A Moonshot is an Attack mission specifically designed to create a moon for yourself or an ally. The goal is to deliberately have the attacking fleet destroyed in battle, providing a debris field that creates a moon at the target planet.
There are two important factors to consider when planning a Moonshot:
Moon creation is not guaranteed; every attack producing a debris field has some chance of creating a moon. The end of every Battle Report lists the likelihood of moon creation as a percentage, and this figure is also provided at the end of a combat simulation. The likelihood of a moon forming from a battle debris increases with the size of the debris field, up to a maximum of 20% per attack. This does NOT mean that 1 in 5 large debris fields will produce a moon. Rather, each attack has a maximum 1 in 5 chance of creating a moon if the debris field is large enough for a 20% chance of moon creation.
Moon sizes are determined at random within a given range, and the range is determined by the size of the debris field. Moon sizes follow a bell curve with sizes between 7 and 9 being near the top of the curve (most common) and sizes smaller or larger being increasingly less likely. The potential range of moon sizes shifts toward the lower end of the curve if the debris field is small, and toward the larger end of the curve with large debris fields. Within this range, however, moon size is still random. This means large debris fields have the potential for larger moons, may still produce smaller ones, but not very small ones; small debris fields have more potential for producing smaller moons, may also produce larger ones, but not very large ones.
The exact code limiting maximum moon sizes makes a 9.8 the absolute maximum size attainable.
Hint: no other factors have any effect on the size of the moon (such as the type of ships in the moonshot fleet, whether they carry cargo or not, etc.). It is always random within the range determined by the size of the debris field.