Oral presentations and abstracts

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.

How to cite: McNutt, R., Wimmer-Schweingruber, R., Gruntman, M., Krimigis, S., Roelof, E., Brandt, P., Mandt, K., Vernon, S., Paul, M., Stough, R., and Kinnison, J.: The Interstellar Probe Study – A Year 2 Update, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-544, https://doi.org/10.5194/epsc2020-544, 2020.

The global nature of the interaction of the heliosphere and the Local Interstellar Medium (LISM) is among one of the most outstanding space physics problems of today. Ultimately, our magnetic bubble is upheld by the expanding solar wind born in the solar corona that is now accessible by Parker Solar Probe. At the other extreme boundary, a completely new regime of physical interactions is at work that shape the unseen global structure of the entire heliosphere. Voyager 1 and 2 are soon nearing their end of operations inside of 170 AU and their payloads dedicated to planetary science have uncovered a region of space that defies our understanding. At the same time, IBEX and Cassini have obtained complementary “inside-out” ENA images of the heliospheric boundary region that cannot be fully explained.

An Interstellar Probe through the heliospheric boundary, in to the LISM would be the first dedicated mission to venture into this largely unexplored frontier of space. With a dedicated suite of in-situ and remote-sensing instrumentation, such a probe would not only open the door for a new regime of space physics acting at the boundary and in other astrospheres, but would also obtain the very first images from the outside of the global structure of the heliosphere that, in context with the in-situ measurements would enable a quantum leap in understanding the global nature of our own habitable astrosphere. Beyond the Heliopause, the Interstellar Probe would offer the first sampling of the properties of the Local Interstellar Cloud and interstellar dust that are completely new scientific territories. Relatively modest contributions across divisions would offer historic science returns, including a flyby of one or two Kuiper Belt Objects, first insights in to the structure of the circum-solar dust disk, and the first measurements of the Extra-galactic Background Light beyond the obscuring Zodiacal cloud. In summary, an Interstellar Probe would represent humanity’s first step in to the galaxy and become the farthest space exploration ever undertaken.

The idea of an Interstellar Probe and a Solar Probe shares a common beginning as two of the “Special Probes” that the Simpson Committee carried forward in their Interim Report to the Space Studies Board in 1960. Since then, an Interstellar Probe has scientifically been highly rated in the Solar and Space Physics Decadal Surveys, but the lack of propulsion technologies and launch vehicles have presented a stumbling block for its realization. However, this bottleneck is now being removed with the development of the Space Launch System (SLS) Block 2 with first launch projected to end of the 2020’s.

A study funded by NASA is now progressing towards its third year of developing realistic mission architectures for an Interstellar Probe using technology ready for launch beginning 2030. An SLS Block 2, with an Atlas Centaur 3^rdstage, a Star 48 4^th stage  could propel a spacecraft up to about 8.5 AU/year, which would be more than twice the fastest escaping spacecraft (Voyager 1 at 3.6 AU/year). The scenario would use a direct inject to Jupiter followed by a Jupiter Gravity assist powered by the 4^th stage. The mission trade space is bound by requirements to be able to operate out to 1000 AU, 600 W of power beginning of mission, and survive up to 50 years.

Here, we discuss the outstanding science questions that could be addressed by a mission to the LISM, notional science payload and report on realistic mission architectures, design concepts and trades, enabling technologies, and programmatic challenges.

How to cite: Brandt, P., Provornikova, E., Runyon, K., Lisse, C., Rymer, A., Mandt, K., McNutt, R., and Paul, M.: Interstellar Probe: Humanity's Journey Into Interstellar Space Begins, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-394, https://doi.org/10.5194/epsc2020-394, 2020.

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 Targets

We 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 (I_sp). 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 Performance

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 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.

How to cite: Alkalai, L., Sauder, J., Preudhomme, M., Mueller, J., Cheikh, D., Sunada, E., Karimi, R., Couto, A., Rapinchuk, J., Arora, N., Peeve, T., Anderson, K., and Panian, J.: Rapid Access to the Interstellar Medium via Solar Thermal Propulsion, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-917, https://doi.org/10.5194/epsc2020-917, 2020.

