Missile design option: picketing.
The affected missile stage behaves as normal, except that during design, a picketing missile stage can have a distance set. This will cause the missile to picket the designated target at a particular distance. This can be useful for active/passive sensor missiles that you want to follow a target around without getting into gauss range, or for spacemines that you want to follow a particular carrier/planetary body/etc.
Requires at least a specific amount of agility on the missile in question before it can be used (with agility being otherwise useless for any picketing missile design), and missiles that picket will use fuel per normal.