Gas Drag (Aerodynamic, Tidal)
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Gas drag is a retarding force operating on a body moving through a gaseous medium, which causes the body to lose momentum and energy. Gas drag is particularly important in a dense gaseous protoplanetary disk, where it causes loss of orbital momentum, and decrease in orbital eccentricity, inclination, and semi-major axis of a body in orbit around the central star. Aerodynamic gas drag functions primarily for small (less than 100 km) bodies, while tidal or gravitational gas drag functions primarily for intermediate-sized (∼Earth-size) bodies.
Gas drag is defined as a force caused by dynamical interactions with the surrounding gaseous material that acts in the opposite direction to the motion of the body. It is primarily important in planetary dynamics because it functions as one of the main processes that remove energy from orbiting bodies, causing their orbits to decrease in eccentricity and inclination and, to a lesser extent, semi-major axis.