low Earth orbit

The term low Earth orbit (LEO) indicates orbits closest to the Earth, a common criteria being they are those with an orbital period of no more than 128 minutes, which for a circular orbit, corresponds to about 2000 km from Earth's surface. (Another criteria sometimes used is that a LEO is an orbit that does not exceed 2000 km, the region within that altitude termed the LEO region.) These include the majority of Earth satellites for communications, surveillance, and research, including a few astronomy satellites, such as the Hubble Space Telescope (HST). As of the end of 2024, on the order of 10,000 satellites are in LEOs. Just a few years ago, this number was on the order of 1,000, but recently-developed satellite-based communication-methods use satellite constellations of hundreds or thousands of small satellites in LEOs, and multiple corporations are independently building such infrastructures, and thousands of additional LEO satellites are currently scheduled for future launch.

Such orbits are the most economical to launch to and are below the Van Allen belts (orbits within the belts are dangerous to humans and equipment, and orbits below them are somewhat protected from dangerous particles), but are subject to orbital decay from atmospheric drag. The only manned space-flights to go beyond low Earth orbits have been the Apollo Moon missions.

Alternatives to LEOs are orbits further from Earth, which include as Mid Earth orbits (MEOs), geosynchronous orbits (GEOs, that have an orbital period of exactly one Earth day), Solar orbits, and orbits of Earth-Sun Lagrangian points (commonly used for astronomy).

Also, Leo is a well-known constellation, used in the designations of galaxies, etc., such as Leo A.