What is escape velocity?

What is escape velocity?

Escape velocity is the speed that an object needs to be traveling to break free of a planet or moon's gravity well and leave it without further propulsion. For example, a spacecraft leaving the surface of Earth needs to be going 7 miles per second, or nearly 25,000 miles per hour to leave without falling back to the surface or falling into orbit.

A Delta II rocket blasting off. A large amount of energy is needed to achieve escape velocity. Photo from Jet Propulsion Laboratory's Planetary Missions & Instruments image gallery http://www-b.jpl.nasa.gov/pictures/browse/pmi.html
Since escape velocity depends on the mass of the planet or moon that a spacecraft is blasting off of, a spacecraft leaving the moon's surface could go slower than one blasting off of the Earth, because the moon has less gravity than the Earth. On the other hand, the escape velocity for Jupiter would be many times that of Earth's because Jupiter is so huge and has so much gravity.

Body	Mass	Escape Velocity in Kilometers/ Second	Escape Velocity in Miles/Hour
Ceres (largest asteroid in the asteroid belt)	1,170,000,000,000,000,000 kg	.64 km/sec	1430.78 mph
The Moon	73,600,000,000,000,000,000 kg	2.38 km/sec	5320.73 mph
Earth	5,980,000,000,000,000,000,000 kg	11.2 km/sec	25038.72 mph
Jupiter	715,000,000,000,000,000,000,000,000 kg	59.5 km/sec	133018.2 mph
Sun	1,990,000,000,000,000,000,000,000,000 kg	618. km/sec	1381600.8 mph
Sirius B (a white dwarf star)	2,000,000,000,000,000,000,000,000,000 kg	5,200. km/sec	11625120 mph

One reason that manned missions to other planets are difficult to plan is that a ship would have to take enough fuel into space to blast off of the other planet when the astronauts wanted to go home. The weight of the fuel would make the spaceship so heavy it would be hard to blast it off of Earth!