Tei Tenga is a fictional moon or planet where Doom was originally supposed to take place during its first and third episodes, according to designer Tom Hall's original design document, the Doom Bible.
Characteristics of the Tei Tenga System
The celestial body is tidally locked and thus the same hemisphere always faces the star it orbits; therefore it does not have day and night cycles. Because of the synchronous rotation, it is divided into two sections, a continually bright hemisphere called the Lightside, and a complementary sunless hemisphere called the Darkside. The anomalies that are being studied are located at Tei Tenga's magnetic poles. There are two main bases on Tei Tenga; the primary military research base is on the Lightside, the secondary base on the Darkside. The secondary base is also the main location of the game's protagonist.
The computer screens in the game, as well as the sky textures, indicate Tei Tenga has a brownish red hue. From this we can easily deduce Tei Tenga has a soil rich in oxides and iron.
Further, the sky textures, the soil oxidation and the lack of any discernible breathing apparatus indicates Tei Tenga has an oxygen rich atmosphere. The presence of magnetic poles also indicate Tei Tenga has an active magnetosphere, which in turn is evidence of a molten, liquid core, consistent with the presence of an atmosphere. Magnetospheres create a protective sheath against solar winds, which tend to ionize and thus deplete any present atmosphere, as is the case with Mars--which has neither magnetosphere nor sufficient atmosphere.
Moon or Planet?
Although it is initially referred to as the giant moon Tei Tenga, it is called a planet in a few other instances. If the celestial body is a planet, it can itself have no moons of substantial size, as these would make tidal lock with the sun an impossibility. The earliest data files with any reference to the celestial body--simply stylized graphics giving the position of the anomalies--names the pictures MOONx0, where x is a letter from A to H.
This would, however, be inconsistent with gravitational physics. Moons can only be tidally locked with their planet, not the central star of the system.
Tei Tenga as Scenario
The bases on Tei Tenga used to be so called 'glory posts', but at the time of the game the bases have fallen into disrepair due to lack of funds, partly due to UAC cost cutting and the lack of research progress. UAC is a military contractor and this explains the military presence at the bases.
According to Tom Hall's Doom Bible, the mining operations on Tei Tenga focused on what is only referred to as "Fire Dust", a volatile combustible mined from the celestial body's crust. No further details are given about the substance, and its chemical composition is unknown. The anomalies are located where Fire Dust was first found and excavated. The bases that monitor the two anomalies on Tei Tenga's poles were initially set up by the military -- initially called UAAF (United Aerospace Armed Forces) -- to research the substance; this research, and the money associated with it, is what had caused the bases to experience their 'glamour days'. At the time of the game, the researchers were focusing their efforts on scrutinizing the polar anomalies.
During the development of Doom the scenario was not Mars, as is the case with retail Doom. Therefore certain maps were known by different names; Phobos Anomaly, for instance, was known as simply Anomaly.
Meaning of Name
The meaning may come from Mandarin Chinese. In Spanish "Te tengo" means "I've got you".
Tei Tenga does appear in the finished Doom games, although somewhat obscurely, being named in some of the computer console display textures. Additionally, some fan-made PWADs expand on the Tei Tenga concept.
The planet makes other appearances as well. In another 3D game--Terminal Velocity by 3D Realms--the planet is encountered as an episode setting. The second episode of the game starts on Tei Tenga. According to the documentation the planet is the second orbiting body around the Ross 154 star; there are no references to any other low-mass celestial bodies in the system. The planet is described as "a desert planet, littered with the fossils of long extinct giant animals". As Doom designer Tom Hall co-produced Terminal Velocity it is safe to assume the thinly veiled and very apparent similarities between the two celestial bodies are more than an accident. It is argued they are, in fact, the same planet but captured at different points in time.
The actual Ross 154 system is located 9.6 light years from Earth, in the Sagittarius constellation.