Orbital Mechanics
Perigee, Apogee, and Trajectory Burns
Orbital mechanics (or astrodynamics) is the application of Newtonian physics and celestial mechanics to the motion of spacecraft. Key terms: Perigee — the closest point of an elliptical orbit to Earth; Apogee — the farthest point. A spacecraft in an elliptical orbit moves faster at perigee and slower at apogee (Kepler's Second Law). To change an orbit, spacecraft fire their engines in controlled burns — the direction, duration, and timing of the burn determine the new orbital parameters.
- Perigee raise burn: Fires engines at or near apogee to raise the perigee — used to prevent unintended re-entry and shape the orbit for subsequent manoeuvres.
- Apogee raise burn: Fires engines at or near perigee to raise the apogee — used to increase the orbit's highest point.
- Delta-v (Δv): The change in velocity produced by an engine burn — the fundamental currency of orbital manoeuvring. Artemis II's TLI burn produces a Δv of ~1,274 ft/sec (~388 m/s).
- Translunar Injection (TLI): A prograde burn (in the direction of motion) that accelerates the spacecraft enough to escape Earth's gravity well and travel to the Moon. Apollo missions used Saturn V's S-IVB stage; Artemis II uses Orion's Service Module engine.
- Free-return trajectory: A figure-eight path around the Moon and back to Earth that requires no additional propulsion after TLI — if engines fail, gravity alone returns the crew safely. Apollo 13 used this principle after its abort.
- Proximity operations: Close-range manoeuvres to demonstrate Orion's ability to rendezvous and dock — critical for future Artemis missions using Gateway or HLS.
● Tracked since April 02, 2026 · last seen July 18, 2026 · updates as the daily brief publishes
See it in today’s brief.
Daily current affairs with every static concept explained in place.
Read the daily brief