## Glossary term: Escape Velocity

**Description:** Escape velocity is the minimum speed that an object must have to permanently escape from the gravitational field of a celestial body. This is a scalar quantity so should more correctly be referred to as escape speed; however, the term escape velocity is commonly used. The simplest case is that of a body that is spherically symmetric – which is an excellent approximation for describing stars and planets. In this case, the escape velocity at a distance r from the center of a body of mass m, is given by √(2Gm/r), where G is the gravitational constant. At the surface of a spherical body the distance from its center is its radius. This means that the escape velocity at the surface of an approximately spherical celestial body depends on the radius and its mass. In the case of the Sun, it is 617.5 kilometers per second (km/s), and for Earth, 11.2 km/s. It is 2.4 km/s for the Moon, which means that an object on the Moon needs to attain a lower speed to escape the Moon's gravity than an object on Earth would need to leave Earth's gravity.

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