The asteroid rules for Fleet Captain can be used as a springboard for designing other space hazards.
First, let's begin by acknowledging that in the real world asteroids are rarely dense enough to pose a threat as a group. Still, we all love the asteroid field sequence in Empire Strikes Back, so we darn well need asteroid rules in our game. From a design point of view, asteroids (and any other terrain) needs to providing insteresting variation to the game without so overwhelming the other rules as to warp overall design. In other words, we want asteroids, they need to be exciting but also nod vaguely towards realism and we have to make sure the game doesn't become about them.
Let's start with a simple assumption: if we are using asteroids then the whole map is a segment of the asteroid field. Every ding dang hex has one or more smallish and uninteresting asteroids in them that for all purposes we ignore mechanically. The asteroid markers we are going to place on the board represent areas of particularly high danger. You can think of those markers as the "big rocks" if you like, but a more sophisticated reading of the simulation at work here would be that those markers indicate crests in the density waves that propogate through the asteroid field.
Anyway, each asteroid marker needs to contain two pieces of information, a direction arrow and a density rating from 1 to 6. Something like this:
You don't need many of these asteroids to jazz up a space battle, one of each size 1 to 6 is probably enough for most space maps.
Okay, let's look at the mechanics for these babies. Each turn asteroids are moved after you place movement markers but before you start flipping cards. Roll d6 for each asteroid marker and move it that many hexes in a straight line. If you move off the map place it opposite where it exited, next turn it will re-enter the map there.
Did you just pass an asteroid through a ship or land on the same hex as one? Good, 'cause now its time to learn what happens when you try to occupy the same space as a few million tons of space rock. Roll a d6 to avoid contact. If you get above the asteroid density rating, you are safe. If you don't, you're in trouble. Roll d20 damage. Shields do not protect you, you are just straight up taking a big pile of Hull Damage here. The one thing that might save you is your Laser(s). Each Laser shaves off d6 worth of incoming asteroid damage. If the asteroid marker tried to end it's movement in your hex, move it one more hex. Nature may abhor a vacuum but Fleet Captain abhors having two objects in the same hex. That's a big pain in the ass when using minis.
The same basic procedure applies when you try to fly through an asteroid hex. The two main differences are that A) you can attempt the Special Maneuver called Avoid Hazard to increase your chances of successfully negotiating the field and B) if you try to end your turn on an asteroid hex it is your vessel that moves one extra hex, not you.
The other important thing to know about asteroids is that they attenuate weapons fire that passes through them. Subtract the density rating from the attack roll of any fire passing through the hex.
Where to initially place the asteroids is something of an art form. One way to do it is to try to guess which rows on the map will be most conducive to screwing up the battle for everyone, then rolling dice to determine the hexes along those rows that the asteroids begin in. The longest rows on the map are usually a good bet, something like this:
But obviously how you set up the fleets will have an impact on choosing where to put the asteroids. In most scenarios you don't want the initial placement to destroy any vessels before the first turn of the cards.
I prefer my asteroids to all float in the same direction, but what do I know?