Sigh. This poor mission, which has certainly suffered its share of slings and arrows, has had its launch postponed until September. The launch slip last week messed things up a bit to start, of course. And now the launch constraints butt up against the launch of the Phoenix Mars probe in early August. Worse, the orbital path of Dawn is somewhat complicated -- it needs to slip into orbit around two separate asteroids, making launch windows tight.
Think of it this way: you are in a car doing laps at a racetrack at 100 kph. You have a bow and arrow, and you have to shoot at a target off to the side. You can wait until the last millisecond and shoot the arrow quickly, or give yourself an extra half-second and shoot the arrow a little more slowly. That range in time is your launch window.
Oh -- did I mention the target is moving? And oh -- you have to hit a bullseye within a tenth of a millimeter of the exact center of the target. And oh -- later on there is a second target you have to hit as well, starting from the first target. That second target is moving too.
That's why the launch constraints on these missions can be so severe. Things in the solar system have to be just so.
So: next time you want to complain about NASA letting launches slip, or that they are a bunch of boobs, try that little experiment above and see how well you do. Those guys can thread a needle from millions of kilometers away, and they do this sort of thing all the time.
So Dawn's launch will be in September, when the targets line up well again. Until then, we have Phoenix to look forward too, and that's a fantastic mission as well.