To access the interstellar medium with current approaches requires 30 to 40 years, significantly longer than most mission lifetimes. The goal of this study was to explore mission concepts that will reach the interstellar medium in a primary mission’s lifetime (15 years or less). Faster access to the interstellar medium would allow high-capability science probes, with many relevant instruments, to explore the galaxy beyond our solar system in-situ. Science targets include the hydrogen wall structure, bow wave/shock, gravitational lens, foreground emissions and interstellar dust, just to name a few. Further, such a capability would enable rapid exploration of Kuiper belt objects in a much shorter time frame than current methods. Finally, distant targets include the solar gravitational lens, which may enable direct imaging of exoplanets.Figure 1: The Interstellar Medium Science TargetsWe examine a solar thermal propulsion (STP) system to rapidly access the local interstellar medium via a solar perihelion burn. This approach uses several Venus and Earth gravity assists to fly out to Jupiter and then would dive towards the Sun. Approaching within 3 solar radii a perihelion burn would be performed, maximizing the spacecraft’s ΔV to achieve high Solar System escape velocities. A unique aspect of the STP mission concept is that the Sun is not only used as a gravity well for an Oberth maneuver, but also to heat the fuel to ultra-high temperatures (>3000 K), enabling a monopropellant burn with high specific impulse (Isp). An in-depth modeling exercise found this approach to be preliminarily feasible, with escape velocities of around 9 AU/yr achievable with current technology, and up to 16 AU/yr with significant future technological advances.While the baseline STP design is capable of providing just under 9±1 AU/yr, Figure 2 highlights areas of key technological improvements that could be explored. Ultimately, if all technological paths could be implemented, the overall performance as a best-case scenario could reach approximately 16 AU/yr. Figure 2 also qualitatively ranks these improvements from most likely to least likely when reading the graph bottom up. For example, implementing turbopumps in the system is likely more readily feasible than reducing the liner thickness in the near future. It is assumed that these upgrades can be implemented in the future without incurring any additional mass penalty over the baseline design. Thus, it predicts best case performance, and actual values would likely be lower. The improvements could be the result of a single point improvement, or a propagation of several developments.Figure 2: Overview of Solar Thermal Propulsion PerformanceAfter reviewing the STP approach, and comparing it to a solid rocket motor (SRM), it was found that with currently available technology, SRM outperforms STP with an escape velocity of approximately 12 Au/yr. However, future advances in heat exchanger lining materials, turbo pumps, and advanced heat exchanger geometries may enable solar thermal propulsion to provide higher escape velocities, which would provide one of the fastest ways to exit the solar system. Of particular importance is heating the hydrogen to 3,500K. Using a perihelion burn as a kick stage for a nuclear electric propulsion system was found to be particularly effective for achieving even higher escape velocities, up to 19.5 Au/yr.

The interstellar medium (ISM) represents the next frontier in space exploration, with many new discoveries to be made. The challenge, being so far away from Earth, the ISM requires many decades to reach. To advance our knowledge of what exists beyond our solar system, new approaches for rapid access are required. One such approach is solar thermal propulsion (STP). The approach uses several Venus and Earth gravity assists to fly to Jupiter and use its gravity well to dive towards the Sun. Approaching within three solar radii a perihelion burn would be performed, maximising the spacecraft’s ΔV to achieve high solar system escape velocities. A unique aspect of the STP mission concept is that the Sun is used not only as a gravity well for an Oberth manoeuvre, but also to heat the fuel to ultra-high temperatures (> 3000 K), enabling a monopropellant burn with high specific impulse (Isp). Prior preliminary studies indicated escape velocities of over 20 astronomical unit (AU)/year would be possible. An in-depth modelling exercise was undertaken to determine how such a system would perform. The model in this paper showed the current STP design is capable of providing just under 9 ± 1 AU/year, but there are many technology developments that could increase escape velocity. The technologies vary from items that could be implemented in the near term, like turbo-pumps driven by the hydrogen, to items requiring more extensive development programs like thin coatings which do not erode in superheated hydrogen. After reviewing the STP approach, and comparing it to a solid rocket motor (SRM), it was found that with currently available technology, SRM outperforms STP with an escape velocity of approximately 10–12 AU/year. However, future advances in heat exchanger lining materials, turbo pumps, and advanced heat exchanger geometries may enable solar thermal propulsion to provide higher escape velocities, providing one of the fastest ways to exit the solar system. Ultimately, if all technology paths could be implemented with minimal side effects, the performance in a best-case scenario could reach up to 16 AU/year.

