Comets are weird and unpredictable. We know the orbits of hundreds of comets, but the problem is that every year, a dozen new comets enter the inner solar system thanks to events that occur way out in the Kuiper Belt beyond the orbit of Saturn. We cannot see bodies out there that are smaller than a few hundred miles across. This means that we can't even see most of the bodies that can damage Earth until they are a few years away from actually colliding with Earth. It can take weeks to get a good orbit determination. In the next 50 years, the known comets will be our friends. It is the hundreds of currently unknown ones discovered by amateur astronomers during this same time that are the most frightening. One of the most spectacular finds by amateurs has been the NASA/ESA Solar and Heliospheric Observatory satellite data. Over 500 previously unknown comets have been discovered in these images. Although there are no known bodies on a direct collision course, as of March 12, 2003, the International Astronomical Union's Minor Planets Center currently has a list of over 498 Potentially Hazardous Asteroids with closest approaches less than 4.6 million miles. Most of these have been discovered since 1998 when careful surveys for them were started by NASA. They can be as small as a few dozen yards, or as large as a few miles across. With a little bad luck, any one of these could get gravitationally tweaked by Jupiter so that their orbits intersect Earth. If you want to keep up with this on-going survey, visit the NASA/JPL NEO pages on the web (http://neo.jpl.nasa.gov/neo.html)