A well-designed radio and plasma wave instrument on Interstellar Probe will measure a variety of key phenomena in the outer heliosphere and interstellar medium, namely few-kHz radio emissions from beyond the heliopause, quasi-thermal noise emissions - hence plasma density and temperature, plasma waves associated local kinetics and beams, and potentially dust impacts on the spacecraft.  Radio emission signatures have been observed as solar transients interact with the local interstellar medium and can give remote measurements of the interaction between the heliosphere and local interstellar medium.  The quasi-thermal noise spectrum is a highly accurate way to measure total density, electron pressure, and potentially bulk flow speed in the interstellar medium.  Nonthermal plasma waves are indication of electron beams and kinetic plasma distributions and can give key diagnostics of shocks, current sheets, and other discontinuities.  And of course, plasma wave measurements have proved to be a simple, robust measure of dust statistics with a relatively large count rate.  All of these science goals can be met with a simple radio and plasma wave instrument, provided that proper consideration is given to sensor design and geometry and spacecraft integration is considered a priori.  We describe the science and instrument trades and resource estimates associated with such an instrument.  

How to cite: Bale, S. and Kurth, W.: Quasi-thermal noise, plasma wave, and radio measurements for an Interstellar Probe Mission, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-913, https://doi.org/10.5194/epsc2020-913, 2020.

Introduction. Interstellar Probe is a NASA Heliophysics Division mission concept that would leave the solar system on a 50-year prime mission with a primary objective to study the heliosphere and the local interstellar medium (>200 au; Brandt et al., 2020). In order to reach interstellar space, the spacecraft must pass through the trans-Neptunian region, presenting the opportunity to study planets and minor bodies (Brandt et al., 2020). We focus here on the opportunities for exploring trans-Neptunian objects (TNOs) with Interstellar Probe. 

TNO Geoscience Investigation. The primary science instrument for TNO flyby geoscience would be a visible/infrared imaging spectrometer to enable studying a wide range of physical, chemical, and geological processes at temperatures <40 K. Reflectance spectroscopy of the infrared region (1-5 μm) is most useful for studying surface composition due to myriad fundamental and overtone absorption bands of volatile (N₂, CH₄, CO) and non-volatile (H₂O, CO₂, NH₃) ices and refractory organics (e.g., C₂H₆, tholins). A visible imager would reveal geological, albedo, and color information to investigate surface characteristics and evolution. 

Opportunities for dwarf planet exploration. The heliophysics science goals will dictate Interstellar Probe’s trajectory, with a consensus coalescing around leaving through the “waist” of the heliosphere between ecliptic longitudes of 300-330° (Brandt et al., 2020; Runyon et al., 2020). There are an estimated 130 dwarf planets—objects >400 km in diameter—beyond Neptune. In or near this region, between -20° and 20° in heliocentric ecliptic latitude, there are five potential dwarf planets that Interstellar Probe could fly by: 2002 MS₄, Quaoar, Gonggong, Pluto, and Ixion (Figure 1).

Potential dwarf planet targets represent a wide range of colors, compositions, and geological processes. Quaoar is dense (~2 g cm^-3; Braga-Ribas et al., 2013) and may therefore have retained enough internal heat to remain geologically active and/or support a subsurface ocean (e.g., Shchuko et al., 2014). Gonggong and its satellite Xiangliu show one of the largest color dichotomies among a primary and satellite (Kiss et al., 2019), and Gonggong may be a contact binary (Pál et al., 2016). Little is known about 2002 MS₄ due to its current position in a crowded star field, frustrating ground-based observations, but it is large enough for possible differentiation and a surface dominated by H₂O ice and the resultant geological features. Ixion has a flat near-IR spectrum (Barkume et al., 2008; Guilbert et al., 2009), meaning it could provide clues to the substrate present beneath the ice on all TNOs. Pluto would benefit from a follow-up 20+ years after New Horizons to reveal surficial and atmospheric changes.

Figure 1. Positions of dwarf planets (black), CCKBOs (red), and extreme TNOs (purple) with respect to the ecliptic on January 1, 2040. Longitudinal regions for the heliospheric nose (pink), tail (yellow), and waist (blue), as well as the heliospheric nose (filled red star) and tail (open red star), are marked. We assumed the trajectory of Interstellar Probe will take it somewhere between ~300-340˚ longitude and -20˚ to 20˚ latitude, opening up the possibility for a flyby of 2002 MS4, Quaoar, Pluto, Ixion, or Gonggong. 

Prospects for a secondary target flyby. New Horizons’ flyby of Arrokoth (2014 MU₆₉) in 2019 provided a valuable glimpse into the formation history of planetesimals and the pristine surfaces present beyond Neptune (Stern et al., 2019). The flyby of this small TNO (~35 km) revealed the building blocks of the largest TNO, Pluto, New Horizons’ other target. Interstellar Probe could complement the Arrokoth and Pluto flybys via a serendipitous encounter with a TNO <400 km in diameter, potentially revealing a new class of TNOs never before visited by spacecraft.

