Semi-Major Axis
Half the longest diameter of an elliptical orbit, a fundamental Keplerian element that determines orbital energy and period.
Explanation
The semi-major axis (a) is the most important of the six Keplerian orbital elements. For a circular orbit, it equals the orbital radius. For an elliptical orbit, it is half the sum of the periapsis and apoapsis distances. The semi-major axis determines the orbital period via Kepler's third law and the total specific orbital energy (energy per unit mass). Two orbits with the same semi-major axis but different eccentricities have the same period but different shapes. Semi-major axis is measured from the center of the primary body, not from the surface. For Earth observation, the semi-major axis determines the ground-track repeat cycle. For communications, it determines coverage area and signal latency. Changes to the semi-major axis require delta-v, making it a critical parameter for station keeping and maneuver planning.
Why It Matters
The semi-major axis defines the orbital regime — LEO, MEO, or GEO — and drives the satellite's period, energy, and coverage characteristics. It is the primary variable in launch targeting and orbit design.
Concept Map
How Semi-Major Axis connects to other glossary terms:
Frequently Asked Questions
Is semi-major axis always the same as orbital radius?
Only for circular orbits (eccentricity = 0). For elliptical orbits, semi-major axis is the average of apogee and perigee distances.
How is semi-major axis changed?
By firing thrusters to change orbital velocity. A prograde burn increases the semi-major axis; a retrograde burn decreases it.
Sources
Last updated: July 1, 2026