A low Earth orbit (LEO) is an Earth-centered orbit with an altitude of roughly 2,000 km (1,200 mi) or less, or with at least 11.25 periods per day (an orbital period of 128 minutes or less) and an eccentricity less than 0.25. LEO is an arbitrary definition. Most of the manmade objects in outer space are in LEO.
The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for circular orbits, the altitude above ground can vary by as much as 30 km (19 mi) (especially for polar orbits) due to the oblateness of Earth’s spheroid figure and local topography. (The Planet Earth is not perfectly spherical)
The LEO region is defined by some sources as the region in space that LEO orbits occupy. Some highly elliptical orbits may pass through the LEO region near their lowest altitude (or perigee) but are not in an LEO Orbit because their highest altitude (or apogee) exceeds 2,000 km (1,200 mi). Sub-orbital objects can also reach the LEO region but are not in an LEO orbit because they re-enter the atmosphere. The distinction between LEO orbits and the LEO region is especially important for analysis of possible collisions between objects which may not themselves be in LEO but could collide with satellites or debris in LEO orbits.
The International Space Station and all prior crewed space stations to date have conducted operations in LEO. The altitude record for human spaceflights in LEO was Gemini 11 with an apogee of 1,374.1 km (853.8 mi).
Apollo 8 was the first mission to carry humans beyond LEO on December 21–27, 1968. The Apollo program continued during the four-year period spanning 1968 through 1972 with 24 astronauts who flew lunar flights but since then there have been no human spaceflights beyond LEO.
