Probing the Outer Planets with Cross-link Radio Observations Enabled by Small Satellites

Small satellites including CubeSats have emerged as viable, low-cost platforms for the remote sensing of Earth and its environment. Their uses in planetary exploration are being actively investigated. Recently, the Mars Cube One (MarCO) twin 6-U CubeSats were launched along with the InSight lander to demonstrate the feasibility of performing deep space radio communication services. With slight modifications, such CubeSats can potentially be used for science observations through precise Doppler tracking. In the past, we have proposed a cross-link radio occultation mission concept consisting of two or more CubeSats carried and deployed on Mars orbits by a large orbiter, to provide large numbers of well-distributed observations of the Martian atmosphere. In this study, we extend the concept further to the outer planets and include the option of probing the interior of the planets through gravity retrievals. As an extreme example, we consider the benefits and feasibility of having small satellites that accompany a "flagship" icy giant mission orbiting Uranus, which may be launched in the 2030 timeframe. We will discuss the science objectives that could be addressed with the small satellites through radio tracking between the small satellites and with the large orbiter, and the requirements imposed at the instrument and system levels. Power, derived from solar cells, is obviously the greatest challenge for exploring the outer planets using small satellites, even though they can operate with much less power than larger spacecraft. We consider scenarios where short measurement periods are interrupted by long sleep/charging periods to quantify the number and duration of measurements attainable with limited power.