Escape velocity is the velocity needed for an object to escape a planet’s (or a different object’s) gravitational pull. It is defined as:
where vesc is the escape velocity [km/s], G is the gravitational constant = 6.67259 x 10-__11__ N m2 /kg2, M is the mass of the planet [kg], and R is the radius of the planet [m].
| Object | Velocity |
|---|---|
| Moon | 2 km/s |
| Earth | 11.2 km/s |
| Sun | 620 km/s |
| White Dwarf | 6,500 km/s (assuming M~2x1030 kg, R=6.4x106 m) |
| Neutron Star | 23,000 km/s (assuming M~4x1030 kg, R=10,000 m) |
| Black Hole | greater than the speed of light! |
In the physical sciences, we usually define a black hole in terms of its escape velocity:
A black hole is an object whose escape velocity is greater than the speed of light.