We consider the possibility of serendipitous flybys of TNOs <400 km, with the highest probability occurring for a spacecraft trajectory in the ecliptic plane among the low-inclination cold classical Kuiper Belt Objects (CCKBOs). CCKBOs have inclinations ≤5° and semi-major axes between 42 and 47 au (Gladman et al., 2008). From surveys at greater depth, predominantly at low ecliptic latitudes (e.g., OSSOS; Bannister et al., 2016), ~35% of known TNOs fit these criteria (JPL/Horizons Small-Body Database); debiased population assessments suggest that the CCKBOs are the most numerous sub-population of the <100 au region (e.g., Fraser et al., 2014).

An estimated 40,000-60,000 new TNOs to absolute magnitudes >8 will be discovered with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST; Schwamb et al., 2018). Assuming 45,000, some 16,000 CCKBOs will be known by the early 2030s. Assuming uniform distribution along the ecliptic between 42-47 au, each CCKBO would “occupy” a volume of 0.0875 au² with a circular radius of ~0.17 au. Defining a reasonable flyby distance of 10^-4 au (~15,000 km, comparable to New Horizons’ flyby distance of Pluto), the probability of serendipitously flying-by a particular CCKBO is 3 𝗑 10^-5 %. For a trajectory radially outward from the Sun, there would be ~30 opportunities for serendipitous flybys, leading to a 1 in 100,000 chance of making a serendipitous flyby.

This estimate assumes a trajectory near the plane of the ecliptic and Gonggong as the primary target. A more oblique trajectory through the classical belt would improve the chances. For trajectories out of the plane of the ecliptic, the probability of a serendipitous flyby drops precipitously.

References.

Bannister, M.T., et al., 2016. AJ 152, 70.

Barkume, K.M., et al., 2008. AJ 135, 55-67.

Braga-Ribas, F., et al,. 2013. ApJ 773, 26.

Brandt, P. C., et al. (2020). NAS white paper for the Solar & Space Physics Decadal Survey 2023.

Fraser, W.C., et al., 2014. ApJ 782, 100.

Gladman, B., et al., 2008. The Solar System Beyond Neptune. University of Arizona Press, Tucson, 43-57.

Guilbert, A., et al., 2009. Icarus 201, 272-283.

Kiss, C., et al., 2019. Icarus 334, 3-10.

Pál, A., et al., 2016. AJ 151, 117.

Runyon, K.D., et al., 2020. NAS white paper submitted to the Planetary Science decadal survey committees.

Schwamb, M.E., et al., 2018. arXiv:1802.01783.

Shchuko, O.B., et al., 2014. P&SS 104, 147-155.

Stern, S.A., et al., 2015. Science 350, aad1815.

Stern, S.A., et al., 2019. Science 364, aaw9771.

How to cite: Runyon, K., Holler, B., and Bannister, M.: Exploring Trans-Neptunian Objects with Interstellar Probe, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-276, https://doi.org/10.5194/epsc2020-276, 2020.

Proposed to pierce the heliopause and explore proximal interstellar space, the Interstellar Probe (IP) mission would have extraordinarily valuable flyby opportunities with other planetary bodies along its flight path. Gas and ice giant encounters are one kind of opportunity, but the most likely such flyby would be with Jupiter, New Horizons style. Jupiter and the Galilean satellites should be well covered scientifically in the years to come, but the jovian magnetosphere and especially the long magnetotail would be prime targets for IP instruments. Superior opportunities for scientific discovery would come with the Kuiper Belt (KB) and beyond, however, as reaching such distant, unexplored worlds remains challenging otherwise. Whatever the heliospheric community decides as the best and most scientifically advantageous outbound asymptote for IP, there will be multiple possible KB encounters, both with dwarf planets and smaller bodies, on either the chosen or very similar trajectories. Given the likely timeframe for the launch of IP (2030 or later), many more potential targets will have been discovered in the intervening years. The discoveries of New Horizons at the Pluto system (dwarf planet) and Arrokoth (cold classical KB object) have proven revolutionary [1,2]; IP would no doubt achieve the same, even before reaching the heliopause, but only if it carries the necessary instrumentation. The minimum would be a visible wavelength camera and a near-infrared imaging spectrometer.

