Around 2168, the Horizon discovered the planet Sigma Iotia II, located on the outer reaches of the galaxy. 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 ship-to-shore 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 served 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 breakdown of Iotian society that had occurred as a result of the Horizon's influence. Because the disruption 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")
Background information Edit
According to the Star Trek Encyclopedia (1st ed., p. 64) the ship, designated as the USS Horizon, was of the Daedalus-class, and had the registries NCC-173 (1st ed., pp. 129-130) and NCC-176 (1st ed., p. 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. According to the Star Trek Encyclopedia (4th ed., vol. 1, p. 347), the authors write in the notes to the entry "Horizon, E.C.S.", "The E.C.S. Horizon was probably not the same ship that made contact with the Ionians, as established in "A Piece of the Action" (TOS), if only because it was a cargo ship and did not seem to operate on the frontier. Nevertheless, based on the information in that episode and in "Horizon", the cargo ship may well have been operating at the same time (2168) for it to have been the ship that left The Book on Sigma Iotia II. In any case, the Star Trek: Enterprise art department made a replica of The Book (actually two replicas, one of which got the title wrong!), and placed them onto book shelves in Mayweather's 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.