A review of cometary sciences

This paper presents an elementary description of comets and their nature. It deals with the contribution from comets to the solid particles that produce the Zodiacal Light and discusses the possibility that some comets in short-period orbits may degenerate into asteroids. The evidence suggests that this may well have happened for the Trojan and other of the outer asteroids but that the near-earth asteroids and the meteorites are largely not cometary in origin. The last section of the paper deals with the possible nature of comets and their origin such that some might become superficially indistinguishable from asteroids. Space missions are clearly needed, to answer some of these basic questions.