Fast flyby speeds are manageable; New Horizons proved that with the Arrokoth encounter. IP will likely be a spinning spacecraft, however, to maximize the capabilities of its primary fields and particle instrument suite, which poses a challenge to any KB object flyby. One strategy is simply to adapt to the spin rate, in the manner that Juno has at Jupiter (i.e., line scanning). Another would be for IP to have the ability to vary (slow) its spin rate, or even to go into 3-axis mode for encounters. In either case, a gimbal in the optical path would greatly reduce (or eliminate) the need for additional spacecraft repointings. Bodies in the so-called detached region of the KB, or inner Oort Cloud (OC), such as Sedna, would be especially tempting targets, as the likelihood of other missions to this region of the solar system are essentially zero. That is, unless there really is a major planet on a distant eccentric orbit shepherding the orbits of the detached/inner OC bodies. This is a big if of course, but if such a planet exists, the drive to visit it will be great, which alone could be used to leverage the launch of IP. Such an encounter would demand a more comprehensive remote sensing suite, in which case a daughter spacecraft to IP might be considered as an engineering option. A fully autonomous RTG-powered daughter craft would probably put too much of a lean on IP’s mass and C3, but a battery-powered 3-axis stabilized daughter craft could, potentially, carry out the critical rendezvous measurements and transmit the results to IP, which from there would be transmitted to the Earth.

Finally, the dream of sending a full IP (1000 AU in 50 yr, which would require speeds of ~100 km/s) should not preclude thinking about tempering ambitions in the face of financial or engineering realities. Even a mission that “simply” doubles the asymptotic velocity of Voyager 1 (the fastest of the 5 interstellar precursor missions) could accomplish much of IP’s science in a similar timeframe. Plus there may be a speed which is “too fast,” even for fields and particles measurements. The technological ability to reach ~35 km/s would also allow us to rendezvous with, or even ride along with interstellar visitors (e.g., 1I/ 'Oumuamua and 2I/Borisov) as they head back into the depths of space.

[1] Stern S. A., Grundy W. M., McKinnon W. B., Weaver H. A., and Young L. A. (2018) The Pluto system after New Horizons. Annu. Rev. Astron. Astrophys., 56, 357–392, doi: 0.1146/annurevastro-081817-051935.

[2] Stern S. A., et al. (2019) Initial results from the New Horizons exploration of 2014 MU₆₉, a small Kuiper Belt object. Science, 364, eaaw9771, doi: 10.1126/science.aaw9771.

How to cite: McKinnon, W. B.: Scientific Opportunities in the Kuiper Belt and Inner Oort Cloud with an Interstellar Probe, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-504, https://doi.org/10.5194/epsc2020-504, 2020.

Solar Lyman-a emission re-radiated from H atoms incoming to the heliosphere from interstellar medium is a powerful tool to probe globally plasma properties both at the heliosphere boundary and near the Sun. H Lyman-α line profiles reflect velocity distributions of low energy H atoms in the heliosphere which hold information about the plasma near the heliopause. H Lyman α intensities as observed at 1 AU serve as diagnostic of global properties of the solar wind. In this talk we will review what we have learned about the global heliospheric interaction from H Lyman-a observations from inside the heliosphere on SWAN/SOHO, Voyages/UVS and New Horizons/Alice missions. Outward trajectory of Interstellar Probe going through the outer heliosphere to the interstellar medium (ISM) up to 1000 AU enable unique science opportunities to explore global interaction between the solar wind and local ISM by observing for the first time Lyman-a emission from outside of the heliosphere. We will report a progress of UV working group in outlining primary science questions on the nature of the global heliosphere and Local Interstellar Cloud, planning observation strategy, measurement requirements and synergies with planetary UV observations for potential KBO fly-by.

How to cite: Provornikova, E., Brandt, P., Roelof, E., Katushkina, O., Baliukin, I., Mayyasi, M., Koutroumpa, D., and Mandt, K.: Understanding our global heliosphere with UV observations: Unique opportunities on Interstellar Probe, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-547, https://doi.org/10.5194/epsc2020-547, 2020.

To push the boundaries of space science, we first need to know more about the real boundary for Terrestrials in space that is heliopause. The heliopause separates solar wind from interstellar matter. This boundary surrounds and contains our heliosphere, the space ruled by the Sun. The state of our current knowledge of the heliosphere, despite a big step forward in the last half-century, requires further work to answer extremely important and at the same time basic science questions. One of the still unsolved and most fundamental question is the structure and shape of the heliosphere. In this paper we shortly discuss selected heliosphere  created so far models and we initite consideration of hybrid-kinetic model using a PIC approach for modeling the heliosphere.

