There has been a time when asteroids and comets were regarded as quite different objects:
"At most meetings of professional astronomers during the 20th century, asteroids were discussed by one group and comets by a generally different group...it has taken decades to realize that these objects might share similar materials, or to allow the possibility of a smooth transaction from one group to the other, based on ice content" (W.K.Hartmann, 1999, chap.23)
The picture we have today is that of "planetesimals" whose composition difference reflects the temperature of the Solar System area where they formed. Comets, accreted beyond the Snow-Line, are ice-dominated object while asteroids are expected to be relatively ice-deficient.
So, traditionally, an object is defined as a comet when it shows a resolved coma at some point in its orbit:
"Deciding whether an object is an asteroid or a comet thus depends critically on the instrumental resolution and sensitivity to low surface brightness coma" (D.C. Jewitt, 2004)
Moreover as we know, comets pass through inactive phases giving rise to the situation where it's not possible to decide the nature of the object by the means of observations. Another clue of cometary nature of an object can arrive from its dynamical path. The Tisserand “invariant” calculated in respect to Jupiter (Tj) is often used as dynamical discriminator to distinguish the nature of Solar System objects. Comets-like objects have Tj < 3.
For all these reasons is not always easy to decide what classification to assign to some objects.
2005 WY3 is such an object. Discovered by Spacewatch Telescope survey, this asteroid is clearly moving on a typical cometary orbit (a= 6.74 AU, e= 0.74,P= 17.5 year, H= 13.4, Tj 2.116) but it never showed cometary activity.
We shoot 2005_WY3 few hours ago remotely from New Mexico (details on image; click on it for a bigger version):
In our stacking this peculiar object has attained a good S/N (nearly 20) and shows no hints of cometary activity: its FWHM is identical to that of nearby field stars, with no traces of coma and/or tail. Either its cometary activity is currently well below the sensibility of our instrument, or there isn't any at all.
Ernesto Guido & Giovanni Sostero
Hartmann, W.K. (1999). Small World: Patterns and Relantionships in "The New Solar System"
Jewitt, D.C. (2004) From Cradle to Grave: The Rise and Demise of the Comets in "Comets II"