is none of it.

Amateur astromers armed with little more than good binoculars or home telescopes and an exceptional knowledge of the sky routinely find comets when they're past the orbit of Jupiter. In this age of the Internet and global communications, there simply is no way that a comet would pass between the Earth and moon tomorrow without the whole world knowing about it.

There have been small asteroids, called Near Earth Objects (NEOs) that have passed that close and even closer. My own interest in the topic came from just such an incident back in the summer of 1983. I read about it in the paper and was intrigued. There was an even closer call back in 1970. An object, believed to be somewhere between 80 feet and 80 meters in diameter, actually skimmed through the Earth's upper atmosphere without actually entering it. I've seen home movies of the event. Had the object hit the ground, estimates put it energy released as somewhere in the range of 1-10 megatons (1-10 million tons of TNT), the same as very big nuclear weapons. A blast that large would be more than enough to destroy a major city.

The Earth has been hit by stuff in the past and, without a doubt, will be hit again in the future. There are some efforts underway to catalog the objects at least a kilometer in diameter that might pose a potential danger to the Earth. Several automated telescopes and tracking systems are aleady working on this effort, and organizations like the Minor Planet Center at Harvard are keeping track of the findings.