Solution:
This post has an explanation of the physics by Sir Alan and an example by Heureka.
Here is a reproduction of Heureka’s math, using the data from this question.
\(\begin{array}{rcl} I &=& \frac{P}{4\cdot \pi\cdot r^2} \qquad P = 23\ \text{watt} \qquad r = 19\ 500\ 000\ m\\ I &=& \frac{23}{4\cdot \pi\cdot 19500000^2} \\ \\ I &=& 4.8\cdot10^{-15}\ \frac{W}{m^2}\\ \end{array}\)
Closest answer from list: 5 x 10−15 W/m2
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It’s usually not necessary to reinvent the wheel, but you must be careful not to inflate the tire on the wheel with hot air from a blarney bag.