Space Debris & Orbital Mechanics Glossary
Essential terminology for space situational awareness, satellite tracking, and orbital operations.
Active Debris Removal (ADR)
Technologies and missions designed to physically remove large debris objects from orbit, such as robotic capture, net systems, or laser ablation.
Apogee
The point in an elliptical orbit farthest from Earth. For circular orbits, the apogee equals the perigee.
B* (B-Star) Drag Term
A parameter in TLE data that represents the atmospheric drag effect on a satellite, incorporating the drag coefficient, cross-sectional area, and atmospheric density.
Collision Probability (Pc)
The statistical probability that two objects will collide during a close approach, typically computed using the Chan or Foster method with position covariance data.
Conjunction
A close approach between two objects in orbit. Conjunctions are assessed for collision risk through conjunction assessment processes.
Conjunction Assessment (CA)
The systematic process of identifying close approaches between orbital objects, computing collision probabilities, and recommending avoidance actions.
Conjunction Data Message (CDM)
A standardized message format containing the orbital states, covariance data, and collision probability for a conjunction event, issued by the 18th Space Defense Squadron.
Eccentricity
An orbital element describing the shape of an orbit. A value of 0 indicates a perfect circle; values approaching 1 indicate highly elliptical orbits.
GEO (Geostationary Earth Orbit)
A circular orbit at approximately 35,786 km altitude above the equator, where a satellite's orbital period matches Earth's rotation, keeping it stationary relative to the ground.
Inclination
The angle between an orbital plane and the equatorial plane, measured in degrees. The ISS orbits at 51.6° inclination.
Kessler Syndrome
A theoretical scenario proposed by Donald Kessler in 1978 where the density of objects in LEO reaches a critical threshold, causing a self-sustaining cascade of collisions generating ever-more debris.
LEO (Low Earth Orbit)
Orbits with altitudes between approximately 200 km and 2,000 km. Home to the ISS, most Earth observation satellites, and mega-constellations like Starlink.
Mean Motion
The average angular velocity of a satellite in its orbit, typically expressed in revolutions per day. The ISS has a mean motion of approximately 15.5 rev/day.
Mega-Constellation
A satellite constellation consisting of hundreds to tens of thousands of satellites, typically in LEO, providing global communications or Earth observation coverage (e.g., Starlink, Kuiper).
MEO (Medium Earth Orbit)
Orbits between LEO and GEO, typically 2,000–35,786 km altitude. Used by navigation constellations (GPS, Galileo, GLONASS).
MOID (Minimum Orbit Intersection Distance)
The minimum distance between two orbital paths, regardless of the objects' positions along their orbits. Used as a first-pass filter in conjunction screening.
NETRA
Network for Space Objects Tracking and Analysis — ISRO's project to develop indigenous space situational awareness capabilities for India.
Orbital Debris
Non-functional human-made objects in Earth orbit, including defunct satellites, spent rocket stages, fragmentation debris, and mission-related objects.
Passivation
The process of depleting all stored energy (propellant, battery charge, pressurized gases) from a decommissioned spacecraft to prevent accidental explosions.
Perigee
The point in an elliptical orbit closest to Earth.
RAAN (Right Ascension of Ascending Node)
An orbital element describing the orientation of the orbital plane relative to the vernal equinox, measured along the equatorial plane.
SGP4 (Simplified General Perturbations 4)
The standard analytical orbit propagation algorithm used with TLE data, accounting for Earth oblateness and atmospheric drag effects.
Space Situational Awareness (SSA)
The comprehensive knowledge of the space environment including the location, trajectory, and characteristics of all objects in orbit, as well as space weather conditions.
Space Traffic Management (STM)
The coordination of all space activities — including launch, orbital operations, and reentry — to ensure safety, sustainability, and efficiency in the use of the space environment.
TCA (Time of Closest Approach)
The predicted moment when two objects in a conjunction event will be at their minimum distance from each other.
TLE (Two-Line Element)
A standardized two-line data format encoding the orbital elements of an Earth-orbiting object, used with the SGP4 algorithm for orbit prediction.