Parking orbit and graveyard orbit

Parking orbit and graveyard orbit are two used Earth orbits. An orbit is the curved path through which objects in space move around a planet or a star. The 1967 Treaty’s regime and customary law enshrine the principle of non-appropriation and freedom of access to orbital positions.

Space Law and International Telecommunication Laws combined to protect this use against any interference. The majority of space-launched objects are satellites that are launched in Earth’s orbit (a very small part of space objects – scientific objects for space exploration – are launched into outer space beyond terrestrial orbits).

It is important to precise that an orbit does not exist: satellites describe orbits by obeying the general laws of universal attraction. Depending on the launching techniques and parameters, the orbital trajectory of a satellite may vary. Sun-synchronous satellites fly over a given location constantly at the same time in local civil time: they are used for remote sensing, meteorology or the study of the atmosphere. Geostationary satellites are placed in a very high orbit; they give an impression of immobility because they remain permanently at the same vertical point of a terrestrial point (they are mainly used for telecommunications and television broadcasting).

Parking orbit and graveyard orbit are different notions. A parking orbit is an orbit around the Earth or the Moon in which a spacecraft can be placed temporarily in order to prepare for the next step in its program. Sometimes called a dump orbit, a graveyard orbit is an Earth orbit into which communications satellites may be moved at the end of their operational lives, where there is no risk of their interference or collision with working satellites in their normal orbits. Let’s have a closer look.

Parking orbit and graveyard orbit

A parking orbit

A parking orbit is a temporary orbit used during the launch of a satellite or other space probe. A launch vehicle boosts into the parking orbit, then coasts for a while, then fires again to enter the final desired trajectory. The alternative to a parking orbit is direct injection, where the rocket fires continuously (except during staging) until its fuel is exhausted, ending with the payload on the final trajectory.

Parking orbit and graveyard orbit are different terms. There are several reasons why a parking orbit may be used. Geostationary spacecraft require an orbit in the plane of the equator. Getting there requires a geostationary transfer orbit with an apogee directly above the equator. Unless the launch site itself is quite close to the equator, it requires an impractically large amount of fuel to launch a spacecraft directly into such an orbit. Instead, the spacecraft is placed with an upper stage in an inclined parking orbit. When the craft crosses the equator, the upper stage is fired to raise the spacecraft’s apogee to geostationary altitude (and often reduce the inclination of the transfer orbit, as well). Finally, a circularization burn is required to raise the perigee to the same altitude and remove any remaining inclination.

Also, in order to reach the Moon or a planet at a desired time, the spacecraft must be launched within a limited range of times known as the launch window (in the context of spaceflight, launch period is the collection of days and launch window is the time period on a given day during which a particular vehicle must be launched in order to reach its intended target). Using a preliminary parking orbit before final injection can widen this window from seconds or minutes, to several hours. For the Apollo program’s manned lunar missions, a parking orbit allowed time for spacecraft checkout while still close to home, before committing to the lunar trip.

Examples of a parking orbit include the Apollo program, which used parking orbits, for all the reasons mentioned above except those that pertain to geostationary orbits. When the American Space Shuttle orbiter launched interplanetary probes such as Galileo, it used a parking orbit to deliver the probe to the right injection spot. The Ariane 5 does not use parking orbits. This simplifies the launcher since multiple restart is not needed, and the penalty is small for their typical GTO mission, as their launch site is close to the equator.

A graveyard orbit

Parking orbit and graveyard orbit are used for different tasks. 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 (either an orbit with a period greater than that of a synchronous orbit, or just an orbit whose apoapsis, apogee in the case of the Earth, 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 barycentre of the orbit) well above geosynchronous orbit. Satellites are typically 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 manoeuvre is too large. For satellites in geostationary orbit and geosynchronous orbits, the graveyard orbit is a few hundred kilometres above the operational orbit. The transfer to a graveyard orbit above geostationary orbit requires the same amount of fuel as a satellite needs for about three months of orbital station-keeping. It also requires a reliable attitude control during the transfer manoeuvre. Most satellite operators try to perform such a manoeuvre at the end of their satellites’ operational lives.