A performance analysis for aerocapture at Uranus and Neptune is presented and considers entry corridor width, peak deceleration, peak heat rate, total heat load, and the effect of postcapture orbit on the design parameters. Aerocapture mass benefit vs chemical capture is also quantified. Design relationships are found that can be used in flight system sizing for future studies involving aerocapture. Results are obtained for interplanetary trajectories for launch years from 2025 to 2036, including both chemical capture and high arrival V-infinity. The results show that based on the current assumption for errors and uncertainties aerocapture at Uranus and Neptune requires a mid-lift-to-drag ratio between 0.6 and 0.8. Technology recommendations are made for aerocapture missions to Uranus and Neptune.

The ice giant planets, Uranus and Neptune, represent an important and relatively unexplored class of planet. Most of our detailed information about them comes from fleeting looks by the Voyager 2 spacecraft in the 1980s. Voyager, and ground-based work since then, found that these planets, their satellites, rings, and magnetospheres, challenge our understanding of the formation and evolution of planetary systems. We also now know that Uranus-Neptune size planets are common around other stars. These are some of the reasons ice giant exploration was a high priority in NASA's most recent Planetary Science Decadal Survey. In preparation for the next Decadal Survey, NASA, with ESA participation, conducted a broad study of possible ice giant missions in the 2024–2037 timeframe. This paper summarizes the key results of the study, and addresses questions that have been raised by the science community and in a recent NASA review. Foremost amongst these are questions about the science objectives, the science payload, and the importance of an atmospheric probe. The conclusions of the NASA/ESA study remain valid. In particular, it is a high priority to send an orbiter and atmospheric probe to at least one of the ice giants, with instrumentation to study all components of an ice giant system. Uranus and Neptune are found to be equally compelling as science targets. The two planets are not equivalent, however, and each system has things to teach us the other cannot. An additional mission study is needed to refine plans for future exploration of these worlds.

The capabilities of a SmallSat-class spacecraft targeting the outer solar system and using a combined chemical and electric propulsion system are explored. The development of compact hybrid rockets has enabled high-thrust engines to be packaged tightly enough to fit on CubeSat and SmallSat spacecraft. These hybrid rockets provide 10's-100 N of thrust depending on the propellant load & >300 s of specific impulse and have been demonstrated in both ambient and vacuum environments. Advancements in low -power long-life Hall thruster technologies have provided the potential for significantly greater propellant throughputs, enabling their use as a primary propulsion element on interplanetary spacecraft. In a recent characterization test campaign, the MaSMi-DM Hall thruster demonstrated power throttling from 150 -1000 W with >1500 s of specific impulse available at >500 W and ≥40% total thrust efficiency available at >300 W; peak values of 1940 s and 53%, respectively, were observed. A notional low-mass spacecraft employing a combined hybrid rocket and low-power electric propulsion system was designed and used for mission concept analysis targeting the outer solar system. Using an imposed wet mass limit of 400 kg, mission trajectories to Saturn and Uranus were generated. Orbit capture with >40% of the launch mass was shown to be possible at either target, with mission transfer times of 7.5 years and 13.5 years for Saturn and Uranus, respectively. Significant follow-on mission activities near Saturn (e.g. to Titan & Enceladus) were also possible by carrying extra propellant mass while remaining under the total wet mass limit.

We examine the current state of readiness of aerocapture at several destinations of interest, to identify what technologies are needed and to determine if a technology demonstration mission is required, before the first use of aerocapture for a science mission. The study team concluded that the current state of readiness is destination dependent, with aerocaptured missions feasible at Venus, Mars, and Titan with current technologies. The use of aerocapture for orbit insertion at the ice giant planets Uranus and Neptune requires at least additional study to assess the expected performance of new guidance, navigation, and control algorithms and possible development of new hardware, such as a mid-lift-to-drag entry vehicle shape or new thermal protection system materials. A variety of near-term activities could contribute to risk reduction for missions proposing the use of aerocapture, but an end-to-end, system-level technology demonstration mission is not deemed necessary before the use of aerocapture for a NASA science mission.

