A graveyard orbit, also called a junk orbit or disposal orbit, is an orbit that lies away from common operational orbits. One significant graveyard orbit is a supersynchronous orbit well beyond geosynchronous orbit. Some satellites are moved into such orbits at the end of their operational life to reduce the probability of colliding with operational spacecraft and generating space debris.
A graveyard orbit is used when the change in velocity required to perform a de-orbit maneuver is too large. De-orbiting a geostationary satellite requires a delta-v of about 1,500 metres per second (4,900 ft/s), whereas re-orbiting it to a graveyard orbit only requires about 11 metres per second (36 ft/s). [1]
For satellites in geostationary orbit and geosynchronous orbits, the graveyard orbit is a few hundred kilometers beyond the operational orbit. The transfer to a graveyard orbit beyond geostationary orbit requires the same amount of fuel as a satellite needs for about three months of stationkeeping. It also requires a reliable attitude control during the transfer maneuver. While most satellite operators plan to perform such a maneuver at the end of their satellites' operational lives, through 2005 only about one-third succeeded. [2] As of 2011, most[ clarification needed ] recently decommissioned geosynchronous spacecraft were said to have been moved to a graveyard orbit. [3]
According to the Inter-Agency Space Debris Coordination Committee (IADC) [4] the minimum perigee altitude beyond the geostationary orbit is:
where is the solar radiation pressure coefficient and is the aspect area [m2] to mass [kg] ratio of the satellite. This formula includes about 200 km for the GEO-protected zone to also permit orbit maneuvers in GEO without interference with the graveyard orbit. Another 35 kilometres (22 mi) of tolerance must be allowed for the effects of gravitational perturbations (primarily solar and lunar). The remaining part of the equation considers the effects of the solar radiation pressure, which depends on the physical parameters of the satellite.
In order to obtain a license to provide telecommunications services in the United States, the Federal Communications Commission (FCC) requires all geostationary satellites launched after March 18, 2002, to commit to moving to a graveyard orbit at the end of their operational life. [5] U.S. government regulations require a boost, , of about 300 km (186 mi). [6]
A spacecraft moved to a graveyard orbit will typically be passivated.
Uncontrolled objects in a near geostationary [Earth] orbit (GEO) exhibit a 53-year cycle of orbital inclination [7] due to the interaction of the Earth's tilt with the lunar orbit. The orbital inclination varies ± 7.4°, at up to 0.8°pa. [7] : 3
While the standard geosynchronous satellite graveyard orbit results in an expected orbital lifetime of millions of years, the increasing number of satellites, the launch of microsatellites, and the FCC approval of large megaconstellations of thousands of satellites for launch by 2022 necessitates new approaches for deorbiting to assure earlier removal of the objects once they have reached end-of-life. Contrary to GEO graveyard orbits requiring three months' worth of fuel (delta-V of 11 m/s) to reach, large satellite networks in LEO require orbits that passively decay into the Earth's atmosphere. For example, both OneWeb and SpaceX have committed to the FCC regulatory authorities that decommissioned satellites will decay to a lower orbit — a disposal orbit—where the satellite orbital altitude would decay due to atmospheric drag and then naturally reenter the atmosphere and burn up within one year of end-of-life. [8]
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a planet, moon, asteroid, or Lagrange point. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets and satellites follow elliptic orbits, with the center of mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion.
In the context of spaceflight, a satellite is an object that has been intentionally placed into orbit. These objects are called artificial satellites to distinguish them from natural satellites such as Earth's Moon.
A geosynchronous orbit is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds. The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky may remain still or trace out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. A circular geosynchronous orbit has a constant altitude of 35,786 km (22,236 mi).
A geostationary orbit, also referred to as a geosynchronous equatorial orbit (GEO), is a circular geosynchronous orbit 35,786 km (22,236 mi) in altitude above Earth's Equator and following the direction of Earth's rotation.
A low Earth orbit (LEO) is an Earth-centered orbit near the planet, often specified as having a period of 128 minutes or less and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never more than about one-third of the radius of Earth.
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits of different radii around a central body in the same plane that is sometimes tangential to both. The Hohmann transfer often uses the lowest possible amount of propellant in traveling between these orbits, but bi-elliptic transfers can use less in some cases.
