There is probably a limit on the space around terra also, much like the air space around Washington DC.
So if a renegade INQ ship tried something like that they'd be blown out of the sky before they got NEAR the holy planet. (Unless they had an appointment with the Emp...which almost NEVER happens ever.)
The moon is basically one giant anti-space craft fortress...it is nearly invincible. Horus's fleet took HUGE losses in the siege of Terra. Also the Emperor thought there was no hope once the moon was over taken...thats stated in some book somewhere but I can't remember which...Taking the moon is like taking the outer walls of a castle, the first step to taking the citadel
Necrons broke the SECOND best planet defense in the galaxy, Mars...but Nercron ships are far, far, far, far better them imperial ones.
HOPE IT HELPS,
BrainDeleted