NASA would have us believe that comets are dirty snowballs.
Supposedly they melt when they get close to the Sun.
If this is the case, it should be obvious which one is the snowball and which one is the rock.
Of course, it is not obvious. It is not obvious because the dirty snowball theory is a bunch of crackpottery.
The ESA’s latest comet hunting probe has conducted a close flyby of an asteroid. One of the images above is from the Rosetta probe, which took pictures of asteroid Lutetia as it passed by. The other is from comet Temple 1, taken by the Deep Impact probe.