Rule number one of defending alien invasions: don't think you can protect your planet by sticking up a couple of slowly-orbiting weapon satellites that can't fire at the same time.

Because that's what the planet on Astroattack has done, and it makes defending your planet against the aliens a bit bloody hard. You control the satellites by taking account of the planet's gravity and firing them at the swarms of aliens streaming at the planet. But there are a bout a million aliens for each of your satellites and it's all a bit hopeless and you'll never win and the planet will be destroyed in the end anyway, so what's the point? What's the bloody point?