How to cite: Ratkiewicz, R.: Will the PIC approach cure the heliosphere modeling?, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-559, https://doi.org/10.5194/epsc2020-559, 2020.

The Interstellar Probe (ISP) will provide the first direct
measurements of interstellar gas and dust when it travels far beyond the
heliopause where the solar wind no longer influences the ambient medium.
We summarize in this presentation what we have been learning about the VLISM
from 20 years of remote observations with the high-resolution spectrographs
on the Hubble Space Telescope. Radial velocity measurements of interstellar
absorption lines seen in the lines of sight to nearby stars allow us to
measure the kinematics of gas flows in the VLISM. We find that the heliosphere
is passing through a cluster of warm partially ionized interstellar clouds.
The heliosphere is now at the edge of the Local Interstellar Cloud (LIC) and
heading in the direction of the slighly cooler G cloud. Two other warm clouds
(Blue and Aql) are very close to the heliosphere. We find that there is a
large region of the sky with very low neutral hydrogen column density, which
we call the hydrogen hole. In the direction of the hydrogen hole is the
brightest photoionizing source, the star Epsilon Canis Majoris (CMa). Extreme
ultraviolet photons from this star produce a Stromgren sphere region of
ionized gas as large as the Local Cavity (extending to 100-200 parsecs)
and produce Stromgren shells at the outer regions of the local warm clouds
including the LIC.

When the ISP passes beyond the hydrogen wall at a distance of about 500 AU,
it will likely enter the outer edge of the LIC where photoionization from
Epsilon CMa plays an important role. Analysis of Hubble observations of
interstellar absorption proves estimates of the densities, temperature,
pressure, and flow properties of the main portion of the LIC, but we have
little informtion on these properties at the LIC's edge. Comparison with the
inflow vector of neutral helium measured by IBEX and Ulysses indicates a
slightly different flow speed and direction than the mean flow of the LIC gas.
ISP will provide direct measurements of the flow and gas properties of this
poorly understood region. In particular, ISP will provide information on
how photoionization from Epsilon CMa influences warm clouds through ionization,
heating, and perhaps pressure balance. This information may resolve questions
concerning the magnetic field surrounding the heliosphere.

How to cite: Linsky, J. and Redfield, S.: What lies immediately outside of the heliosphere in the very local interstellar medium (VLISM): What will ISP detect?, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-68, https://doi.org/10.5194/epsc2020-68, 2020.

Introduction. We present the scientific potential for using an Interstellar Probe (ISP) spacecraft traveling to > 500 AU from the Sun to study the dust in the solar system's debris disks and the nearby ISM, as well as to map the surface of a flyby KBO and study the Cosmic Background Light (CBL).

Figure 1 – Interstellar Probe Explorer payload at its design goal location of 1000 AU with respect to the planets, the heliopause, Alpha Centauri, and the Oort Cloud.

Discussion. The solar system is known to house two planetesimal belts, the inner Jupiter Family Comet (JFC) + Asteroid belt and the outer Edgeworth-Kuiper Belt (EKB), and at least one debris cloud, the Zodiacal Cloud, sourced by planetesimal collisions and comet evaporative sublimation. However, these are poorly understood in toto because we live inside of them. It is not understood well how much dust is produced from the EKB since the near-Sun comet contributions dominate the inner cloud and only New Horizons (NH) has ever flown a dust counter through the EKB. New estimates from NH put the EKB disk mass at at 30 – 40 times the inner disk mass [1]. Better understanding EKB dust production will improve our estimates of the number of EKB bodies, especially the smallest ones, and their dynamical collisional state. For the innermost Zodiacal cloud, questions remain concerning its overall shape and orientation with respect to the invariable plane - these are not explainable from perturbations caused by the known planets alone.

Imaging Studies. Using new technologies and passively cooled detectors, a suitable low system size/mass/power VISNIR spectrometer/FIR imager + 10 cm class primary has been specified using a CubeSat study baseline design [2]. The VISNIR spectrometer could provide maps of the cloud's dust particle size and composition, while FIR imagery would map the dust cloud's density. 3-D cloud mapping would occur during flythrough via tomographic inversion, and via lookback imaging once the s/c is beyond 200 AU. The lookback imaging will allow ISP to measure for the 1^st time in history the entire extent of the Zodiacal Cloud, and determine whether its inner JFC/asteroidal & outer KBO parts connect smoothly, as predicted by Stark & Kuchner [3-4] and detected by Piquette, Poppe et al. [5-8] (Figs. 2-3). This would also allow direct comparison of the solar system’s debris disks with those observed around other nearby stars, and test theories that suggest that our solar system is planet rich but dust-poor [9].

