Frictional force opposing motion. Generally speaking, the term is applied to the aerodynamic drag exerted by a planetary atmosphere on a spacecraft. it is the drag effect which causes the decay of the orbits of close Earth satellites; the resistance of the tenuous outer layers of the atmosphere, even at altitudes of a few hundred kilometres, is sufficient to reduce the KINETIC ENERGY of a spacecraft and eventually cause it to descend into denser layers and burn up. Aerodynamic drag is also used as a deliberate means of braking a re-entering spacecraft prior to a soft-landing with the aid of parachutes, RETRO-ROCKETS or a combination of both.
