Practical Interstellar Probe Concepts: Mission Study Results

From the beginning of space exploration, humans have looked forward to escaping the solar system into interstellar space. As early as 1958, before NASA was established, mission concepts for an Interstellar Probe have been proposed. None have been attempted, mainly because the technologies required to do this mission have not been developed. However, with the development of the Space Launch System (SLS), the main difficulty - how to launch a system with the necessary speed to reach interstellar space in a reasonable time - has been addressed. In 2018, NASA asked The Johns Hopkins University Applied Physics Laboratory to develop a practical near-term mission concept that could finally achieve the goal of exploring interstellar space. In this study, we have identified three classes of trajectories that could achieve an escape speed of greater than 7 Astronomical Units (AU)/year, about twice the speed of the Voyager spacecraft which allows for transit into interstellar space well within a 50-year mission lifetime. These trajectory classes are: (i) launch on SLS with solid rocket motor upper stage followed by a ballistic Jupiter gravity assist, (ii) SLS launch followed by a powered Jupiter gravity assist (JGA) using a solid-rocket motor fired at Jupiter, and (iii) SLS launch followed by a JGA to target a deep dive into the Sun's gravity well for a Solar Oberth Maneuver (SOM) to achieve escape velocity. Each of these trajectory classes imposes significant requirements on the launch vehicle and spacecraft, and represents increasing levels of risk and difficulty. The powered JGA trajectory class would require carrying a large solid rocket motor to Jupiter such that it can successfully fire during the Jupiter flyby, which imposes requirements on thermal control of the system, as well as the ability to target the flyby accurately with a significantly larger flight system than for the unpowered JGA option. The SOM trajectory option imposes even more difficult requirements on the flight system, given that the maneuver requires a closest approach of 3–4 solar radii (Rs) to achieve a significant escape speed. This perihelion is well beyond that planned for Parker Solar Probe, and will require a heat shield capable of withstanding even higher temperatures than existing heat shields. Preliminary development work in this area has provided a potential path forward, which we have used to develop a heat shield design that can be employed to study whether such a mission is possible, the constraints and requirements on the flight system, and risks associated with an SOM mission concept. In this work, we present the three trajectory classes and associated example flight system configurations. We compare two example mission concepts along with science goals for each one, discussing the advantages and risks of both. We conclude by identifying the mission concept that represents the best option for a practical Interstellar Probe.