Where have all the comets gone?

Comets are the fossils of the solar system. From their chemical composition to the path they cut across the sky, they convey a wealth of information about how the planets first formed. Events such as the demise of Comet Shoemaker-Levy 9 offer clues about whether a similar collision is in Earth's future. But because sightings of these objects are so rare, astronomers must use computer simulations to fill the knowledge gap. Planetary scientist Hal Levison and his colleagues at the Southwest Research Institute (SWRI) in San Antonio, Texas, recently drew on two computer models to solve the mystery of why we see so few comets.