Low Earth orbit

A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of artificial objects are in LEO. LEO is at an altitude less than about one third of the Earth's radius, or about 2000 km. 

No human spaceflight took a place beyond LEO except of the Apollo program, see Apollo program overview. LEO is the easiest orbit to get to and stay in. 

Low Earth Orbit (LEO) and Geosynchronous Orbit (GEO) 

The mean orbital velocity needed to maintain a stable LEO is about 7.8 km/s, which is 28,000 km/h. 

This speed depends on the exact altitude of the orbit. For example, if we calculate the orbital speed for a circular orbit of 200 km, the orbital velocity is 7.79 km/s. For example, for a higher 1,500 km orbit the velocity is reduced to 7.12 km/s. 

The pull of gravity in LEO is only slightly less than on the Earth's surface. This is because the distance to LEO from the Earth's surface is much less than the Earth's radius. However, an object in orbit is in a permanent free fall around Earth, because in orbit the gravitational force and the centrifugal force balance each other out. As a result, spacecraft in orbit continue to stay in orbit, and people inside or outside such spacecraft continuously experience weightlessness.

Objects in LEO encounter atmospheric drag from gases in the thermosphere (approximately 80–600 km above the surface) or exosphere (approximately 600 km and higher), depending on orbit height. Orbits of satellites that reach altitudes below 300 km decay fast due to atmospheric drag. Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner Van Allen radiation belt.

Examples of objects on LEO:

The International Space Station (ISS) is in a LEO about 400 km to 420 km above Earth's surface, and needs re-boosting a few times a year due to orbital decay.

The Chinese Tiangong space station was launched in April 2021, and currently orbits between about 340 km and 450 km.

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