User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-3217145-20130204112603/@comment-3217145-20130206111011

[Since Tiredman2 never did it, here's a massive post!]

You *are* talking about AA guns though, you mentioned the ones on Virmire specifically. What I said holds true for any gun that can hit spacecraft near the planet, including in orbit. They stop enemies from engaging wherever they want, and force them to fight away from the planets surface. Case in point: Koris's ship getting shot down when it flew toward Rannoch with no recon about the guns.

A point about Tuchanka: for one, they're Krogans, so they like big guns; and two, they're demilitarized, meaning they have no fleet, so it makes sense that they would maintain a weapon capable of killing cruisers. It's their only option. IIRC that gun was from before they were demilitarized, so I default to the "Krogans <3 BFGs" explanation.

About projectiles hitting the planet, here's some info:

1. Smaller projectiles slow down faster when they enter the atmosphere, because they're less resistant to changes in momentum. Depending on the size of the projectile being fired, it would lose a lot of power entering an atmosphere and may not be enough to damage the PDC (you would assume that it has shields and armor of some kind). Since dreadnoughts fire 20-kilo projectiles, you would assume smaller ships would fire smaller projectiles; it doesn't make sense to cart around hundreds of kilos of big fat projectiles and use a weaker mass effect field to fire them, when you could carry many more smaller projectiles and use a stronger mass effect field. This would make them much faster and more practical (more energy in a smaller object means more speed). This is the principle hand-held guns follow; smaller projectiles means you can carry a lot more of them. 2. The Codex entry "Starships: Dreadnought" says that "As a rule of thumb, each Earth-atmosphere of air pressure saps approximately 20% of a projectile's impact energy." This supports the above point, though the 20% figure may be different for different-sized shots; you'd lose a lot of energy firing on something from space, so low-power guns might not be capable of taking down the PDCs. 3. Projectiles, even in space, don't actually go in "straight lines." They're affected by lots of things, like magnetic fields and gravity from the target planet and everything else in the system (Tiredman2 detailed this). 4. Especially when projectiles hit the atmosphere, it would be like firing a bullet into water; the sudden change from an empty vacuum to a dense atmosphere would ruin the projectile's trajectory, and mostly in unpredictable ways. There would be a million things to account for. It might change angle (like light through glass or a bullet into water), or it might bank so hard it bounces off the atmosphere. Temperature would affect air density, which would affect the resistance of air to the projectile and alter its course. Cloud cover would mean lots of water, which would also affect resistance. Wind would blow it to the side, and possibly different sides as it traveled through the entire atmosphere and hit different wind currents. All of this is information you couldn't possibly have unless you had sensors on the ground or in orbit checking all these things, and if you're already there, the PDCs are already firing on you. If you're a dreadnought firing projectiles that hit as hard as than nukes, you don't need to worry about accuracy so much, but if you're a frigate firing on a single defense tower, you do. 5. Targetting technology isn't perfect in the first place. If it was, Garrus wouldn't be calibrating all the time. An error of just 0.5% in your accuracy would put you waaaaaay off your target if you were hundreds or thousands of kilometers away, and in at least one conversation between Garrus and Legion, they were actively trying to improve their accuracy by such small amounts. Hitting a single building on a planet would be like trying to hit a grain of sand with a molecule-sized bullet.

One last thing: speaking of dreadnoughts, firing on garden worlds is forbidden by the Council. This means that you can count on not being firebombed from space if the enemy is Turian, Salarian, or Asari, or anyone who wants to stay on the Council's good side (Alliance, Hanar, etc). Anyone who broke this rule would make enemies of every Council race, and they wouldn't want to do that; even the Batarians didn't risk it (the ones in Bring Down the Sky were terrorists, and even many of them didn't want to do it). The Space Combat entry mentions this, saying that one of the big problems with trying to attack a garden world is that you can't fire on the defending fleet if they're sitting in front of the planet, in case you miss and hit the planet. So against most opponents who even HAVE the capability, you wouldn't have to worry about them using high-powered ordnance to snipe your towers out.

Thank you for asking this question! I never thought about this stuff before...I agree, it's really interesting. Got me to do research.