Learn About Orbits

There are many different types of orbits. In this post, we will mainly look at Geocentric orbits. A Geocentric is simply an orbit around the planet Earth, such as that of the Moon or of artificial satellites. Orbits may be classified in many ways.

By Altitude:

  • Low Earth orbit (LEO): geocentric orbits with altitudes below 2,000 km (100–1,240 miles).[2]
  • Medium Earth orbit (MEO): geocentric orbits ranging in altitude from 2,000 km (1,240 miles) to just below geosynchronous orbit at 35,786 kilometers (22,236 mi). Also known as an intermediate circular orbit. These are “most commonly at 20,200 kilometers (12,600 mi), or 20,650 kilometers (12,830 mi), with an orbital period of 12 hours.”[3]
  • Geosynchronous orbit (GSO) and geostationary orbit (GEO) are orbits around Earth matching Earth’s sidereal rotation period. Although terms are often used interchangeably, technically a geosynchronous orbit matches the Earth’s rotational period, but the definition does not require it to have zero orbital inclination to the equator, and thus is not stationary above a given point on the equator, but may oscillate north and south during the course of a day Thus, a geostationary orbit is defined as a geosynchronous orbit at zero inclination. Geosynchronous (and geostationary) orbits have a semi-major axis of 42,164 km (26,199 mi).[4] This works out to an altitude of 35,786 km (22,236 mi). Both complete one full orbit of Earth per sidereal day (relative to the stars, not the Sun).
  • High Earth orbit: geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,240 miles).[3]

By Inclination:

  • Inclined orbit: An orbit whose inclination in reference to the equatorial plane is not 0.
    • Polar orbit: An orbit that passes above or nearly above both poles of the planet on each revolution. Therefore, it has an inclination of (or very close to) either 90 degrees or ?90 degrees.
    • Polar Sun-synchronous orbit (SSO): A nearly polar orbit that passes the equator at the same local solar time on every pass. Useful for image-taking satellites because shadows will be the same on every pass.
  • Non-inclined orbit: An orbit whose inclination is equal to zero with respect to some plane of reference.
    • Ecliptic orbit: A non-inclined orbit with respect to the ecliptic.
    • Equatorial orbit: A non-inclined orbit with respect to the equator.
  • Near equatorial orbit: An orbit whose inclination with respect to the equatorial plane is nearly zero. This orbit allows for rapid revisit times (for a single orbiting spacecraft) of near equatorial ground sites.

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