I humbly ask for people's opinions on the following hypothetical setup: A solid black body sphere in space is supplied by an internal nuclear energy source with a constant power flux to its surface of 459 W/m2. It's surface temperature is accordingly 300K. The power released (to space) balances exactly the power supplied (from the nuclear source) at the sphere's surface. The temperature thus remains constant. Then we place a shell around the sphere, enclosing it totally. For argument's sake, the temperature of the shell can be considered to be equal to that of the surrounding space (0 K - impossible, yes, but bear with me). The surface of the sphere and the shell is equidistant all around and the gap between the two bodies is a vacuous space. Now, let's say the radius of the sphere is 1000 km and the radius of the shell 1001 km. This means that the distance between the surface of the sphere and the inner surface of the shell is 1 km, allowing us, for simplicity's sake, to disregard the difference in area between the two facing surfaces (amounting to 0,2 %). (If it turns out to matter at all.) My question is this: What would happen to this sphere/shell system if the entire inner surface of the shell were a perfect mirror, i.e. a perfect insulator of thermal radiation? Cheers.