What is the difference between a depleted comet and an asteroid? When a periodic comet flies by the Sun enough times, it can become depleted of its volatile elements. Once it is depleted, what distinguishes it from an asteroid, other than records of past sightings showing that it used to have a tail when it got close to the Sun?
Extinct comets are a sub-category of asteroids.
Minor solar system bodies are classed as &quot;Comets&quot; if they show a coma, or &quot;Asteroids&quot; if they don't.  A depleted comet that has never shown any coma would be categorised as an asteroid.
It is hard to determine if an asteroid is an extinct comet. Some may be distinguished from other asteroids by its spectrum. F and B type Asteroids with low albedo may be extinct comets;  comets are very dark.  Asteroids with eccentric orbits at a high inclination are also often suspected of being extinct comets.
There is another meaning of &quot;depleted&quot;, it means that the comet (with a coma, and perhaps a tail) is low in one or more species of molecule, which suggests it formed in an atypical location, or has been modified by some process since its formation.  For example, C/2016 R2 is rich in CO and N2, but depleted in HCN. (Which suggests that it formed in a region of space at ~50K, so much closer to the sun than the outer Oort cloud.