Radio Science at Jupiter: Past Investigations, Current Results, and Future Prospects

Over the last 40 years, several missions of the National Aeronautics and Space Administration (NASA) and European Space Agency (ESA) have explored the largest planet in the solar system, Jupiter. Radio Science has been a key component of each mission to the planet, where radio signals between the spacecraft and the Earth-based observing antennas have been utilized to determine the physical properties of Jupiter and its moons, their atmospheres, ionospheres, and gravity fields. As a spacecraft passes through or around the Jovian system, small changes in phase, frequency, and amplitude of the radio signal are induced by the surrounding environment. These changes are detectable and form the basis for determining gravitational fields, planetary motion, and reconstruction of atmospheric and ionospheric profiles. The era of outer-planet exploration began in 1973 and 1974 with the Pioneer 10 and 11 flybys of Jupiter. Several missions have used Jupiter's massive gravitational well to perform a gravity assist to reach farther in the solar system, starting with Pioneer 11 in 1974 and Voyager 1 and Voyager 2 in 1979 with subsequent flybys from Ulysses in 1992, Cassini-Huygens in 2000, and New Horizons in 2007. Jupiter has hosted two dedicated orbiting spacecraft thus far: Galileo from 1995-2003 and Juno from 2016 to present. Throughout this time, instrumentation advances in both spacecraft technology and technology developed for ground stations have improved the precision and accuracy of the radiometric measurements, leading to improved results from radio science investigations. The latest mission to Jupiter, Juno, includes the most advanced radio science instrumentation to date. With Juno's unique polar orbit and dual frequency radio links, it is able to probe the planet's deep interior structure and zonal wind profile with measurements of the gravitational field and probe the electron densities in the Io plasma torus, a doughnut-shaped ring around Jupiter charged with particles emitted by the volcanic activity on Io. Upcoming missions, such as the planned NASA's Europa Clipper multiple flyby mission in 2022, potential follow-on Europa Lander, and the ESA's Jupiter Icy Moons Explorer mission in 2022, may make further strides in the study of the planet and its moons utilizing radio science.