Orbit

An **orbit** is a continuous movement around a celestial body, where it movement is fast enough to where the object does not fall back. There are two points in an orbit — apoapsis and periapsis. Apoapsis is the highest point in an orbit and periapsis is the lowest point in an orbit.

To achieve an orbit around the earth, launch the rocket and burn the engines until you see the achivement *Reached low Earth orbit*. Click this link on a tutorial on how to get to orbit.

In-game, an orbit is stable as long as it does not intersect the atmosphere or cross a celestial body's orbit. If it intersects the atmosphere, the orbit will decay until it reaches the ground. When it crosses another celestial body's orbit (such as the moon), the gravity of that celestial body will change the orbit until it escapes the celestial body's sphere of influence or it enters the atmosphere/crashes into the planet.

To change the orbit, fire the engines until you get to an optimal orbit. Burning prograde makes the opposite side of the orbit higher, and burning retrograde makes it lower.

## Types of orbits

There are many types of orbits. Here's a list of them:

### Suborbital Trajectory

A suborbital trajectory is an "orbit" that intersects the atmosphere of a planet, with its apoapsis reaching space. It can also refer to a collision course with a planet.

### Low orbit

A low orbit is an orbit close to a planet. For Earth, both apsides must be between 30 and 202.5 kilometers high. Here the player can put rockets into a parking orbit for waiting for interplanetary transfers on space missions. They can also put space stations and other artificial satellites.

### Transfer orbit

A transfer orbit (or medium orbit) is an orbit at a medium distance from a planet. For Earth, both apsides must between 202.5 to 547.5 kilometers high. Medium orbits can be used for similar purposes as low orbits, although they may be less useful for space stations due to the higher delta-v required.

### High orbit

A high orbit is an orbit at a large distance from a planet. On Earth, both apsides are between 547.5 kilometers high and the boundary of Earth's sphere of influence. If the orbit is higher than 8000 kilometers, the satellite could experience gravitational perturbations from the Moon.

## Orbital properties

### Apsides

The points in an orbit where the altitude is the highest (apoapsis) and the lowest (periapsis).

### Eccentricity

This value tells how non-circular the orbit is. 0 is circular, between 0 and 1 is elliptical, 1 is parabolic, above 1 is hyperbolic.

### Velocity

How fast an object is currently traveling relative to the planet's rotation.

## Orbital maneuvers

### Hohmann transfer

A Hohmann transfer is a way of moving from one orbit to another orbit, using two rocket burns. For example, to raise an orbit from 100 km to 600 km, burn prograde until the apoapsis is at 600 km, wait a half-orbit, and make a prograde burn (at the apoapsis) to raise the periapsis to 600 km. Something similar can be done to lower the orbit; burn retrograde to lower one side, wait a half-orbit, and burn retrograde again to lower the first side.

### Deep space maneuvers

*Main article: Deep Space Maneuvers*

Deep space maneuvers (or DSMs) are maneuvers that are done far away from Earth or other bodies, so as to correct the trajectory of a rocket towards the correct destination.

### Bi-elliptic transfer

This is a lot like the Hohmann transfer, but with 3 engine burns. The first burn will "overshoot" the target orbit. The second engine burn will raise the original side to the target altitude. The final engine burn will "correct" the first burn, until the target orbit is achieved. This can sometimes require less delta-v than a Hohmann transfer.

### Oberth effect

*Main article: Oberth Effect*

This is the effect of when a rocket falls a celestial body's gravitational well, the rocket accelerates in the periapsis to increase its speed to get to the target orbit sooner. It is named after Hermann Oberth, an Austro-Hungarian-born German physicist and a founder of modern rocketry. It is more effective at higher speeds.

### Gravity assists

*Main article: Gravity Assists*

This uses a planet's gravity to slow down or speed up a rocket and alter its trajectory to save fuel in most missions. This is commonly used during a Mercury mission or an interstellar mission.

### Rendezvous

*Main article: Rendezvous*

Rendezvous is the process of getting two rockets close to eachother, typically for docking.

