Antares-class
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The Antares is an older class of unknown size and purpose.

Antares is the brightest star in the constellation of Scorpio, as observed from Earth.  There is a lot of confusion about ships and classes named for Antares, for the writers are obviously a bit too fond of the name.

The name Antares for a starship was first used in the Original Series episode "Charlie X", but screen evidence is missing and the registry remained unknown for the next 25 years.  It is possible that the NCC-501 given in the Star Trek Encyclopedia was never supposed to be the Antares's registry in the old Original Series script and was just made up by Okuda, due to popular request.  In the entry about the USS Antares the Star Trek Encyclopedia says that "writer/consultant Naren Shankar suggested that the Bajoran cargo vessels in 'Ensign Ro' (The Next Generation) be designated as Antares-class ships, a tip of the hat to 'Charlie X'".  This is merely a backstage information and has no effect on the canon Star Trek Universe.

Antares-class ships are frequently used by civilian (human and alien) owners since The Next Generation.  There is plenty of screen evidence of the ships.  The problem is that they are actually at least three considerably different designs!  The only thing they have in common is that they are usually described as obsolete and don't look like Starfleet designs at all.  The four designs verbally identified as "Antares-class" are:
1.) the Talarian ship Batris (TNG: "Heart of Glory")- a modular vessel carrying cargo containers - named "Antares-class" in the Star Trek Encyclopedias I-III.
2.) Kasidy Yates's ship Xhosa (DS9: "Way of the Warrior", "For the Cause")- a similar, yet different design - named "Antares-class" on the ship's dedication plaque, according to Mike Okuda.
3.) The Corvallen freighter (TNG: "Face of the Enemy")- an equally boxy, but entirely different design - called "Antares-class" in the episode.
4.) The Bajoran Antares whose warp drive is defunct (TNG: "Ensign Ro")- a ship with a triangular basic shape - called "Antares-class" in the episode.

The entry about the "Antares-class carrier" in the Encyclopedia doesn't distinguish between these designs at all and suggests that all the ships should look like the Batris.  Moreover, the annotations to the entry speculate: "We further wonder if this might be what the ship in 'Charlie X' (TOS) looked like."  This would make the USS Antares NCC-501 the class ship of the Talarian ship type, although it doesn't sound imperative.

The Star Trek Fact Files just reproduced the errors arleady made in the Encyclopedia.  In addition, the Fact Files explicitly call the Erstwhile and even the huge ancient sublight freighter "Antares-class" in a separate file dedicated to this ship type.

Starfleet also has an Antares-class as of 2368.  The USS Hermes NCC-10376 is the only known ship of its class, and it could not be identified on screen, but was only listed with its name and registry as participating in the generation of the tachyon grid (TNG: "Redemption").  It is interesting that there is neither a reference to the USS Antares NCC-501 nor to the Antares-class cargo ships.  Nevertheless, the entirety of Encyclopedia entries reads as if all the ship types are supposed to be the same.

Chakotay's ship (the Maquis raider) is classified as Antares-class in Jeri Taylor's novel Pathways.  This doesn't comply with any of the above Encyclopedia entries at all, and it hasn't found its way into the Encyclopedia or Fact Files so far, although Jeri Taylor's books are allegedly canonical considering that she was the co-producer of Star Trek: Voyager.  One big problem is that Chakotay's raider doesn't look like a TOS ship.  Actually, with its hundreds of obviously improvised hull extensions it doesn't look like a Starfleet ship at all.

It is hard to accept that ships which are supposed to be the same, namely Antares-class, look actually different.  On the other hand, the same models have been used for ships of considerably different sizes, purposes and ages which can't be the same.  Explaining this mess is virtually impossible.

The Batirs is explicitly identified as "Antares-class" in teh Star Trek Encyclopedias.  The studio miniature of the freighter has been modified and re-used several times.  No uniform size can be determined, but 200-280m seems to be a good estimate for most of its incarnations.  The ship are (in order of appearance):
1.) THe Batris - the original design with a pointed bow section (The Next Generation)
2.) Ornaran - a modification of the  Batris with several additional parts (The Next Generation)
3.) Okona's ship Erstwhile - visually identical to the Ornaran freighter (The Next Generation)
4.) The ancient sublight freighter - close to the original Batris version, additional parts removed (The Next Generation)
5.) The Norkova - an extensive modification, if not an entirely new model, with a blunt bow but apparently the same tail section as the Batris (Deep Space Nine)
6.) A ship carrying Skrreean emigrants - probably the same model as the Norkova, but flying upside down (Deep Space Nine)
7.) Kasidy Yates's ship Xhosa - a modification of the Norkova with a blunt bow, without some of the thruster pods, without the big cargo containers, and with the tail section flipped uspide down (Deep Space Nine)
8.) an unnamed ship - hardly visible, but essentially the same as the Xhosa (Deep Space Nine_
9.) an Akritirian freighter - obviously the same model as the sublight freighter (Voyager)