Expert: Paul Soderman - 1/15/2007

Question
Why are the rockets launched only in the morning? Does earth's momentum or atmosphere has anything to do with it?

Answer
Hi Kevin
Good question - launch windows.  You can launch a rocket anytime you like, but rocket engineers like to take advantage of anything that might help.  You are right, launching toward the east gives them added momentum and speed due to the earth' rotation.  And launching in the early morning gives them added speed relative to the sun because of the earth's motion around the sun.   See http://spaceplace.nasa.gov/en/kids/ds1_mgr.shtml for a graphical illustration of those effects.

A morning launch also allows for good visibility and a fresh crew to get things going.  That said, launches from California Vandenburg often occur at sundown.  Launch windows are somewhat flexible and really depend on someone's analysis of all the variables, both earthwise and celestial.
Paul