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Eccentricity is the measure of how circular an orbit is. An elliptical orbit’s eccentricity can range from zero to one. An orbit with eccentricity of zero is a perfect circle, and an eccentricity of one is a parabola, which means the orbiting object will escape the gravity of the object being orbited. For example, the International Space Station’s orbital eccentricity is 0.000172, close to zero, while Apollo 11’s orbit when catapulting from the Earth to the moon (called a translunar injection) was 0.97696, almost 1.