Long-Term Orbital Evolution of Halley-Type and Long-Period Comets

Luke Dones (SJSU Foundation and NASA Ames), Sesha K. Dassanayake (Wabash College)

Halley-type comets (HTCs) are defined by Levison and Duncan ( Icarus 127, 13-32 (1997)) as comets with semi-major axes a < 40 AU and encounter velocities with Jupiter > Jupiter's orbital velocity. HFCs have a wide range of inclinations, and include retrograde objects such as Halley and Swift-Tuttle. Intermediate long-period comets (ILPCs), such as Hale-Bopp and IRAS-Araki-Alcock, have slightly larger orbits with tex2html_wrap_inline29 100-200 AU. Most HTCs and ILPCs probably come from the Oort Cloud, but the long-term orbital evolution of few ILPCs has been investigated in detail. Previous studies (Bailey et al., Astron. Astrophys. 257, 315-322 (1992); Levison and Duncan, Icarus 108, 18-36 (1994); Bailey and Emel'yanenko, MNRAS 278, 1087-1110 (1996); Bailey et al., MNRAS 281, 916-924 (1996); Levison and Duncan (1997)) indicate that HTCs and Hale-Bopp are more likely to become Sun-grazing than are most low-inclination, ``ecliptic'' comets. We are presently integrating the orbits of hundreds of clones of real HTCs and ILPCs, using Levison and Duncan's RMVS3 code, to determine the likelihood that these objects will be ejected from the solar system, or collide with the Sun or a planet. The comets are followed for times up to 10 Myr. For half of the clones, the dynamical model includes all nine major planets, while for the other half, we include only the giant planets and Pluto. We will discuss the implications of our results for the prevalence of extinct HFCs and ILPCs in the region of the terrestrial planets. In addition, we will compare our numerical results with the theory of Chambers (Icarus 125, 32-38 (1997)), which predicts that HFCs can be captured into high-order mean-motion resonances with Jupiter, while long-period comets cannot.