While the standard geosynchronous satellite graveyard orbit results in an expected orbital lifetime of millions of years, the advent of large mega constellations of thousands of satellites after the late 2010s necessitated new approaches for deorbiting to assure earlier removal of the objects once they have reached end-of-life. Both OneWeb and SpaceX have committed to regulatory authorities that they will move the satellites 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.

List of orbits

A geocentric orbit or Earth orbit involves any object orbiting Planet Earth, such as the Moon or artificial satellites. Geocentric (having the Earth as its centre) orbits are organised as follow:

1) Low Earth orbit (LEO): geocentric orbits with altitudes (the height of an object above the average surface of the Earth’s oceans) from 100 to 2 000 kilometres. Satellites in LEO have a small momentary field of view, only able to observe and communicate with a fraction of the Earth at a time, meaning a network or constellation of satellites is required in order to provide continuous coverage. Satellites in lower regions of LEO also suffer from fast orbital decay (in orbital mechanics, decay is a gradual decrease of the distance between two orbiting bodies at their closest approach, the periapsis, over many orbital periods), requiring either periodic reboosting to maintain a stable orbit, or launching replacement satellites when old ones re-enter.

2) Medium Earth orbit (MEO), also known as an intermediate circular orbit: geocentric orbits ranging in altitude from 2 000 kilometres to just below geosynchronous orbit at 35 786 kilometres. The most common use for satellites in this region is for navigation, communication, and geodetic/space environment science. The most common altitude is approximately 20 000 kilometres which yields an orbital period of twelve hours.

3) Geosynchronous orbit (GSO) and geostationary orbit (GEO) are orbits around Earth at an altitude of 35 786 kilometres matching Earth’s sidereal rotation period. All geosynchronous and geostationary orbits have a semi-major axis of 42 164 kilometres. A geostationary orbit stays exactly above the equator, whereas a geosynchronous orbit may swing north and south to cover more of the Earth’s surface. Communications satellites and weather satellites are often placed in geostationary orbits, so that the satellite antennae (located on Earth) that communicate with them do not have to rotate to track them, but can be pointed permanently at the position in the sky where the satellites are located.

4) High Earth orbit: geocentric orbits above the altitude of 35 786 kilometres.

The freedom of use of orbits and frequencies

Near-Earth space is formed of different orbital layers. Terrestrial orbits are limited common resources and inherently repugnant to any appropriation: they are not property in the sense of law. Orbits and frequencies are res communis (a Latin term derived from Roman law that preceded today’s concepts of the commons and common heritage of mankind; it has relevance in international law and common law).

It’s the first-come, first-served principle that applies to orbital positioning, which without any formal acquisition of sovereignty, records a promptness behaviour to which it grants an exclusive grabbing effect of the space concerned. Geostationary orbit is a limited but permanent resource: this de facto appropriation by the first-comers – the developed countries – of the orbit and the frequencies is protected by Space Law and the International Telecommunications Law. The challenge by developing countries of grabbing these resources is therefore unjustified on the basis of existing law.

Denying new entrants geostationary-access or making access more difficult does not constitute appropriation; it simply results from the traditional system of distribution of access rights. The practice of developed States is based on free access and priority given to the first satellites placed in geostationary orbit.

The geostationary orbit is part of outer space and, as such, the customary principle of non-appropriation and the 1967 Space Treaty apply to it. The equatorial countries have claimed sovereignty, then preferential rights over this space.

These claims are contrary to the 1967 Treaty and customary law. However, they testify to the concern of the equatorial countries, shared by developing countries, in the face of saturation and seizure of geostationary positions by developed countries. The regime of res communis of outer space in Space Law (free access and non-appropriation) does not meet the demand of the developing countries that their possibilities of future access to the geostationary orbit and associated radio frequencies are guaranteed. New rules appear necessary and have been envisaged to ensure the access of all States to these positions and frequencies.