In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero. It is the speed needed to "break free" from a gravitational field without further propulsion.
For a spherically-symmetric body, escape velocity is calculated by the formula
where G is the universal gravitational constant (G=6.67×10−11 m3 kg−1 s−2), M the mass of the planet, star or other body, and r the distance from the center of gravity.
In this equation atmospheric friction (air drag) is not taken into account. A rocket moving out of a gravity well does not actually need to attain escape velocity to do so, but could achieve the same result at walking speed with a suitable mode of propulsion and sufficient fuel. Escape velocity only applies to ballistic trajectories.
The term escape velocity is actually a misnomer, and it is often more accurately referred to as escape speed since the necessary speed is a scalar quantity which is independent of direction (assuming a non-rotating planet and ignoring atmospheric friction).