Orbits
An orbit is the curved path a satellite follows around Earth (or another celestial body), governed by gravity and velocity. The shape, altitude, and orientation of an orbit determine what a satellite can see, who it can talk to, and how long it lasts. From the 90-minute laps of low Earth orbit to the fixed hang of geostationary slots, understanding orbits is the foundation of space operations.
Terms in this Category
LEO (Low Earth Orbit)
The orbital region generally extending up to roughly 2,000 kilometers above Earth.
MEO (Medium Earth Orbit)
The orbital region between LEO and GEO, often used for navigation and some communications systems.
GEO (Geostationary Earth Orbit)
An orbit where satellites move at Earth's rotational rate and appear fixed over one longitude.
GSO (Geosynchronous Orbit)
A class of orbits with a period equal to Earth's rotation; geostationary orbit is a circular equatorial subset.
Geostationary Transfer Orbit (GTO)
A highly elliptical Earth orbit used to transfer a spacecraft from a low Earth parking orbit to geostationary orbit.
Sun-Synchronous Orbit (SSO)
A near-polar low Earth orbit where the satellite passes over any given point on Earth at the same local solar time on every pass.
TLE (Two-Line Element Set)
A standard orbital data format used to describe tracked objects for propagation and monitoring.
Inclination
The tilt of an orbit relative to Earth's equator.
RAAN (Right Ascension of the Ascending Node)
An orbital element describing where an orbit crosses the equatorial plane heading north.
Apogee
The point in an Earth orbit where a spacecraft is farthest from Earth.
Perigee
The point in an Earth orbit where a spacecraft is closest to Earth.
Orbital Period
The time a satellite takes to complete one full revolution around its primary body.
Mean Motion
The average angular speed of a satellite in its orbit, expressed as revolutions per day.
Semi-Major Axis
Half the longest diameter of an elliptical orbit, a fundamental Keplerian element that determines orbital energy and period.
Orbital Eccentricity
A dimensionless parameter between 0 and 1 that describes how much an orbit deviates from a perfect circle.
Nodal Precession
The gradual rotation of an orbit's ascending node caused by gravitational perturbations, primarily from a planet's equatorial bulge.
Hohmann Transfer
A fuel-efficient orbital maneuver that moves a spacecraft between two circular orbits using two engine burns: one to raise the apogee and another to circularize at the target altitude.
Transfer Orbit
An orbit used to move a spacecraft from one regime or altitude to another.
Apoapsis
The farthest point in an orbit around any central body; for Earth specifically, apogee is the more common term.
Periapsis
The point in an orbit where an object is closest to the central body; for Earth specifically, perigee is more common.
Argument of Perigee
An orbital element that defines the orientation of an elliptical orbit within its plane by measuring the angle from the ascending node to perigee.
True Anomaly
The angular position of a satellite along its orbital path, measured from periapsis as the angle between the periapsis direction and the satellite's current position.
Delta-V
A measure of the velocity change required or achieved by a spacecraft maneuver.
Ephemeris
A time-based set of position and velocity data describing an object's path.
Catalog Object
A tracked satellite, payload, rocket body, or debris object identified in an orbital catalog.
Conjunction
A close approach between two tracked objects that may require additional monitoring or maneuver planning.
Maneuver
A deliberate change in a spacecraft's trajectory, altitude, or orientation.
Propulsive Disposal
The planned use of onboard propulsion to deorbit or relocate a spacecraft at end of life.
Station Keeping
The periodic maneuvers performed by a satellite to maintain its designated orbital position against perturbing forces.
Cislunar
The region of space between Earth and the Moon, including lunar transfer and operational space.
Attitude Control
The system used to orient a spacecraft in three-dimensional space.
Reaction Wheel
A spinning flywheel used for precision attitude control of a spacecraft without expending propellant.
RCS (Reaction Control System)
Thrusters or control devices used to orient or fine-tune a spacecraft's motion.
Star Tracker
A optical sensor that determines a spacecraft's orientation by photographing star fields and matching them against an onboard star catalog.
Star Sensor
An optical device that detects star positions to determine a spacecraft's orientation, typically simpler than a full star tracker.
Momentum Wheel
A spinning flywheel that stores angular momentum to maintain spacecraft attitude stability without expending propellant.
Reaction Control System (RCS)
A system of small thrusters used for attitude control and small trajectory adjustments on spacecraft and launch vehicle upper stages.
Frequently Asked Questions
What is the most common orbit for satellites?
Low Earth Orbit (LEO) — roughly 200–2,000 km altitude — hosts the largest number of satellites, including Earth-observation, communications constellations, and the ISS.
Why do some orbits take 24 hours?
Geosynchronous orbits (GSO) match Earth's rotation period of 24 hours. When circular and equatorial (GEO), the satellite appears fixed over one longitude.
What is a Sun-synchronous orbit used for?
SSO keeps the Sun angle constant over every pass, making it ideal for Earth-observation imaging where consistent lighting simplifies change detection.