Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995mnras.273.1091s&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 273, Issue 4, pp. 1091-1096.
Astronomy and Astrophysics
Astronomy
11
Methods: Numerical, Celestial Mechanics, Stellar Dynamics, Minor Planets, Asteroids
Scientific paper
The probability of a collision with the Earth is calculated for each of the 152 Apollo and 17 Atem asteroids known as at 1994 June 30. For the Apollos the mean impact probability (P_c) is 9.3 per billion (10^9) years; for the Atens, 24.3 per billion years; and, for all 169 Earth-crossers, the mean is 10.8 per billion years. The mean impact speeds, individual values being weighted by their collision probabilities, are 15.7, 15.1 and 15.5 km s^-1 respectively. 1991 VG was assumed to be a returned spacecraft, and therefore excluded from these averages; due to its very Earth-like orbit its P_c is extremely high, and its inclusion would more than quadruple the mean P_c derived for all Earth-crossers. Since the first Apollo was found in 1932 the mean P_c has varied significantly as our inventory has grown, currently having its highest-ever value. The present large value is largely due to the operation over the past five years of the Spacewatch telescope of the University of Arizona, which has added to the discovered population a number of small (<100 m) asteroids with low eccentricities and inclinations, these having high values of P_c. Using only the 99 Earth-crossers with absolute magnitude H<18 (sizes >~1 km), the mean P_c is 4.9 per billion years, with a weighted mean impact speed of 18.0 km s^-1. The `real' mean collision probability and speed are highly dependent upon the distribution of orbits amongst the near-Earth population, such as the relative numbers of Apollos and Atens, the fraction of Amors that evolve into Earth-crossing orbits, and in particular the number of e<0.2 objects.
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