The Antares Class(es)
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 and model builders are obviously just too fond of the name.
Antares as a Ship (Class) Name
1. The name Antares for a starship was first used in the TOS episode "Charlie X", but screen evidence of the vessel is missing and the registry remained unknown for the next 25 years. Most likely the NCC-501 given in the Star Trek Encyclopedia was never supposed to be the Antares's registry in the old TOS script and was just made up by Michael 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' (TNG) 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.
The previously unseen Antares was finally visible in the remastered version of the TOS episode of 2007. The CGI design is a modification of the cargo drone from TAS: "More Tribbles, More Troubles".
2. Antares-class ships are frequently used by civilian (human and alien) owners since TNG. 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:
Talarian ship Batris (TNG: "Heart of Glory") - a modular vessel carrying cargo containers - named "Antares class" in the Star Trek Encyclopedias I-IIIbut not in the episode itself
Kasidy Yates's ship Xhosa (DS9: "Way of the Warrior", "For the Cause") - a similar, yet different design - named "Antares Class Cruiser" on the ship's dedication plaque
Corvallen freighter (TNG: "Face of the Enemy") - an equally boxy, but entirely different design - called "Antares class" in the episode
Bajoran Antares (TNG: "Ensign Ro") - a ship with a triangular basic shape whose warp drive is defunct - 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 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. And at latest since TOS-R (provided we accept the new visual evidence) this idea has been refuted.
The Star Trek Fact Files just reproduce the inaccuracies from the Encyclopedia. In addition, the Fact Files explicitly call the Erstwhile (see below) and even the huge ancient sublight freighter "Antares Class", in a separate file dedicated to this ship type.
3. Starfleet also has an Antares class as of 2368, according to LCARS displays from TNG. The USS Hermes NCC-10376 is the only known ship of this 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 important to note that there is neither a connection 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.
4. A ship model of the "Frankenstein Fleet" built for DS9 by Adam Buckner is labeled "USS Antares NCC-9844". It is a modification of the Miranda class and appears on screen in DS9: "Favor the Bold". This could be easily the class ship of Starfleet's 24th century Antares class.
5. 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 ostensibly 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.
Side note Pathways obviously continues to be ignored. An article in Star Trek: The Magazine tells us of a model constructed of Chakotay's Maquis ship, as well as several displays of the ship where it is called a Ju'day-class ship. The name comes from Penny Juday, wife of Anthony Fredrickson, who built the Maquis model for "Repression". And on the personnel files of the ship from the same episode we can even read its name "Val Jean", although it is called "Liberty" in the novel. The Star Trek Fact Files, on the other hand, call the ship "Zola", presumably taken from an early draft of "Parallax".
5. Just for the sake of completeness, a USS Antares is also mentioned in "Star Trek (2009)", which has no bearing on the above Prime Universe ships.
Alien Ship Design Families
A "class" is a family of ships with the same design principles. Going by this definition, it is hard to accept that various freighters with the same class name, namely Antares class, are actually different designs. On the other hand, the same models have been used for ships of considerably different sizes, purposes and ages, and in different corners of the galaxy, so even some ships with identical appearance can't be the same class.
The Batris family
The Batris is explicitly identified as "Antares class" in the Star Trek Encyclopedias. The studio miniature of the freighter has been modified and re-used several times. No uniform size can be determined, but 250-300m seems to be a good estimate for most of its incarnations. The ships, in the order of their appearance, are analyzed in the article Redresses of the Batris.
Considering that the bow section of the Norkova, the Xhosa and their derivates looks considerably different than the pointed bow of the original Batris, they may easily represent a completely different class. Depending on the extent of modifications, it is obvious to suggest that some of them could be of the same class - the Antares class.
Since the Sanction, the Erstwhile and the Arcos (as can be confirmed in TNG-R: "Legacy") are the same design, and the Arcos belongs to the Deneva class of Starfleet, we would not refer to these ships as Antares class.
