NASA hosted a conference to address the question of detecting objects that come near the Earth. They concluded that a network of six two- or three-meter telescopes would detect 90% of the asteroids that pose the threat of global catastrophe (those half-a-mile across and larger) and 35% of the comets (discovered at least three months before a potential impact) over the course of a 25-year search. Astronomers expect that such a program, called the Spaceguard Survey, would not find any object heading straight for Earth, but might find objects which, sometime within a hundred years or so, could pose a threat. The Spaceguard Survey would cost $50 million for the telescopes, and another $10 million a year to operate, a 4-cent investment for each American in the early warning system. Since asteroid collisions are an international problem, international cooperation in such a survey could reduce the cost to each country.

What if we do detect an object headed straight for Earth? Another conference held by NASA dealt with interception of threatening objects. The farther away an object is intercepted, the easier it is to deflect or destroy (a closer one has to be given a bigger push to get it to miss the Earth). So far, the only source of energy powerful enough to deflect an asteroid like the one that struck the Earth 65 million years ago is nuclear. A nuclear blast on an asteroid's surface would blow off some of the its material. The motion of the material away from the surface acts almost like a ``kick,'' pushing the asteroid in the direction opposite to the moving material. Comets, on the other hand, could fragment into many dangerous pieces if a bomb exploded on their surfaces. But a nuclear-tipped rocket exploded nearby would melt some of the comet's frozen gases turning them into ``jets'' that act like rocket exhaust, nudging the comet away from its deadly path. No doubt, entirely new technologies will exist hundreds or thousands of years in the future when we will need to deflect an incoming cosmic rocket.