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IAA Transactions, No. 8, ``Celestial Mechanics'', 2002
Analytical constraints on comet nucleus rotation
A. I. Neishtadt 1 , D. Scheeres 2 , V. V. Sidorenko 3 , A. A. Vasiliev 1
1 Space Research Institute, Moscow, Russia
2 University of Michigan, Ann Arbor, USA
3 Keldysh Institute of Applied Mathematics, Moscow, Russia
Nucleus rotation essentially affects many processes studied in cometary phy­
sics. Hypotheses on the nucleus rotation are obligatory in mathematical models
being developed to solve complicated ballistic and navigation problems arising in
spacecraft comet studies. Hence, it is important to realize typical parameters of
nucleus rotation for various types of comets.
In the classical model of a comet nucleus suggested by Whipple [6], anisotrop­
ic ice sublimation under solar radiation produces in general reactive torque. It
should result in slow variations of a nucleus' rotation parameters. In [3--5, 7] the
spin evolution of comet nuclei was investigated by numerical integration of the
equations of nucleus rotation. We seek to develop a more systematic approach
to the problem by studying the rotational evolution of a cometary nucleus using
the averaging method [1]. It allows us to extract the relevant physical parame­
ters that control the evolution of a comet's rotation state and to describe several
scenarios qualitatively different in behavior of the motion parameters.
In particular, we found that the long--term evolution of the comet's angular
momentum vector is a function strongly dependent on the distribution of active
regions over its surface. We obtained that the fraction of active area may lead
to certain values of nutation angle and cause its angular momentum direction to
align in specific directions related to its perihelion. This result can be used to
discriminate between competing theories of comet outgassing based on a nucleus'
rotation state. It also allows 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.
According to modern ideas about the physical properties of the upper layer of
a comet nuclei, standard cometary activity formulas [2], used in our studies, are
very rough model of the real sublimation process. Nevertheless, it is remarkable,
that this simple model allows the detailed analysis of reactive torque effects on
the nucleus spin evolution.
This work was supported by NASA JURRISS Grant NAG5­8715.
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