A mission that traverses through our solar system, past the boundaries of our heliosphere, and out of our habitable astrosphere to the very local interstellar medium (VLISM) provides a unique opportunity for various in-situ and remote observations during this long journey. The Interstellar Probe mission concept explores a near term, pragmatic basis for designing such a mission, prioritizing critical science measurements while identifying and working with the engineering constraints that come with a long duration mission operating far away from Earth. One of the many challenges of such a mission is selecting instrumentation that will collectively meet science requirements over a long baseline. In order to accomplish this, a variety of instruments will be need to be included in the payload, while keeping in mind size, mass, and power constraints for the mission. These may include particle and field sensors, imaging spectrometers, spectrographs, mass spectrometers, and dust analyzers.Magnetometers (MAG), placed on a boom away from the spacecraft, will be one of the most critical instruments in the payload. With the exception of composition analysis and particle detection, magnetometers are capable of answering many questions related to the nature of the heliosphere, VLISM, and interactions between the two. While both vector helium magnetometers and fluxgate magnetometers have heritage, due to the lengthy duration of this mission fluxgates may provide a more reliable instrument.Another set of critical instruments will be a particle suite that covers a wide range of energies. Particle sensors will play a key role in learning more about our heliosphere and VLISM, providing insight into everything but the neutral hydrogen wall. The suite would most likely include four sensors. First, a plasma system (PLS) would detect thermal ions and electrons up through light pick-up ions (PUI) with energies in the 10s-10000s eV. Detecting energetic ions, electrons, inner source PUIs, and PUI in the ISM would require an energetic particle system and dedicated pick-up ion instrument (EPS and PUI) for particles with energies 10s-1000s keV. A cosmic ray system (CRS) would account for the highest energy particles, observing anomalous cosmic rays (ACRs) and galactic cosmic rays (GCRs) with energies most likely ranging from 1-1000 MeV. Each of these systems would need as close to full coverage of the sky as possible, most likely achieved through angular coverage provided by a spinning spacecraft.The final particle and field sensor that might be included on such a mission is a plasma wave instrument (PWI). This would support measurements made by the magnetometers and particle suite, enabling a better understanding of the size and shape of the heliosphere, particle acceleration in shock regions and the heliosheath, the structure and nature of the heliopause, and properties of the VLISM and GCR spectra outside the heliopause. While the measurements would most likely be made with four components spaced 90° from each other, all perpendicular to ram direction, determining the length and type of antenna used for this instrument is a trade between plasma wave science, guidance navigation and control capabilities, and mission operations.Another critical sensor suite would involve energetic neutral atom (ENA) imagers, where the suite might include one or more imagers designed to image at different energy levels (the low energy ENA-L at 10-2000 eV, medium energy ENA-M at 0.5-15 keV, and high energy ENA-H at 1-100 keV). ENA imagers would result in a better understanding of the force balance and ENA ribbon, as well as solar/heliosphere/VLISM interaction and influence on each other. In particular, an ENA-H that has the capability to point back at our heliosphere once we are well into the VLISM would allow scientists to gain insight into what our astrosphere looks like from the outside. While the two lower energy ENA imagers would only require noseward hemisphere angular coverage, in order to perform the study of the heliosphere from the outside the ENA-H would need full sky coverage with a sun exclusion zone.A neutral mass spectrometer (NMS) would provide key compositional insight during the mission by measuring neutral gas and dust in the VLISM, as well as the neutral hydrogen wall and neutral ISM gas and dust inside the heliosphere. Direct measurements of elemental and isotopic gas compositions of the VLISM would place an important constraint on models of stellar nucleosynthesis which holds implications for the formation of matter in the galaxy. This would enable a much better understanding of the properties and potential history of the ISM as a whole. The instrument would be placed facing the ram direction. Co-boresighted to perform complementary measurements to the NMS would be an Interstellar Dust Analyzer (IDA), which would further establish properties of the VLISM and how it affects our heliosphere. It would also provide important insight into the formation of planetary systems through the examination of interplanetary dust.There are additional choices that could augment these core instruments, including a Lyman-alpha spectrograph (LYA) to provide vital information about interplanetary and VLISM hydrogen phasespace density, imaging spectrometers in the ultraviolet/visible/infrared (UVS/VIR) to study planet formation in the solar system by examining the debris disk and potential nearby Kuiper Belt objects and dwarf planets, and a visIR spectral mapper (IRM) to observe the diffuse red-shifted light emitted by the universe beyond the dominant Zodiacal cloud foreground that obfuscates such studies when performed within our heliosphere.Taking the science objectives into account along with size, mass, and power constraints, two example payloads were developed for the Interstellar Probe concept study: one baseline payload which focuses on heliophysics objectives and an augmentation payload which accommodates a visNIR imager and the visIR mapper for performing a dwarf planet flyby and studying the extragalactic background light in addition to core heliophysics instrumentation. This presentation provides an overview of these example payloads, their accommodation on the spacecraft, and reliability issues associated with requiring up to 50 years of functionality.

