Or. . . just land and start mining it themselves? There are a lot of shared CMC sites in Sol system; if a non-hostile power started their own colony in Sirius would it be worth a war to not share the planet? Mars would still get a million tons of Duranium without fighting.
Mars would have to allow them to land, though (at least if they were willing to risk a war over it). And if their requirement for allowing a colony were something like "no ground forces allowed" (to prevent someone else from using it as an excuse to land their own ground forces), then anyone colonizing it would know they were basically just giving colonists and infrastructure to Mars in the case of war.
I think that the Sirius colony in particular is going to need a Custodes unit stationed there though, and soon. It's such an obvious target for a Duranium strapped polity to try and take, or another nation to try and deny to the Martians. It's a Jungle Rifts Valley planet though, so... rather hard to assault.
This got me thinking, how would a Custodes unit compare to the more common warship units available.
First, some assumptions; a Custodes is described as having 6 particle beams and 6 twin gauss turrets. We don't get the actual stats, but I assume they're comparable to matching weapons on Martian warships, so we get:
6 STO Particle Cannons:
Tiberius Weapon Systems TW6-240 Particle Beam Range 240,000km TS: 5,335 km/s Power 15-4 ROF 20
Titus-Felix TF-256 Beam Fire Control Max Range: 256,000 km TS: 5,000 km/s
HP: 30? (Ceramic Composite armor tech)
Armor: 10-30? (ditto, and I'm guessing 30)
6 Twin Gauss Turrets:
Twin R3 Gauss Cannon Turret (1x6) Range 30,000km TS: 20000 km/s Power 0-0 RM 30,000 km ROF 5
Titus-Felix TF-20 Point Defence Fire Control (1) Max Range: 64,000 km TS: 20,000 km/s
HP: 30?
Armor: 10-30?
Assuming 30 armor, a 9 damage hit is necessary to ensure a kill (ouch), 4 damage would have a 44% chance of a kill, and 1 damage would have a ~5% chance of a kill. The terrain gives a fortification/accuracy multiplier of 12 (ie 1 in 12 attacks will miss) and the STO weapons will likely have a fortification of either 3 (self fortified) or 6 (with engineers/construction factory assistance). Assuming full fortification (worst case scenario) that gives the following hits per kill (on average):
9+ damage: 72 hits
4 damage: 162 hits
2 damage: 365 hits
1 damage: 1,440 hits
Just making a Fermi estimate, I'd say that puts
each individual gun as roughly two thirds tougher than a Martian Battlecruiser, if not higher (and proportionately better against smaller and very large hits). The firepower is a lot lower, of course (or at least it would be if the Martian battlecruisers had particle beams), but still, it shows how tough STO can be on a high fortification multiplier planet.
IIRC, the rules for firing on revealed STO units are considerably different from a general bombardment of ground forces, so it at least avoids the need for FFD to make a proper effort.
Scenario 1: Saturation missile bombardment
The Jovian federation uses WH 9 missiles. With a sufficient force of battlecruisers and patrol craft, they could overwhelm the point defense and land unanswered hits. They'd still need an average of 432 missile hits to destroy the 12 guns of a Custodes unit, which would make a mess of the planet, and the point defenses would be shooting down an average of nearly 34 missiles per salvo. Not completely impossible, but extremely expensive and likely to end up basically reducing the planet to a radioactive wasteland. Also, they'd need to provoke the STO weapons into revealing themselves first.
Scenario 2: Long Range Beam engagement
At current tech, none of the other nations appear to have weapons that can outrange the particle cannons, making bombarding them from out of range impossible.
However, if one nation did develop the tech, likely a large high powered laser, then it would come down to weapons failure and MSP costs. A theoretical next gen bombardment weapon might look like this:
20cm Far Ultraviolet Laser: Damage 10 (2 at 241,000) Max range 500,000 km
Max fire control range: 320,000 km
Bombarding from 241,000 km, they'd need approximately 365 hits per gun, and have an accuracy of ~33%. This means 1,104 shots per gun, or roughly 22 failures, each costing 158 MSQ, or 3,475 MSP per gun (once the particle beams were down the fleet could move closer). This would create a lot of dust but not radiation, and would still require the attackers to provoke the STO weapons into firing first. By far the most practical solution, but it requires a tech advantage.
Scenario 3: Short range bombardment (gun duel)
The 6 particle cannons put out 36 damage every 20 seconds. Range doesn't matter, so it would make the most sense to engage them relatively close, either point blank or just outside the gauss range of 30,000 km.
The most suited to this sort of short range bombardment would be the Shogunate, with their mass driver heavy ships. Their shields also mean they could minimize incoming damage by moving ships in and out of range. Each strike cruiser is capable of 32 shots of 1-4 damage depending on range every 15 seconds.
In theory a single strike cruiser could take out all the guns without damage by dancing in and out of range as its shields recharged, but this would take forever and be a micromanagement nightmare. In practical terms, the 8 strike cruisers of the Shogunate navy, or even perhaps half of them, could probably handle the defenses reasonably well with a similar technique. It would still create levels of dust similar to the long range laser bombardment, and require a lot of micromanagement as well as probably resulting in some armor damage to the strike cruisers, but it would work.
