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Journal of the British Interplanetary Society

VOLUME 72 NO.6 JUNE 2019 General interstellar issue

THE MOTIVATION AND FREQUENCY OF INTERSTELLAR MIGRATIONS: A Possible Answer to Fermi’s Paradox Gregory L. Matloff TOWARDS A COMPREHENSIVE BIBLIOGRAPHY FOR SETI: Alan Reyes & Jason Wright ULTRAHIGH ACCELERATION NEUTRAL PARTICLE BEAM-DRIVEN SAILS James Benford & Alan Mole THE IMPLICATIONS OF NON-FASTER-THAN-LIGHT TYPE-3 KARDASHEV CIVILIZATIONS Agustín Besteiro HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER : The Interstellar Probe Mission Pontus Brandt et al.

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Published by the British Interplanetary Society. Registered Company No: 402498. Registered Charity No: 250556. Printed by Latimer Trend & Company Ltd, Estover Road, Plymouth, PL6 7PY, England. © 2019 British Interplanetary Society. No part of this magazine may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying or recording by any information storage or retrieval system without prior permission from the Publishers. contents VOLUME 72 NO.6 JUNE 2019

182 THE MOTIVATION AND FREQUENCY OF INTERSTELLAR MIGRATIONS: A Possible Answer to Fermi’s Paradox Gregory L. Matloff

186 TOWARDS A COMPREHENSIVE BIBLIOGRAPHY FOR SETI Alan Reyes & Jason Wright

190 ULTRAHIGH ACCELERATION NEUTRAL PARTICLE BEAM-DRIVEN SAILS James Benford & Alan Mole

198 THE IMPLICATIONS OF NON-FASTER-THAN-LIGHT TYPE-3 KARDASHEV CIVILIZATIONS Agustín Besteiro

202 HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER STAR: The Interstellar Probe Mission Pontus Brandt et al.

OUR MISSION STATEMENT The British Interplanetary Society promotes the exploration and use of space for the benefit of humanity, connecting people to create, educate and inspire, and advance knowledge in all aspects of astronautics.

JBIS Vol 72 No.6 June 2019 181 JBIS VOLUME 72 2019 PAGES 181–185

THE MOTIVATION AND FREQUENCY OF INTERSTELLAR MIGRATIONS: a Possible Answer to Fermi’s Paradox

GREGORY L. MATLOFF Physics Dept., New York City College of Technology, CUNY, 300 Jay St., Brooklyn, NY 11211, USA

Email [email protected]

Human space exploration and discoveries raise Fermi’s Paradox (“Where is Everybody?”) to new heights. The author suggests that technologically advanced interstellar civilizations might defer interstellar migration until their host enter the post-main-sequence ~108-year phase. Increased stellar luminosity will then enhance the performance of solar-photon-sail spacecraft. A scenario is presented for a 109-kg generation ship propelled by a graphene sail. The best destination for such a migration is a widely separated giant-dwarf binary. The giant would be used for deceleration; the vicinity of the dwarf would be colonized. At least one such system would likely closely approach the host star during its subgiant phase.

Keywords: Fermi’s Paradox, Solar Sails, Stellar Evolution, Binary Stars, Interstellar Migrations

