Low-Earth orbit

A low Earth orbit (LEO) is an orbit around Earth with an altitude between 160 kilometers (99 mi) (orbital period of about 88 minutes), and 2,000 kilometers (1,200 mi) (about 127 minutes). Objects below approximately 160 kilometers (99 mi) will experience very rapid orbital decay and altitude loss.

With the exception of the 24 human beings who flew lunar flights in the Apollo program during the four-year period spanning 1968 through 1972, all human spaceflights have taken place in LEO or below. The International Space Station conducts operations in LEO. The altitude record for a human spaceflight in LEO was Gemini 11 with an apogee of 1,374.1 kilometers (853.8 mi). All crewed space stations to date, as well as the majority of satellites, have been in LEO.

Objects in LEO encounter atmospheric drag from gases in the thermosphere (approximately 80–500 km up) or exosphere (approximately 500 km and up), depending on orbit height. Due to atmospheric drag, satellites do not usually orbit below 300 km. Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner Van Allen radiation belt.

The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude. Calculated for circular orbit of 200 km it is 7.79 km/s and for 1500 km it is 7.12 km/s. The delta-v needed to achieve low Earth orbit starts around 9.4 km/s. Atmospheric and gravity drag associated with launch typically adds 1.3–1.8 km/s to the launch vehicle delta-v required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h).

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