To those who think this is just a copy of Desktop Tower Defense, please Google "Tower Defense Games" and then stop complaining. There are 216 TDs. This is NOT copying. All this needs is a 1.5 with a menu button, a reworked economy, and badges.
(Great game btw, I say again, 5/5) Inertia applies here, the planets aren't that far away, just fling yourself there and wait all of 10 or so seconds. If you have an issue with the size, press F11 to expand your browser.