Introduction

In the 1994 book edited by Gehrels, Hazards due to Comets and Asteroids, chapters by Ahrens and Harris (1994), Shafer et al. (1994), Simonenko et al. (1994), Solem and Snell (1994), and Melosh et al. (1994) present and study a number of ways of preventing an oncoming asteroid from colliding with the Earth. Most methods considered nudging it sufficiently at 10 or so years before the impending collision to change its course so it would miss the Earth.

The methods studied include the use of conventional or nuclear explosives on or below the surface, the impact by large masses at high velocities, the blowing off of material by standoff nuclear weapons or by the concentration of solar energy using giant mirrors or by zapping it with lasers, and more gentle methods such as simply attaching a propulsion rocket, a solar sail or launching surface material off at sufficient velocity to escape the asteroid.

The analyses of the different methods rely primarily on data and estimates accumulated for cratering and disruption using the material properties of terrestrial materials. In most cases those were silicate materials with mass densities of ∼3 g cm–3 or iron asteroids of density ∼8 g cm–3. However, it is becoming generally accepted that many of the asteroids are re-accumulated rubble pile bodies of very low density and strength, and comets have been thought for some time to have that structure.