One of the most interesting thing about old arcade games is that all of them are incredibly fatalistic by nature. When you insert your coin, all you're doing is playing to delay the inevitable - Death either by the enemies or, if you're good, the machine crashing. Quite a cheery outlook; it doesn't matter what you do, the aliens will still kill you and invade Earth. Have fun.

From Brad HD, defending-the-earth-from-the-outside

And the description of your utterly futile mission