From the Super Mario Wiki, the Mario encyclopedia
The objective of the game is to drag the Earth through an obstacle course in space. First, pairs of watermelon-like planets fire several small ammo at Earth, which it must avoid. Some comets fly in after, and the player must then capture a stranded astronaut (which is being blocked by some giant asteroids) in the earth's gravity. This allows what appears to be the moon to orbit the Earth and shoot small beams. The player must use these to shoot down attacking planets (two of which resemble Jupiter and Saturn and orbit the larger, third planet), which shoot great quantities of ammo. Only shooting down the main enemy planet is necessary as its defeat takes down the other two planets as well. If the Earth gets hit once, the player loses.
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