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.

Persephone is a NASA concept mission study that addresses key questions raised by New Horizons' encounters with Kuiper Belt objects (KBOs), with arguably the most important being "Does Pluto have a subsurface ocean?". More broadly, Persephone would answer four significant science questions: (1) What are the internal structures of Pluto and Charon? (2) How have the surfaces and atmospheres in the Pluto system evolved? (3) How has the KBO population evolved? (4) What are the particles and magnetic field environments of the Kuiper Belt? To answer these questions, Persephone has a comprehensive payload, and would both orbit within the Pluto system and encounter other KBOs. The nominal mission is 30.7 years long, with launch in 2031 on a Space Launch System (SLS) Block 2 rocket with a Centaur kick stage, followed by a 27.6 year cruise powered by existing radioisotope electric propulsion (REP) and a Jupiter gravity assist to reach Pluto in 2058. En route to Pluto, Persephone would have one 50- to 100-km-class KBO encounter before starting a 3.1 Earth-year orbital campaign of the Pluto system. The mission also includes the potential for an 8-year extended mission, which would enable the exploration of another KBO in the 100- to 150-km-size class. The mission payload includes 11 instruments: Panchromatic and Color High-Resolution Imager; Low-Light Camera; Ultra-Violet Spectrometer; Near-Infrared (IR) Spectrometer; Thermal IR Camera; Radio Frequency Spectrometer; Mass Spectrometer; Altimeter; Sounding Radar; Magnetometer; and Plasma Spectrometer. The nominal cost of this mission is 3.0B, making it a large strategic science mission.

Since the beginning of space exploration, one of the most ambitious goals has been to explore beyond the boundaries of our solar system. Ground and Earth-orbit based systems have given a deep understanding of the overall characteristics of the heliosphere in the local interstellar medium and how the characteristics of our solar system are similar to and different from other systems. Viewing the heliosphere from outside will allow, for the first time, a more complete understanding of how a star system evolves and interacts with the Universe. Interstellar missions have been studied for decades. The primary reasons we have not yet explored this region are critical limitations in technology. These include a lack of propulsion that can achieve the high speeds needed to get to the heliospheric boundary in reasonable time, reliable systems that can function for the long lifetime needed, reasonable communications capabilities at interstellar range, and constraints on mission resources such as power when more than 100 AU from the Earth. Recent developments in launch systems, execution of long-lived missions such as New Horizons, new radioisotope power systems, and advanced communications systems have for the first time allowed for a practical, feasible near-term mission that can achieve the goal of exploring outside the solar system. We present recent results of a concept study that examined possible missions that could be launched as early as 2030 using existing, or near-existing, technology. These possible missions support significant payloads for heliospheric science, with the potential for additions to the payload for planetary investigations or astrophysics. The concept study includes a detailed look at possible trajectories in launch years from 2030 to 2040 with flyout speeds at least twice that of Voyager 1 and 2, and significant opportunities for tuning the flyout direction to maximize the heliophysics return as well as allow encounters with outer planets or Kuiper Belt Objects. We present a summary of trade studies performed to investigate the constraints and design space for an interstellar probe. These trades include a comparison of trajectories that include gravity assists at Jupiter and at the Sun to increase speed, optimization of the telecommunications architecture to balance data downlink rate with power usage, and spacecraft control methods to allow precise pointing for telecommnications while minimizing propellant usage for an extremely long-lived mission. We present a spacecraft design that can support the potential payloads designed to operate reliably for 50 years, while allowing for communications from 1000 AU.

Exploration of the heliosphere and the far reaches of our solar system by the Voyagers and New Horizons and near-Earth observations of the Kuiper Belt and exoplanetary systems have all profoundly changed how we view our own home in space. An Interstellar Probe escaping beyond the solar-system boundaries with new observational techniques would be a bold move in space exploration, enabling detailed, new understanding of the global heliosphere in the context of other astrospheres, further discoveries of unexplored Kuiper Belt Objects, and the first observations of our circumsolar dust disk. These would offer insight into the evolution of the solar system and our understanding of exoplanetary systems. With the power supplies on the Voyagers nearing their end of life, the rapid pace of discovery of exoplanets in other stellar systems, and the evolution in our own understanding brought about by results from the Voyager, Cassini, and Interstellar Boundary Explorer (IBEX) spacecraft, the time is right for looking at the next steps we can make into our stellar neighborhood. The question facing us today is what the appropriate next step - a true first step - is in negotiating the transition from science fiction to engineering reality. The scientific imperatives associated with reaching the near interstellar medium in order to understand our origins and our own current locale in the Milky Way trace to the beginnings of the Space Age, and they have been debated and refined since that time and into the current epoch. The subjects of interstellar travel, interstellar probes, and interstellar "precursor" missions are not new but have lacked traction with policy makers and the scientific community at large because of the states of both scientific knowledge and engineering realities. The next step in reaching to the stars will require the recognition of engineering limits, scientific trades, and scientific compromises, but this is new neither in science nor exploration. Such a step would be an "Interstellar Probe." The time for that step has come.