Abstract
The origin of short-period comets, usually believed to arise by planetary perturbations acting on nearly parabolic orbits, has been unclear for many years1. Given the observed near-parabolic flux, the predicted number of such comets is too small by a factor of ∼100 (refs 2–15). It has been suggested, therefore, that the adopted near-parabolic flux is in error16, that a physical effect such as fragmentation3,17,18 could operate, or that short-period comets do not come from the outer Oort cloud at all9,10,13,19. It has been proposed13–15,20,21 that the problem could be resolved by postulating a comet belt or massive inner core to the Oort Cloud. Here I recalculate the flux of nearly parabolic orbits from the Oort Cloud, including the recently discovered dominant injection of comets by the galactic tide22–25. The observed number of short-period comets is consistent with the Oort Cloud having a massive inner core26.
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