Article on everything2 describing the topology of the Asteroids universe as a torus (donut shape).

…So theÂ topological part is this: when youÂ fly up off the top edge of the screen, youÂ magically appear at the same position on the bottom of the screen, and vice-versa. The same is true of the left and right edges. So consider this: from theÂ pilot’s perspective, he or she is flying around in aÂ 2-dimensional universe with noÂ edge, ie: where every spot the ship is in looks locally like two-dimensionalÂ Euclidean space. Mathematicians call this sort of thing aÂ manifold, specifically aÂ 2-manifold. I’m going to represent it like this, as it is represented on the game screen:

The edges ‘a’ and ‘b’ are labelled to indicate that the top and bottom are the sameÂ location in space (a), as are the left and right (b). In fact (when you think about it) the four corners areÂ actually the same point! If you were to try to connect this up as a realÂ physical surface (this is called anembedding), you could think about it as a sheet of paper where you firstÂ glued edge a-top to a-bottom (giving you a rolled-up paperÂ tube), and then bent the resulting tube around gluing b-left to b-right. You would end up with…wait for it…a donut! Or, in topological jargon, aÂ torus. So when you are playing Asteroids, you are actually playing it on a torus, mathematically speaking. (The advantage to this explanation is that in a bar, there’s always a napkin around that you can use to demonstrate. Sometimes there are even videogames.)…