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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:
Geosynchronous orbit (GSO) andgeostationary orbit (GEO) are orbits around Earth matching Earth'ssidereal rotation period.[1][9] 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 asemi-major axis of 42,164 km (26,199 mi).[10] 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).
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.[11] Above 800 km, solar radiation pressure causes the largest orbital perturbations.[12] 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.
Polar orbit: An orbit that passes above or nearly above both poles of the planet on each revolution. Therefore, it has aninclination of (or very close to) either 90degrees or −90 degrees.
Near equatorial orbit: An orbit whose inclination with respect to theequatorial plane is nearly zero. This orbit allows for rapid revisit times (for a single orbiting spacecraft) of near equatorial ground sites.
Prograde orbit: An orbit that is in the same direction as the rotation of the primary (i.e. east on Earth). By convention, theinclination 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 aninclination angle of more than 90°. Apart from those inSun-synchronous orbit, few satellites are launched intoretrograde 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 ofvelocity equal to the rotational velocity of the planet at its launchlatitude.
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.
Ballistic capture orbit: a lower-energy orbit than aHohmann transfer orbit, a spacecraft moving at a lowerorbital 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.[13]
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 arecoplanar andconfocal. 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".[14]
Parabolic orbit: An orbit with the eccentricity equal to 1. Such an orbit also has avelocity equal to theescape velocity and therefore will escape the gravitational pull of theplanet. If the speed of a parabolic orbit is increased it will become a hyperbolic orbit.
Hyperbolic orbit: An orbit with the eccentricity greater than 1. Such an orbit also has avelocity in excess of theescape velocity and as such, will escape the gravitational pull of theplanet and continue to travelinfinitely until it is acted upon by another body with sufficient gravitational force.
Radial orbit: An orbit with zeroangular 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 closedelliptic orbit where the object is moving at less than theescape 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 hyperbolic orbit: An openhyperbolic orbit where the object is moving at greater than theescape 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.
Geostationary orbit as seen from the northcelestial pole. 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 whoseperiod is a rational multiple of the averagerotational 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 onesidereal day, which is Earth's average rotational period of 23hours, 56minutes, 4.091seconds. For a nearly circular orbit, this implies an altitude of approximately 35,786 kilometers (22,236 mi). 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 ananalemma (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 begeostationary. Also known as aClarke orbit after the writerArthur C. Clarke.[8]
Geostationary orbit (GEO): A circulargeosynchronous orbit with aninclination of zero. To an observer on the ground this satellite appears as a fixed point in the sky. "All geostationary orbits must be geosynchronous, but not all geosynchronous orbits are geostationary."[8]
Tundra orbit: A synchronous but highlyelliptic orbit with significantinclination (typically close to 63.4°) andorbital period of onesidereal day (23 hours, 56 minutes for the Earth). Such a satellite spends most of its time over a designated area of theplanet. The particular inclination keeps the perigee shift small.[15]
Semi-synchronous orbit: An orbit with anorbital period equal to half of the averagerotational 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 analtitude of approximately 20,200 km (12,544.2 miles) if the orbit is circular.[16]
Molniya orbit: A semi-synchronous variation of aTundra orbit. For Earth this means anorbital period of just under 12 hours. Such a satellite spends most of its time over two designated areas of theplanet. An inclination of 63.4° is normally used to keep the perigee shift small.[15]
Pyramid orbit: An orbit near a massiveblack hole at the center of a triaxial galaxy.[17] The orbit can be described as a Keplerian ellipse thatprecesses about the black hole in two orthogonal directions, due totorques from the triaxial galaxy.[18] 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 massiveblack 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.[18]
Beyond-low Earth orbit (BLEO) and beyond Earth orbit (BEO) are a broad class of orbits that areenergetically farther out thanlow Earth orbit or require an insertion into aheliocentric orbit as part of a journey that may require multipleorbital insertions, respectively.
Near-rectilinear halo orbit (NRHO): an orbit currently planned incislunar space, as a selenocentric orbit that will serve as a staging area for future missions.[19][20] Planned orbit for the NASALunar Gateway in circa 2024, as a highly-elliptical seven-day near-rectilinearhalo orbit around the Moon, which would bring the small space station within 3,000 kilometers (1,900 mi) of thelunar north pole at closest approach and as far away as 70,000 kilometers (43,000 mi) over thelunar south pole.[21][22][23]
Distant retrograde orbit (DRO): A stable circularretrograde 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.[24] This was proposed[by whom?] in 2017 as a possible orbit for theLunar Gatewayspace station, outside Earth-Moon L1 and L2.[20]
Decaying orbit: A decaying orbit is an orbit at a low altitude that decreases over time due atmospheric resistance. Used to dispose of dying artificial satellites or toaerobrake an interplanetary spacecraft.
Earth-trailing orbit, a heliocentric orbit that is placed such that the satellite will initially follow Earth but at a somewhat slower orbital angular speed, such that it moves further behind year by year. This orbit was used on theSpitzer Space Telescope in order to drastically reduce the heat load from the warm Earth from a more typical geocentric orbit used for space telescopes.[25]
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 abovegeosynchronous orbit.[26][27]
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.[28]
A diagram showing the five Lagrangian points in a two-body system with one body far more massive than the other (e.g. the Sun and the Earth). In such a system,L3–L5 are situated slightly outside of the secondary's orbit despite their appearance in this small scale diagram.
Libration point orbits such ashalo orbits andLissajous orbits: These are orbits around aLagrangian 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 theL1 point are used by spacecraft that want a constant view of the Sun, such as theSolar and Heliospheric Observatory. Orbits aroundL2 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 theWilkinson Microwave Anisotropy Probe and theJames 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.
^Orbital periods and speeds are calculated using the relations 4π2R3 = T2GM andV2R = GM, whereR = radius of orbit in metres,T = orbital period in seconds,V = orbital speed in m/s,G = gravitational constant ≈ 6.673×10−11 Nm2/kg2,M = mass of Earth ≈ 5.98×1024 kg.
^Approximately 8.6 times when the Moon is nearest (363,104 km ÷ 42,164 km) to 9.6 times when the Moon is farthest (405,696 km ÷ 42,164 km).
^abParker, Sybil P. (2002).McGraw-Hill Dictionary of Scientific and Technical Terms Sixth Edition. McGraw-Hill. p. 1772.ISBN007042313X.
^McDowell, Jonathan (24 May 1998)."Jonathan's Space Report".Transatmospheric orbit (TAO): orbital flight with perigee less than 80 km but more than zero. Potentially used by aerobraking missions and transatmospheric vehicles, also in some temporary phases of orbital flight (e.g. STS pre OMS-2, some failures when no apogee restart)
^abcd"Orbit: Definition".Ancillary Description Writer's Guide, 2013. National Aeronautics and Space Administration (NASA) Global Change Master Directory. Archived fromthe original on 11 May 2013. Retrieved29 April 2013.
