The Ballistic Missile is a type of Support Asset. It allows the commander to designate a target for a guided ballistic missile, which is very accurate but does not do as much damage as artillery does. The Ballistic Missile is useful for taking out certain targets, like bridges.
The Ballistic Missile is an expensive weapon though, and only a couple can be fired each game. That being said, the Missile can be very useful for taking out point targets, as it is impossible for it to be shot down or stopped once it is fired, unlike the Flying Bomb.