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Project Progress

Despite the differences in their composition, structure, and the mechanisms of formation, the giant planets of our solar system have one common feature: they all host irregular satellites. Marked by their highly eccentric orbits, and/or high orbital inclinations, irregular satellites revolve around their host planets at large distances. The dynamics of these objects is affected by perturbation from the Sun, and their precessions are controlled by solar tugs.

Although several of the dynamical characteristics of irregular satellites have already been studied, there is one interesting feature that has recently been observed in the distribution of Jovain irregulars and has not yet been accounted for. The region extending from the orbit of Callisto, the outermost Galilean satellite, to approximately 80 Jupiter-radii is void of irregulars. To understand the reason for this phenomenon, in collaboration with David Jewitt from NAI/Hawaii, Haghighipour completed an extensive numerical study of the orbital stability of small bodies in the region between 30 to 80 Jupiter-radii. Haghighipour and Jewitt simulated the dynamics of a large battery of small objects and mapped their parameter-space for different values of their semimajor axes, eccentricities, and orbital inclinations. Results of their simulations indicate that the region between Callisto and Themisto is indeed intrinsically unstable. Figure 1 shows the lifetimes of 500 test particles in this region. As shown here, objects in orbits with high eccentricities and/or inclinations have shorter lifetimes. As shown in figure 2, the perturbative effects of Ganymede and Callisto distabilize the orbits of irregular satellites and clear the region between Callisto and Themisto from small objects. The results of this project have been presented in a review chapter that will be published in the August 2007 issue of the Annual Review of Astronomy and Astrophysics, and have also been submitted for publication in the Astrophysical Journal.

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