Astronomy and Astrophysics – Astronomy
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dda....39.1401h&link_type=abstract
American Astronomical Society, DDA meeting #39, #14.01
Astronomy and Astrophysics
Astronomy
Scientific paper
For asynchronous binary asteroids, the primary is spinning at close to the rate where centrifugal force cancels gravity at the equator. The time varying tidal acceleration from the satellite may nearly cancel, or even reverse, the total acceleration vector on the equator as it passes directly under the satellite. We numerically investigate mass motion of loose regolith under these conditions, using parameters of binary NEA (66391) 1999 KW4, with coefficients of static and sliding friction typical of regolith materials. The onset of mass motion and even levitation off the surface begins when the tidal acceleration is not quite enough to reverse the static acceleration at the sub-satellite point. Sliding motion in the direction of the rising satellite begins when the satellite has risen about half way to the zenith. The added velocity results in the material levitating off the surface and following a ballistic trajectory for a time, re-impacting on the surface after the satellite has passed overhead and is now retarding the levitated matter, causing it to fall back. From the surface of the satellite, the levitated regolith follows a looping trajectory, falling back to a point behind where it launched, opposite the direction of rotation. Viewed from the satellite, the mass motion is a sort of standing wave, like sand saltation at the crest of a sand dune. It also resembles a tidal bulge, but with a "lag angle” close to 45 degrees, maximum for transfer of angular momentum from the spin of the primary to the orbit of the secondary. We suggest that this is the main process by which YORP torque received by the primary is transferred to the orbit of the secondary, thereby regulating the primary spin to very near the critical spin rate, and also producing the smooth "racetrack” profile of the equator.
Fahnestock Eugene G.
Harris Alan W.
Pravec Petr
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