Direct high-resolution investigations of a potentially habitable exoplanet may result in finding extra-terrestrial life, arguably the raison d’etre of space exploration. This can be achieved, if a modest astronomical telescope is delivered to the focal region of the Solar Gravitational Lens (SGL), some 650 AU from the Sun. Given the current state of space flight technologies, this can be done in ~25 years. The payoff from such a novel spacebased facility would be enormous: it is the only practical way to achieve a multi-pixel image at a kilometers-scale resolution on the surface of a potentially habitable exoplanet. Instrument requirements are a telescope, a coronagraph and a spectrometer. Although programmatically, exoplanet science resides in the NASA’s Astrophysics Division, an SGL imaging mission addresses the science objectives of three Divisions including Astrophysics (science), Heliophysics (flying through the Interstellar Medium) and Planetary (deep space flight with multi-year observations of a dedicated target, analogous to a planetary orbiter), which would be a major benefit to multiple science technology programs at NASA. See conceptual video description at https://youtu.be/Hjaj-Ig9jBs

The remarkable optical properties of the solar gravitational lens (SGL) include major brightness amplification (~1e11 at wavelength of 1 um) and extreme angular resolution (~1e-10 arcsec) in a narrow field of view. A mission to the SGL carrying a modest telescope and coronagraph opens up a possibility for direct megapixel imaging and high-resolution spectroscopy of a habitable Earth-like exoplanet at a distance of up to 100 light years. The entire image of such a planet is compressed by the SGL into a region with a diameter of ~1.3 km in the vicinity of the focal line. The telescope, acting as a single pixel detector while traversing this region, can build an image of the exoplanet with kilometer-scale resolution of its surface, enough to see its surface features and signs of habitability. We report here on the results of our initial study of a mission to the deep outer regions of our solar system, with the primary mission objective of conducting direct megapixel high-resolution imaging and spectroscopy of a potentially habitable exoplanet by exploiting the remarkable optical properties of the SGL. Our main goal was to investigate what it takes to operate spacecraft at such enormous distances with the needed precision. Specifically, we studied i) how a space mission to the focal region of the SGL may be used to obtain high-resolution direct imaging and spectroscopy of an exoplanet by detecting, tracking, and studying the Einstein ring around the Sun, and ii) how such information could be used to detect signs of life on another planet. Our results indicate that a mission to the SGL with an objective of direct imaging and spectroscopy of a distant exoplanet is challenging, but possible. We composed a list of recommendations on the mission architectures with risk and return tradeoffs and discuss an enabling technology development program.

A bounded, time transformation, based on vercosine of the change in eccentric anomaly, is introduced. This transformation, coupled with the F and G functions, explicitly defines the velocity vectors for a pair of position vectors. Using this property, a discretization strategy is formulated where continuous or impulsive thrusting arcs are represented by set of impulses, implicitly realized by maintaining spatial continuity. Time discontinuity is propagated forward and removed at the last grid point either via explicit constraints or a Lambert arc. The trajectory is transformed into a NLP which is solved using existing solvers. Algorithm performance is studied and compared to JPL's Mission Analysis Low-Thrust Optimizer(MALTO).

The Solar Gravitational Lens (SGL) allows for major brightness amplification ($\sim 10^{11}$ at wavelength of $1~\mu$m) and extreme angular resolution ($\sim10^{-10}$ arcsec) within a narrow field of view. A meter-class telescope, with a modest coronagraph to block solar light with 1e-6 suppression placed in the focal area of the SGL, can image an exoplanet at a distance of 30 parsec with few kilometer-scale resolution on its surface. Notably, spectroscopic broadband SNR is $\sim 10^{-6}$ in two weeks of integration time, providing this instrument with incredible remote sensing capabilities. A mission capable of exploiting the remarkable optical properties of the SGL allows for direct high-resolution imaging/spectroscopy of a potentially habitable exoplanet. Such missions could allow exploration of exoplanets relying on the SGL capabilities decades, if not centuries, earlier than possible with other extant technologies.

Interplanetary Trajectory options for missions to Uranus and Neptune, launching between 2025 and 2037, are presented. Trajectories using Chemical Propulsion, Solar Electric Propulsion and Radioisotope Thermoelectric Generator Electric Propulsion, with up to four planetary flybys are investigated. The effect of different launch vehicles with or without an optimal kick stage, on flight time, inserted mass and propellant throughput, is quantified. To enable simultaneous exploration of both planets, dual-spacecraft trajectories that deliver one spacecraft to each planet from a single launch, are presented. Attractive trajectories and mission opportunities for different multi-element mission architectures are presented.

The increase in performance resulting from optimization of the magnetic field in a low-power magnetically shielded Hall thruster is investigated. The magnetic circuit of the MaSMi-60 Hall thruster was modified to improve the magnetic field topology while increasing the strength of the field across the discharge channel gap. Direct thrust measurements were then taken to assess the changes to thruster efficiency, thrust, and specific impulse. The MaSMi-60's total efficiency increased by nearly 30% as a result of the improved magnetic field, resulting in a peak value of 32.1% (38.6% anode efficiency). Peak thrust and total specific impulse values of 35.8 mN and 1,440 s (1,550 s anode specific impulse) were observed. To demonstrate the thruster's enabling capabilities when paired with a smallsat-class spacecraft, three example mission trajectories to 118401 LINEAR, an icy asteroid-belt comet, were calculated. For each trajectory, the MaSMi-60's experimentally demonstrated performance was used for the throttling table inputs. The trajectory solutions show a delivered mass fraction of between 35-49% for an initial spacecraft mass of up to 350 kg, a solar array power of up to 2.0 kW, and a total transfer time of ~6.5 years.

