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I always see lots of information about asteroid impacts, but very little is said about comet impacts. As I see, a long period comet impact is much more dangerous because a) we wouldn't have time to prepare for it, and b) even small bodies would have high kinetic energy, equivalent to large asteroids.

On the flip side, I think that a long period comet would have trouble hitting the Earth, as it wouldn't have time to interact with our planet as an asteroid does. It would just hit us if, by chance, it was crossing the ecliptic plane at 1 AU from the Sun and we happened to be at the same place as well. So, my question boils down to the following:

  • Do long period comets cross the ecliptic plane at a preferential distance from the sun (or: what is the chance of them crossing at approximately 1 AU)?
  • What is the area of the hitting plane, versus the cross section of our planet?
  • Does the assumption that a comet wouldn't have time to interact with us holds?
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  • $\begingroup$ Interesting, I was just watching Impact TV miniseries on Czech TV Prima Cool. $\endgroup$ – Luboš Motl Nov 10 '12 at 21:08
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An estimate of impacts from long-period comets is available at this NASA JPL site. They appear to be randomly distributed . Given detection at about 5 AU, we'd only have a year of warning for potential impactors. Their mean impact velocity is in the order of 52 km/s with an impact probability crossing Earth's orbit of 2.2 - 2.5x${10^{ - 9}}$ per perihelion passage. Nuclei large enough to initiate global climate disturbances occur on average, every 16 Myr.

With our current technology, it appears we would have no way to avoid disaster from a long-term comet should we be in the cross-hairs of one of them. Fortunately, impact probability is very small.

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    $\begingroup$ Thanks for the excellent reference! Although it is a 1995 report (inspired by Shoemaker-Levy, probably?), the conclusions would probably be the same post-WISE, that considerably increased the number of known comets. $\endgroup$ – Bruno Kim Nov 12 '12 at 14:45

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