Exposing cohesion forces in asteroids using fast rotating bodies

TYPE	Astrophysics Seminar
Speaker:	David Polishook
Affiliation:	Weizmann Institute of Science
Date:	18.11.2015
Time:	14:30
Location:	Lidow 620

Abstract:

Asteroids are considered to be collections of rocks separated by voids with no tensile strength to hold their components. When an asteroid is spinning-up, its gravity supposed to be the only force resisting the centrifugal acceleration before the body breaks apart. This notion is argumented by the observation that asteroids larger than ~300 m do not rotate faster than 2.2 hours per cycle. Smaller asteroids (<300 m) that reach faster rotations are therefore strong monolithic bodies from which the larger asteroids are composed.

In recent years, theoreticians argue that friction and electrical (Van der Wals) forces can maintain the weak asteroid body by increasing its cohesion value and this is the force that allows small asteroids to rotate fast and not their supposed monolithic nature. However, the clear spin barrier at 2.2 hours for asteroids larger than ~300 m serves as an argument against the non-zero cohesion model since it requires a correlation between size and spin rate.

We report an observation of a 2 km size asteroid, (60716) 2000 GD65, with a lightcurve indicating a rotation period of 1.9529±0.0002 hours. This adds to a handful of asteroids, recently observed by the Palomar Transient Factory (PTF) survey (Chang et al. 2014, 2015), with diameters between 0.5-1.5 km and lightcurves indicating rotation periods of 1.2-1.9 hours.

We apply the non-zero cohesion model (Holsapple 2007) to these asteroids and find that the fast rotating asteroids must have internal cohesive strength of at least 25 to 250 Pa in order to prevent disruption against centrifugal acceleration. Similar cohesion values have been found within weak lunar soils (~100 Pa). Among the population of small asteroids only 10% must have substantial internal cohesion of over 1000 Pa to prevent disruption, however none of them are rotating fast enough to require a fully monolithic body, i.e. cohesion >10 kPa.