A science and exploration program is proposed in which traditional Planetary, Heliophysics, Astrophysics and Exoplanet sciences are served with robotic missions that explore the far reaches of our solar system and eventually, embark on a long road to visit an exoplanet. As we learn more about our own solar system we can apply that knowledge to the observations of distant stars and resident exoplanets. This paper describes a program that consists of a series of missions to deploy robotic probes to explore the interstellar medium (ISM) as a pathway towards one-day reaching an exoplanet. We divide this program of ISM probes into 4 elements: 1) Exploration of the Local ISM (LISM) using groups of small satellite explorers for the in-situ exploration of the ISM at distances of 50-200 AU from the Sun; 2) deep-ISM probes to explore the pristine ISM and travelling at > 20AU/year reaching distances of 200 400 AU from the Sun in ~20 years from launch; 3) probes to deliver imaging telescopes to the Solar Gravity Lens Focus area of our Sun at distances of 500 – 800 AU, for the multi-pixel high-resolution imaging of exoplanets prior to sending a dedicated probe towards an exoplanet; 4) technology development program to develop and demonstrate technologies that will one day allow our robotic explorers to leave our solar system at increasing higher velocities and reach an exoplanet that was previously imaged by the SGL observatory emplaced by the probes developed under item 3 above. INTRODUCTION As of today (2017), the international space science community and the international space agencies can claim that we (humanity) have visited all planets and Pluto (with the New Horizons [2] spacecraft) in our solar system, we have landed and returned samples from comets and asteroids, and pierced the Heliosphere and sensed the interstellar medium (Voyager-1), the space between stars in our galaxy. We have continuous robotic presence on Mars for the past 20 years, with plans to return samples and send humans to the red planet. However, as evidenced by recent breakthrough discoveries by the Cassini spacecraft at Saturn and its moons [20], and by the Juno spacecraft at Jupiter and its moons [19], as well as by other robotic explorers and telescopes currently operating in space, our understanding of the origins, evolution and workings of our own solar system are still at an early stage of discovery. As we plan to send robotic explores to sample the geysers of Saturn’s moon Enceladus [9], land in the lakes on Saturn’s moon Titan [18], or reach the liquid oceans of Juno’s moon Europa [10], we will look for evidence of past and present forms of life, and we will continue to seek the answer to the question: How did our solar system evolve from a primordial proto-stellar nebula of dust and particles, into a life bearing solar system intelligent enough to investigate its own past and scientifically forecast its future? 68th International Astronautical Congress, Adelaide, Australia. Copyright 2017 by Caltech/JPL. Published by the IAF, with permission and released to the IAF to publish in all forms. 2 Among the most stunning developments of the past decade in space exploration is the fact that we now know that planets around other solar systems are in abundance. At the latest count, Kepler [3] and other space observatories as well as ground-based telescopes have found over four thousand exoplanets, some of which are deemed to be Earth-like in what is referred to as the ‘Goldilocks zone’. The pursuit of knowledge of our own solar system’s evolution and the further observation of distant solar systems provides for an intra-galactic scientific testbed by which our own solar system acts as a control-case for the understanding of distant solar systems. The more we understand our own solar system and the processes that were essential in its formative years and subsequent evolution to the present state, the better we can understand the same for other solar systems in our galaxy. Therefore, it is reasonable to assume that in the future, these two, otherwise disjoint fields will continue to overlap and inform each other as new discoveries are made. There are, after all, only eight planets in our solar system, and so far over four thousand planets discovered in neighboring solar systems. The discovery of plethora of exoplanets was accompanied by another remarkable event in space exploration history. Voyager-1, launched 40 years ago, survived long-enough to transition from a planetary flyby mission within our solar system, traveling through the Heliosphere into the Heliopause, to become the first functioning inter-stellar explorer sensing the medium between stars, that is, the interstellar medium (ISM). In 2013-2014, just as Voyager-1 [1] was exiting the Heliosphere, Stone, Alkalai and Friedman co-led a team of scientists and engineers in a study funded by the Caltech Keck Institute for Space Studies (KISS) [4, 5] called “The Science and Enabling Technologies for the Exploration of the Interstellar Medium.” This study focused on answering 3 questions, summarized below: 1. Is there compelling science in the exploration of the interstellar medium? The answer was a strong endorsement detailed with goals and objectives that spanned Planetary, Heliophysics, Astrophysics, and Exoplanets science. The team identified overlapping science regions of interest based on the distance from the Sun: a. Planetary science 1 – 70 AU; b. Zodiacal dust science 1 10 AU; c. Kuiper Belt Objects 50 – 700 AU; d. Heliophysics, Local ISM 100 – 200 AU; e. Heliophysics, Pristine ISM 200 – 400 AU; f. Exoplanet science 550 – 800 AU. g. Astrophysics, astrometry: 100 – 700 AU; h. Fundamental physics: 100 – 200 AU; 2. What is a meaningful first step in the exploration of the ISM, as a pathway towards another star? The team agreed that exploration of the local and deep (pristine) ISM are reasonable first steps towards reaching another star. The Sun’s supersonic solar wind and magnetic field create a protective bubble (Heliosphere) around the Sun with a frontal bowshock as it travels through the ISM. It was noted that waiting 40 years to reach the ISM where Voyager is today, was clearly not acceptable. If the cruise time were reduced to < 8-10 years, it would indeed become very attractive to the science community. Thus, a strong recommendation was made to explore innovative and advanced mission design and advanced propulsion technologies to be able to escape the pull of our Sun at higher and Technology Development Program: Propulsion, Power, Telecom., Autonomy Small Satellite Probes to the Local ISM: 100 – 200 AU Deep ISM Probes to the Pristine ISM: 200 – 400 AU Probes carrying telescope to the SGL: 500 – 800 AU Interstellar Program Elements Figure 1: Interstellar Precursor proposed Program Elements : Probes to the Local ISM, Pristine ISM, Solar Gravity Lens (SGL) telescopes, and a long-term technology development program. 68th International Astronautical Congress, Adelaide, Australia. Copyright 2017 by Caltech/JPL. Published by the IAF, with permission and released to the IAF to publish in all forms. 3 higher velocities. Currently, Voyager is traveling at 3.6 AU/year. A near-term goal was set to reach 20 AU/year with technologies that are under development and can be matured in ~10 years. 3. The team also endorsed the goal to study a mission to deploy an optical telescope at the Solar Gravity Lens (SGL [4, 5, 17]) focal line (> 500 AU from the Sun) to obtain high-resolution images of exoplanets using the gravitational lensing of our Sun, before any in-situ mission to an exoplanet is considered. In this paper, we propose a program for the exploration of the ISM as a first step on a pathway to one-day send a robotic explorer to an exoplanet. In the following section, we outline the scope of each of the four program elements which includes: i) sending multiple small probes to the Local ISM (LISM); ii) sending a smaller set of larger deep-ISM probes to sample the pristine ISM and demonstrate escape velocities of > 20 AU/year; iii) deliver imaging telescopes to the SGL region in 25 – 30 years from launch to provide detailed images of a target exoplanet prior to sending a probe to that exoplanet; iv) a dedicated technology development program to develop advanced technologies in areas such as: propulsion, power, mission and trajectory design, telecommunications, miniaturized instruments, guidance navigation and control, avionics and autonomy.

