Low Earth orbit, commonly abbreviated LEO, is the region of Earth orbit in which satellites travel relatively close to the planet, generally from about 160 kilometres above Earth's surface to approximately 2,000 kilometres in altitude. The exact boundary used for LEO can vary slightly among technical and regulatory contexts, so it is better understood as an orbital regime rather than a single precisely defined altitude.
Satellites in LEO travel at several kilometres per second and complete an orbit in roughly 90 minutes to a few hours depending on altitude. Because they are close to Earth, they can provide relatively low communication latency and high-resolution observations compared with satellites in much higher orbits. LEO is used for Earth observation, crewed spacecraft, scientific missions, reconnaissance, technology demonstrations, communications constellations and many other applications.
The reason a spacecraft remains in orbit is that its forward velocity causes it to continually fall towards Earth while its trajectory curves around the planet. At a typical LEO altitude, orbital velocity is around 7.5 to 8 kilometres per second, depending on altitude and orbital parameters. Gravity provides the centripetal acceleration required for orbital motion.
A spacecraft does not need to fire its engines continuously to remain in a circular orbit; after achieving the appropriate velocity, it follows a largely ballistic trajectory. Atmospheric drag, however, remains significant in LEO because even very high altitudes contain extremely thin traces of atmosphere. Drag gradually removes orbital energy and lowers a satellite's orbit unless the spacecraft performs manoeuvres to compensate.
LEO satellites have relatively short orbital periods because they travel close to Earth and therefore experience stronger gravitational acceleration than satellites at higher altitudes. A satellite at several hundred kilometres altitude can circle Earth approximately every hour and a half. Earth rotates beneath the orbital plane, so a satellite's ground track moves across the planet. Depending on inclination, a LEO spacecraft may cover particular latitudes or eventually pass over much of Earth's surface.
Polar and sun-synchronous LEO orbits are especially useful for Earth observation because they can provide systematic global coverage. The combination of altitude, inclination, orbital plane, eccentricity and local crossing time determines the geometry of a LEO mission and strongly affects its observations or communications coverage.
One of the principal advantages of LEO is reduced signal propagation distance. A radio signal travelling between a user and a LEO satellite generally has a much shorter path than one travelling to a geostationary satellite, making LEO attractive for communications services that require relatively low latency. LEO also allows Earth-observation instruments to obtain fine spatial detail because the sensor is comparatively close to the surface. These advantages come with trade-offs.
A single LEO satellite has a limited field of view and moves quickly relative to locations on Earth, so continuous coverage of a particular region requires multiple satellites or an appropriate orbital design. Communications constellations consequently use many spacecraft arranged across multiple orbital planes to provide frequent or continuous access.
Atmospheric drag is a defining operational concern in LEO. The atmosphere becomes progressively thinner with altitude but does not abruptly disappear, and the remaining particles collide with spacecraft surfaces. Solar activity can heat and expand the upper atmosphere, increasing density at a given altitude and thereby increasing drag. Operators therefore monitor space weather and orbital decay. Satellites with propulsion can perform reboost manoeuvres, while spacecraft without sufficient propulsion may eventually descend into denser atmosphere and undergo destructive re-entry.
The lifetime of a satellite can therefore depend strongly on its altitude, ballistic coefficient, solar conditions and available propulsion. Atmospheric drag also causes orbital debris to behave differently at different altitudes, with some objects naturally re-entering relatively quickly and others remaining for decades or longer.
LEO is heavily used because spacecraft can be launched into it with less energy than is required to reach higher orbits, although the exact launch energy depends on inclination and destination. The region contains a large and growing population of satellites and debris. Earth-observation spacecraft, crewed orbital stations, scientific instruments and communications constellations all operate there.
The International Space Station is a prominent example of a large crewed facility in LEO, while many commercial broadband constellations deploy large numbers of satellites in the same general orbital regime. This concentration makes orbital coordination, collision avoidance, tracking and responsible end-of-life disposal increasingly important. Space operators use tracking networks and conjunction-assessment systems to identify objects that might pass close to operational spacecraft.
LEO is sometimes incorrectly treated as a synonym for any satellite that is close to Earth or for any low-altitude spacecraft. The technical concept refers specifically to an orbit around Earth, not merely to a spacecraft's physical proximity at one instant. A suborbital vehicle can travel through the LEO altitude range without entering orbit because it lacks sufficient horizontal velocity to remain continuously above Earth.
Similarly, a spacecraft may temporarily pass through LEO altitudes during an ascent or descent without operating in LEO. LEO should also be distinguished from medium Earth orbit, highly elliptical orbit and geostationary orbit, which have different altitude ranges and operational characteristics. Altitude alone is not sufficient to describe every important orbital property because inclination and eccentricity can dramatically alter coverage and mission behaviour.
LEO has become increasingly important in modern space technology because advances in small satellites, electric propulsion, reusable launch vehicles, mass-produced spacecraft, onboard computing and automated operations have made large constellations more practical. The same characteristics that make LEO attractive also create engineering and regulatory challenges. Thousands of satellites can increase communications and observation capacity, but they also increase the number of potential conjunctions and the importance of space-traffic coordination.
Operators must plan reliable command and control, collision avoidance and end-of-life disposal. LEO therefore represents both a highly useful orbital environment and a finite shared operational region. Its future development depends not only on launch and satellite technology but also on spectrum coordination, orbital safety, international rules and responsible debris management.