Alas, the only two ships looking much like the original Batris (none is exactly the same) are the ones which logically can't be of the same class. The sublight freighter is said to be at least 300 years old and of unknown origin. Moreover, it is supposed to be much larger than the Enterprise-D, as opposed to the Batris which is most likely a ship in the usual 250m range of most freighters shown next to the Enterprise. The Akritirian ship, on the other hand, was encountered in the Delta Quadrant, and it would be hard to explain why two ships of inferior designs appear some 70,000 light years apart. One might conceive ideas of literally universal ship designs in various sizes that were handed down by an ancient civilization to the Akritirians as well as the Talarians and other civilizations. However, this speculation goes too far, considering that none of this ships was ever supposed to play a prominent role.
The Corvallen design
This ship from TNG: "Face of the Enemy" was called "Antares class" too, but it is definitely a different model than the Batris. In fact, the Corvallen freighter is a re-use of the Straleb patrol ship from TNG: "The Outrageous Okona". It is not plausible why a freighter should employ the same basic design as a patrol ship. On the other hand, the civilizations of Straleb and Altec (the latter re-used the Merchantman from "Star Trek III") were not very advanced. Their own technology being limited, they might be lucky to purchase whatever starship they can afford. The Corvallen design might have at least the size in common with the Batris, although visual evidence from TNG: "The Outrageous Okona" points to a size well below 200m for the Straleb ship. The complete analysis of the Straleb/Corvallen miniature can be found in Redresses of the Straleb Vessel.
The Bajoran design
The triangular Bajoran ship in TNG: "Ensign Ro" was called "Antares class" too. The according model showed up several more times, with considerably different assignments each time. The model was first built as the Kivas Fajo's Jovis for TNG: "The Most Toys". The characteristic "hammerhead" as well as a couple of pods were added for the Bajoran ship. The differences are significant enough to justify that the Jovis is another class. The Numiri patrol ship from the Gamma Quadrant (VOY: "Ex Post Facto"), however, looks exactly like the Bajoran Antares class. Like the connection between the above Batris, sublight freighter and Akritirian ship, the statement that the Bajorans and the Numiri use the same ship design is hard to maintain. It is difficult to give a size for the triangular Antares variant, but it seems to be roughly 250m long. More about the history of the model in the article Redresses of the Husnock Ship.
Note Only the Fact Files depiction of the Jovis shows approximately correct proportions of the model, whereas the Numiri ship is bizarrely elongated and the Bajoran ship too short. The latter might have happened because the artist thought the two Bajoran ship types (the other one being much more compact) should have the same basic shape. Anyway, there is no excuse that the same drawing was overproportionally stretched, as if this were possible with a studio miniature.
All the above facts raise lots of questions, and it is obvious that not everything stated above can be true. There are several theories, none of which is completely satisfactory:
A: The old TOS Antares (1.), as it eventually appeared in TOS-R, is actually the prototype vessel for Starfleet's Antares class (3.). The design has probably been upgraded several times. It is similar as with the Miranda class, which is not much newer. All the various alien Antares-class ships (2.) are naturally completely different designs, also because Starfleet wouldn't share their technology, however old, with whoever requires a cargo ship, let alone with enemies like the Talarians. The individual USS Antares from DS9 (4.) is another class, essentially just a standard Miranda with some modifications of that name. Solution A would cover all evidence, provided that Starfleet does use other class designations than civilians do. Considering the different designs, "Antares class" would most likely denote a certain size of a civilian cargo ship anyway, rather than a certain type. There might be a "Sirius class" and a "Betelgeuse class" too. Still, the connection between the old USS Antares and the new Starfleet Antares class in this theory seems unnecessary. Chakotay's raider (5.) is a distinct design, as Pathways is not really canon.
