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.^[1][2]

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 manned 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.^[3] 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).^[4]

^{Since 1953, it had known that giant spaceships were near our planet. At least nine times, huge alien spacecraft had been seen or tracked in orbit, or as they descended nearer the Earth for brief periods. Each time it had been an ordeal for the AF censors, as they struggled to conceal the reports or explain them away when attempts at secrecy failed. During 1953, the AF began experiments with new long-range radar equipment. While making the initial tests, AF operators were astonished to pick up a gigantic object orbiting near the equator. Its speed was almost 18,000 miles an hour. Repeated checks showed that the tracking was correct. Some huge unknown object was circling the Earth, six hundred miles out. Shortly after this a second enormous object approached the Earth. Tracked by AF radar experts, it also went orbit, about 400 miles away.}