Thanks for answering.
Yes, that would of course be my guess too. Ever-increasing temperature.
But what actually happens to the energy returning from the mirror to the sphere? And how is the temperature actually raised? By what mechanism? Is it just from piling up of energy of equal level...
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...