The following is a list of types of
orbits:
Centric classifications •
Galactocentric orbit: An orbit about the center of a
galaxy. The
Sun follows this type of orbit about the
Galactic Center of the
Milky Way. •
Heliocentric orbit: An orbit around the
Sun. In the
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 object. •
Geocentric orbit: An orbit around the planet
Earth, such as that of the
Moon or of
artificial satellites. •
Selenocentric orbit (named after
Selene): An orbit around Earth's
Moon. •
Areocentric orbit (named after
Ares): An orbit around the planet
Mars, such as that of
its moons or
artificial satellites. For orbits centered about planets other than Earth and Mars and for the dwarf planet Pluto, the orbit names incorporating Greek terminology are not as established and much less commonly used: • Mercury orbit (
Hermeocentric orbit, named after
Hermes): An orbit around the planet
Mercury. • Venus orbit (
Cytherocentric orbit, named after
Cytherea, or Aphrodiocentric, after Aphrodite): An orbit around the planet
Venus. • Jupiter orbit (
Zenocentric orbit, named after
Zeus, or Latin equivalent Jovicentric): An orbit around the planet
Jupiter. • Saturn orbit (
Kronocentric orbit, named after
Cronus, •
Very low Earth orbit (VLEO) is defined as altitudes between approximately 100 - 450 km above Earth’s surface. •
Low Earth orbit (LEO): geocentric orbits with altitudes below . •
Medium Earth orbit (MEO): geocentric orbits ranging in altitude from to just below
geosynchronous orbit at . Also known as an
intermediate circular orbit. These are used for
Global Navigation Satellite System spacecraft, such as
GPS,
GLONASS,
Galileo,
BeiDou. GPS satellites orbit at an altitude of with an orbital period of almost 12 hours. 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 . This works out to an altitude of . 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 (). For Earth orbiting satellites below the height of about 800 km, the atmospheric drag is the major orbit perturbing force out of all non-gravitational forces. Above 800 km, solar radiation pressure causes the largest orbital perturbations. However, the atmospheric drag strongly depends on the density of the upper atmosphere, which is related to the solar activity, therefore the height at which the impact of the atmospheric drag is similar to solar radiation pressure varies depending on the phase of the solar cycle.
Inclination classifications •
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.
Directional classifications •
Prograde orbit: An orbit that is in the same direction as the rotation of the primary (i.e. east on Earth). By convention, the
inclination of a Prograde orbit is specified as an angle less than 90°. •
Retrograde orbit: An orbit counter to the direction of rotation of the primary. By convention, retrograde orbits are specified with an
inclination angle of more than 90°. Apart from those in
Sun-synchronous orbit, few satellites are launched into
retrograde orbit on Earth because the quantity of fuel required to launch them is 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.
Eccentricity classifications There are two types of orbits: closed (periodic) orbits, and open (escape) orbits. Circular and elliptical orbits are closed. Parabolic and hyperbolic orbits are open. Radial orbits can be either open or closed. •
Circular orbit: An orbit that has an
eccentricity of 0 and whose path traces a
circle. •
Elliptic orbit: An orbit with an
eccentricity greater than 0 and less than 1 whose orbit traces the path of an
ellipse. •
Geostationary or geosynchronous transfer orbit (GTO): 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. •
Hohmann transfer orbit: An
orbital maneuver that moves a
spacecraft from one
circular orbit to another using two engine
impulses. This maneuver was named after
Walter Hohmann. •
Ballistic capture orbit: a lower-energy orbit than a
Hohmann transfer orbit, a spacecraft moving at a lower
orbital velocity than the target celestial body is inserted into a similar orbit, allowing the planet or moon to move toward it and gravitationally snag it into orbit around the celestial body. • Coelliptic orbit: A relative reference for two spacecraft—or more generally,
satellites—in orbit in the same plane. "Coelliptic orbits can be defined as two orbits that are
coplanar and
confocal. A property of coelliptic orbits is that the difference in magnitude between aligned radius vectors is nearly the same, regardless of where within the orbits they are positioned. For this and other reasons, coelliptic orbits are useful in [spacecraft]
rendezvous". •
Parabolic orbit: An orbit with the eccentricity equal to 1. Such an orbit also has a
velocity equal to the
escape velocity and therefore will escape the gravitational pull of the
planet. If the speed of a parabolic orbit is increased it will become a hyperbolic orbit. •
Escape orbit: A
parabolic orbit where the object has
escape velocity and is moving away from the
planet. •
Capture orbit: A
parabolic orbit where the object has
escape velocity and is moving toward the
planet. •
Hyperbolic orbit: An orbit with the eccentricity greater than 1. Such an orbit also has a
velocity in excess of the
escape velocity and as such, will escape the gravitational pull of the
planet and continue to travel
infinitely until it is acted upon by another body with sufficient gravitational force. •
Radial orbit: An orbit with zero
angular momentum and eccentricity equal to 1. The two objects move directly towards or away from each other in a straight-line. •
Radial elliptic orbit: A closed
elliptic orbit where the object is moving at less than the
escape velocity. This is an elliptic orbit with semi-minor axis = 0 and eccentricity = 1. Although the eccentricity is 1, this is not a parabolic orbit. •
Radial parabolic orbit: An open
parabolic orbit where the object is moving at the
escape velocity. •
Radial hyperbolic orbit: An open
hyperbolic orbit where the object is moving at greater than the
escape velocity. This is a hyperbolic orbit with semi-minor axis = 0 and eccentricity = 1. Although the eccentricity is 1, this is not a parabolic orbit.
