Gravity
Gravity is the attraction between masses. It pulls objects toward Earth, keeps satellites in orbit, governs planetary motion, and shapes nearly every trajectory used in spaceflight.
Technical meaning
For many mission-design problems, engineers begin with Newtonian gravity: the acceleration caused by a body decreases with the square of the distance from its center and depends on the body’s mass. Gravity does not disappear in orbit; an orbiting spacecraft is continuously accelerated toward the body it circles.
Why it matters
Gravity determines launch-energy requirements, orbital velocity, transfer trajectories, escape conditions, landing dynamics, and the stability of operational orbits. It also creates useful opportunities, including gravity-assist flybys, while producing perturbations that may require station-keeping.