Chapter 29, Problem 72P

The table below shows the intensity of the radio signal received at Earth from a spacecraft on its way to the outer solar system, as a function of its distance from Earth. Distances are in astronomical units (AU, with 1 AU being the mean Earth-Sun distance; see Appendix E). Determine a quantity that, when you plot S versus that quantity, should give a straight line. Make your plot, establish a best-fit line, and use it to determine the spacecraft’s transmitter power.

Distance (AU)	1.56	1.81	2.14	2.78	3.17	4.25
Intensity, $\bar{\text{S}}$(10−23 W/m2)	22.5	17.8	11.6	7.10	5.63	3.0!

Proposals have been made to “sail" spacecraft to the outer solar system using the pressure of sunlight, or even to propel interstellar spacecraft with high-powered, Earth-based lasers. Sailing spacecraft would need no fuel—a great advantage because fuel constitutes much of the initial weight of any space mission. NASA’s Sunjammer, launched in 2015, is the first sailing spacecraft deployed beyond Earth orbit. Figure 29.15 shows what a similar solar sail might look like in space.