While there, the crew of the Horizon made first contact with the planets inhabitants, the Iotians; a civilization that was in the beginning stages of industrialization. It was also reported that the Iotians were an extremely intelligent and somewhat imitative species. The Horizon's orbit-to-ground transport did not include transporters, but rather required planetary landing in some form.
The Horizon's contact came before the non-interference directive went into effect, and the crew inadvertently interfered with the normal evolution of the planet. The source of cultural contamination was a number of textbooks on how to make radio sets and the like. The most damning was a text published in 1992, called "Chicago Mobs of the Twenties". This text would serve as the "Bible" of the Iotian culture for the decades to come, developing a society of "cold-blooded killers".
The Horizon was only equipped with conventional radio, and as such it predated subspace communications. It was lost shortly after leaving Sigma Iotia II. The Federation finally received her radio reports a century later, in early 2268.
A month later, the USS Enterprise was dispatched to investigate the Horizon's reports, only to discover the break down of Iotian society that had occurred as a result of the Horizon's influence. Because the breakdown of Iotian society had been caused by a member of the Federation, the crew of the Enterprise was obligated to correct the Horizon's mistake. (TOS: "A Piece of the Action")
According to the Star Trek Encyclopedia (1st ed., pp. 64, 129-130, 320) the ship, designated as the USS Horizon, was of the Daedalus-class, and had the registries NCC-173 (1st ed., pp. 129) and NCC-176 (1st ed., pp. 320; studio model). Indeed, an ad hoc model, bearing the name of the USS Essex and registry of the Horizon (to represent both ships in these publications), was originally created by Greg Jein for the Star Trek Chronology, before eventually creating a second model solely dedicated to the Horizon. The writers added that "the class and registry designations of the Horizon are conjectural, but a desktop model bearing that name and number has been seen as set decoration on Star Trek: Deep Space Nine." (Star Trek Encyclopedia, 1st ed., pp. 129-130) Despite the effort of fabricating this background information for a Daedalus-class Horizon, at no time was it ever successfully tied back to the Horizon mentioned in "A Piece of the Action", as originally intended.
The production staff later made a tongue-in-cheek reference suggesting that the ECS Horizon may be connected to this Horizon, with the placement of a similarly titled book, Chicago Gangs, as a set decoration on the bookshelf Travis Mayweather's old quarters.
According to the Pocket ENT novel Kobayashi Maru, the Horizon was indeed the freighter ECS Horizon mentioned throughout Star Trek: Enterprise and seen in the episode "Horizon". The Horizon made first contact with Sigma Iotia II in the 2150s. The fate of the Horizon was unclear; by the end of the book it was deemed missing and presumed destroyed by Romulan action.