Official rule text
77.7 STEP 1—ROLL DICE: Each player may use the “Space Cannon” ability of each of their units on the invaded planet by rolling a specific number of dice for each of those units; this is called a space cannon roll. A hit is produced for each die roll that is equal to or greater than the unit’s “Space Cannon” value. a If a unit has a “Space Cannon” ability, it is present on its faction sheet and technology cards. b “Space Cannon” is displayed as “Space Cannon X (xY).” The X is the minimum value needed for a die to produce a hit, and Y is the number of dice rolled. Not all “Space Cannon” abilities are accompanied by a (Y) value; a space cannon roll for such a unit consists of one die. c Game effects that reroll, modify, or otherwise affect combat rolls do not affect space cannon rolls. d Game effects that allow the use of “Space Cannon” abilities against ships in adjacent systems have no effect during Space Cannon Defense.