Apo-eup

Apo-eup, also known as apogeu-up, describes a specific type of lunar or planetary orbit raising maneuver in spaceflight. It refers to a powered change in a spacecraft's trajectory performed at or near the apogee (the point in an orbit farthest from the central body) designed to increase the overall energy and altitude of the orbit.

Specifically, an apo-eup burn raises the perigee (the closest point in an orbit to the central body). When performed at apogee, the thrust primarily increases the velocity vector in the direction of motion, resulting in a higher, more circular orbit. Repeated apo-eup maneuvers can be used to progressively raise a spacecraft's orbit until the desired altitude is achieved. This is a common technique for transferring spacecraft from a low Earth orbit (LEO) to a higher geostationary transfer orbit (GTO), or from a GTO to geostationary orbit (GEO).

The effectiveness of an apo-eup burn relies on Oberth effect principles, which state that a change in velocity (delta-v) is more effective at increasing orbital energy when performed at a lower gravitational potential (i.e., when the spacecraft is traveling faster, which typically occurs near the periapsis or, in this case, near apogee). While not technically at the lowest point of potential, apogee offers a compromise between efficiency and available window of time to perform the burn compared to a periapsis burn for raising the opposite point in the orbit.

Compared to a similar maneuver performed at perigee (a peri-eup burn), an apo-eup burn is often chosen for its strategic benefit of directly affecting the perigee altitude, which can be critical for avoiding atmospheric drag in lower orbits or achieving specific orbital characteristics. The choice between apo-eup and peri-eup maneuvers depends on the specific mission requirements and constraints.