1 INTRODUCTION: WHERE IS EVERYBODY? [6]. Current radio-astronomy searches have been funded in an effort to resolve this paradox [7]. The Space Age is only seven decades old. In this short time, have lived in space for over one year, visited our Moon, Starting in the 1990’s, advanced observational techniques dispatched robotic probes to all major in our solar sys- have demonstrated the existence of thousands of planets with- tem and many smaller celestial objects and launched gravi- in the . Very recently, it has been shown that ty-assisted probes on slow voyages into our galaxy. Although Proxima Centauri, the nearest star to our , is likely attended chemical rockets have propelled most of these ventures, some by a small orbiting in this ’s habitable zone [8]. craft have been propelled by solar-electric propulsion. Three Potentially habitable planets seem common in our galaxy. spacecraft to date have successfully tested the principle of pure photon propulsion by unfurling solar-photon sails. If current scientific speculation regarding the origin of life approximates reality, many of these planets support or have Experiments with beamed propulsion have been performed supported active biospheres. Intelligence may have evolved on and the construction and launch of wafer-thin interstellar some of these distant worlds. Some extraterrestrial societies probes propelled by the pressure of photons in microwave or may be technological. laser beams does not seem to be an unreasonable goal for the not-too distant future [1]. Yes, based upon humanity’s own sorry history, many devel- oping extraterrestrial civilizations might self-destruct through Self-sufficient space habitats constructed using lunar or -as nuclear war, pandemics, climate change, over-population, po- teroid material have been studied in depth [2,3]. It seems that litical turmoil, etc. But certainly, some among the millions or deep-space craft supporting hundreds or thousands of humans billions of habitable worlds in our galaxy would have achieved and the ecology required to support them are far from impos- stability. It is not impossible that such a stable society might sible. Propelled by thermonuclear fusion or photon sails, these conquer mortality by uploading the population’s “essences” craft (and their populations) could almost certainly survive the into a computer network not far in advance of our own [9]. But millennium-duration crossings to the nearest extra-solar star some might instead choose the stars. system [4,5]. Even at our early stage of in-space development, we can But advances in astronomy provide us with a serious par- envision “thousand-year arks” carrying small populations be- adox. Coined by physicist Enrico Fermi, this interstellar co- tween stars. After developing the destination , nundrum, “Fermi’s Paradox”, is stated: “Where is Everyone?” some might elect to continue interstellar expansion, exploring anew until, after less than 100 million years, the entire galaxy is occupied by one or more space-faring extraterrestrial civili- An early version of this paper was presented at the 10th International zations. Discounting UFOs and alien-abduction theories, one Academy of Astronautics Symposium on the Future of Space might wonder why humanity seems to be alone, at least in our Exploration: Towards the Moon Village and Beyond, Turin, Italy, solar system. June 27-29, 2017.

182 Vol 72 No.6 June 2019 JBIS THE MOTIVATION AND FREQUENCY OF INTERSTELLAR MIGRATIONS: a Possible Answer to Fermi’s Paradox

