# Q & A: Picking a launch site

Q:
How does choosing a diiferent launch site affect the escape velocity of a rocket?
- Anonymous
Australia
A:
The Earth turns on its axis, once a day. Objects on the surface of the Earth, therefore, are moving eastward at a speed which depends on their latitude. At the north or south poles, an object at rest on the surface is not moving, while an object on the equator is moving with a speed of 465 meters/second eastward. To go into orbit, spacecraft are nearly always launched going eastward, to take advantage of the fact that they are already going in this direction (some spacecraft take a different path and go over the poles -- it takes more fuel to get into this orbit, however).

To escape the Earth's gravity entirely, the spacecraft needs to be going at the escape velocity in a non-rotating frame. Launching the spacecraft going eastward again increases the speed of the spacecraft with no additional expense in fuel. Launching westward takes more fuel to escape.

The speed at which something's going while on the spinning Earth is that 465 meters per second times the cosine of the latitude (latitude=0 at the equator, and +- 90 degrees at the poles). That way you can pick up more speed for free by launching closer to the equator. But the cosine function doesn't change rapidly near zero latitude, so you have some freedom to choose a good launch site.

Tom J.

(published on 10/22/2007)