Decent, but the task of defending Earth from asteroids get's old pretty fast. If there was an asteroid storm as well as something else that was a hazard (aliens, our own space junk, satellites, etc.), it could become more intriguing, especially if you gained different amounts of $ for each type of item.
Also, some bonus items could also make it more fun. Making your little Earth Defender device into a tiny temporary black hole would be fun, or an item that cleared the screen of all current hazards.