Atmosphere

An **atmosphere** is a layer of gases surrounding a celestial body. They can slow down or destroy a rocket passing through them. The act of entering the atmosphere of a celestial body is called atmospheric entry.

## Characteristics

Atmospheres have their characteristics. They can be edited by the planet editor.

### Height

The **height** of the atmosphere tells how high it is. The celestial body that has the smallest atmosphere is Europa; the highest one is the sun. Earth's atmosphere is 30 kilometers tall.

### Atmospheric pressure

The **atmospheric pressure** is how much force is exerted on an object in the air. Earth's atmospheric pressure is 101,325 pascals or 0.005 in the planet editor. It is known as `density` in the files.

### Upper atmosphere

The **upper atmosphere** is the layer where the atmosphere starts to fade out and feels like a vacuum. Earth's upper atmosphere starts at around 10 kilometers; this is also the height at which Earth's atmosphere fades from bluish to pitch black.

### Curve

The **curve** is a coefficient on how fast the atmosphere fades when going up. The higher the number, the faster the atmosphere fades; lower values indicate the atmosphere is fading slower. Earth's *curve* is 10.

### Scale height

The **scale height** is the height where the atmosphere fades by a factor of *e,* or 2.71828. Earth's scale height is roughly 3200 meters.

## Atmospheres of celestial bodies

As of now, there are five celestial bodies with an atmosphere. They are the Sun, Venus, Earth, Mars, Jupiter, and Europa.

### Sun

The sun has an atmosphere. The layers of the sun are the photosphere, the chromosphere, and the corona. It's even the part where sunspots, coronal holes, and solar flares occur. The sun's upper atmosphere is the corona. In-game, the sun's corona extends up to 800,000 km. At around 300 km, your spacecraft will be destroyed instantly; this is called the *crush depth*. The sun's surface is 5778 kelvin; however, the sun's atmosphere is hotter than the surface; the photosphere is 5,800 kelvin and the corona is 5780 kelvin.

### Venus

Venus's atmosphere is thick and toxic. Venus's atmosphere is so thick that the planet's surface cannot be seen because of its opaque clouds. The temperature is a total of 740 kelvin, and the pressure is 93 bars or 1350 psi. The atmosphere is made out of carbon dioxide and nitrogen, and the clouds are made out of sulfuric acid. The pressure on Venus's surface is so high that it becomes a supercritical fluid. In-game, Venus' atmosphere has a height of 40 km, but in reality, it has a height of 250 km. Parachutes are usually deployed at 100 meters above the surface, and the atmospheric pressure is 0.025 in SFS. Venus's atmosphere, when seen on the Venusian surface, is yellowish. In-game, you can easily enter the atmosphere and land, but launching on the planet will be hard because of the density, like pushing harder in a non-Newtonian fluid. Also, Venus, when zoomed out, will blur the planet, which is a unique feature in the game. Venus's atmosphere is some kind of light brown, but when on the planet's surface, it's yellow. The clouds of Venus are thick, and they extend up to several kilometers above its surface.

### Earth

We all know the Earth's atmosphere. Earth's atmosphere has five layers: the troposphere, stratosphere (where the ozone layer is located), mesosphere, thermosphere (where auroras appear), and exosphere. The Kármán Line is where the boundary of space begins. Earth's atmosphere is light blue. Earth's atmosphere is composed of nitrogen, oxygen, argon, carbon dioxide, and others. Because of human activity, the Earth gets hotter because of the greenhouse effect, which includes burning fossil fuels and emitting more carbon dioxide. Mostly, the Earth's atmosphere controls the weather and where clouds appear. The clouds have a composition of water droplets and ice crystals, while in the thermosphere at around 100 km, it is still part of the atmosphere, and where the International Space Station (ISS) orbits, it has an atmosphere up to 10,000 km. In-game, the height is 30 km, and reentry starts at 25 km. The Earth's density is 0.005 in SFS. Parachutes are deployed at most at 2500 m and are fully deployed at 250 m. Earth's clouds are almost transparent, making it look like a mostly clear day on Earth.

### Mars

Mars's atmosphere is very thin. It has an atmospheric pressure that is around 0.00636 times as thick as Earth's atmosphere. It is mainly made of carbon dioxide and nitrogen. In the game, Mars's atmosphere extends 20 kilometers from the surface, and its atmospheric density is 6¼ times less than Earth's. Parachutes can be deployed as high as 4.5 kilometers above the surface. Unlike Earth, Mars usually doesn't sufficiently slow down a rocket with a single parachute; multiple or bigger parachutes must be used. Mars's atmosphere is reddish or yellowish depending on the environment. Mars has no clouds, unlike in real life.

### Jupiter

Jupiter has a thick atmosphere that reaches up to 5,000 kilometers above its surface. Its atmosphere is made up mostly of hydrogen gas and helium gas, just like the sun's. In the game, the atmosphere is 50 times smaller, making it extend 100 kilometers above the surface. The atmosphere can be seen up to 240 kilometers above Jupiter's surface. Jupiter's atmosphere has an orange hue, with blue in the upper layers of the atmosphere. Trying to enter Jupiter's atmosphere can result in the rocket burning up too quickly, with or without a heat shield.

#### Europa

Europa has a very tenuous atmosphere that extends up to fifteen kilometers above its surface. It is also the only moon that has an atmosphere. In-game, you cannot land on the surface of Europa with parachutes because of the glow from the atmosphere that can trick anyone. The atmosphere's texture, `Atmo_Europa`, can even be used for a nighttime mode. In Europa's code, the `ATMOSPHERE_PHYSICS_DATA` is not present, and thus only the atmospheric glow is present.

## Trivia

* The celestial bodies in-game—Mercury, the Moon, Ganymede, Io, and Callisto—even have atmospheres, but they're extremely tenuous and nearly turning into a vacuum.
* A celestial body must have enough gravity to hold an atmosphere.
* Atmospheres usually have five layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.
* A celestial body can form craters on its surface due to the lack of an atmosphere.
* Atmospheres can shape the terrain of the planet because of winds.