Comets are weird and unpredictable.
Of course we know the orbits of the known comets discovered every year, 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 Jupiter and Saturn. We cannot see bodies way out there than are smaller than a few hundred kilometers, which means that most of the bodies that can damage the Earth are not detectable until they are a few months out from the inner solar system. It can takes weeks to get a good orbit determination. Dr. Sten Odenwald Return to Ask the Astronomer .