A comet close to the Romulan Neutral Zone through whose tail a cloaked Romulan bird-of-prey passed in 2266 while being hunted by the Enterprise.
Icarus IV appears in the original series episode Balance of Terror. As the Enterprise shadows the Romulan vessel that has destroyed several Earth outposts, Spock notes that the invisible ship's course will take it through the tail of the comet.
Kirk hopes the ionised particles will reveal the bird-of-prey's outline and plans to attack at that moment. The Romulan commander anticipates the trap and turns as he enters the tail, intending to strike the Enterprise from behind, while Kirk makes a matching manoeuvre of his own.
Both captains are left impressed by an opponent who thinks as they do, and the Enterprise is reduced to firing phasers blindly into the area, damaging the Romulan ship and beginning the long duel that follows.