Satellite drag

Satellite drag is the decelerating force on an orbiting object caused by collisions with residual atmospheric gas. It removes orbital energy, typically lowering the orbit and shortening the satellite’s lifetime.

Atmospheric drag: A resistive force caused by an object moving through a gas. Satellite drag is atmospheric drag acting where the atmosphere is extremely tenuous.
Low Earth orbit: An Earth-centered orbit with an altitude generally below about 2,000 kilometers. Residual atmospheric gas is dense enough here to make drag operationally significant.
Orbital mechanics: The study of the motion of bodies under gravity, especially spacecraft and celestial objects. Satellite drag is a small perturbation to motion otherwise governed mainly by gravity.
Solar radiation pressure: A force exerted on an object when it absorbs or reflects sunlight. It changes satellite orbits through photon momentum rather than atmospheric collisions.
Aerodynamic drag equation: An equation estimating drag force from gas density, speed, reference area, and drag coefficient. It relates orbital conditions and spacecraft properties to the force producing orbital energy loss.
Spacecraft reentry: The passage of a spacecraft from orbit into the atmosphere, where aerodynamic heating and forces grow rapidly. Drag can lower a satellite until atmospheric reentry begins.
Specific orbital energy: An orbiting body's kinetic plus gravitational potential energy per unit mass. Drag removes this energy, changing the size and shape of the orbit.
Earth's oblateness: The Earth's departure from a perfect sphere, which alters gravitational forces around the planet. Its gravity perturbs orbits without dissipating energy like satellite drag.
Thermosphere: The atmospheric layer above the mesosphere, where temperatures rise with altitude and many low-orbiting satellites travel. Its variable density supplies most of the gas responsible for drag on low-Earth-orbit satellites.
Orbit determination: The estimation of an object's orbit from tracking measurements and dynamical models. Unmodeled drag causes predicted positions to diverge from tracking observations.

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