An atmosphere is a collection of gases that surrounded a celestial body that possessed sufficient mass, and therefore sufficient gravity, to hold it. Examples included many planets, moons, and large asteroids. Almost all sentient species in the Milky Way needed some form of atmosphere to breathe. The United Federation of Planets used a system of classification for the atmospheres of celestial bodies found throughout the galaxy. This system, which by the 32nd century had been adopted by nearly all other governments in the galaxy, separates atmospheres into five distinct categories: Type 1, Type 2, Type 3, Type 4, and Type 5.
Type 1 atmosphere Edit
Type 1 atmospheres had appropriate levels of oxygen, nitrogen, and hydrogen to be safely breathed by the majority of sentient species in the galaxy. Some contaminants or other trace gases could have had detrimental effects during long-term occupation. Planets with these types of atmospheres almost always had some form of indigenous life. Plant life or some analog that frees oxygen was also extremely common on planets with a Type I atmosphere. Most Class-M planets had Type 1 atmospheres. Earth, the human homeworld, is a classic example of a planet with a Type 1 atmosphere.
Type 2 atmosphere Edit
Type 2 atmospheres usually had appropriate levels of atmospheric oxygen and nitrogen. Due to lack of sufficient pressure or the presence of contaminants and/or of other trace gases, however, these atmospheres usually caused detrimental effects to humans and most other species over a shorter period of time. Many species, however, could tolerate Type 2 atmospheres without any trouble depending on their physiology. Planets with a Type 2 atmosphere often supported some form of indigenous life. Balaho, the homeworld of the Unggoy, is an example of a planet with a Type 2 atmosphere.
Type 3 atmosphere Edit
Within Type 3 atmospheres, due to either the lack of appropriate levels of atmospheric oxygen and nitrogen, sufficient pressure, and the presence of contaminants and/or of trace gases, a breath mask had to be worn in order to function properly. For humans and most other species, these atmospheres caused immediate or eventual impediments or even incapacitation or death. Some species could breathe these types of atmospheres depending on their physiology. Planets with a Type 3 atmosphere sometimes supported native life. Irune, the homeworld of the Volus, is an example of a planet with a Type 3 atmosphere.
Type 4 atmosphere Edit
Environment suit required
Type 4 atmospheres were often extremely toxic, flammable, or subject to extremely high pressure; these types of atmospheres caused immediate incapacitation or death to most species of the galaxy. Few species could survive unaided in this type of atmosphere; planets with a Type 4 atmosphere rarely support life. Venus, in the Sol system, is an example of a planet with a hazardous Type 4 atmosphere.
Type 5 atmosphere Edit
Type 5 atmospheres were either extremely thin or nonexistent. Planets and other bodies with Type 5 atmospheres provided very little protection from harmful cosmic radiation. Luna, Earth's moon, was an example of a celestial body which possessed a Type 5 atmosphere. The vacuum of space was also considered the equivalent of a Type 5 atmosphere.
Other uses Edit
The word "atmosphere" is also used to represent the capacity of a starship to hold a determined atmosphere in its decks. A broken hull could produce a sudden loss of atmosphere (called "decompression").