Huge Nebula & Asteroid Field

Hi Guys I was hoping to get some help with a project I am currently involved in

I am creating a space game which is kind of the same effect as Freelancer

I know how to make asteroid fields and nabula's but the problem comes with size.

I need to be able to create massive asteroid fields and nebula's within my scene, thing is when i do it the old fasioned way and crank up the numbers things slow down massively , I can use a nabula' cloud that stays with the ship as the nebula should only be in a certain arewa of the scene. I would like to know does anyone have any ideas on how to get the same effect as freelancer as they have nailed it Red nebula

Also I would like to create massive asteroid belts now my idea is to used billboards and then change them into meshes when the ship moves into range 3rd image

and before the warnings , yes I know the implications of creating huge scenes , and I am using occlusion culling as well as found a clever way to avoid floating point precision problems :)