[tshiggins]

I think it's safe to say that, no matter how much armor you pile on a space craft, the sheer energy produced by any particle moving at relativistic velocities will swiftly penetrate. If enough mass of particles hit, the space craft will get vaporized, no matter what.

So, that means Battlestar Galactica-style armor plating has only limited use. Yeah, it's nice to keep out hard rads from background radiation and near-misses, but a direct hit from a nuke or a strike from a ball bearing moving at relativistic velocity will punch through it.

So, what if you have a softer defense, rather than a harder one?

I can think of two possible systems, and we might want to consider both in tandem. In both cases, you use the reality of kinetic damage at relativistic velocities to help defend the space craft.

Firstly, even though you kind of resist the notion, an aegis-style point-defense is just too useful to pass up. I'd put in a combination of automatic turrets sporting high-velocity rotary cannons, and maybe some BSG-style turrets that fire AAA that puts out an ungawdly amount of shrapnel.

Secondly, I'd pull in a trick I read in one of David Brin's Uplift novels. In that one, a dolphin-piloted starship destroyed a pursuer by dumping much of the water in its habitat-tanks in a cloud behind, as it whipped through a sling-shot orbit around a gas giant. The water immediately froze into a cloud of fine ice crystals.

The vector of the pursuing craft, also in a sling-shot orbit, took it straight through the cloud of ice, and it promptly vaporized.

So, what if you do something similar? What if the space craft have Traveller-style sand-casters that put up a cloud of fine debris between the defending craft and the incomiing ordinance?

They could consist of high-albedo metallic dust that disperses lasers reasonably well, particle beams less well, and catastrophically ablates incoming ordinance of a more traditional sort.

Use the anti-missiles to stop stuff further out, AAA for what gets through that, and dust-clouds to (more or less) create useful cover.