He has a Ph.D. in Planetary Sciences from the University of Colorado and worked on the Voyager project as a graduate student. His research focuses on the origin and evolution of the planets, and in particular on the small denizens of the solar system: asteroids, moons, comets, planetary rings, and dust. He has flown experiments on the space shuttle to understand collisions in the solar system and has flown more than 280 zero-gravity parabolas to perform experiments on planet formation. His study of collisions in the solar system led him to a position as science advisor on the film "Deep Impact" in which he has a (very) small part. He has been working on the Cassini-Huygens mission since 1991 and is a Co-Investigator on the Ultraviolet Imaging Spectrograph on Cassini with a responsibility for observations of Saturn's rings with that instrument.