The radial speed which a particle or larger body must attain in order to escape from the gravitational field of a planet or star. When friction is neglected, the escape velocity is
G is the universal gravitational constant (see gravitation);
m is the mass of the planet or star; and r is the radial
distance from the center of the planet or star. Also called escape speed.
Escape velocity from Earth is 7 miles/sec; from Mars it is 3.2 miles/sec; and from the Sun it is 390 miles/sec. In order for a celestial body to retain an atmosphere for long periods of time, the mean velocity of the atmospheric molecules must be considerably below the escape velocity. </dd>
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use