Eccentricity measures how stretched an orbit is: 0 is a perfect circle, values approaching 1 are long ellipses, and 1 or more means escape. It is one of the six classical orbital elements, shaping the difference between an orbit's highest and lowest points.
Understanding Eccentricity
Reading shape from one number
Together with the semi-major axis (size), eccentricity fully fixes an orbit's geometry: perigee radius = a(1−e), apogee radius = a(1+e). A quick mental model: at e = 0.1 the orbit is already visibly oval with apogee about 22% higher than perigee; at e = 0.5 apogee is three times perigee's radius; by e = 0.9 the orbit is a long cigar spending nearly all its period near apogee.
Eccentricity in mission design
Designers choose eccentricity to buy behaviour: transfer orbits use it to reach altitude cheaply; science missions use elongated ellipses to sample plasma environments across radial distance each revolution; and communications designers exploit apogee dwell — the Molniya and Tundra families park their slow apogee segment over a chosen hemisphere, delivering hours of high-elevation coverage per orbit to latitudes that geostationary satellites see poorly.