Words have meaning: (Launch) Window
Word today that WISE launch is now scheduled for Monday, December 14, 2009 (6:09:33 – 6:23:51 a.m. PST ). For those of you paying attention, you may notice that while the date changes, the time of the launch is still the same. This time range is the “launch window.”
The timing for the launch window can be as short as minutes, and as long as weeks. It all depends on where you are trying to put your spacecraft. For any mission that must operate in a particular orientation with respect to the Earth (this would be pretty much anything that directly orbits the Earth), the window is on the order of minutes. That’s because the Earth turns relatively quickly, rotating 15 degrees per hour. The more precise the needed orbit, the shorter the window becomes. It’s hard to compensate for an orbital offset once you’re in space.
Missions with more distant targets, like the moon, other planets, comets, or one of the Lagrangian points, have a better chance of compensating for changes in launch time. Small perturbations of the path early on translate into much larger shifts in position after they’ve travelled millions of miles. This means the timing of the launch window is typically a function of the relative positions of bodies in the solar system. In fact, more important than knowing where the planets/moons/etc. are now, is knowing where they WILL be at the time you want the spacecraft to rendezvous with them.
After all, if they miss, there’s no turning back…
(Hat tip to palmerin for the ability to sleep in!)