Orbital space and its regions
Most anthropogenic space activity is between altitudes of 100km to 36,000km. For the purposes of this article, we refer to this as “near Earth orbital space”, traditionally classified in three broad regions.
Low Earth Orbit (LEO) is generally understood to be at altitudes of 100 to 2,000km, with many anthropogenic space objects (ASOs) at around 500km. LEO has traditionally been dominated by scientific, earth observation and military missions, with some communications systems. The orbital period at these altitudes is around 90-120 minutes, and so, seen from Earth, any one satellite moves across the entire sky in a few minutes. Orbits in this region decay due to atmospheric drag, but the timescale varies significantly from a few months at the lowest altitudes to hundreds of years above ~1,200km.
Medium Earth Orbit (MEO) is at altitudes around 20,000 km, within a broad range. This is the regime of global navigation satellite systems such as GPS and GLONASS, but also recently of some internet communications systems such as O3B. Satellites in MEO take around 12 hours to orbit the Earth, and each satellite can communicate with a fairly large portion of the Earth’s surface. From the perspective of a person on Earth, a spacecraft in MEO will move across the sky much more slowly, and remain in the field of view for an hour.
Geosynchronous Orbit (GSO) is at altitude 35,786 km above mean sea level, where the orbital period is matched to Earth’s rotation rate about its spin axis. From the perspective of a person on Earth, a spacecraft in GSO will appear as a faint, stationary point source in the sky. GSO satellites can be seen from a large fraction of the Earth's surface. This orbit is traditionally where communications satellites have been placed, including those providing internet or phone services to remote locations. It takes a minimum of 0.24 seconds to send a signal from Earth to a satellite in GSO and back. GSO, in a limited range of orbital inclinations, has long been overcrowded and international regulations restrict its use. With large constellations, we are heading towards similar overcrowding in LEO.
Anthropogenic space objects
In late 2018, there were around 2,000 active satellites. SpaceX launches have already almost doubled the number of active satellites over the last two years, and all of these in LEO. From published proposals of various companies and states, it seems likely there will be a population of 100,000 or more by the end of the decade [3,4], and a recent filing with the ITU requests 327,000 satellites in a single project. The growth of all tracked anthropogenic space objects (ASOs) is shown in Fig. 1.
