Solar System interiors, atmospheres, and surfaces investigations via radio links: goals for the next decade

From Mercury to the outer reaches of the solar system, the past six decades have witnessed a vast set of discoveries utilizing radio science (RS) methods. For example, based on key gravitational evidence, sub-surface oceans have been inferred at Titan, Enceladus, and Europa, where potential future missions may search for life. The ability to precisely measure properties of spacecraft radio signals — frequency, phase, delay, amplitude, polarization — provides unique leverage to extract new information about atmospheres, ionospheres, rings, surfaces, shapes, and internal structure of solar system bodies (Asmar et al., 2019). In addition to planetary sciences, RS observables such as precision Doppler and ranging are critical to studies in fundamental physics and solar dynamics such as observing effects on signal passage through the Sun’s gravitational field and solar wind, investigating gravitational waves, and monitoring planetary motion to study gravitational theory and solar mass loss (Armstrong et al., 2003; Genova et al., 2018; Smith et al., 2018; and Woo, 1993; Armstrong, 2006). RS remains a powerful and cost-effective tool for many solar system investigations planned or conceived in the coming decade. Additional science discoveries could be enabled by developing new technologies and mission concepts such as: • Deployment of small spacecraft missions for high spatial and temporal resolution of atmospheric and gravitational mapping (see cover page illustration), • Novel instrumentation and calibration techniques to improve data quality by up to an order of magnitude over current levels, and • Exploitation of uplink transmissions from Earth such as those used by New Horizons at Pluto, to improve sensitivity by orders of magnitude.