On the Relationship Between Comets and Minor Planets

Abstract

Although the majority of the minor planets evidently arose as the result of fragmentation of larger planetary objects through collisions, there is a certain amount of evidence that some of them may be defunct cometary nuclei. Öpik (e.g. 1963) has long felt that the Apollo asteroids, with their perihelia inside the orbit of the earth, are especially likely to be ex-comets. Their aphelia are well inside the orbit of Jupiter (unlike those of the short-period comets), but this could be explained by the action of nongravitational forces when they were comets, just as the secular acceleration of P/Encke has apparently caused that comet’s aphelion distance to decrease to the present 4.1 AU. The radii of these objects, 1 to 5 km or so, are quite comparable with those of cometary nuclei (Roemer, 1966); further, Wetherill (1968, 1969) has shown that large meteoroids, and in particular chondritic meteorites, apparently come from typically cometary orbits with perihelia near the orbit of the earth and aphelia near that of Jupiter.