Problem: Suppose a spacecraft is in a circular orbit about a distant planet. The spacecraft emits a continuous radio signal with a wavelength of 6 m. You observe the signal's wavelength to vary between 5:99969 m and 6:00031 m; the period of variation (i.e., the full period of the signal) is 5 hours. Calculate the mass of the planet. Assume that you are located in the plane of the spacecraft's orbit. It may help to know that the circumference of a circle with radius a is 2[tex]\pi[/tex]a, and to recall that speed equals distance divided by time. I'm pretty sure I have to use the Doppler effect in some way because I'm given wavelengths but I am unsure how to use it. Can anyone help?