 
Summary: NearEarth object velocity distributions and consequences for the
Chicxulub impactor
S. V. Jeffers,1,2
S. P. Manley,1
M. E. Bailey1P
and D. J. Asher1
1
Armagh Observatory, College Hill, Armagh BT61 9DG
2
School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS
Accepted 2001 May 25. Received 2001 May 9; in original form 2000 October 27
A B S T R AC T
An OĻ pikbased geometric algorithm is used to compute impact probabilities and velocity
distributions for various nearEarth object (NEO) populations. The resulting crater size
distributions for the Earth and Moon are calculated by combining these distributions with
assumed NEO size distributions and a selection of crater scaling laws. This crater probability
distribution indicates that the largest craters on both the Earth and the Moon are dominated by
comets. However, from a calculation of the fractional probabilities of iridium deposition, and
the velocity distributions at impact of each NEO population, the only realistic possibilities for
the Chicxulub impactor are a shortperiod comet (possibly inactive) or a nearEarth asteroid.