A geosynchronous transfer orbit or geostationary transfer orbit (GTO) is a type of geocentric orbit. Satellites that are destined for geosynchronous (GSO) or geostationary orbit (GEO) are (almost) always put into a GTO as an intermediate step for reaching their final orbit.
Delta-v, symbolized as ∆v and pronounced delta-vee, as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or landing on a planet or moon, or an in-space orbital maneuver. It is a scalar that has the units of speed. As used in this context, it is not the same as the physical change in velocity of the vehicle.
Space debris is defunct artificial objects in space—principally in Earth orbit—which no longer serve a useful function. These include derelict spacecraft—nonfunctional spacecraft and abandoned launch vehicle stages—mission-related debris, and particularly numerous in Earth orbit, fragmentation debris from the breakup of derelict rocket bodies and spacecraft. In addition to derelict man-made objects left in orbit, other examples of space debris include fragments from their disintegration, erosion and collisions or even paint flecks, solidified liquids expelled from spacecraft, and unburned particles from solid rocket motors. Space debris represents a risk to spacecraft.
A geocentric orbit or Earth orbit involves any object orbiting Earth, such as the Moon or artificial satellites. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. More than 16,291 objects previously launched have undergone orbital decay and entered Earth's atmosphere.
A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is an orbit arranged so that it precesses through one complete revolution each year, so it always maintains the same relationship with the Sun.
In astrodynamics and aerospace, a delta-v budget is an estimate of the total change in velocity (delta-v) required for a space mission. It is calculated as the sum of the delta-v required to perform each propulsive maneuver needed during the mission. As input to the Tsiolkovsky rocket equation, it determines how much propellant is required for a vehicle of given empty mass and propulsion system.
In astrodynamics, orbital station-keeping is keeping a spacecraft at a fixed distance from another spacecraft or celestial body. It requires a series of orbital maneuvers made with thruster burns to keep the active craft in the same orbit as its target. For many low Earth orbit satellites, the effects of non-Keplerian forces, i.e. the deviations of the gravitational force of the Earth from that of a homogeneous sphere, gravitational forces from Sun/Moon, solar radiation pressure and air drag, must be counteracted.
AsiaSat 3, previously known as HGS-1 and then PAS-22, was a geosynchronous communications satellite, which was salvaged from an unusable geosynchronous transfer orbit (GTO) by means of the Moon's gravity.
The Kessler syndrome, proposed by NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) due to space pollution is high enough that collisions between objects could cause a cascade in which each collision generates space debris that increases the likelihood of further collisions. In 2009 Kessler wrote that modeling results had concluded that the debris environment was already unstable, "such that any attempt to achieve a growth-free small debris environment by eliminating sources of past debris will likely fail because fragments from future collisions will be generated faster than atmospheric drag will remove them". One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges difficult for many generations.
A supersynchronous orbit is either an orbit with a period greater than that of a synchronous orbit, or just an orbit whose apoapsis is higher than that of a synchronous orbit. A synchronous orbit has a period equal to the rotational period of the body which contains the barycenter of the orbit.
A Tundra orbit is a highly elliptical geosynchronous orbit with a high inclination, an orbital period of one sidereal day, and a typical eccentricity between 0.2 and 0.3. A satellite placed in this orbit spends most of its time over a chosen area of the Earth, a phenomenon known as apogee dwell, which makes them particularly well suited for communications satellites serving high-latitude regions. The ground track of a satellite in a Tundra orbit is a closed figure 8 with a smaller loop over either the northern or southern hemisphere. This differentiates them from Molniya orbits designed to service high-latitude regions, which have the same inclination but half the period and do not loiter over a single region.
Intelsat 2, formerly PAS-2, was a communications satellite operated by Intelsat which spent most of its operational life serving the Pacific Rim market from a longitude of 169° East. Launched in July 1994, the satellite was operated by PanAmSat until it merged with Intelsat in 2006. The spacecraft was renamed, along with the rest of PanAmSat's fleet, on 1 February 2007.
Spacecraft collision avoidance is the implementation and study of processes minimizing the chance of orbiting spacecraft inadvertently colliding with other orbiting objects. The most common subject of spacecraft collision avoidance research and development is for human-made satellites in geocentric orbits. The subject includes procedures designed to prevent the accumulation of space debris in orbit, analytical methods for predicting likely collisions, and avoidance procedures to maneuver offending spacecraft away from danger.
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