Orbits are ellipses, not circles. The closest point to the body being orbited is the periapsis, and it is where the spacecraft moves fastest. The farthest point, the apoapsis, is where it moves slowest.

Operators plan manoeuvres around these points because they offer the best fuel efficiency. A burn at periapsis raises the apoapsis. A burn at apoapsis raises the periapsis. Choosing the right point saves propellant that cannot be replaced once the tank runs dry.

Named variants

• Perigee around Earth
• Periapsis around the Moon or a planet
• Perihelion around the Sun

Aerobraking uses the periapsis deliberately. The spacecraft dips into the upper atmosphere at its closest approach, shedding speed through drag, then climbs back out. Repeat the process enough times and the orbit shrinks without burning fuel.