**DPS 2001 meeting, November 2001**

*Session 49. Outer Planet Satellites*

Oral, Chairs: R. Johnson, C. Dumas, Friday, November 30, 2001, 4:30-5:50pm, Regency GH
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## [49.01] On the Orbital Distribution of Irregular Satellites

*J.A. Burns, V. Carruba, P.D. Nicholson (Cornell University), B.J. Gladman (Obs. de la Cote d'Azur), M.J. Holman (SAO)*

Clusters of irregular satellites, moons that occupy large
orbits of significant eccentricity {\em e} and/or
inclination {\em I}, circle Jupiter, Saturn and Uranus. The
irregulars often extend close to the orbital stability
limit, about 1/3-1/2 of the way to the edge of their
planet's Hill sphere. These distant, elongate and inclined
orbits suggest capture, perhaps by energy loss (gas drag by
a circumplanetary nebula or collisions) or by rapid
planetary growth. Capture, however, should provide random
orbits and yet no known irregulars have inclinations
(relative to the ecliptic) between 47^{\circ} and
141^{\circ}. Secular (Kozai) perturbations allow two kinds
of high inclination ({\em I} >39.2^{\circ}) orbits:
circulating and librating. In circulating orbits, the
argument of pericenter {\em \omega} sweeps through all
possible values (0-2\pi). In contrast, for libration, {\em
\omega} oscillates around 90^{\circ} (or 270^{\circ}).

Circulating objects suffer appreciable periodic changes in
eccentricity, and reach a maximum value of their
eccentricity when \omega=90^{\circ}). As a result,
particles of high inclination (~70^{\circ}<{\em I} <
110^{\circ}) are found to penetrate the realm of the
regular satellites where collisions and scatterings are
likely to remove them from planetocentric orbits. In
addition, long-term (10^{9}~yr) orbital integrations of
hypothetical satellites show that solar and planetary
perturbations considerably broaden this zone of avoidance to
~55^{\circ}<{\em I} < 130^{\circ}. Such objects either
escape, or collide with their parent planet.

Librating bodies can in theory be stable for larger values
of {\em I}. However, our integrations show that tidal
perturbations from the Sun and the other jovian planets also
restrict the region of phase space accessible to these
bodies. Interactions with the Galilean satellites will
reduce it further. Several N-body simulations have allowed
us to estimate the size of the region where librating
objects can be stable for periods of ~10^{7}-10^{8} years.

The author(s) of this abstract have provided an email address
for comments about the abstract:
vc27@cornell.edu

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