During its evolution, the Sun and its protective magnetic bubble – the heliosphere - has completed nearly twenty revolutions around the Galactic Core. During this “Solar Journey” it has plowed through widely different interstellar environments that have all shaped the system we live in today. The orders-of-magnitude differences in interstellar properties have had dramatic consequences for the penetration of interstellar material and have affected elemental and isotopic abundances, atmospheric evolution and perhaps even conditions for habitability. As far as we know, only some 60, 000 years ago, the Sun entered what we call the Local Interstellar Cloud (LIC), and in less than 1,900 years the Sun will be entering a very different interstellar environment that will continue to shape its evolution and fate.The Interstellar Probe is a pragmatic mission with a possible launch already in the next decade that would explore the heliospheric boundary and how it interacts with the Very Local Interstellar Medium (VLISM) to understand the current state along this Solar Journey and, ultimately understand where our home came from, and where we are going. During its 50-year nominal design life, it would go far beyond where the Voyager missions have gone, out to about 400 astronomical units (au) and likely survive out to 1000 au. Therefore, the Interstellar Probe mission would represent humanity’s first explicit step in to the galaxy and become NASA's boldest step in space exploration.When the Voyager missions traversed the heliospheric boundary with their very limited payload it became clear that we are faced with a whole new regime of space physics that is not only decisive for our own heliosphere, but also for understanding the physics of other astrospheres as well. Today we still do not understand the force that is upholding the magnetic shell (the heliosheath) around our heliosphere, or the mechanisms that shield the solar system from galactic cosmic rays, and many other mysteries. Once beyond where the furthest Voyager spacecraft will cease operations (likely at ~170 au), Interstellar Probe would step in to the unknown, traverse the hydrogen wall and the complex magnetic topology at the very edge of the Sun’s sphere of influence, and then directly sample for the first time the interstellar material that has made all of us. There, measurements of the unperturbed gas, plasma, and fields would allow accurate determination of the current state of the LIC and how it affects the global heliosphere. Measurements of unshielded interstellar dust and galactic cosmic rays would provide unprecedented information on stellar and galactic evolution. The physical processes that occur as the solar wind and magnetic field interact with VLISM would also provide the only directly measurable prototypes for understanding the astrospheres surrounding other stars that control the atmospheres and habitability of their exoplanets. All this newly acquired knowledge would then enable an understanding of the current state of the heliosphere and the VLISM, and how they interact, which ultimately can be used to extrapolate the understanding of our system back to the past and into the future.At the same time, the outward trajectory is a natural opportunity for exploring one of the ~4,000 Kuiper Belt Objects or ~130 dwarf planets similar to and beyond Pluto and determine the large-scale structure of the circum-solar dust disk to provide the ground truth for planetary system formation in general. Once beyond the obscuring dust, the infrared sky would open a window to early galaxy formation.An Interstellar Probe has been discussed and studied since 1960, but the stumbling block has always been propulsion. Now this hurdle has been overcome by the availability of new and larger launch vehicles. An international team of scientists and experts are now in the final year of a NASA-funded study led by The Johns Hopkins University Applied Physics Laboratory (APL) to develop pragmatic example mission concepts for Interstellar Probe with a nominal design lifetime of 50 years. Together with the Space Launch System (SLS) Program Office at NASA’s Marshall Space Flight Center, the team has analyzed dozens of launch configurations and demonstrated that asymptotic speeds in excess of 7.5 au per year can be achieved using existing or near-term propulsion stages with a powered or passive Jupiter Gravity Assist (JGA). These speeds are more than twice that of the fastest escaping man-made spacecraft to date, which is Voyager 1 currently at 3.59 au/year. Launching near the nose direction of the heliosphere, Interstellar Probe would therefore reach the Termination Shock (TS) in less than 12 years and cross the Heliopause into the VLISM after about 16 years from launch.In this presentation we provide an overview and update of the study, the science mission concept, the compelling discoveries that await, and the associated example science payload, measurements and operations ensuring a historic data return that would push the boundaries of space exploration by going where no one has gone before.

