## Eclipse Phase | AUSTIN, TEXAS | 2015-2019 A.D.

## Orbits and Position

Real-life space navigation is a complex undertaking, and well beyond what can be incorporated into a roleplaying game. However, space travel is a large part of characters’ lives within the Eclipse Phase universe. This section presents a simplified system of space navigation that allows the Players and Gamemaster to help establish what is happening to the Player Characters. Beginning with a simplified explanation of orbits and orbital mechanics, further sections describe how vessels move and relate to other objects within space under this simplified system. A description of space traffic control and the “rules of the road” give Players and Gamemasters an idea of what the people of the Solar System are doing when they direct their vessels between the planets. The section concludes with some discussion on how to use the previous sections with the game itself.

**Orbits and Position**

Orbits are described using four conic sections: circle, ellipse, parabola and hyperbola. If the spacecraft’s path follows the curve of the orbit, then the focus of the orbit is the center of mass and a source of gravitational influence. The central body – planet, moon, sun or other – is used as a reference point for describing orbits. As orbits change, so can the central body used as the reference point. For example, the planet is used as a reference for orbits about the planet and making the transfer to an interplanetary orbit, but the interplanetary orbit is described relative to the sun. One or more elliptical or hyperbolic arcs define transfer orbits between planets, between orbits or between two points in space.

Planet-synchronous orbits place the craft above a fixed point on the planetary surface. Planet-synchronous orbits are restricted to planets because planets have sufficient gravity to have practical synchronous orbits. For example, a synchronous orbit above the Moon places a vessel in an orbit that would cause it collide with the Earth. Sun-synchronous orbits place the vessel in a fixed orbit with respect to the sun about a planet. For example, Mercurian colony cylinders are in a sun-synchronous orbit that places the cylinders constantly in Mercury’s shadow. Polar orbits are perpendicular to equatorial orbits. The major advantage of a polar orbit is the vessel’s ability to observe the entire surface of the orbited planet or moon. While this coverage is not complete or constant, the vessel will track across the entire surface area after a number of orbits.

All of these orbits have altitudes described as being low, middle or high. Low orbits are the lowest safe zone in which a vessel can maintain an orbit without spending significant energy to maintain its altitude. High orbits are from a planet-synchronous altitude and higher. Middle orbits fall between low and high orbits. Continuous orbits about planets and moons are either circular or elliptical. Circular orbits place the vessel at a constant altitude, while elliptical orbits vary the altitude from within a few kilometers to thousands of kilometers from a circular orbit. The section on Flight Rules details some of the navigation regulations for the orbits described.

When a vessel is not orbiting a planet, the simplest method for pinpointing the vessel’s position is by the use of polar coordinates. Polar coordinates position a point by the number of degrees from an axis and the distance from the origin. In the case of navigation, the origin is the Sun, and the axis is measured counter-clockwise from a line drawn from the Sun to the Earth’s position at the vernal (or Spring) equinox; the axis is 0" celestial longitude. The simplest measure used to denote a third dimension is to refer to the inclination of the plane the vessel is traveling along. The orbit plane, or plane of travel, is the plane defined by any three unique points in the orbit, and is expressed by its inclination relative to the ecliptic. (See Flight Rules in the following pagesfor more information about orbit planes and orbit inclination.) Using a standard ephemeride table that gives a planet’s celestial longitude for a certain date, realism-minded Players and Garnemasten can make reasonably accurate guesses about interplanetary orbits and positions.

**SOME USEFUL TERMS**

TERM |
DEFINITION |

Apoapsis |
The point on an orbit furthest from the central body |

Periapsis |
The point on an orbit closest to the central body |

Conjunction |
The situation where (or time at which) two bodies are either the same celestial longitude or 180" apart |

Eccentricity |
A parameter that defines the shape of a conic section, circle, ellipse, parabola; hyperbola Eccentricity is a mesure of how an orbit deviates from circular. A perfectly circular orbit has en eccentricity of zero: other numbers denote increasingly deviating orbits. |

Ecliptic |
The plane of the Earth’s edit around the Sun inclined to the Earth’s equator by about 23.4’ |

Injection Point |
The point at which spacecraft velocity is adequate to enter a planetary orbit |

Transfer Point |
The point at which spacecraft velocity is adequate to move to a new orbit. either planetary or interplanetary |

Ascending |
Travelling toward the ecliptic from below, or travelling away from the ecliptic from above |

Descending |
Travelling toward the ecliptic from ebove, or travelling away fmm the ecliptic from below |

Inclination |
The angle betweenn the orbal plane and the reference plane (usually the ecliptic or the equator of the body) |