowly increasing elongations of non-spherical asteroids caused by collisions

“…There is a set of mechanical properties of the target and the projectile, which can be varied in a physically plausible range. We did this to some extent in our previous paper, Henych & Pravec (2013), and we found that these values affect the size of an impact crater formed on the surface of the target asteroid. Therefore, they affect the timescale of increasing elongation of the target's shape.…”

Asteroid Interiors and Morphology

“…The degree of dissipation is not fully understood, and depends on the assumed parameters of rigidity, µ, which measures the stiffness of the body; the quality factor, Q, which is inversely proportional to the rate of energy dissipation due to non-uniform deformations; and the rate at which tumblers are created. We note that tumblers can be created in several ways: catastrophic disruptions (Asphaug and Scheeres, 1999;Scheeres et al, 2000b), planetary flybys (Scheeres et al, 2000b(Scheeres et al, , 2005Pravec et al, 2014), YORP-induced fission (Sánchez and Scheeres, 2014), and, potentially, YORP spindown effects either in isolation (Vokrouhlickỳ et al, 2007) or in combination with impacts (Marzari et al, 2011;Henych and Pravec, 2013).…”

The tumbling rotational state of 1I/‘Oumuamua

“…Tumbling can be brought about by collisions 14 , tidal torques in planetary close encounters 15 , cometary activity 16 , or the YORP effect 17 . It is eventually damped by internal friction and stressstrain forces removing the excess of rotational energy above that of the basic rotational state around the body's principal axis with largest moment of inertia.…”