The maps need to be done in increments. If you made massive changes it has a tendency to corrupt the save file. When I work on a map I do a little and save it (as map001). Then do some more and save it as a new map (map002). When I save the map, I also then load it back in from the save file to check that the map file is still being read by the editor (which is the same as loading it into a game). This way if the game has problems loading the map you can go back to a previous file and try loading it. At least you'll have some point of reference to which save was corrupted.

Once you get the hang of it, you'll be saving your maps a little more often. It seems as though when you do massive changes or deletions without saving and reloading, that the map file gets corrupted and the game will not load it. Once you finish your map and the game loads it, you can delete all the saves up to the current map.

Also, don't get carried away with asteroids as too many in a system (anything over 6-8) will start to cause problems.