Defying Gravity: The Many Uses of Radio Doppler Data for Planetary Science

All deep space exploration probes communicate using radio signals to large-aperture antennas such as NASA’s Deep Space Network. The received frequency at the Earth-based antenna is affected by the Doppler shift, a consequence of the relative motion between the spacecraft and the receiving antenna. Because the primary force driving the motion of the spacecraft is due to gravitation, one can perform radiometric tracking on the received signals and use the measured Doppler shift to calculate the mass and gravitational field of celestial objects. The radio signal is also modified by propagation effects, for which one can also take advantage of to measure planetary atmospheres, ionospheres, and magnetospheric plasma features. In planetary science, this is referred to as the discipline of radio science. One of the primary objectives of the Juno mission, in orbit around Jupiter since 2016, is to precisely measure the gravity field of Jupiter using this technique; and although not originally intended or designed to do so, the radio science team has adapted the radio system to also make precise measurements of the atmosphere and ionosphere of Jupiter and it’s moons, along with measuring plasma properties of magnetospheric features on Jupiter such as the Io Plasma Torus and the Io Alfven wings. This work aims to discuss the many uses of radio Doppler data in the context of the Juno mission and how Juno adapted the radio system – designed for radio communications, navigation and gravity measurements – into a complete radio science investigation.