Terms of Reference

Earth orbits

  • Low Earth orbit (LEO) is generally circular or near-circular orbits about the Earth with average altitudes ranging from about 185 km (100 nm) to about 500 km (270 nm). Satellites or other space systems placed into such orbits are able to remain in orbit for weeks to years. Below approximately 185 km, the effects of atmospheric drag, while still small, will lead to reentry into the atmosphere within hours-to-days. Above about 600 km, the lowest part of the radiation belt, with charged particles trapped within the Earth’s magnetic field, will become a design consideration.
  • Medium Earth orbit (MEO) is the range of orbits between LEO and geostationary orbit. Many observation, scientific, and navigation satellites are located in MEO.
  • Geostationary orbit (GEO) is a circular orbit, above the Earth’s equator, with an orbital radius of 42,164 km (22,752 nm). A satellite placed in this orbit rotates about the Earth once each sidereal day (23.9344696 hrs) so that, from the ground, it appears to be in a fixed position. Most communications satellites are placed into geostationary orbits. This may be the orbital position used by space solar power satellites.
  • Global positioning satellite (GPS) orbit is a circular orbit, classified as a MEO, with an altitude of approximately 20,200 km (10,900 nm) in orbital planes inclined 55° to the equator.
  • The International Space Station (ISS) is located in LEO at an inclination of 51.6° to the equator. The ISS orbit altitude ranges from a maximum of about 425 km (229 nm) and a minimum of 278 km (150 nm). To enable a micro-gravity environment to support scientific experimentation, the ISS’s orbital altitude slowly degrades due to atmospheric drag and gravity. Several times a year, the ISS is reboosted to a higher altitude to regain its lost altitude.
  • A LEO logistics orbit is a circular orbit with an inclination-specific orbital altitude that generates a repeating ground track. With the proper location of the orbit’s ascending node (essentially orienting the position of the ground track relative to the 0° longitude), the ground track can be made to pass over the primary terrestrial spaceport. Each day, at least one and, at times, two minimum ascent time launch windows will exist to access space logistics depots located in each LEO logistics orbit. Because of the need to maintain a specific orbital altitude, minor reboosting of facilities, satellites, and spaceships located in a LEO logistics orbit will be frequently needed to make up altitude lost to atmospheric drag. For this reason, a LEO logistics orbit is not suitable for scientific investigations requiring multi-orbit micro-gravity environments.
LaGrange Points in Earth-Moon system.
(© Spacefaring Institute LLC)