Icarus IV was a comet which passed close to the Romulan Neutral Zone in 2266. This comet, with a magnitude of seven, was an ionized mass that left a trail of frozen vapor particles. Information on this comet was recorded in the Table of Comets.
In that year, the commander of an experimental Romulan Bird-of-Prey ordered his cloaked ship into the tail of the comet, in an attempt to elude the Federation Starship Enterprise, which had been pursuing them. Unfortunately for the Romulans, James Kirk, the Enterprise's commanding officer, anticipated this move, and used the Bird-of-Prey's disturbance of the comet's tail to calculate the Romulan vessel's position, and mount an attack. (TOS: "Balance of Terror")