The giant planets within our Solar System present an exceptional opportunity for studying the physics of high-pressure, rotating convective systems with density and compositional variations. Among them, Jupiter and Saturn have been relatively well-studied compared to the more distant Uranus and Neptune, which remain the least explored solar system planets to this day, having been visited only once by the Voyager II spacecraft in late 1980s. These missions are not only significant for planetary science, but also an exceptional opportunity for exploiting the non-planetary science potential of the spacecraft, due to long spacecraft cruise periods and travel distances. Radio communication between Earth and two distant spacecraft presents a unique case for investigating various phenomena via Doppler tracking, with which we can measure variations in the light travel time between Earth and the spacecraft and deviations in the spacecraft trajectory as it travels to the outer planets. Measurements of the Doppler signal can allow us to utilize the ~ 10 year travel time of these missions to constrain the dark matter content in the Solar System, discover exoplanets around compact binaries in the Milky way, and detect low-frequency gravitational waves from supermassive black hole systems at cosmological distances.