## College Physics (4th Edition)

We can assume that electromagnetic waves are used for communication. We can use the Earth-sun distance to find the minimum distance between Earth and Mars: $d = (2.28\times 10^{11}~m)-(1.5\times 10^{11}~m)$ $d = 7.8\times 10^{10}~m$ We can find the time for EM waves to travel twice this distance: $t = \frac{2d}{c}$ $t = \frac{(2)(7.8\times 10^{10}~m)}{3.0\times 10^8~m/s}$ $t = 520~s$ The shortest possible time to wait for a response is 520 seconds.