Low Earth orbit

Lore & Background

Low Earth orbit is defined by an orbital period of 128 minutes or less, corresponding to at least 11.25 orbits per day, and an eccentricity under 0.25. While many sources define LEO by altitude, this measure is inherently ambiguous: altitude varies along an elliptic orbit, and even circular orbits can differ by up to 30 kilometers due to Earth’s oblate shape and local topography. Most altitude-based definitions, however, fall within the range implied by the 128-minute period, which Kepler’s third law links to a semi-major axis of about 8,400 kilometers—roughly 2,000 kilometers above Earth’s mean radius, near the inner Van Allen radiation belt.

The term “LEO region” refers to the space below this altitude, about one-third of Earth’s radius. This region is distinct from LEO orbits: highly elliptical orbits may pass through it at perigee but are not in LEO if their apogee exceeds 2,000 kilometers, and sub-orbital objects can reach it but re-enter the atmosphere. This distinction is critical for collision risk analysis, as objects not in LEO can still threaten satellites and debris within the region. The mean orbital velocity for a stable circular LEO is about 7.8 kilometers per second, though this varies with altitude—higher orbits require lower speeds.

Gravity in LEO is only slightly less than on Earth’s surface, but objects experience weightlessness due to perpetual free fall, where gravitational and centrifugal forces balance. Atmospheric drag from the thermosphere and exosphere causes orbital decay below roughly 600 kilometers, requiring orbit-raising technologies for very low Earth orbits (VLEO) below about 450 kilometers. LEO hosts the majority of artificial objects in space, all space stations, and the only human spaceflights beyond it were Apollo lunar missions and the Artemis II flyby.

Defining the Middle Band

What makes this band conceptually interesting is that its lower boundary is not a physical phenomenon but a convention agreed upon by the orbital mechanics community, while its upper boundary is anchored to something far more concrete: the altitude at which a satellite completes one revolution in exactly 24 hours, matching Earth's own rotation. Every satellite dwelling in this middle region circles the planet in less than a full day, with the fastest circular orbits at the lowest MEO altitudes completing a lap in approximately two hours. The region goes by several names depending on context—mid Earth orbit, intermediate circular orbit, or simply MEO—but all refer to the same swath of space. It is a transitional zone, neither hugging the atmosphere nor locked in step with the planet's spin, and that in-between quality shapes both its engineering challenges and its applications.

The Debris Problem and Orbital Permanence

Perhaps the most sobering aspect of the MEO region is the permanence of objects once placed there. Space debris in medium Earth orbit stays practically permanently in orbit around the planet, never decaying away as it might in lower altitudes. This means every defunct satellite, spent stage, or collision fragment becomes a long-term resident in a region already populated by operational navigation and communications constellations.

The debris population does not stop at the MEO boundary; it extends upward into the lowest high Earth orbits, the same altitude band where geostationary satellites are parked in so-called graveyard orbits after their useful lives end. The result is a continuous corridor of residual objects stretching from the upper edge of MEO into the geostationary belt. For operators maintaining GPS, GLONASS, Galileo, BeiDou, and the newer broadband constellations in this altitude range, the persistent clutter elevates the importance of collision avoidance and long-term orbital sustainability to a degree that distinguishes MEO from both lower and higher orbital regimes.

Reader's Guide

Low Earth orbit is the region of space where the vast majority of artificial objects reside, with the highest concentration occurring around 800 kilometers in altitude and the upper boundary of the region defined as 2,000 kilometers, near the start of the inner Van Allen radiation belt. Objects in LEO orbit Earth between the denser thermosphere and this radiation belt, and those below approximately 300 kilometers experience rapid orbital decay due to atmospheric drag from the thermosphere and exosphere. The mean velocity required to maintain a stable circular orbit in LEO is about 7.8 kilometers per second, though this varies with altitude, being higher at lower altitudes. The gravitational pull in LEO is only slightly less than at Earth’s surface, but spacecraft and their occupants experience continuous weightlessness because the gravitational force is balanced by centrifugal force in free fall.

All human space stations have operated within LEO, and the only human spaceflights beyond it were the Apollo lunar missions and the Artemis II lunar flyby. The LEO region is also defined as the area below 2,000 kilometers, and objects passing through this zone, even if their orbits are highly elliptical or sub-orbital, are tracked due to collision risks with the many satellites there. Equatorial low Earth orbits allow rapid revisit times over low latitudes and require less launch energy by using Earth’s rotation, while polar and Sun-synchronous orbits provide coverage for higher latitudes. Very low Earth orbits, below about 450 kilometers, have recently been considered but require novel technologies to counteract rapid orbital decay.

Did You Know?

• The International Space Station orbits at about 400 to 420 km and decays by about 2 km/month, requiring re-boosting a few times a year.
• The oldest piece of space debris in LEO is the Vanguard 1 satellite and its third-stage.