When you "zoom out" and scaling the tiles down, you are effectively making more of them visible. So, instead of showing a grid of 30x30 tiles (or whatever) you will be showing 40x40 tiles (you will have to calculate this). Then you have to draw them closer together.
This is fairly simple if your ship stays in the middle of the screen. So your zoom point is constant.
This is fairly simple if your ship stays in the middle of the screen. So your zoom point is constant.