Official rule text
77.6 SPACE CANNON DEFENSE “end of combat round” effects occur during the same timing During the invasion step of a tactical action, after ground forces window. have been committed to land on planets, players other than the active player can resolve the “Space Cannon” ability of their units To resolve a space combat, players perform the following steps: on those planets by performing the following steps:
View official source
Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 77.6