Space
===

Space forms a huge part of *The Expanse*. There are so many vessels operating inside the System that it is impossible to list all of them. Small ships, big ships, medium ships. Old ships rarely fall out of service too, they are regularly refitted and patched.

# Space Travel

While the Epstein Drive has overcome some of the limits of space travel, it is far from previous imaginary visions of spaceships able to zip from place to place without any regard for the laws of physics. A lot of space travel during the game is handled during Interludes.

## Motion

Understanding space travel depends on your understanding of motion. There are two key measures of motion:

- An object's velocity (how fast it's moving).
- The object's relative velocity (how fast it's moving compared to another object).

For example, when you stand on a planet, you appear still relative to your own body, but you're still moving through space. Your body is moving in several ways: it's rotating beneath you, it may be orbiting around a planet, which is orbiting around a star.

## Velocity

When you're standing still your relative velocity is 0 metres per second (m/s). All travel, including space travel, requires changes in velocity, symbolized by Δv (or "delta-vee"). You change your velocity by apply force, such as taking a step forward or accelerating a car.

Space travel is the same, whether you're launching from an object or manoeuvring in space.

> To accelerate to low Earth orbit, you need a Δv change of 9,400m/s. Or a change of 9,400m/s in velocity from the ground.
>
> To then transfer from Earth orbit to Luna requires an additional 3,260m/s of Δv.

Think of Δv like the cost of any manoeuvre in space, which you pay for using force, typically powered by an engine.

## Thrust and Mass

While the amount of Δv required to perform a manoeuvre is constant, the amount of *thrust* depends on the *mass* of an object you're trying to move. The more massive an object, the more thrust that is required to achieve the same amount of Δv.

## Mass, Spin, and Thrust Gravity

Three things cause masses to have weight:

1. The gravity of another large **mass** (planets).
2. The centrifugal force of a **spinning** object (space station).
3. The momentum of an object under **thrust** (acceleration).

All of these are measured in terms of numbers of "g", a measurement of weight at Earth-normal gravity. A planetoid or station may only have 0.3g for example (things weigh 1/3 that they would on Earth). Alternatively a ship may be under 6g or more thrust, meaning something that weighs 80kg on Earth now weighs 480kg.

## :rocket: Rocketry and Epstein Drive

Prior to the advent of the Epstein Drive, space travel required vast amounts of fuel to generate thrust for the required Δv. This led to the "tyranny of rocketry", where more fuel meant more mass, meaning you need to generate more thrust, and so on.

The Epstein Drive changed all that. A modified fusion reactor, the Epstein Drive could generate nearly limitless thrust without the need for enormous stores of fuel, drastically reducing the mass of ships. The solar system as a result, opened up to Humanity.

## Launching Rockets

Even with the advent of the Epstein Drive, it's still important to understand how rockets are launched.

Rockets begin by launching directly up, that is, away from the centre of gravity. As you climb, you will eventually read the speed of sound (320m/s). At this speed, most launches do a 'gravity turn', turning in the same direction the body is rotating. This adds both the rotational body's velocity with the rocket's velocity.

## Apoapsis and Periapsis

All orbits have two points, measured from the surface of the object you're orbiting.

- The **Apoapsis** - the highest (farthest) point from the object.
- The **Periapsis** - the lowest (closest) point from the object.

## Circularizing Orbit

The most efficient way to change a ship's apoapsis or periapsis is to burn at the opposite point.

## Prograde and Retrograde

There are two basic directions a ship can burn:

- **Prograde** - Burning in the direction of the orbit (speeding up).
- **Retrograde** - Burning against the direction of the orbit (slowing).

## Brachistochrone Trajectories and Hard Burns

The Epstein Drive removed Δv limitations to space travel and there is no longer a need for conventional orbital manoeuvres.

To transfer, ships can simple burn prograde at a constant rate, then flip around and decelerate by burning retrograde. This is known as a Brachistochrone Trajectory, from the Greek meaning "shortest time".

The time it takes to execute a flight path along a Brachistochrone Trajectory depends on two factors:

- The distance between two points.
- The acceleration of the ship.

The greater the acceleration, the less time it takes to travel, but the multiple g-forces created by hard-burns are stressful on the human body. Crash couches and pharmaceutical cocktails like "the juice" alleviate some, but not all, of the damage inflicted by the hardest burns.

## Sphere of Influence

Every object has its own gravitational pull, based on its mass, or a Sphere of Influence (SOI).

For example, your ship could be orbiting a station and inside it's SOI; while in Ganymede's SOI, as the station orbit Ganymede; while in Jupiter's SOI; while in Sol's SOI.

# System Communication and Travel Time

One of the few constants in the universe remains the speed of light. Whether communications are sent as radio waves or on the laser of a tight-beam, they travel 300,000 kilometres per second.

Ship transit times are even more variable, as the distances between planets, planetoids and station change as they follow their own orbits around the sun. The follow tables provide a good guideline of information about these two topics.