Compute the square of the distance instead of the actual distance, and compare that to the square of the collision threshold. That should be a bit faster. If most asteroids are the same size, you can reuse the same value for the collision threshold without recomputing it.
Another useful trick is to first do a simple check based on bounding boxes, and compute the distance only if the bounding boxes intersect. If they don't, then you know (for cheap) that the two objects aren't colliding.