Ralph McNutt, Robert Wimmer-Schweingruber, Mike Gruntman, Stamatios Krimigis, Edmond Roelof, Pontus Brandt, Kathleen Mandt, Steven Vernon, Michael Paul, Robert Stough, and James Kinnison Johns Hopkins University Applied Physics Laboratory, Space Exploration Sector, Laurel, Maryland, United States of America Christian-Albrechts-Universität zu Kiel, Germany (wimmer@physik.uni-kiel.de) University of Southern California, Los Angeles, California USA (mikeg@usc.edu) Office of Space Research and Technology, Academy of Athens, Athens, Greece NASA Marshall Space Flight Center, Spacecraft / Payload Integration and Evolution (SPIE) Office Huntsville, Alabama, USA (robert.w.stough@nasa.gov)

The interaction of our protective heliosphere and the Very Local Interstellar Medium (VLISM) is the least explored and most rewarding frontier of space physics.New evidence amplifies the central role of the heliosphere in the evolution of the solar system along its 4.6billion-year journey around the galaxy.In addition to the dense clouds of plasma, gas and dust seeding the early proto solar nebula, recent supernovae have left the entire solar system exposed to extreme fluxes of interstellar material and cosmic radiation with far-reaching implications.Our current knowledge lacks the direct measurements necessary to understand how our star upholds its vast heliosphere and its potentially game-changing role in the evolution of our galactic home.Interstellar Probe provides new, required measurements over more than a solar cycle to uncover the physical processes starting near the Sun responsible for creating our dynamic heliosphere.In April 2022, the pragmatic Interstellar Probe Mission Concept Study was completed after four years, detailing a Large Strategic heliophysics mission that would transect the heliosphere from 1 au to the VLISM.Its journey provides rich science for generations across heliophysics and presents an opportunity to push the frontier of space exploration farther than ever done before.Modest crossdivisional investments enable high-value planetary science and astrophysics, deepening our understanding of the emergence of our habitable planetary system.A trajectory through the forward hemisphere of the heliosphere would be accomplished by a launch in the 2036-2042 timeframe using conventional chemical propulsion and a heavy-lift launch vehicle, such as the Space Launch System (SLS).A Jupiter Gravity Assist could propel an 860-kg spacecraft with an 87-kg payload of ten instruments delivering a unified view of the global heliosphere, reaching the VLISM after 16 years.The spacecraft is designed to a 50-year nominal lifetime using modern-day technology based on successful missions like New Horizons.Two next-generation Radioisotope Thermal Generators (RTGs) would ensure 300 We at end of nominal mission at 375 au and could enable exploration even beyond 500 au.

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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.

The Interstellar Probe mission presents the ambitious challenge of launching a spacecraft for interstellar exploration by 2030. Achieving interstellar distances via either a Solar Oberth Maneuver (SOM) or Jupiter gravity-assist (JGA), state-of-the-art technologies are pushed to provide a system capable of surviving 50 years through the extreme hot temperatures of the solar environment and extreme cold of deep space, while meeting communications requirements at distances up to 1000 AU. Previous published work summarized the telecommunications tradespace and baseline design for an Interstellar Probe mission based on a Jupiter gravity assist (JGA). This paper furthers this work by discussing design changes necessary to support a Solar Oberth Maneuver, presenting options on extending mission life beyond the 50-year nominal mission in an effort to reach the goal of 1000 AU, and presents implications on the choice of exit target due to the reliance on primarily Northern Hemisphere ground assets. Compared to the Jupiter gravity assist mission design, the SOM option significantly reduces the amount of aperture available for communications. The heat shield used to provide thermal protection during the SOM dictates the aperture available for communications. Operating at higher frequency is immediately advantageous in an aperture-limited case. However, the longevity of the mission does not allow for use of typical mechanical pointing technologies like reaction wheels. Given these size and pointing restrictions on spacecraft communications, investigation into alternative pointing technologies and comparison between X-band, Ka-band, and optical communication systems are presented. Ultimately, the concept study chose the JGA option as baseline. To support communications to and through interstellar space, ground systems require increasing aperture. This work, as well as previous work, explores the notional case of using the Next-Generation Very Large Array (ngVLA) as a ground asset for the science downlink. Such expansion of operational communications ground capability is a key enabling concept for supporting communications and retrieval of science data from 1000 AU.

