The traditional difference between comets and asteroids based
on the type of orbits (e.g. the Tisserand parameter) and/or on
whether they show or not gaseous and/or dust activity has become
increasingly blurry with the unexpected discovery of activity on
some typical main-belt asteroids, the so-called main-belt comets
(Hsieh and Jewitt, 2006), and the discovery of objects on typical
cometary orbits that do not show any activity at all.