Everyone knows the old saying, but does it always hold true? Generally, yes it does. You throw a ball up, it comes right back down to you. Birds fly in the sky, but eventually they too return to the ground. But what about rocket ships, which launch right out of the Earth’s atmosphere? What is so different from throwing a fast ball straight up and launching a space shuttle? The answer lies in speed. An object must be traveling at a certain velocity in order to escape the pull of the Earth’s gravity. That speed is called escape velocity, and on Earth it is 11 kilometers per second. On other planets in our solar system, the escape velocity is different. The exact speed needed to breach the gravitational pull of the planet depends on factors such as the size of the planet. For example, if you happened to be out for a run on the surface of Mars’ moon Deimos, you just might launch yourself out into space! The escape velocity for Deimos is only 4.7 meters per second.
For more on the factors that determine a planet's gravitational pull see Page 4.