The idea of an "Outer solar system probe: to be aimed away from the Sun in the plane of the ecliptic" dates from a report of the "Simpson Committee" of the Space Science Board of the National Academy of Sciences in March of 1960. After many studies and name changes, what is now known as "Interstellar Probe" has matured as a concept for making new discoveries that can be made in no other way, by going places yet to be explored. The central technical question has always been propulsion with "near-future" capabilities taken as the backdrop for defining the mission requirements. However, the real issue has always been to unite compelling science with engineering and technical reality. With that perspective in mind the Johns Hopkins University Applied Physics Laboratory (APL) has been tasked by the NASA Heliophysics Division to (re-)study the mission and provide a Technical Report to be delivered late 2021 for input to next Solar and Space Physics Decadal Survey. This "pragmatic Interstellar Probe" of the study is a mission through the outer heliosphere and to the nearby "Very Local" interstellar medium (VLISM), uses today's technology to take the first explicit step on the path of interstellar exploration, and can pave the way, scientifically, technically, and programmatically for more ambitious future journeys (and more ambitious science goals). To enforce these goals broadly-based engineering requirements include (1) readiness to launch no later than January 1, 2030; (2) capability to transmit useful scientific data from 1000 au; (3) powered by no more than 600 W (electric) at the beginning of the mission and no more than half of that at mission's end; and, (4) lifetime of no less than 50 years. To travel as far and as fast as possible with available technology, the use of the Space Launch System Block 2 (SLS B2) cargo version is enabling: carrying the spacecraft as well as a 3rd and 4th stage, solar system escape speeds of at least twice that of Voyager 1 (i.e., up to 7.2 au/yr) should be possible. We provide a top-level summary of work accomplished to date, focusing on how the science goals drive and are affected by telecommunication options, guidance and control requirements, trajectory options, and the baseline system architecture and approach. "It isn't about where we are going. It's about the journey out there."

In the early development stages for Parker Solar Probe (PSP), the Johns Hopkins University Applied Physics Laboratory (JHU/APL) team mapped the mission’s top-level requirements to mission success criteria to ensure that a failure of any one science instrument would not translate to a failure of the entire mission. This paper details this analytical process and shows how the results were folded into the reliability assessments. The Interstellar Probe study team is also using this process to define mission success criteria. Interstellar Probe faces the added challenge of being a 50+ year mission with science objectives at each epoch during its flight. Designing the science objective – instrument interface will increase the mission reliability without paying the mass penalty of simply using a redundant payload.

An Interstellar Probe mission to the Very Local Interstellar Medium (VLISM) would bring new scientific discoveries of the mechanisms upholding our vast heliosphere and directly sample the unexplored Local Interstellar Clouds that our Sun is moving through in relatively short galactic timescales. As such, it would represent Humanity's first explicit step in to the galaxy and become perhaps NASA's boldest step in space exploration. Such a mission has been discussed and studied since 1960, but the stumbling block has often been propulsion. Now this hurdle has been overcome by the availability of new and larger launch vehicles. An international team of scientists and experts are now progressing towards the final year of a NASA-funded study led by The Johns Hopkins University Applied Physics Laboratory (APL) to develop pragmatic example mission concepts for an Interstellar Probe with a nominal design lifetime of 50 years. Together with the Space Launch System (SLS) Office at the NASA Marshall Space Flight Center, the team has analyzed dozens of launch configurations and demonstrate that asymptotic speeds in excess of 7.5 Astronomical Units (AU) per year can be achieved using existing or near-term propulsion stages with a powered or passive Jupiter Gravity Assist (JGA). These speeds are more than twice that of the fastest escaping man-made spacecraft to date, which is Voyager 1 currently at 3.59 AU/year. An Interstellar Probe would therefore reach the Termination Shock (TS) in less than 12 years and cross the Heliopause into the VLISM after about 16 years from launch. In this presentation we provide an overview and update of the study, the science mission concept, discuss the compelling discoveries that await, and the associated example science payload, measurements and operations ensuring a historic data return that would push the boundaries of space exploration by going where no one has gone before.

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.

“Interstellar Probe” is a mission through the outer heliosphere and to the nearby “Very Local” interstellar medium (VLISM) that uses today’s technology to take the first explicit step on the path of interstellar exploration. As such, it can pave the way, scientifically, technically, and programmatically for more ambitious future journeys (and more ambitious science goals). This mission concept is being studied by the Johns Hopkins Applied Physics Laboratory for the NASA Heliophysics Division to help inform the next Space and Solar Physics Decadal Survey, planned by the National Academies for 2022.

