In simple terms
Orbital eccentricity is a single number that says how far an orbit departs from a perfect circle.
It is written as e. A value of 0 is a circle, and the larger the number, the more stretched the path.
How it works
NASA’s Jet Propulsion Laboratory defines eccentricity as the ratio of half the distance between the two foci of the orbit ellipse to the semi-major axis, or e = c divided by a.
The value sorts orbits into families. At e = 0 the orbit is circular. Between 0 and 1 it is an ellipse, which is what planets, moons and most comets follow. At e = 1 the path is a parabola.
Above 1 the path is a hyperbola. The object is not bound by the gravity of the body it passes, so it arrives once and leaves for good.
That threshold is how astronomers spot visitors from other star systems. Planets can nudge a local comet onto a slightly hyperbolic path, so a value just over 1 is not proof on its own. Oumuamua, found in October 2017, had an eccentricity of about 1.2, well beyond anything a planetary nudge could explain.
Why it matters
Eccentricity fixes how much an object’s distance and speed change across one orbit. That shapes satellite coverage, spacecraft fuel budgets and how strongly a comet heats up near the Sun.
It is also the test that separates objects belonging to our solar system from objects passing through it.
Where you’ll see it
- Orbit tables for asteroids and comets
- Satellite mission planning and ground station coverage
- News about interstellar objects such as 3I/ATLAS
- Descriptions of exoplanet orbits
Example
NASA says 3I/ATLAS was on a hyperbolic trajectory, meaning it moved too fast to be held by the Sun’s gravity and never followed a closed orbital path.
Often confused with
Eccentricity is not inclination. Eccentricity describes the shape of the orbit. Inclination describes its tilt relative to a reference plane.
Key facts
- JPL defines eccentricity as the ratio of half the distance between the foci of the orbit ellipse to the semi-major axis, e = c/a.1
- An eccentricity of 0 is a circular orbit, values between 0 and 1 are elliptical, and a value of 1 is parabolic.1
- An eccentricity greater than 1 means an unbound hyperbolic trajectory.2
- Oumuamua, discovered on 19 October 2017, had an eccentricity of about 1.2, which showed it originated outside the solar system.2
- NASA describes 3I/ATLAS as travelling on a hyperbolic trajectory, too fast to be bound by the Sun's gravity.3
Related concepts
In the news
Quick checkWhat eccentricity value tells astronomers an object is not bound to the Sun?Show answer
Greater than 1. That is a hyperbolic path, so the object passes once and does not come back.
Sources
- NASA Jet Propulsion Laboratory, Solar System Dynamics. Glossary: Eccentricity. Undated (accessed 16 September 2026)
- European Space Agency, Near-Earth Object Coordination Centre. Oumuamua: the first interstellar object found in our Solar System. Undated (accessed 16 September 2026)
- NASA Science. 3I/ATLAS Facts and FAQs. 28 May 2026 (accessed 16 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 16, 2026Researched and drafted with AI assistanceReport an issue