Comets account for a small but very significant fraction of impactors on the Earth. Although the total number of Earth-crossing comets is modest as compared with asteroids, the more eccentric and inclined orbits of the comets result in much higher encounter velocities with the planet. Additionally, some Earth-crossing comets are significantly larger than any current near-Earth asteroids (NEAs); comets 1P/Halley and C/1995 O1 Hale-Bopp are good examples of this. Thus, the most energetic impacts on the Earth likely result from comets and not NEAs. The mean impact probability for long-period comets is 2.4× 10−9 per comet per perihelion passage, assuming the perihelion distribution of Everhart (1967), with a most probable encounter velocity of 53.5 km sec−1. There are 21 known Earth-crossing Jupiter-family comets with a mean impact probability of 1.6× 10−9 per comet per year and a most probable encounter velocity of 17.0 km sec−1. For the 16 known Earth-crossing Halley-type comets the mean impact probability is 1.2× 10−10 per year with a most probable encounter velocity of 51.3 km sec−1. The poor knowledge of the size distribution of cometary nuclei makes it difficult to estimate actual impact energies at this time, though that situation is slowly improving, in particular for the Jupiter-family comets.
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