Atmosphere

An **atmosphere** is a collection of gases that surround a celestial body that possesses sufficient mass, and therefore sufficient gravity, to hold it. Examples include many planets, moons, and large asteroids. Almost all sentient species in the Milky Way need some form of atmosphere to breathe. The United Federation of Planets uses a system of classification for the atmospheres of celestial bodies found throughout the galaxy. This system separates atmospheres into five distinct categories: Type 1, Type 2, Type 3, Type 4, and Type 5.

## Type 1 atmosphere

**Breathable**

Example: Earth

Type 1 atmospheres have 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 can have detrimental effects during long-term occupation. Planets with these types of atmospheres almost always have some form of indigenous life. Plant life or some analog that frees oxygen are also extremely common on planets with a Type 1 atmosphere. Most Class-M planets have Type 1 atmospheres. Earth, the human homeworld, is a classic example of a planet with a Type 1 atmosphere.

## Type 2 atmosphere

**Respirator suggested**

Example: Balaho

Type 2 atmospheres usually have 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 cause detrimental effects to humans and most other species over a shorter period of time. Many species, however, can tolerate Type 2 atmospheres without any trouble depending on their physiology. Planets with a Type 2 atmosphere often support 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

**Respirator required**

Example: Irune

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 has to be worn by most species in order to function properly. For humans and most other species, these atmospheres cause immediate or eventual impediments or even incapacitation or death. Some species can breathe these types of atmospheres depending on their physiology. Planets with a Type 3 atmosphere sometimes support native life. Irune, the homeworld of the Volus, is an example of a planet with a Type 3 atmosphere.

## Type 4 atmosphere

**Environment suit required**

Example: Venus

Type 4 atmospheres are often extremely toxic, flammable, or subject to extremely high pressure; these types of atmospheres cause immediate incapacitation or death to most species of the galaxy. Few species can 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

**Nonexistent**

Example: Luna, or space

Type 5 atmospheres are either extremely thin or nonexistent. Planets and other bodies with Type 5 atmospheres provide very little protection from harmful cosmic radiation. Luna, Earth's moon, is an example of a celestial body which possess a Type 5 atmosphere. The vacuum of space is also considered the equivalent of a Type 5 atmosphere.

## Other uses

The word "atmosphere" is also used to represent the capacity of a starship to hold a determined atmosphere in its decks. A broken hull can produce a sudden loss of atmosphere (called "decompression").