lowperigee

In orbital mechanics, low perigee refers to an orbit in which the perigee—the point of closest approach to the central body—is at a relatively small distance, often near the planet’s surface. This configuration is typical of highly elliptical orbits where the perigee is much lower than the apogee. While the term is most commonly applied to Earth orbits, the concept applies to any body with a substantial atmosphere or other drag forces.

The perigee distance rp is given by rp = a(1 − e), where a is the orbit’s semi-major axis and e is eccentricity. The altitude at perigee above a planet’s surface is hp = rp − RE, with RE being the planet’s radius. A low perigee therefore corresponds to a small hp. Even when the apogee is far away, a low perigee keeps the spacecraft on each pass within or near the upper atmosphere, subjecting it to increased drag and heating.

Consequences and uses: In low-perigee orbits, atmospheric drag becomes a dominant force, progressively lowering the orbit’s altitude and shortening its lifetime. This factor is critical for mission planning, influencing deorbit maneuvers, end-of-life disposal, and reentry predictions. Solar activity can expand the upper atmosphere, enhancing drag and accelerating decay. Conversely, operators may raise perigee to extend lifetime or lower it for controlled reentry.

Earth-specific notes: For Earth, perigees below roughly 150–200 kilometers experience substantial drag, leading to rapid decay. In airless environments, such as the Moon, the absence of atmosphere means drag is negligible, and low perigee behavior is governed mainly by gravity and orbital perturbations.

See also: perigee, apogee, eccentricity, orbital decay, deorbit burn, reentry.