Comet and Asteroid Hazard to the Terrestrial Planets

We estimated the rate of comet and asteroid collisions with the terrestrial planets by calculating the orbits of 13000 Jupiter-crossing objects (JCOs) and 1300 resonant asteroids and computing the probabilities of collisions based on random-phase approximations and the orbital elements sampled with a 500 yr step. The Bulirsh-Stoer and a symplectic orbit integrator gave similar results for orbital evolution, but may give different collision probabilities with the Sun. A small fraction of former JCOs reached orbits with aphelia inside Jupiter's orbit, and some reached Apollo orbits with semi-major axes less than 2 AU, Aten orbits, and inner-Earth orbits (with aphelia less than 0.983 AU) and remained there for millions of years. Though less than 0.1% of the total, these objects were responsible for most of the collision probability of former JCOs with Earth and Venus. We conclude that a significant fraction of near-Earth objects could be extinct comets that came from the trans-Neptunian region.Comment: "Advances in Space Research" (Proc. of COSPAR-2002 (10-19 October 2002, Houston, TX, USA), COSPAR02-A-00845), final tex

Migration of Small Bodies and Dust to Near-Earth Space

The orbital evolution of Jupiter-family comets (JFCs), resonant asteroids, and asteroidal, trans-Neptunian, and cometary dust particles under the gravitational influence of planets was integrated. For dust particles we also considered radiation pressure, Poynting-Robertson drag, and solar wind drag. The probability of a collision of one former JFC with a terrestrial planet can be greater than analogous total probability for thousands other JFCs. If those former JFCs that got near-Earth object (NEO) orbits for millions of years didn't disintegrate during this time, there could be many extinct comets among NEOs. The maximum probability of a collision of an asteroidal or cometary dust particle with the Earth during its lifetime was for diameter $d$$\sim$100 microns. At $d$$<$10 micron, the collision probability of a trans-Neptunian particle with the Earth during a lifetime of the particle was less than that for an asteroidal particle by only a factor of several.Comment: Submitted to Advances in Space Research (Proceedings of COSPAR-2004

Evolution of the orbits of the objects P/1996 R2 (Lagerkvist) and P/1996 N2 (Elst-Pizarro)

Two new interesting objects were observed recently in the asteroid belt. The object P/1996 R2 (Lagerkvist) has a Jupiter-crossing orbit. On August 7, 1996 Elst reported his discovery of the object with a tail, which looked like a comet. This object was named P/1996 N2 'Elst-Pizarro' has a typical asteroid orbit, and was identified with the object 1979 OW7