Published online by Cambridge University Press: 12 April 2016
It is now a generally accepted concept that comets are a residue of the early history of the solar system from the time when the planets were forming. Because of the approximately 0.1% loss of material from the nucleus during perihelion passage near 1 A.U., lifetimes of short period comets are limited to 104-105 years. This requires an astronomically recent source of the comets seen at the present epoch. From the statistics of the aphelia of parabolic and long period comets, Oort (1951) proposed the existence of a comet cloud between 50,000 and 100,000 A.U. which serves as a reservoir from which presently observed comets have recently been perturbed. Although there are various difficulties with populating the cloud (Opik, 1973) and its subsequent evolution (I.A.U. Symposium 45, 1972; Everhart, 1974) it is the basis for nearly all current studies on the origin and evolution of comets.