Synchronicity classifications . To an observer on the rotating Earth, the red and yellow satellites appear stationary in the sky above Singapore and Africa respectively. •
Synchronous orbit: An orbit whose
period is a rational multiple of the average
rotational period of the body being orbited and in the same direction of rotation as that body. This means the track of the satellite, as seen from the central body, will repeat exactly after a fixed number of orbits. In practice, only 1:1 ratio (geosynchronous) and 1:2 ratios (semi-synchronous) are common. •
Geosynchronous orbit (GSO): An orbit around the Earth with a period equal to one
sidereal day, which is Earth's average rotational period of 23
hours, 56
minutes, 4.091
seconds. For a nearly circular orbit, this implies an altitude of approximately . The orbit's inclination and eccentricity may not necessarily be zero. If both the inclination and eccentricity are zero, then the satellite will appear stationary from the ground. If not, then each day the satellite traces out an
analemma (i.e. a "figure-eight") in the sky, as seen from the ground. When the orbit is circular and the rotational period has zero inclination, the orbit is considered to also be
geostationary. Also known as a
Clarke orbit after the writer
Arthur C. Clarke. •
Areosynchronous orbit (ASO): A
synchronous orbit around the planet
Mars with an
orbital period equal in length to Mars'
sidereal day, 24.6229
hours. •
Areostationary orbit (AEO): A
circular areosynchronous orbit on the
equatorial plane and about 17,000
km (10,557
miles) above the surface of Mars. To an observer on Mars this satellite would appear as a fixed point in the sky. •
Subsynchronous orbit: A drift orbit close below GSO/GEO. •
Semi-synchronous orbit: An orbit with an
orbital period equal to half of the average
rotational period of the body being orbited and in the same direction of rotation as that body. For Earth this means a period of just under 12 hours at an
altitude of approximately 20,200 km (12,544.2 miles) if the orbit is circular. •
Molniya orbit: A semi-synchronous variation of a
Tundra orbit. For Earth this means an
orbital period of just under 12 hours. Such a satellite spends most of its time over two designated areas of the
planet. An inclination of 63.4° is normally used to keep the perigee shift small. The orbit can be described as a Keplerian ellipse that
precesses about the black hole in two orthogonal directions, due to
torques from the triaxial galaxy. The eccentricity of the ellipse reaches unity at the four corners of the pyramid, allowing the star on the orbit to come very close to the black hole. •
Tube orbit: An orbit near a massive
black hole at the center of an axisymmetric galaxy. Similar to a pyramid orbit, except that one component of the orbital angular momentum is conserved; as a result, the eccentricity never reaches unity. Planned orbit for the NASA
Lunar Gateway in circa 2024, as a highly-elliptical seven-day near-rectilinear
halo orbit around the Moon, which would bring the small space station within of the
lunar north pole at closest approach and as far away as over the
lunar south pole. •
Distant retrograde orbit (DRO): A stable circular
retrograde orbit (usually referring to Lunar Distant Retrograde Orbit). Stability means that satellites in DRO do not need to use station keeping propellant to stay in orbit. The lunar DRO is a high lunar orbit with a radius of approximately 61,500 km. This was proposed in 2017 as a possible orbit for the
Lunar Gateway space station, outside Earth-Moon L1 and L2. •
Graveyard orbit (or disposal, junk orbit) : An orbit that satellites are moved into at the end of their operation. For geostationary satellites a few hundred kilometers above
geosynchronous orbit. •
Parking orbit, a temporary orbit. •
Transfer orbit, an orbit used during an
orbital maneuver from one orbit to another. •
Lunar transfer orbit (LTO) accomplished with
trans-lunar injection (TLI) •
Mars transfer orbit (MTO) also known as trans-Mars injection (TMI) orbit •
Repeat orbit: An orbit where the ground track of the satellite repeats after a period of time. • Gangale orbit: a solar orbit near Mars whose period is one Martian year, but whose eccentricity and inclination both differ from that of Mars such that a relay satellite in a Gangale orbit is visible from Earth even during solar conjunction.
Pseudo-orbit classifications •
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. •
Libration point orbits such as
halo orbits and
Lissajous orbits: These are orbits around a
Lagrangian point. Lagrange points are shown in the adjacent diagram, and orbits near these points allow a spacecraft to stay in constant relative position with very little use of fuel. Orbits around the point are used by spacecraft that want a constant view of the Sun, such as the
Solar and Heliospheric Observatory. Orbits around are used by missions that always want both Earth and the Sun behind them. This enables a single shield to block radiation from both Earth and the Sun, allowing passive cooling of sensitive instruments. Examples include the
Wilkinson Microwave Anisotropy Probe and the
James Webb Space Telescope. L1, L2, and L3 are unstable orbits[6], meaning that small perturbations will cause the orbiting craft to drift out of the orbit without periodic corrections. •
P/2 orbit, a highly-stable 2:1
lunar resonant orbit, that was first used with the spacecraft TESS (
Transiting Exoplanet Survey Satellite) in 2018. ==See also==