Periapsis

Periapsis is the point in an orbit where the orbiting body is closest to its primary. Its specific name depends on the primary, such as perihelion for an orbit around the Sun.

Orbit: The path of an object moving under the gravitational influence of another body or system. Periapsis is one location on an orbit.
Specific angular momentum: An object's angular momentum per unit mass, conserved in motion under a central force. Its conservation helps fix the orbit's closest-approach distance.
Apoapsis: The point in an orbit farthest from its primary. It is periapsis's opposite point in an orbit.
Hohmann transfer orbit: An idealized transfer orbit using two engine burns to move between coplanar circular orbits. Its transfer ellipse has periapsis at one endpoint orbit.
Primary (astronomy): The main body around which another object orbits. The primary defines which point in the orbit counts as closest.
Vis-viva equation: An equation relating orbital speed, distance from the focus, and the orbit's semimajor axis. It gives the body's speed when its distance equals periapsis distance.
Perihelion: The point in an orbit where an object is closest to the Sun. This is the specific name for periapsis in a solar orbit.
Aerobraking: A technique that uses atmospheric drag to reduce a spacecraft's orbital energy. A spacecraft's low periapsis determines where atmospheric drag can act.
Ellipse: A closed plane curve whose points have a constant sum of distances from two foci. Bound two-body orbits are ellipses, with the primary at one focus.
Specific orbital energy: The sum of an orbiting body's kinetic and gravitational potential energy per unit mass. Together with angular momentum, it determines the orbit's periapsis.

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