On August 12, 2018, Parker Solar Probe (PSP) began its seven-year exploration of the inner heliosphere and the Sun's corona. Using specially designed systems to protect from the Sun's intense environment and power the spacecraft, PSP has already provided game-changing insights into coronal heating, the source of the solar wind, and how the solar wind is propagated into the interplanetary medium to interact with Earth. More than a decade in the making, PSP was designed, built, operated and managed by The Johns Hopkins University Applied Physics Laboratory (JHU/APL) as part of NASA's Living With a Star program. PSP launched into a solar orbit, and used a Venus gravity assist to lower the orbit perihelion to complete the planned solar encounters (where the solar distance is less than 0.25 AU), currently with perihelion as close to the Sun as 0.17 AU. PSP will use six more Venus gravity assists over the next six years to reduce its perihelion from 0.17 AU to 0.046 AU in the last three encounters. During solar encounter, measurements to characterize the in situ environment will be made by four instrument suites: FIELDS (electromagnetic fields), SWEAP (solar wind plasma), IS⊙IS (energetic particles) and WISPR (imaging of the coronal structure). We discuss the scientific motivation for PSP and the instrument suite used to make these ground-breaking measurements. We describe the development of PSP since inception in 2007, the extreme environments to which the observatory is exposed and how these environments drove design, technologies that enable the mission, and the final configuration of both the mission and the observatory. Finally, we give an account of post-launch commissioning and operations in early orbits, and an assessment of observatory performance over these first orbits.

The Johns Hopkins University Applied Physics Laboratory (APL) has been tasked by the NASA Heliophysics Division to (re-)study a robotic Interstellar Probe mission. The top-level requirement is to provide input to support the next Solar and Space Physics “Decadal Survey” in the United States, with a nominal time frame of performance from 2023 to 2032.The study approach is to look widely across both relevant scientific and technical communities and assemble a “Menu” of what has been, and what can be, done with respect to Interstellar Probe desires and concepts past. By its nature this assemblage is a “superset” of what might be implemented; “ordering” from the menu will be a charge to a future Science Definition Team – at NASA’s discretion. This approach has been adopted successfully in the past, with the emphasis on informing the Survey participants of valid possibilities, while not dictating a “best” solution.The components of such a menu are not random. They must flow from compelling top-level science goals, through explicit measurement requirements, instrumentation to make those measurements, and an assessment of how those measurements have provided “closure” to the investigation, i.e. have addressed the science goals.For this type of engineering “menu” study, payload instrument possibilities and capabilities, with representative masses and power requirements, are required to help assess the overall spacecraft cost, mass, and achievable speed for a given launch vehicle configuration. Details have and will continue to evolve as the engineering aspects mature and as more people throughout the international scientific community continue to contribute their ideas.Neither critical trade-offs nor enabling technologies are new. For example, with a given launch system, the total energy that can be imparted to the spacecraft is fixed. Thus, with a given set of planetary gravity assists there is a trade between the maximum asymptotic escape speed from the Sun and the total mass of the spacecraft, which, in turn, tends to scale with the payload mass. As a starting point, a range of spacecraft masses from ~300 to 800 kg, corresponding roughly to Pioneer (251.8 kg) through Voyager (825.4 kg), has been considered. Communication has focused on microwave downlink (X-band or Ka-band), which is well developed, known, and robust. While optical laser comm might achieve far higher downlink rates, it requires extreme pointing stability, and the associated lifetime also needs continuing investigation. The need and capability for radioisotope power systems (RPS) for powering deep-space robotic spacecraft operating far from the Sun is well established  and will be required here as well. Also, in this study the use of the Space Launch System (SLS) cargo version is in use along with upper stages. Other “lunar capable” launch vehicles have also been “spot checked,” but more complete performance data is needed from their respective vendors for a definitive assessment.As with any other study of this type, initial engineering requirements must be imposed to begin an inherently iterative process of design. Engineering requirements are needed to frame the engineering study and “bound the box” – but allow for trades. As with other aspects of this study, these are also still evolving:(1)       Enable a mission that can be launched no later than 1 January 2030 (technology driver)(2)       Have the capability to operate from a maximum range of not less than (NLT) 1000 astronomical units (a.u.) from the Sun (communications system driver)(3)       Require no more than 600 Watts of electrical power (We) at the beginning of mission (BOM) and be able to operate at no less than half of the BOM amount at the end of mission (EOM) (power system driver)(4)       Achieve a mission lifetime of NLT 50 years with an “acceptable” probability of success (drives parts program, physics of failure analyses, programmatics, and policy)Given the requirement of near-term flight, we focus entirely upon ballistic solutions with Jupiter Gravity Assists (JGA) in order to maximize the asymptotic solar system escape speed. Three options are under study: The first two use prograde gravity assists at Jupiter, one passive and one active with an upper stage burn in Jupiter’s gravity well; the third uses a retrograde gravity assist and a powered “Oberth maneuver” near the Sun.The current focus is upon a medium-mass, all-heliophysics payload to obtain a first cut of basic architecture approach and initial subsystem master equipment list (MEL) with power requirements and margins and reserves, as appropriate. This is being used to size the communication system, avionics, and guidance and control and data systems. It will also serve as a baseline for looking at both upscale and downscale payload and capability options, and trades with planetary and astrophysics science options, including effects on required spacecraft concept of operations and the payload mass, power, and cost tradespaces.The current effort is well into this initial concept definition via seven focus studies:1) Longevity - Spacecraft lifetimes/failures, long-lasting systems, failure modes, ground operations, staff renewal, and knowledge documentation and retention 2) Instruments - Candidate payload components with parameters + operating requirements 3) Trajectory and launch vehicle trades including Jupiter’s radiation and dust ring keep-out zone(s) 4) Communication and guidance and control trades, achievable pointing accuracy 5) Thermal shield requirements and possibilities for a near-Sun maneuver, including quantifying shield mass, radiation pressure effects, and center-of-mass migration and attitude control requirements during a near-Sun, rocket-motor burn 6) Overall mechanical layout, fairing clearances, payload adaptors, instrument clear fields of view, boom and antenna options, deployments, and associated pointing control 7) Power requirements, including payload and spacecraft systems, parasitic heating of propellant lines, surge capability trades, and shunt requirements for use with a Next Generation (NG)-RPS The study continues to solicit input on all of these topics from the space science and engineering communities via meetings and workshops. We remain on track for delivering the final report to NASA’s Heliophysics Division in late calendar year 2021.

