If one wants to be honest about what happens to any object with basically no atmosphere that gets close enough to the Sun to heat up its surface, they are all comets according to our definition of comets, even if we can't see the ejected material. That intense radiation heat is going to result in the ejection of material off the surface of such objects, the only limiting factor is whether our telescopes and cameras can see the objects that don't eject very much material so we humans can call them a comet or not. An asteroid is either an almost burnt-out comet or an object waiting to become a comet depending on its orbit. If one is lucky enough to catch one of these rather non-active objects when it happens to eject a little material due to the intense solar radiation falling on the right spot on its surface at the right time and get an image of that activity, then we humans call it a comet. Everything is subjective, even determining what is or isn't a comet, based on our own limitations of observation and the timing of such observations. As always, the truth is in the eye of the beholder.
Mike Olason