What's a launch window?
Kurt Warner - Super Bowl Champion Quarterback: (Kurt Warner standing in front of a chalkboard that shows a football play) Good quarterbacks try to make sure conditions are right when they show a pass. They want everyone on the team to be in just the right place, at the just the right time for a play to be successful. NASA does the same thing when they plan launch windows for a mission.
(Video of a shuttle launching) A "launch window" is that time period in which a launch has to happen for a space craft to meet its target. If you were supposed to rendezvous with the spacestation, the Moon, or a distant planet, (Animation showing an object following a trajectory from an inner orbiting object to the outer orbiting object) your launch has to be timed just right so that your ship's path intersects with your target's orbit, and they meet in space at the exact same time and place. Just like NASA, I don't aim for where my target is when I throw the ball, (Video of a football play showing Kurt Warner passing the football to his teammate), I aim for where he is going to be when the ball intersects his path.
If you miss your launch window, you might miss your target completely. For me, that could mean an incomplete pass, or even worse, an interception. But when the timing works just right, we're talking touchdown! (Kurt Warner throws a football pass.)
For more about launch windows and orbital mechanics, visit NASA.gov