Subsurface color always shows though what should be in shadow.

(I moved this from “Rendering”)

I have a scene set up with a series of asteroids and debris.

The asteroids are using a subsurface color to create a pleasing “ice” style effect, but; For some reason, asteroids beyond a certain distance away are no longer shaded, with the sun behind them that is. As i move closer to the 'roids in question, the shading appears and they go back to looking as they should.

It seems as if the subsurface color is showing through when they should be in shadow.


I’ve taken a couple of screenshots to show what i mean:

Green circles are on the 'roids which look right, red are the ones that are being “weird” As i move further into the scene (pic 2) you can see that now some of them are shaded as they should be.

Have i missed something simple/obvious (as is usually the case), or is there a rational explanation for this phenomena?