Eric is right. Don't put them too far from Y=0 or the players won't see them. Also, if the helmsperson is used to avoiding asteroids by looking at his/her console display, then he/she will be dodging asteroids that are above or below the ship.
This is even more hilarious with mines. I used it in "The Pegasus Passage." Unfortunately that mission script relied so heavily on the old DIRECT block that it no longer works.
"Damn the torpedoes! Four bells, Captain Drayton!"
(Likely actual words of Admiral David Farragut, USN, at the battle of Mobile Bay. Four bells was the signal for the engine room to make full steam ahead).