This really isn't an asteriods clone, the gameplay is very different with a lot of features missing such as asteroids splitting into two smaller ones when shot, bullets wrapping around the screen and momentum based movement with an engine that pushes in one direction. I would recommend playing asteriods and analysing the design of the game.
The way this game is currently, there is no reason to move from the bottom of the screen as you can just blow away all incoming threats.