Dormant Comets

Don Quixote was discovered in 1983 and although it has a perfectly comet-like orbit, no cometary activity (a tail or a coma around the object) have been observed. According to protocol, the object was listed as an asteroid. In 2009, we found from observations with the now defunct Spitzer Space Telescope that there actually is a faint coma around Don Quixote that most likely consists of molecular band emission from either CO or CO2. This is discussed in detail here and in this paper. ...

One long-standing question is: how many asteroids - especially near-Earth asteroids (NEAs) - were comets in the past? In the classical conception, comets have surface ices and form tails and comae in the vicinity of the Sun; asteroids are simply rocks in space. In the recent years, the two classifications became less clear than that. Comets can end up on orbits that bring them close to the Earth. We can observe them. But what happens to them after spending a while there? For some comets, like ISON, being close to the Sun is fatal; they simply disintegrate and disappear. For other comets in near-Earth space, the strong degree of insolation forces them to lose their surface ices quickly and eventually their activity stops - they are dead or dormant comets. Since comet surfaces have very low albedos, these dead comets simply look like low-albedo asteroids and are really hard to identify. ...