Escape velocity is the minimum speed needed for a free, non-propelled object to escape from the gravitational influence of a primary body, such as a planet or moon, and reach an infinite distance from it. It is typically stated as an ideal speed, ignoring atmospheric friction. Although the term "escape velocity" is common, it is more accurately described as a speed than a velocity because it is independent of direction. More generally, escape velocity is the speed at which the sum of an objects kinetic energy and its gravitational potential energy is equal to zero. An object which has achieved escape velocity is neither on the surface nor in a closed orbit of any radius.

Escape velocity depends on the mass of the planet or moon that an object is leaving. For example, a spacecraft leaving the surface of Earth needs to be going about 7 miles per second, or nearly 25,000 miles per hour, to leave without falling back to the surface or falling into orbit. The escape velocity for Jupiter would be many times that of Earths because Jupiter is so huge and has so much gravity. The formula for escape velocity is vesc = sqrt(2GM/r), where G is the gravitational constant, M is the mass of the attracting mass, and r is the distance from the center of that mass. Escape velocity decreases with altitude and is equal to the square root of 2 times the velocity necessary to maintain a circular orbit at the same altitude.

Escape velocity is important in space exploration because it determines the amount of energy needed to launch a spacecraft from a planet or moon. If the escape velocity is too high, it may be difficult or impossible to launch a spacecraft from the surface. On the other hand, if the escape velocity is too low, a planet or moon cannot long retain an atmosphere if its escape velocity is near the average velocity of the gas molecules making up the atmosphere.