## Orbits around celestial bodies

### Mercury

Mercury orbits are hard to get to due to the high delta-v requirement to both get to Mercury and slow down enough for orbit insertion. Venus is typically used in Mercury missions for gravity assists.

### Venus

Venus orbits are much easier to get to. Aerobraking can be used for orbit insertion to save fuel, although you may need a heat shield to avoid burning up in Venus' thick atmosphere.

### Moon

The moon is the closest celestial body to Earth, so lunar orbits are quite easy to get to.

### Mars

Mars orbits are easier to get to, due to Mars' proximity to Earth. Getting to Mars orbit from the Martian surface is quite easy and can be done with a very small rocket due to the low gravity and small atmosphere. A satellite's orbit can sometimes cross Phobos' or Deimos' orbit if it's eccentric. They can study the composition of the atmosphere and surface of Mars. Much like Venus, Aerobraking can be used to enter an orbit, although Mars' atmosphere is much thinner

#### Mars's moons

Mars's moons are tiny and have very little gravity — so it requires a very low velocity to orbit them. It is usually more practical to land directly on their surfaces.

### Jupiter

Getting to low Jupiter orbit is one of the hardest tasks to do in the game because it consumes so much fuel. A low orbit can require high velocity (up to 10 km/s) because of its size and gravity — but high orbits require much less velocity than that. This is why many Jupiter probes orbited Jupiter at a high orbit, not only to not consume as much fuel but also to decrease radiation levels on the spacecraft.

#### Galilean moons

The Galilean moons are similarly sized as the Moon. When transferring into an orbit around one of these moons, Relative velocity can be very high depending on the transfer orbit around Jupiter. However, due to their low gravities, transferring from a nearby orbit is trivially easy.

## Activities

*Main article: Earth Orbit Activities*

There are many activities to do in orbit around the celestial body because the rocket is in zero gravity.

### Parking

A rocket can orbit a planet while it waits for planetary alignment before executing a transfer. They are usually placed in a low orbit, to spend as little fuel as possible.

### Satellites

Satellites can be used for atmospheric research, studying space, the Internet, and many more. Geostationary satellites bring GPS, navigational services, and many more services. Most satellites are midway between a low orbit and high orbit, while others are eccentric for thermal regulation.

### Space stations

Space stations can be built using multiple pieces — or "modules". They can be used for transferring crew to other destinations, an orbital outpost, wet workshop, scientific research, and many more. Typically space stations are located in a low to transfer orbit.

### Orbiters

Orbiters, just like satellites, orbit a celestial body apart from Earth. They study a celestial body's properties, such as surface features, magnetic field, atmosphere, etc.

## Bugs

1. Orbits are locked during time warp. This is due to physics simplification during the time warp.
2. Orbits of rockets that intersect a planet's atmosphere cannot decay unless you switch to that rocket. However, those rockets will still heat up.
3. Some orbits of planets are not to scale. This is due to estimation.
	1. For example, the moon's orbit is around 10,900 kilometers away; if scaled to 20, it will orbit it at around 19,200 kilometers.

## Trivia

* Orbits in the game do not decay, since there is not atmospheric drag past the atmosphere boundary to slow the rocket down.
* Because of the timewarpHeight of Venus, Earth, Mars, and Jupiter, you can make an orbit at their upper atmospheres without atmospheric drag using time warp. However, the height was increased to the Karman line in a later update.
* In older versions of the game, apoapsis, periapsis, and eccentricity can be viewed by turning on *Orbit Info*. In 1.4 and later, this was removed, and apoapsis and periapsis are now viewed in the orbit itself.
* In real life, there is also inclination. At 0 degrees, the rocket's latitude does not change through the orbit. At 90 degrees, it crosses both poles. Between 90 and 270 degrees, it is a retrograde orbit.
* In real life, the points of orbit are called various things depending on the orbited object: apogee and perigee on Earth, apolune and perilune on the Moon, aphelion and perihelion around the Sun, apoareon and periareon at Mars, and apojove and perijove on Jupiter.