The Earth orbits the Sun in a sort of cosmic shooting gallery, subject to impacts from comets and asteroids. It is only fairly recently that we have come to appreciate that these impacts by asteroids and comets (often called Near Earth Objects, or NEOs) pose a significant hazard to life and property. Most of the hazard is from asteroid impacts; comets make only a minor contribution. Although the annual probability of the Earth being struck by a large asteroid or comet is extremely small, the consequences of such a collision are so catastrophic that it is prudent to assess the nature of the threat and prepare to deal with it.

Studies have shown that the risk from cosmic impact increases with the size of the projectile. The greatest risk is associated with objects large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. Such an event could depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Such global catastrophes are qualitatively different from other more common hazards that we face (excepting nuclear war), because of their potential effect on the entire planet and its population. Various studies have suggested that the minimum mass impacting body to produce such global consequences is several tens of billions of tons, resulting in a groundburst explosion with energy in the vicinity of a million megatons of TNT. The corresponding threshold diameter for an asteroid is between 1 and 2 km. Smaller objects (down to tens of meters diameter) can cause severe local damage but pose no global threat.

Today we are addressing this impact hazard by carrying out a comprehensive telescopic search for potentially hazardous near-Earth asteroids (NEAs). This program is called the Spaceguard Survey, and it has already resulted in the discovery of more than half the NEAs larger than 1 km diameter. The objective is to find 90% of the NEAs this large by the end of 2008. Today no known asteroid is on a collision course with the Earth, and the Spaceguard Survey does not expect to find any large asteroid that directly threatens us. If, however, such a rock is discovered on a collision course, then we anticipate that we would apply appropriate technology to deflect it before it hits. Asteroid impacts are the only natural hazard that we can, in principle, eliminate entirely.

David Morrison, September 2004