(Jeff Root) writes:
Are you assuming that infrared from an RTG is radiated only
perpendicular to the surfaces? When I look at an RTG from near
the body of the spacecraft, I see a good deal of surface area
of the fins and of the cylinder itself. About 20% of the total
surface area, as I said.

Perhaps, but the surface emissivity does decrease as the cosine of the
viewing angle to the normal. Say the angle was 85 degrees, then
cos(85 deg) = 0.087, which when multiplied by your 20% figure is 1.7%.
I have no idea what the real angles are, but the radiation transfer is
more compilcated than just estimating the solid angle.

Craig