For one, you can only see a portion of the planet at any one time, so you have to keep spinning it, ever mindful of income threats. (writingsofmassdeduction.com)
Future projects, like the Large Synoptic Survey Telescope and NASA's NEOCam, will scan the sky for incoming threats. (discovermagazine.com)
This means you not only need to be constantly scanning to unlock more guns and complete the level, but you also need to think about how many shots you have left in your gun and if you have enough to deal with incoming threats. (pocket-lint.com)