Solar Probe Plus (SPP), currently in Phase B, will be the first mission to fly into the low solar corona, revealing how the corona is heated and the solar wind is accelerated, solving two fundamental mysteries that have been top priority science goals since such a mission was first proposed in 1958. The scale and concept of such a mission has been revised at intervals since that time, yet the core has always been a close encounter with the Sun. SPP uses an innovative mission design, significant technology development and a risk-reducing engineering development to meet the SPP science objectives: 1) determine the structure and dynamics of the magnetic fields at the sources of the fast and slow solar wind, 2) trace the flow of energy that heats the corona and accelerates the solar wind. and 3) determine what mechanisms accelerate and transport energetic particles. In this paper, we present the Solar Probe Plus mission along with a brief comparison with some previous concepts for such a mission, and discuss the trade studies that led to the SPP implementation. We present a summary of the challenges associated with operation in the solar encounter environment and discuss the technology development and engineering trade studies to compose a mission that will not only survive this environment, but will provide the data needed to answer the science questions that have remained unanswered to date.

Solar Probe Plus will be the first mission to pass into the solar corona to study how the corona is heated and the solar wind is accelerated. Solving these two fundamental mysteries has been a top-priority science goal for over five decades. The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, is managing the mission for NASA’s Living with a Star Program, including the development, build, and operation of the spacecraft. SPP will launch in 2018, performing 24 orbits of the Sun over a 7-year duration. The mission design utilizes seven Venus gravity assists to gradually reduce perihelion from 35 solar radii in the first orbit down to 9.86 solar radii for the final three orbits. Science data are collected during each perihelion pass and transmitted to Earth between passes. The SPP spacecraft is 665-kg at launch, almost 3 meters in height and 2.3 meters in diameter. A thermal protection system composed of Carbon-Carbon and carbon foam protects the spacecraft from the extreme solar environment near perihelion. At 9.86Rs, the solar intensity is 475 times that at 1AU. Hiding behind the thermal protection system provides the spacecraft a benign thermal environment while the TPS experiences temperatures of 1400°C on its sun-facing surface. Solar Probe Plus is solar powered with water cooled solar arrays for power generation that maintain the solar cell assemblies within their required temperature limits. This paper presents the science overview, mission concept, and spacecraft description as the mission approaches its Preliminary Design Review.