Low Earth orbit

Low Earth orbit is the region of Earth orbits at roughly 160 to 2,000 kilometers above the planet’s surface. Objects there travel quickly and experience measurable atmospheric drag.

Orbital velocity: The speed an object needs to follow a particular orbit around a celestial body. Objects in low Earth orbit circle Earth at about 7.8 kilometers per second.
International Space Station: A multinational crewed research laboratory assembled in orbit around Earth. Its roughly 400-kilometer orbit is a prominent example of sustained human activity in low Earth orbit.
Geostationary orbit: A circular equatorial orbit in which a satellite appears fixed above one point on Earth. Its much greater altitude gives persistent regional coverage, unlike low-orbit satellites.
Space debris: Nonfunctional human-made objects and fragments orbiting Earth. Abandoned hardware and collision fragments accumulate in heavily used low orbits.
Sputnik 1: The first artificial satellite of Earth, launched by the Soviet Union in 1957. Its orbit demonstrated that a human-made object could remain in low Earth orbit.
Atmospheric drag: A force opposing motion through a fluid, including the tenuous upper atmosphere. Even sparse atmospheric particles gradually slow satellites at these altitudes.
Earth observation satellite: A satellite designed to collect information about Earth’s land, oceans, atmosphere, or activity. Low altitude supports detailed imaging and frequent observation of the planet.
Medium Earth orbit: The range of Earth orbits between low Earth orbit and geostationary orbit. It is the intermediate altitude regime used by systems such as GPS.
Kessler syndrome: A proposed cascade in which collisions generate debris that triggers further collisions in orbit. Dense traffic in low Earth orbit makes collision cascades a major concern.
Explorer 1: The first artificial satellite launched by the United States, in 1958. Its instruments discovered the Van Allen radiation belts during an early low-orbit mission.

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