Are there rules for attacking ground targets from orbit, using ships weapons?

Not yet, maybe in the upcoming ship expansion pack.

Lance cannons have been described as being capable of cracking continental crusts, though that may be hyperbole. Macro-batteries may well impact with the power of a mid sized asteroid, and it is not unknown for Imperial Fleets to destroy planets from space without employing exterminatus level weaponry.

Marine ships have special bombard weaponry to make "surgical" strikes possible amid combat, and cruise into "low" orbit to do it.

What I get from this is that YES it is possible to attacks planets, but NO it is not a "tactical" weapon. Its kind of like dropping a nuke.

So my players are free to use it, but it would be unwise to use it in the middle of a ground battle which they are participating in (because they would be vaporised).

Using something like a lance weapon on low power has been done. In 40k, grey knights get a heavy support lance battery, which is a ship in low orbit firing a lance strike, melta torpedo or anti-infantry torpedo. I wouldn't call it precision though, as the chance to hit the target is less than the chance that the shot will scatter.

If you're talking about using a ship's lance weaponry to crack a governor's palace, I'd allow it. Vaporize a village - sure. Take out that dude on the hill with the sniper rifle - a little too precise, and will most likely take out the dude, the hill, and anyone in range of the rifle.

For macrocannons, I'd picture their effects like smaller versions of the mass drivers used in babylon 5. Prolonged use would kick up enough dust to cause a nuclear winter minus the nuclear. Then again, there's the whole debate on whether a macrocannon shell specifically designed for ship to ship combat in space could survive re-entry into an atmosphere. I suck at science, so I'll decline to comment on that one.

-Thulis

Thulis said:

I would take this as additional evidence of special weaponry being necessary to use tactical strikes. With the proper weaponry a ship can be used for "fire support", but your usual ship to ship weapons seem to be unsuitable for the task.

So yeah, ships make great nuclear strikes, but bad fire support.

In general I let anyone with a lance to make orbital strikes that on a good hit are only off by a city block or so. In general I require that the PCs require special ammo to use the macro cannons. In general they haven't bother as they have their lance....

That said my players rarely try the orbital strike thing as they are afraid of getting attacked by planetary defenses or the like. The thing to remember is most worlds have orbital defenses, system ships, and ground batteries. It's hard to imagine a world in the Expanse lasting long if anyone with a frigate could "nuke them from orbit". Even a planet without defences may have availble shuttles for hit and run raids.

Dalnor Surloc said:

Undiscovered worlds might be safe, until someone finds them.

The way I see it, a lance strike has enough energy behind it to take out at least the building it hits (one of the Caiphas Cain books has one used to take out a planets military command bunker). The beam isn't that wide, but it causes whatever it hit to be heated up to really high temperatures. So it expands violently, causing an explosion.

A macrobattery barrage would take out the city.

Then there is the destructive potential from the ships exhaust, which I will post about later today.

Technically, macrobateries would need divine BS roll to hit a city from orbit, as atmosphere, gravity and planetary magnetic field (the natural one) are great hindrance to such "attack" (physical rounds or laser ones should suffer the same). Trying to use them to nuke a city would probably result in nuking a neighbor one.

Stellar lance would probably be far easier to use as orbital bombardment weapons. They are very focused and precise weapon, so with a little luck and LOTS of MATHS (I give bonus/minus to ship weapon operator on their BS roll depending on the result of a challenging logic test), it should hit. But (there is always a but), the massive energy ray would have some nasty effects :

- Greatly rising the heat a few kilometers around the impact, instant death to anyone under 500m to 1000m radius, depending on the power output of the lance, due to lung burning...

- A violent breath due to air dilatation, with non-nuclear proof building "washed away" some hundred meters around the impact.

- Possible tectonic reactions (a volcano appearing, earthquake, etc.)

- Most probably a nuclear reaction as the ray overheat the few, yet present, rare metals in the crust. <= This one is more a long term curse on your PC for using orbital bombardment

- Most probably a massive production of Ozone (O3), at the expanse of the quite required to breath O2...

- Probably other nasty consequences (durable atmospheric change, etc.)

So many reasons for your PC to thing about OTHER solutions before wasting a planet (wasted planets generally don't generate profit).

Technically, macrobateries would need divine BS roll to hit a city from orbit, as atmosphere, gravity and planetary magnetic field (the natural one) are great hindrance to such "attack" (physical rounds or laser ones should suffer the same). Trying to use them to nuke a city would probably result in nuking a neighbor one.

Thing is, anything on a planet is effectively stationary so it should be much easier to hit than something which can change course under its own power The atmosphere might require thicker armour on the shells, gravity can be easily compensated for and the magnetic field isn't going to be strong enough to matter on a missile, let alone a laser. But since macrobatttries are designed to fill a volume of space, I'd say that any barrage would hit the target city and the neighbouring ones.

As for your side effects of a Lance strike, they are all good. Except for setting off the radioactive elements, those won't be present in large enough amounts to matter, unless you are targeting somewhere with a large concentration of them (eg a mine, reactor or nuke silo).