Orbital Eccentricity
A dimensionless parameter between 0 and 1 that describes how much an orbit deviates from a perfect circle.
Explanation
Eccentricity (e) is one of the six Keplerian orbital elements. A perfectly circular orbit has e = 0. An elliptical orbit has 0 < e < 1. A parabolic escape trajectory has e = 1, and hyperbolic flybys have e > 1. The eccentricity determines the difference between a body's closest approach (periapsis) and farthest distance (apoapsis). Highly eccentric orbits are used for Molniya communications (e ≈ 0.74), where a satellite lingers for hours near apogee over high latitudes. Most LEO constellations operate near-circular orbits with very low eccentricity. In practice, eccentricity is perturbed by gravitational anomalies, third-body effects, and solar radiation pressure. Station-keeping maneuvers periodically correct these perturbations. For Earth observation, eccentricity drift changes ground-track repeat patterns; for communications, it alters coverage timing.
Why It Matters
Eccentricity determines whether a satellite maintains constant altitude (circular) or varies altitude dramatically (elliptical). This affects everything from coverage patterns to radiation exposure and propellant budgets.
Concept Map
How Orbital Eccentricity connects to other glossary terms:
Frequently Asked Questions
Can eccentricity be zero in practice?
No. Perturbations from Earth's gravity field, the Moon, and solar radiation pressure always introduce some tiny eccentricity, though operators can keep it very close to zero with station keeping.
What is the most eccentric Earth orbit used operationally?
Molniya orbits with eccentricities around 0.74 are among the most eccentric operational orbits.
Sources
Last updated: July 1, 2026