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From Nicholas Cross:
A mysterious event has disrupted the orbit of thousands of asteroids sending them on a collision course with the fledgling Mars colonies.Tap to aim the orbital defence cannon, vaporise the debris and help the settlers escape before the shield fails!Once aimed tap to fire or hold down to charge the cannon further for a bigger explosion. Be careful each shot will cause heat to build and if the cannon overheats then you will have to wait for a coolant flush to restore operation.