When the Voyager 2 spacecraft passed Neptune in 1989, it was 4.5 × 10⁹ km from the earth. Its radio transmitter, with which it sent back data and images, broadcast with a mere 21 W of power. Assuming that the transmitter broadcast equally in all directions,
a. What signal intensity was received on the earth?
b. What electric field amplitude was detected?
The received signal was somewhat stronger than your result because the spacecraft used a directional antenna, but not by much.