We know the orbits of about 7000 minor bodies in the solar system, and these run to sizes as small as a few kilometers across. But their orbits can and do get perturbed over decades and centuries. Some of these which end up on orbits intersecting the Earth called 'Apollo-Amor' asteroids have been know for decades. But it is cometary bodies launched from out by the orbit of Jupiter and beyond which are troublesome. Hyakutake and IRAS-Alcock discovered in 1983 were nearly upon us before they were detected at all.
Comets are a real problem, and can do nearly as much damage as asteroids.
We have, however, known about it for several years now because it is big enough and its orbit is optimal enough to have been tracked beyond the orbit of Mars. This is serious business, and the extent to which we all live on a knife edge is very unsettling. Copyright 1997 Dr. Sten Odenwald Return to Ask the Astronomer .