Not a bad start, though feels like it could use some more polish. For example limit lives, less it not go on eternally, and maybe levels where the speed/amount of asteroids overtime increase? Seems like the easiest way to balance gameplay. Unfortunately there's also a big luck factor here in how if the asteroid just so happens to be below you when you spawn there's no way to avoid it.
Not a bad start though! Works well just: isn't all that captivating after a while.
-cd-