Comets don't have a single, universal nickname. However, they are sometimes called:

* "Dirty snowballs": This refers to their composition of ice, dust, and rock.

* "Cosmic snowballs": Similar to the above, highlighting their icy nature.

* "Heavenly messengers": This is a more poetic term, reflecting their historical significance and connection to celestial events.

It's important to note that these are not official nicknames, but rather common ways to refer to comets. The specific name given to a comet depends on its discoverer, its characteristics, and the context in which it's being discussed.