Official rule text
15.1 STEP 1— The active player chooses which planet each of their units that has a “Bombardment” ability will bombard. Then, that player rolls dice for each of those units; this is called a bombardment roll. A hit is produced for each die roll that is equal to or greater than the unit’s “Bombardment” value. a A unit’s “Bombardment” ability is presented along with a unit’s attributes on faction sheets and unit upgrade technology cards. b The “Bombardment” ability is displayed as “Bombardment 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 “Bombardment” abilities have a (Y) value; a bombardment roll for such a unit consists of one die. c Game effects that reroll, modify, or otherwise affect combat rolls do not affect bombardment rolls. d Multiple planets in a system may be bombarded, but a player must declare which planet a unit is bombarding before making a bombardment roll. e The L1Z1X’s “Harrow” ability does not affect the L1Z1X player’s own ground forces. f Planets that contain a unit with the “Planetary Shield” ability cannot be bombarded.