1. Shoemaker, E. M., Wolfe, R. F. & Shoemaker, C. S. in Global Catastrophes in Earth History (eds Sharpton, V. L. & Ward, P. D.) 155–170 (Geol. Soc. Am. Pap. 247, 1990).
2. Chapman, C. R. & Morrison, D. Nature 367, 33 (1994).
3. Morrison, D. Acta astronautica 30, 11–16 (1993).
4. Ahrens, T. J. & Harris, A. W. Nature 360, 429–433 (1992).
5. Harris, A. W., Canavan, G. H., Sagan, C. & Ostro, S. J. in Hazards Due to Comets and Asteroids (ed. Gehrels, T.) (University of Arizona Press, Tucson, in the press).