Throw a ball hard enough and it never comes back. Escape velocity is the minimum speed needed to break free from a body's gravity without further propulsion. For Earth at the surface, that figure is about 11.2 kilometres per second.

The number depends on the mass and radius of the body. The Moon's escape velocity is only 2.4 kilometres per second, which is why early lunar missions could return with relatively small rockets. Jupiter's is over 59 kilometres per second, a major obstacle to exploring its moons.

Related figures

• Earth surface: 11.2 km per second
• Moon surface: 2.4 km per second
• Mars surface: 5.0 km per second
• Sun surface: 617 km per second

Rockets rarely reach escape velocity directly. They climb into orbit first, then add velocity with a final burn. This staging approach saves enormous amounts of fuel.