So, sometimes I ask dumb, pointless questions; it's a weakness of mine, and this is probably going to fall into that category. What are the actual defenses of Earth itself probably like? If I wanted to deal a crippling blow to the Imperium, Terra would be an ideal target. This is irrelevant, I confess, as nothing in the RPGs could go further from where they happen than back to Terra, but if a plot was discovered, it could be an interesting twist. The Emperor rests there, but some of its protection might be "who'd be so nuts?", meaning some is a bit of a bluff. Now, if I was some roving Chaos Warband, with a big fleet of nasty, I wouldn't hold out much hope for my chances, but what if I was one mad person, either a psyker tainted by the Hive Mind, a servant of Choas willing to use "exotic" methods, or even a ship-established Genestealer Cult? What's to stop one small ship from landing on Earth, out in one of the arid slums, and let loose one or more Genestealers I might be carrying? There has to be limitless traffic above the ruined planet that was once the cradle of Mankind, with numerous warships of the Battlefleet Solar, Black Ships dropping off their psyker-quota, various Adepta craft ferrying envoys to or away, and pilgrimage ships bringing the faithful, or their descendants, to bask in the glory of the birth planet of the Immortal God-Emperor of Mankind. My ship would hardly draw the notice of the Battlefleet, and Earth probably doesn't have too many Space Marines. One little ship wouldn't seem to hard to land, or force-crash, and let loose a plague that could spell the doom of the Imperium. One Genestealer laying low + 30 years of peace among the crime-ridden slums of Earth (for being the capitol of the Imperium, Terra is no garden spot to live) could equal a very hard to get rid of Genestealer Cult, and while it might not topple Earth, as it would many colonies, it could draw a Hive Fleet. Such a force might even, through it's Shadow in the Warp, blot out the Astrnomicon at it's source, crippling much of the Imperium for as long as it takes to dislodge the Tyranid menace. I'm not saying Earth would fall, but it MIGHT, and it would be a big hit to the Imperium everywhere else, from logistics to morale.
One might say "yeah, well if it was that easy, someone would've already done it, you dimwit. They've been writing this stuff for several decades now", and that would be true, but I don't see the hard part for the aggressor, if their threat is so small, and seemingly trivial. Thoughts?