The impact does no damage, but it causes nearby units and buildings to catch fire, which will in turn spread to other nearby units and buildings. This makes it a great weapon to use against bases where buildings are close together. There doesn't appear to be a limit to how long the fire can spread, as it seems to spread to every single unit and building within range of a burning target, with the exception of some air units. A target that has already been burned once will not catch fire again from a fire started by the same missile. However, it can catch on fire again from a different missile. Fires don't always kill the target, as there is a time limit to how long they burn. But it seems that a fire missile can burn long enough to destroy turrets and AA turrets at full health, kill anything weaker than a heavy soldier, and severely damage stronger units. One advantage over the nuclear missile is that it is able to go through the shields created by shield generators. In addition, its range of effect is slightly larger than a nuclear missile's.