At first, I thought the hitbox in front of the player was too large, but then I noticed the asteroid images are apparently being resized, so I suspect the hitbox for the asteroids is assuming the original dimensions, causing collisions in apparently open space. I'm only assuming this is what's happening, since I don't want to wade through that much javascript to find out.

Since this seems to be a demo i'm not going to get too pedantic about the gameplay, but that definitely is something you would want to address in an actual game.