B: The old USS Antares (1.) is not the class ship of the Starfleet class of the 24th century (3.). It is no class ship at all. The new Antares class might have been named in honor of the destroyed ship though. Very likely the USS Antares NCC-9844 (4.) is the class ship. The alien Antares (3.) class might be anything else, probably a generic designation like in theory A. This solution is favored by those who don't trust in Okuda's possibly haphazard numbering and classifying of ships in the Encyclopedia. After all, there was no according hint concerning the USS Hermes. Chakotay's raider (5.) is a distinct design, as Pathways is not really canon.
C (obsolete): The old USS Antares (1.) is the namesake of (one of) the civilian Antares classes (5.), as suggested in the Encyclopedia. If, however, these ships are the same class and are both Antares class, there can't be another Antares class (USS Hermes) of Starfleet (3.), unless Starfleet re-used class names. It is obvious that theory C neither provides a necessary distinction between old and new ships nor supports what was stated in Pathways. It is only possible to maintain theory C if the alien Antares is a spin-off class of Starfleet which, for some reason, was named in honor of the destroyed Starfleet ship and became a generic name for other ships of the size. With the appearance of the USS Antares in the form of a new and very different design in TOS-R this theory cannot be maintained any longer.
D: The TNG Starfleet Antares class (3.) and the contemporary alien/civilian class (2.) of this name are actually identical. The individual USS Antares from DS9 (4.) is just a standard Miranda with some modifications of that name. Although this solution avoids a naming conflict within Starfleet, the same problems as with solution C arise. It is unlikely that either Starfleet shares their technology with potential enemies or Starfleet uses totally obsolete ships in a possible battle (TNG: "Redemption"). Furthermore, for what it's worth, Pathways would be wrong in this case. Like theory C, this suggestion doesn't take into account the different designs of the civilian Antares-class ships, and why the name of a certain Starfleet class is used by Starfleet to designate a variety of civilian vessels.
E: The Batris, let alone the sublight freighter and the Akritirian ship on one hand, have nothing in common with the (upgraded) Norkova and the (old-fashioned) Xhosa (2.), on the other hand. The latter might be actually be the Starfleet Antares class (3.), an old design from the 23rd century which was (like the Norkova shows on the inside) upgraded several times. The USS Hermes would look like that too, while Chakotay's raider (5.) and the USS Antares NCC-9844 (4.) must be something else. Certain similarities of alien and Starfleet ships are accidental, or they indicate that at least the first three alien ships might be "downgraded" or "basic" versions of the Starfleet class. The reason for (informally) calling alien ships "Antares" is because they look similar to the Starfleet design and are of roughly the same size.
Summarizing, it is obvious that "Antares" as name for an alien/civilian starship must be a generic human/Federation designation. Perhaps it is a common name for all cargo ships that share a certain capacity, speed or other characteristics. Considering that Antares is one of the brightest stars in Earth's sky, there might be other generic ship classes like "Sirius" or "Betelgeuse", although rating ships by visual magnitudes as seen from Earth would be geocentric.
Overall, I have a preference for theory B. Starfleet has an Antares class, with a different design than any of the various alien freighters. It is very well possible that the class is named for the USS Antares NCC-9844, a Miranda modification just like the Soyuz class. The old TOS Antares is just a ship of that name, while Chakotay's raider is not a member of any Antares class.
Redresses of the Batris - modifications for Ornarans, sublight freighter, Norkova
Redresses of the Husnock Ship - modifications for the Jovis, Bajoran freighter and even the black smuggler ship
Redresses of the Straleb Vessel - modifications for the Angosian transport
Some screen caps from the Neutral Zone Starship Database. I am grateful to Timo Saloniemi and the members of the ASDB for invaluable discussion and to Dukhat for new information from Adam Buckner. Thanks to Jörg for new and much better screen caps and to William R. York for the suggestion that (cargo) ship classes may be rated by star magnitudes.
Ode to the Antares by M. Bartel @Star Trek Minutiae