Recent results in cometary astronomy: Implications for the ancient sky

Abstract

The discovery that a significant number of distant cometary nuclei and icy planetesimals have diameters in excess of 100 km and revolve around the Sun on chaotic orbits with lifetimes in the order of 1 million years, shows that the inner Solar System is subject to the passage of exceptionally large comets with masses ranging from 1000 to 10,000 times that of Halley's comet. The observed cometary flux suggests that the mean interval between the injection of large ( d ≳ 100 km) comets into short-period orbits is in the order of 100,000 years, sufficiently short compared with the expected physical lifetime of comets that effects due to fragmentation of the most recent giant comet captured into a short-period orbit might still be observable. The evolution of comets into both Halley-type and Jupiter-family short-period orbits, particularly those passing close to the Sun or Jupiter, is reviewed and the astronomical evidence for giant-comet break-up is presented. Encounters of the Earth with cometary decay products (asteroids, boulders and dust) have important implications for the evolution of life on earth, and on shorter timescales the development of civilization. The existence of streams of cometary debris on Earth-crossing orbits during past millennia would have produced a much more active astronomical environment than that currently observed, with possibly important implications for the study of ancient myths and religions.