Implemented in v1.0.2 to allow values up to 5000. However, also implemented a hard cap on the asteroid spawn probability (per asteroid) so that it cannot be increased past a certain point.
Technically, the more common asteroids would hit the cap before the others, so 5000 was chosen as the maximum multiplier for this reason. Some of the lowest asteroid probabilities that I checked were around 0.0008, so multiplying by 5000 would get even these low-probability asteroids up to the limit (which was around 1 or 2; anything higher than this would fill up the screen and lag the game, especially with more than one space platform)