ESCAPE VELOCITY

The radial speed which a particle or larger body must attain in order to escape from the gravitational field of a planet or star. The escape velocity from Earth is approximately 7 miles per sec.; from Mars, 3.2 miles per sec.; and from the Sun, 390 miles per sec. In order for a celestial body to retain an atmosphere for astronomically long periods of time, the mean velocity of the atmospheric molecules must be considerably below the escape velocity.