Sunlight fingerprints on shattered
asteroids
Updated: June 2014
Observations
from the Wise Observatory show for the first time that loose bound "rubble
pile" asteroids, can break apart due to spin-up. The discovery, by Dr.
David Polishook (MIT, USA) highlights one of the proposed mechanisms for the
formation of pairs of asteroids. An asteroid pair consists of two unbound
objects with almost identical heliocentric orbital. Models suggest that the
pairs' progenitors gained their fast rotation from the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP)
effect – a torque applied on asteroids by the momentum carried by
sunlightÕs photons. Since it was shown that the spin axis vector can be aligned by the YORP effect, such a behavior should be
seen on asteroid pairs if they were indeed formed by YORP. Alternatively, if
the pairs were formed by a collision, the spin axes should have random
orientations while small or young bodies might even show tumbling rotation.
The
dataset on which this discovery is based was collected using exclusively Wise
ObservatoryÕs 1m and 0.46m telescopes during 110 observation nights from 2007
to 2014. The light curve inversion method was applied in order to derive the
rotation axis vectors and shape models of six asteroids in pairs. Three
asteroids resulted with polar-directed spin axes and three objects with
ambiguous results. In addition, the secondary member of pair 44612 presents the
same sense of rotation as its primary member 2110, and its spin is not tumbling
refuting a formation by collision. Finally, a rotational fission model was
used, based on the assumption of angular momentum conservation. This model was
matched to the measured spin, shape, and mass ratio parameters in order to
constrain the density of the primary members in the pairs. Using this method in
an unprecedented way, low density values as expected
for Òrubble pileÓ asteroids are derived. This forms the first observational
linkage between the Òrubble pileÓ structure to
separated pairs of asteroids. All these results lead to the conclusion that the
disruption of asteroid pairs was most likely the outcome of the YORP effect
that spun-up Òrubble pileÓ asteroids.
The
results are presented in a paper accepted for publication by the journal Icarus (see http://arxiv.org/abs/1406.3359).