There are a few options to make missiles. If you want the missile to fly at a unit you can use a dummy unit attacking or using an ability. In your case you probably want it to attack the ground, right? A very good and easy solution is using the meat wagon's disease cloud ability. It creates a unit (disease cloud) at the target area upon impact. When a unit casts the ability, create a dummy that attacks the target area (artillery attack type) and set targets allowed to none, so no damage is dealt. The meat wagon ability creates a dummy unit upon impact. Use a unique unit type for this dummy and catch its creation with unit enters entire map, unit type = ... .This way you have a missile + are able to detect when the missile hits the ground.
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