|MadSci Network: Astronomy|
There are 3 main reservoirs for comets and asteroids in our Solar System: the asteroid belt, the Kuiper belt, and the Oort cloud. Once objects are removed from these locations, anything is possible! I'll summarize the main characteristics of each of these reservoirs then answer your questions more specifically. The Asteroid Belt is located between 2.1 and 4 AU (1 AU = 93 million miles) from the Sun. It has several zones called Kirkwood gaps that are devoid of asteroids. These zones were formed when gravitational perturbations in or close to the Kirkwood gaps caused changes in the orbits of asteroids, causing them to leave the zones. The orbits became destabalized and eventually evolved into orbits similar to those of comets (Hartmann, et al, 2000). The Kuiper Belt is located between 40 and 400 AU from the Sun. Only main-belt Kuiper Belt objects (KBOs) have stable orbits around the sun. KBOs in resonant orbit with Neptune and the scattered KBOs that have been flung into deep space after a gravitational encounter with Neptune account for the short-period comets (P < 200 years) that we see in the inner Solar System (Hartmann, et al, 2000). The Oort Cloud is located out to 50,000 AU or more. Most of the comets here are vaporized or thrown into other orbits by interactions with Jupiter, so between 10^11 and 10^14 objects are thought to exist here (Zeilik and Gregory, 1998). In most cases, comets enter the inner Solar System once and never return. Encounters with Jupiter, however, can disrupt orbits, leading to other interactions (such as planetary impact or ejection). Weissman (1990) reports that cometary showers and long- and short-period comets account for approximately 17% and 12%, respectively, of terrestrial craters that are 10 km in diameter. Hartmann (1999) reports that over 96% of the encounters with Jupiter lead to ejection from the Solar System in 1 to 100 million years. Zeilik and Gregory (1998) report that computer simulations of the Oort cloud indicate that about 65% of the objects are ejected from the Solar System, 30% are randomly disrupted, and the rest are lost by processes such as loss of volatiles (vaporization) and collisions with the sun and planets. I think it's almost impossible to know how many comets actually formed in the Oort Cloud and how many ended up there because we haven't made any observations of objects in the Oort Cloud, just sitting there, to know what they look like (our telescopes aren't big enough for that yet). Our only observations come from comets, and there are a lot of differences in their chemistries. By knowing their orbits, we can only know from where they were perturbed, not if they originated someplace else, got perturbed, ended up in a stable orbit (like the Oort Cloud), then got perturbed again to become the comet we see. I think it is safe to say, though, that some probably did come to reside in the Oort Cloud via interactions with Jupiter, Neptune, and the other giant planets. Since computer models show that ojects can be ejected from the Solar System by interactions with Jupiter, it's likely that there are some Solar System objects floating around in deep space. Are the objects from other star systems floating around in our space? So far, none have been detected, but a number of authors are using computers to try to model the liklihood of finding them. Valtonen and Innanen (1982) report that a significant accumulation of interstellar comets could occur only when the Solar System moved through a relatively dense cloud of interstellar debris having a certain velocity. Sen and Rana (1993) say that non-detection is entirely expected, given the current understanding of Oort Cloud formation, but McGlynn and Chapman (1989) say that we should expect to find 6 interstellar comets over a 150-year period. I'm sure there are others... As far as safety for interstellar travel, there's probably not _that_ many rogue comets out there. Even the Asteroid Belt is mostly empty space. And I think that once we have the know-how to travel to other star systems, we should be able to figure out a way to avoid these kinds of "road" hazards! REFERENCES: Hartmann, W. K., 1999, "Moons and Planets", California: Wadsworth Publishing Company. Hartmann, W. K., et al. 2000, in "Origin of the Earth and Moon", ed. R. Canup and K. Righter, 493-512. McGlynn, T. A. and Chapman, R. D., 1989, "On the nondetection of Extrasolar Comets", Astrophysical Journal, vol. 346, L105. Sen, A. K. and Rana, N. C., 1993, "On the missing intersteallar comets", Astronomy and Astrophysics, vol. 275, 298-300. Valtonen, M. J. and Innanen, K. A., 1982, "The Capture of Interstellar Comets", Astrophysical Journal, vol. 255, 307-315. Weissman, P. R. 1990, in "Global Catastrophes in Earth History", GSA Special Paper 247, ed. V. L. Sharpton and P. D. Ward, 171-180. Zeilik and Gregory, 1998, "Introductory Astronomy and Astrophysics", Florida: Saunders College Publishing [Moderator's Note: There's a series of articles addressing these questions in the May 18, 2001 issue of "Science" magazine (vol. 292, no. 5520). Check it out if you want to see some of the most recent research!]
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