Sometimes the shortest route to somewhere _is_ right through an asteroid belt (even with the movement penalty), especially with ships that don't move many spaces per turn.
Here's how it should work, (if I remember correctly):
Moving into an asteroid field cancels all the rest of your movement for that turn, and all but one of your movement points for the following turn.
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Ex:
So, for example, if you had a ship that moved 3 spaces per turn and set it's destination someplace beyond a belt of asteroids, and were 1 tile away from the asteroid belts, then here's what would happen: you'd move 1 space towards your destination and have 2 of 3 movement points left. Since you are now on an asteroid, you'd lose the rest of your moves for this turn. On your next turn, you'd only have 1 of 3 movement points left, because of the next-turn penalty for moving through an asteroid.
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If this is not what is happening, and you can get the error repeatedly from a gamesave within a few moves, please zip it up with details on how to make it happen (be as detailed as possible) and email it to us. We'll take a look at it in the debugger and make sure things are working as intended, or hopefully discover what is causing the wrong behavior.
Thanks,
Charles