Hi,

At some point the pc's will decide to blast the crap out of a planet, now are the Lances and Macrobatteries not strong enough to actually destroy a planet but they should be strong enough to level a city.

If the planet has a atmosfeer the Thunderstrike and Mars Pattern Macrocannons are likely to be useless since their projectiles would possibly burn up in orbit (though because of their size they just might not).
But all Laser and Plasma weapons are likely cause massive destruction.

How difficult would it be to:
Target a settlement or small ruin?
Target a Hive?
A Battlefield?

And what type of defense weapons do some planets have, are they roughly the equivelent of ship weapons or perhaps even heavier?

Santiago said:

Ah, Ortillery ( Or bital Ar tillery ) a subject close to the heart of any psychopath with a starship...

Battlefleet Gothic actually covers this to a reasonable extent - there is, afterall, a planetary assault mission in the rulebook.

Projectile weapons are hindered by atmosphere, as you've surmised, but so are directed energy weapons (a Lance in hard vacuum doesn't have to deal with atmospheric distortion; firing at a planet adds that problem, and similarly plasma weapons tend to suffer from the fact that they're firing through a medium that conducts heat, rather than through empty void). For all purposes, Battlefleet Gothic penalised gunnery through an atmosphere when firing at ground-based defences.

Targeting ground targets is difficult - as shown by the Epic: Armageddon rules, nothing smaller than a super-heavy tank can be accurately targeted by orbital lance batteries (the distortion and diffusion caused by the atmosphere lessening their effects to the point where they hit about as hard as terrestrial Volcano Cannons and similar anti-titan weapons), while weapons batteries (including the specialised Bombardment Cannons used by the Adeptus Astartes) tend to bombard areas several hundred metres across, causing massive carnage where the shells fall. There are specialised anti-surface torpedoes, called barrage bombs, which contain several small warheads which scatter over a wider area for dealing damage to ground targets (where an anti-ship torpedo wouldn't spread its damage widely enough).

The types of planetary defences Imperial worlds are likely to have (depending on world type) are covered from page 140 of the Battlefleet Gothic rulebook (for the relevant section of the online Living Rulebook, look here ) - covering both high orbit defences (orbital defence platforms, minefields, system ships, space stations) and low orbit defences (defence laser silos - essentially ground-based lance batteries), surface-to-orbit missile silos, and airbases. A much larger form of defence, the Ramilies-class Starfort (a huge structure of similar mass to 4-5 battleships) can be found on pages 30-34 of Battlefleet Gothic: Armada (for the online version, look here ). Strength values for weapons in BFG translate roughly on a 1:1 basis with those in Rogue Trader, which is a considerable blessing for those of us working to convert things over to Rogue Trader...

Anything you can spot from orbit you can hit from orbit. However, the slugs probably won't make it through. On the other hand, flinging rocks down upon it from a forward facing cargo bay...

Lasers are likely to decolimate, and not do significant disruption (but they will generate weather changes).

Torpedoes should hit. You're talking tactical nuclear warheads, there.

If you have a teleportarium, you can beam warheads down...

Looking at source materials, ortillery in WH40KRT was inaccurate, expensive, and thus probably specialized munitions.

Most macrobatteries aren't going to work as they fire shells. The shells are going to burn up in the atmosphere. Laser weapons are going to be deflected and dissipated to a degree, but usable. Lances will work fairly well. Most torps simply aren't going to work as they are designed to home into ships. Which doesn't mean you couldn't use specialized torps.

Id say lances would suffer similarly to laser batteries, though it wouldnt be as bad. but im not sure, id just make them have to make a couple toughish BS tests and do it narritivly.

How they would work with physics aside, planetary bombardment is a staple event in the 40k universe. Although not exactly pinpoint, lances are still used for more precise strikes above say, a general area bombardment in preparation for an assault.

As for shells burning up, a heat shielded bombardment shell is probably used, and these shells are not exactly small. Some idea of the effect is given by some of the rules in the TT game expansion Planetstrike, and the Firestorm