Depends of what you mean by "simple" . There's no other elements introduced after you unlock the full game (yet). Still, the challenge comes from you sending more and more satellites. My personal high score is 37, and there is the alternative objective to unlock all the achievements, including all the "combos", which you unlock when you shoot down a couple of satellites with a single rocket...

So yes, Sputnik Panik is a "simple game" in the sense that there are not a lot of content, but we think that the gameplay is deep enough to keep you engaged for a while, if you enjoy this kind of things