[Previous] | [Session 15] | [Next]
D.J. Scheeres (Univ. of Michigan, Ann Arbor), V.V. Sidorenko (Keldysh Inst. of Applied Math., Moscow), A.I. Neishtadt, A.A. Vasiliev (Space Research Inst., Moscow)
The rotational dynamics of outgassing cometary nuclei are investigated analytically. We develop a general theory for the evolution of a comet nucleus' rotation state using averaging theory and assuming that the outgassing torques are a function of solar insolation and heliocentric distance. The resulting solutions are a function of the nucleus inertia ellipsoid, its outgassing properties, its heliocentric orbit, and the assumed distribution of active regions on its surface.
We find that the long-term evolution of the comet nucleus rotation is a strong function of the distribution of active regions over its surface. In particular, we find that nuclei with nearly axisymmetric inertia ellipsoids and a uniformly active surface will tend towards a rotation state that has a nutation angle of ~ 55 degrees and its angular momentum perpendicular to the sun-perihelion direction. If such a comet nucleus has only one isolated active region, it will tend towards a zero nutation angle with its approximate symmetry axis and rotational angular momentum aligned parallel to the sun-perihelion direction.
In the general case for an inertia ellipsoid that is not close to being axisymmetric we find a much richer set of possible steady-state solutions that are stable, ranging from rotation about the maximum moment of the inertia axis, to SAM and LAM non-principal axis rotation states. The resulting stable rotation states are a strong function of outgassing activity distribution, which we show using a simplified model of the comet Halley nucleus. Also, we demonstrate that comet Borrely observations are consistent with a stable rotation state.
Our results can be used to discriminate between competing theories of comet outgassing based on a nucelus' rotation state. They also allow for a range of plausible a priori constraints to be placed on a comet's rotation state to aid in the interpretation of its outgassing structure.
This work was supported by the NASA JURRISS program under Grant NAG5-8715. AIN, AAV, and VVS acknowledge support from Russian Foundation for Basic Researches via Grants 00-01-00538 and 00-00-00174 respectively. Also they acknowledge support from INTAS via Grant 00-221. DJS acknowledges support from the PG&G program via Grant NAG5-9017.
[Previous] | [Session 15] | [Next]
Bulletin of the American Astronomical Society, 34, #3
© 2002. The American Astronomical Society.