[Pitch]

Ok, deselect your asteroid (Crtl + D) and you will notice a white border around your asteroid.

The best thing to do is take your eraser, set it to "Brush mode" about 15 Pixels wide and go around all the edges. This will neaten the whole thing up for you. It helps to zoom in close when doing this.

Again, make sure you are working on the texture layer.

Once done, it should look something like this.