Atmospheric drag

Atmospheric drag is the force exerted by air on a moving object, opposing its motion relative to the surrounding atmosphere. It persists in the thin upper atmosphere, where it can alter satellite orbits.

Drag equation: An equation estimating fluid resistance from density, speed, reference area, and a drag coefficient. It expresses how atmospheric density and speed set the scale of drag.
Orbital decay: The gradual lowering of an orbit caused by energy loss or other perturbations. Drag removes orbital energy from satellites passing through residual atmosphere.
Fluid: A substance that continually deforms under applied shear stress, including liquids and gases. Air is a fluid, so its motion and contact with objects generate drag.
Lift: A force exerted by a fluid on a moving object, directed perpendicular to the relative flow. Unlike drag, lift acts across the flow and can support or redirect an object.
Drag coefficient: A dimensionless measure of an object's resistance to fluid flow under specified conditions. It captures how shape and flow regime affect atmospheric resistance.
Atmospheric reentry: The passage of a spacecraft or other object from space into a planetary atmosphere. Drag slows reentering objects and helps determine their trajectory and heating.
Viscosity: A fluid's resistance to deformation and internal flow. Viscous stresses contribute to drag, especially near an object's surface.
Buoyancy: The net upward force on an immersed object caused by pressure differences in a fluid. Buoyancy arises from displaced fluid, whereas drag opposes relative motion.
Dynamic pressure: The kinetic energy per unit volume of a fluid flowing relative to an object. It combines air density and relative speed in common drag calculations.
Ballistic coefficient: A measure relating an object's mass to its drag area and drag coefficient. It summarizes how strongly an object responds to atmospheric drag.

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