Actually, in this case the scripting was done not by distance, but by self-destruction. I saw a massive wave of missiles incoming, I saw that they all had time to impact under 30 seconds; I went after them, trying to destroy as many as I could, but it seemed pretty hopeless - there was more than ten, so some would get through either way.For me it is the other way round. Missile launchers are invisible ships in the engine that have only one weapon ( a capmiss launcher). They are spawned somewhere, assigned a target, and despawned after a period of time (normally after the time it takes to shoot one or two missiles).
If the missile launchers were placed further away the missiles wouldn't be invincible that often and you could shoot them down much more easily. So easy missile missions are not more scripted than the others, just a bit differently. Placing the launchers further away.
(Actually I'm not sure whether Keldor and Tolwyn did balance it that way, but it is the easiest way to to it.)
...Only, none of them did, because they started exploding. For an instant, I wondered if the other pilots were doing such a great job. Then I wondered if maybe the transport turrets did it. Then I realised that no: the missiles just aren't allowed to make impact. Later in the mission, your convoy is attacked by a pair of destroyers. I thought my job was to shoot down the missiles fired by the destroyers until someone could put them out of commission. I started doing this, but then I noticed the same thing again: when a missile got past me, it still didn't get anywhere. So I decided not to bother with missiles any more, and got on with dogfighting the Kilrathi fighters.
What can I say, concealing a script is the toughest problem a designer can encounter.