The Solar Probe Plus fact-sheet states that the craft will approach to the distance of 9 solar radii to the surface of the Sun (Approx 6.26e6 km) and its heat shields must withstand 1644K of heat.
I wonder how did they arrive at this number? Based on that, how can I calculate the max temperature an object has to withstand at 3.68e6 km (about 5 solar radii)?
Of course I know that at some point simple formula wouldn’t work as we’ll enter corona.