There are many variables that go into gauging asteroid risk, including the object's current position, movement, mass, and proximity to other objects in the solar system. The biggest challenge for researchers is calculating possible orbital perturbations that can, down the road, set a Near Earth Object (NEO) on a new and potentially dangerous course.

A series of such gravitational tugs -- from Earth, Jupiter or some other body -- can cause an asteroid to make a close pass in, say 20 years, then be sucked into a different orbit and make an even closer pass many decades later.

All these calculations result in impact probabilities that are typically very small, on the order of one or less chance in a hundred million. Nonetheless, these teensy odds of doom have made big headlines.

As recently as April 1999, asteroid 1999 AN10 found its way into major American newspapers after researchers in Italy calculated that it had a 1-in-a-billion chance of bothering us in 2039. Follow-up observations eliminated the threat.

There are some 800 known Near-Earth Asteroids, most of which pose no danger (comets are tallied separately). Of the 800, nearly 200 have been identified as Potentially Hazardous Asteroids (PHAs), which might come within 5 million miles of Earth in the future. Most of these pose no real threat, but a few have the potential of one day being perturbed into a dangerous orbit.