perigee

Perigee is the point in the orbit of an object around the Earth where the object is closest to Earth's center. It is the Earth-centered form of the general term periapsis. In elliptical orbits, the perigee distance, r_p, is the smallest distance to Earth and is given by r_p = a(1 − e), where a is the semi-major axis and e is the eccentricity. If the orbit is circular (e = 0), every point is technically a perigee and the distance remains constant.

In orbital mechanics, speed is greatest at perigee due to conservation of angular momentum, as described by the vis-viva equation. Perigee is contrasted with apogee, which is the farthest point in the orbit from Earth. The perigee distance is a key parameter in mission planning, affecting atmospheric drag, gravitational interactions, and the delta-v required for orbital maneuvers.

Nomenclature-wise, perigee is specific to orbits around Earth. The analogous closest-approach term for orbits around other bodies is periapsis, with the farthest point called apoapsis or apogee. The Moon’s perigee, for example, varies over time due to perturbations from the Sun and planets, as do the perigees of many artificial satellites.