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S. I. Ipatov (NRC/NAS Senior Research Associate, NASA/GSFC; Inst. Appl. Math., Moscow), J. C. Mather (NASA/GSFC)
The orbital evolution of about 8000 and 5400 objects with initial orbits close to those of Jupiter-family comets (JFCs) was integrated with the use of the Bulirsh-Stoer and symplectic methods, respectively. The gravitational influence of planets (except for Pluto, and sometimes Mercury) was taken into account. In the case of close encounters with the Sun (e.g., for Comet Encke 2P) the probability of collisions with the Sun was different for different methods and different accuracy per integration step, but all other obtained results were similar. Decoupling from Jupiter was found for less than 1% of the simulated JFCs. About 0.1% of the migrating studied JFCs reached orbits with a semi-major axis a<2 AU for more than 0.5 Myr. Several former JFCs moved in such orbits for tens or even hundreds of Myrs, and even reached Aten orbits and inner-Earth orbits, which are located inside the orbit of the Earth. Based on orbital elements sampled with a 500 yr step, we calculated the mean probabilities of collisions of objects with planets. If we exclude a few bodies with the largest probabilities, the mean probability 'P' of a collision of a former JFC with the Earth during the lifetime of the object was about 4*10-6, enough for delivering an amount of water similar to the mass of Earth oceans during the formation of the giant planets. For some runs this probability was smaller (the same as that obtained by other scientists, who considered relatively small number of objects and did not obtain decoupling from Jupiter), but a few objects increased the above mean value of 'P' by more than an order of magnitude. The probability of a collision with Earth or Venus for a single former JFC moving in a typical orbit of a near-Earth object (NEO) for millions of years could be greater than the total for thousands of other objects. Based on the runs of orbital evolution of JFCs and using the results of migration of trans-Neptunian objects to the orbit of Jupiter, we concluded that up to several tens of percent of NEOs could be extinct comets that came from the trans-Neptunian region. Some former comets that have moved in typical NEO orbits for millions or even hundreds of millions of years, and might have had multiple close encounters with the Sun, could have lost their mantles, which caused their low albedo, and so change their albedo and would look like typical asteroids. Several our papers on this problem were put in http://arXiv.org/format/astro-ph/, the last one is astro-ph/0303219. This work was supported by NRC (0158730), NASA (NAG5-10776), INTAS (00-240), and RFBR (01-02-17540).
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.