Orbits

The first satellite, Sputnik 1, was put into orbit around Earth and was therefore in geocentric orbit. This is the most common type of orbit by far, with approximately 2,787 active artificial satellites orbiting the Earth. Geocentric orbits may be further classified by their altitude, inclination and eccentricity.

The commonly used altitude classifications of geocentric orbit are Low Earth orbit (LEO), Medium Earth orbit (MEO) and High Earth orbit (HEO). Low Earth orbit is any orbit below 2,000 km. Medium Earth orbit is any orbit between 2,000 and 35,786 km. High Earth orbit is any orbit higher than 35,786 km.

Centric classificationsedit

• Galactocentric orbit: An orbit around the centre of a galaxy. The Sun follows this type of orbit about the galactic centre of the Milky Way.
• Geocentric orbit: An orbit around the planet Earth, such as the Moon or artificial satellites. Currently there are over 2,787 active artificial satellites orbiting the Earth.
• Heliocentric orbit: An orbit around the Sun. In our Solar System, all planets, comets, and asteroids are in such orbits, as are many artificial satellites and pieces of space debris. Moons by contrast are not in a heliocentric orbit but rather orbit their parent planet.
• Areocentric orbit: An orbit around the planet Mars, such as by moons or artificial satellites.

Altitude classificationsedit

• Low Earth orbit (LEO): Geocentric orbits ranging in altitude from 180 km – 2,000 km (1,200 mi)
• Medium Earth orbit (MEO): Geocentric orbits ranging in altitude from 2,000 km (1,200 mi) – 35,786 km (22,236 mi). Also known as an intermediate circular orbit.
• Geosynchronous orbit (GEO): Geocentric circular orbit with an altitude of 35,786 kilometres (22,236 mi). The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3,000 metres per second (9,800 ft/s).
• High Earth orbit (HEO): Geocentric orbits above the altitude of geosynchronous orbit 35,786 km (22,236 mi).

Inclination classificationsedit

• Inclined orbit: An orbit whose inclination in reference to the equatorial plane is not zero degrees.
  • 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) 90 degrees.
  • Polar sun synchronous orbit: A nearly polar orbit that passes the equator at the same local time on every pass. Useful for image taking satellites because shadows will be nearly the same on every pass.

Eccentricity classificationsedit

• Circular orbit: An orbit that has an eccentricity of 0 and whose path traces a circle.
  • Hohmann transfer orbit: An orbit that moves a spacecraft from one approximately circular orbit, usually the orbit of a planet, to another, using two engine impulses. The perihelion of the transfer orbit is at the same distance from the Sun as the radius of one planet's orbit, and the aphelion is at the other. The two rocket burns change the spacecraft's path from one circular orbit to the transfer orbit, and later to the other circular orbit. This maneuver was named after Walter Hohmann.
• Elliptic orbit: An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
  • Geosynchronous transfer orbit: An elliptic orbit where the perigee is at the altitude of a Low Earth orbit (LEO) and the apogee at the altitude of a geosynchronous orbit.
  • Geostationary transfer orbit: An elliptic orbit where the perigee is at the altitude of a Low Earth orbit (LEO) and the apogee at the altitude of a geostationary orbit.
  • Molniya orbit: A highly elliptic orbit with inclination of 63.4° and orbital period of half of a sidereal day (roughly 12 hours). Such a satellite spends most of its time over two designated areas of the planet (specifically Russia and the United States).
  • Tundra orbit: A highly elliptic orbit with inclination of 63.4° and orbital period of one sidereal day (roughly 24 hours). Such a satellite spends most of its time over a single designated area of the planet.

Synchronous classificationsedit

• Synchronous orbit: An orbit where the satellite has an orbital period equal to the average rotational period (earth's is: 23 hours, 56 minutes, 4.091 seconds) of the body being orbited and in the same direction of rotation as that body. To a ground observer such a satellite would trace an analemma (figure 8) in the sky.
• Semi-synchronous orbit (SSO): An orbit with an altitude of approximately 20,200 km (12,600 mi) and an orbital period equal to one-half of the average rotational period (Earth's is approximately 12 hours) of the body being orbited
• Geosynchronous orbit (GSO): Orbits with an altitude of approximately 35,786 km (22,236 mi). Such a satellite would trace an analemma (figure 8) in the sky.
  • Geostationary orbit (GEO): A geosynchronous orbit with an inclination of zero. To an observer on the ground this satellite would appear as a fixed point in the sky.
    • Clarke orbit: Another name for a geostationary orbit. Named after scientist and writer Arthur C. Clarke.
  • Supersynchronous orbit: A disposal / storage orbit above GSO/GEO. Satellites will drift west. Also a synonym for Disposal orbit.
  • Subsynchronous orbit: A drift orbit close to but below GSO/GEO. Satellites will drift east.
  • Graveyard orbit: An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation.
    • Disposal orbit: A synonym for graveyard orbit.
    • Junk orbit: A synonym for graveyard orbit.
• Areosynchronous orbit: A synchronous orbit around the planet Mars with an orbital period equal in length to Mars' sidereal day, 24.6229 hours.
• Areostationary orbit (ASO): A circular areosynchronous orbit on the equatorial plane and about 17000 km (10557 miles) above the surface. To an observer on the ground this satellite would appear as a fixed point in the sky.
• Heliosynchronous orbit: A heliocentric orbit about the Sun where the satellite's orbital period matches the Sun's period of rotation. These orbits occur at a radius of 24,360 Gm (0.1628 AU) around the Sun, a little less than half of the orbital radius of Mercury.

Special classificationsedit

• Sun-synchronous orbit: An orbit which combines altitude and inclination in such a way that the satellite passes over any given point of the planets' surface at the same local solar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites.
• Moon orbit: The orbital characteristics of Earth's Moon. Average altitude of 384,403 kilometers (238,857 mi), elliptical–inclined orbit.

Pseudo-orbit classificationsedit

• Horseshoe orbit: An orbit that appears to a ground observer to be orbiting a certain planet but is actually in co-orbit with the planet. See asteroids 3753 (Cruithne) and 2002 AA₂₉.
• Suborbital spaceflight: A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.
• Lunar transfer orbit (LTO)
• Prograde orbit: An orbit with an inclination of less than 90°. Or rather, an orbit that is in the same direction as the rotation of the primary.
• Retrograde orbit: An orbit with an inclination of more than 90°. Or rather, an orbit counter to the direction of rotation of the planet. Apart from those in sun-synchronous orbit, few satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude.
• Halo orbit and Lissajous orbit: Orbits "around" Lagrangian points.