Oort Cloud - probable limits Comets too far from the sun could easily get "dislodged" by passing stars. Most long-period comets seem to have aphelia at around 10 4 AU (where they have an average tempthey have an average temperature around 3K!). 1 parsec is around 2 10 5 AU, so the comets in the Oort cloud can be up to 5% of the way to the nearest star. Consider, however, that other stars do pass closer. In the Sun's vicinity there are about 0.044 stars per cubic parsec (556 stars in a 10 pc radius). The Sun's velocity through this medium is about 20km s-1 , or about 20 pc My-1 . If we take a c/s area of A pc 2 travelling at v pc My-1 through a medium of d (pc-3 ) density then we will have a mean time between encounters of T (My) where T is given by: T = 1/(Avd) If r = 20,000 AU (0.1pc) then T is around 38My. One such collision would remove all comets out to 0.05pc,or 10,000 AU. 12 such collisions (200My) will sweep away any material outside 0.05pc. The closest encounter in 4.6Gy would be expected to be about 1800 AU, but this is most likely to
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