Figure 2 – Predicted dust cloud morphologies arising from solar system JFC (JFC) & Oort Cloud (OCC) comets & Kuiper Belt (EKB) sources. (Top) Looking down on the solar system. (Bottom) Looking through the plane of the solar system. After [1].

Looking back towards the Sun from >100 AU, ISP will perform deep searches for the presence of rings and dust clouds around discrete sources, like Planet X, the Haumea family of icy collisional fragments, the rings of the Centaur Chariklo, or dust emitted from spallation off the larger KBOs. The same instrument will map the surfaces of KBOs encountered along the way. Measurement of the cloud’s total brightness will allow removal of its signal from near-Earth CBL measurements looking at all the starlight ever formed in the Universe, and the same instrument will return its own remote CBL measurements.

Figure 3 – NH in situ measurements (black data pts) and predicted dust flux contributions (colored curves) for the solar system’s debris disks [1,8]. Measured PIONEER 10 dust fluxes are in the upper left corner of the plot, so the predicted crossover at ~10 AU from JFC dominated to EKB dominated is seen. ISP will help us determine if another crossover from EKB dominated to OCC dominated occurs at ~100 AU, and if the EKB dust is ice, rock, & organics rich like KBOs and comets.

First Ever Outer Solar System In Situ Dust Characterization. ISP can also carry the first ever in situ dust chemical analyzer past Saturn. Based on the Europa Clipper SUDA instrument [10], it will compositionally and directionally characterize the solar system’s dust clouds and will help isolate their sources, like the rocky asteroidal dust bands and the icy Haumea family fragments. Using measured dust particle masses and velocities, dust input & loss rates from these sources will be derived. Direct dust sampling will return the first ever in situ chemical analysis of EKB dust, the first ever in situ sampling of dust beyond 200 AU, and provide calibrated ground truth for cloud models produced from our imagery. It should also resolve the tension between the expected makeup of inflowing ISM microdust as determined by remote sensing and the mesasured ISM dust component found at Jupiter and Saturn by Galileo, Ulysses, and Cassini ([11] & Figure 4).

Understanding a G2V’s Astropause ISM Bowshock. At the outermost solar system edges, the role dust plays in shaping and energizing the heliosphere’s boundary with the local galactic medium is almost completely unknown. Estimates range up to 1/3 of the heliopause and heliosheath energy density is in the dust. Current heliopause/sheath models do not allow for dusty plasma physics because the dust component is so poorly known. We do know that submicron sized dust is streaming into the solar system's ram direction through the local ISM, and the large deficit between remote sensing models of local ISM dust and ISM dust measured inside the solar system suggests a large amount of energy is involved in diverting much of the impinging dust around the edges of the solar system.

Figure 4 – Disconnect between the nearby ISM dust size distribution predicted from remote sensing measurements (blue) and ISM dust counts measured inside the solar system (red). After [11].

References: [1] Poppe+2019, ApJLett881, L12 [2] Zemcov+2019, AASMeeting#233, id.#171.06 [3] Stark&Kuchner 2009, ApJ707, 543 [4] Stark&Kuchner 2010, AJ140, 1007 [5] Poppe+2010, Geophys.Res.Lett.37, L11101 [6] Poppe&Horányi 2012, Geophys.Res.Lett. 39, 1 [7] Poppe2016, Icarus264, 369 [8] Piquette+2019, Icarus321, 116 [9] Greaves&Wyatt 2010, MNRAS404, 1944 [10] Kempf+2014, EPSCAbstracts9, EPSC2014-229 [11] Weingartner&Draine, ApJ548, 296; Draine&Hensley 2016, ApJ831, 59

How to cite: Lisse, C., Zemcov, M., Poppe, A., Szalay, J., Draine, B., Horanyi, M., Sterken, V., Beichman, C., McNutt, R., Corcoros, A., Brandt, P., and Paul, M.: What Science Can an Interstellar Probe Mission at Large Heliocentric Distances Achieve With Remote Imaging and In Situ Dust Measurements?, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-845, https://doi.org/10.5194/epsc2020-845, 2020.