Trajectory design trades for a near-term Pluto orbiter, launching between 2022 and 2030, are presented. Both, chemical and nuclear-powered electric propulsion (EP) trajectories are investigated. Low-thrust EP trajectories powered by either Radioisotope Thermoelectric Generators (RTG) or a high power nuclear reactor are found to be mission enabling. Trajectories using NASA Evolutionary Xenon Thruster (NEXT) or the Xenon Ion Propulsion System (XIPS) are studied and compared. While the XIPS prefer trajectories require lower power and longer flight times, the NEXT enable a shorter, high-powered, mission to Pluto. The effects of using different launch vehicles (including NASA's Space Launch System (SLS)) on flight time, delivered mass and propellant throughput are also studied.

Trajectories for rapid access to the interstellar medium (ISM) with a Kuiper Belt Object (KBO) flyby, launching between 2022 and 2030, are described. An impulsive-patched-conic broad search algorithm combined with a local optimizer is used for the trajectory computations. Two classes of trajectories, (1) with a powered Jupiter flyby and (2) with a perihelion maneuver, are studied and compared. Planetary flybys combined with leveraging maneuvers reduce launch C-3 requirements (by factor of 2 or more) and help satisfy mission-phasing constraints. Low launch C-3 combined with leveraging and a perihelion maneuver is found to be enabling for a near-term mission to the ISM.