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Geological Society, London, Special Publications; 1998; v. 140; p. 11-17;
DOI: 10.1144/GSL.SP.1998.140.01.03
© 1998 Geological Society of London

The Flux of Extraterrestrial Material to the Earth: Determination by Astronomical and Statistical Techniques

Cometary capture and the nature of the impactors

M. E. Bailey1 & V. V. Emel’yanenko2

1 Armagh Observatory, College Hill, Armagh BT61 9DG, UK
2 Department of Theoretical Mechanics, South Ural University, Chelyabinsk 454080, Russia

The inclination-averaged capture probability from nearly parabolic orbits to Halley-type orbits with periods P < 200 years and q < 1.5 AU is 0.0128 for an isotropic long-period source flux with perihelion distances q < 4 AU. The observed near-parabolic flux brighter than visual absolute magnitude H10 = 7 is approximately 0.2 comets AU–1 a–1, and the dynamical lifetime of the captured Halley-type comets is on the order of 0.3 Ma. The predicted steady-state number of such comets is thus about 3000, many times more than the number of observed Halley-type comets. Provided that long-period comets do not totally disintegrate during the capture process (e.g. into streams of small bodies such as cometary meteoroids and dust), this indicates that there should be a large population of undiscovered Halley-type ‘asteroids’ in the inner Solar System. The predicted number of dark Halley-type objects is sufficient to produce a terrestrial cratering rate comparable with that from observed near-Earth asteroids, and hence could explain the reported Galactic signal seen in the geological and biological records. The assessment of the terrestrial impact hazard should consider the possibility that a significant number of dark Halley-type objects may exist.