2 CONSIDERING THE MOTIVATION FOR INTERSTELLAR planetary system must contemplate either extinction or migra- EXPANSION tion. As the next section illustrates, prospects for interstellar mi- gration during subgiant and giant stellar phases is enhanced by But might the answer to the lie in the rationale the greatly increased stellar luminosity. But if the space habitat for interstellar expansion and galactic colonization. Construct- populations in the ’s planetary system elect to stay at ing a galactic empire in a militaristic fashion does not seem home, their long-term survival prospects may be limited as the reasonable in light of the tremendous distances between stars declining star crosses the main sequence once again on its way and long travel times. Barring an enormous (unexpected but to the final, sub-luminous stage. certainly not impossible) breakthrough in propulsion tech- nology, it seems very unlikely that an extraterrestrial Caesar 3 EFFECTS OF INCREASED STELLAR LUMINOSITY ON or Napoleon would commit himself/herself/itself and all of PERFORMANCE OF INTERSTELLAR SOLAR-PHOTON its descendants to the uncertain rigors of a millennial journey SAILS through the interstellar void. In-person exploration does not seem to be a reasonable motivation, as well. After all, robots We assume that a solar-system wide civilization will use in situ aren’t phased by the rigors of long-duration . They resources as much as possible. For space-habitat in-system pro- do not require life support, company or entertainment as they pulsion, the solar-photon sail will be the preferred technique. venture deeper into the celestial void. And they seem to ac- Also, it is assumed that an absorptive graphene sail is used, complish their goals at least as efficiently as astronauts. since this may be the best-performing sail material suggested to date [15]. Answering the question of motivations might be complicat- ed if a technologically advanced galactic civilization discovers The sail used in this hypothetical migration scenario is a a similar or more advanced civilization inhabiting a nearby disc-shaped hollow-body sail with a radius of 1,500 km and solar system. Even in this case, an exchange of intelligent ro- an areal mass density of 5 x 10-6 kg/m2 [15]. Total sail mass is botic probes or uploaded “essences” might be preferred over therefore 3.5 x 107 kg. an exchange of biological entities. This would avoid such com- plexities as bio-contamination and resistance of populations to Since departure is from a Sun-mass star with enhanced lu- aliens. minosity, we introduce the factor L, which is the ratio of stellar to present-day solar luminosity. Equation (1) of Ref. 15 pre- Another possibility is presented by evidence that red dwarf sents the lightness factor (η), the ratio of solar radiation pres- stars, the most common galactic stellar population, can have sure force on the sail to solar gravitational force on the space- multiple planets orbiting in stars’ habitable zones [10]. It is not craft. This is corrected for a post-main sequence Sunlike star: impossible that cases might exist of multiple independent tech- nological societies orbiting the same star. Although such an (1) eventually might be very rare, it would almost certainly result in either a “war of the worlds” or development of a symbiotic relationship. where R is sail fractional reflectivity to sunlight (0.05) and A is sail fractional absorption of sunlight (0.4); both of these values Perhaps the only motivation that might lead an interplane- are from Ref. 15. In Eq. (1), σ s/c is the areal mass density of the tary civilization to cross to another star is species / biosphere spacecraft. Sail emissivity ε is equal to 0.44, the value calculated survival. During most of star’s main sequence life, biosphere in Ref. 15. survival does not seem to be a driver for interstellar migration. As host stars age on the main sequence and increase in lumi- It is assumed that the spacecraft follows a parabolic pre-per- nosity, habitable planets can be protected from fatal climate ihelion trajectory, with the sail oriented normal to host-star change by large sunshades situated at gravitationally stable light at perihelion. The perihelion distance is Dau Astronomical Lagrange points between the star and planet [11]. The planet’s Units. Equation (3) of Ref. 15 is modified to calculate the inter- orbit around the aging host star can be altered using repeat- stellar cruise velocity (Vfin) for the case of a graphene sail craft ed controlled fly-bys of asteroids or comets [12]. By the time a departing from a post-main-sequence Sun-like star: habitable planet circling a Sun-like star is a billion years or so older than the Earth, many asteroids and comets in the plane- tary system may have been converted to a “Stapledon / Dyson km / s (2) shell” of space habitats containing elements of the host planet’s biosphere [13]. It would be a comparatively simple matter to adjust the orbits of these habitats around the host star to com- The perihelion temperature in degrees Kelvin of the sail, pensate for increased stellar luminosity. Tperi, is estimated using Eq. (7) of Ref. 15, modified for the case of departure from a post-main-sequence Sunlike star: But at the end of the star’s main sequence lifespan (perhaps 5 billion years in our Sun’s future) things begin to change. The (3) star expands in both size and luminosity as it enters the sub- giant phase of its existence. Ultimately, much of the in As discussed in Ref. 15, acceleration stress on the sail is not a the stellar interior is converted into carbon in an event called major problem because of the high tensile strength of graphene the “helium flash”. Since carbon acts as a catalyst, greatly speed- and the relatively low accelerations that can be tolerated by ing the fusion of into helium, the star’s luminosity and biological entities (assuming that these are the migrants, not size both increase dramatically during the resulting giant phase conscious machines). of the star’s existence. During these phases of increased lumi- nosity, which probably have a cumulative duration less than 200 Before considering details of spacecraft performance, some million years [14], any technologically advanced life in the star’s attention will be devoted to numerical values of luminosity ra-

JBIS Vol 72 No.6 June 2019 183 GREGORY MATLOFF

TABLE 1 Post-Main Sequence Stages and Close Stellar Encounters for a Sunlike Star

Phase Approx. Duration Star Size Luminosity Number Close Stellar Encounters Subgiant 108 yr 3 Sol Radii 0.015 AU 10x Sun ~1,000 Helium Flash 105 100 0.5 1000 ~1 Horizontal Branch 5x107 10 0.05 100 ~500 Asymptotic Giant 104 500 0.25 10,000 ~0.1 tio L at various stages of a Sunlike star’s post-main sequence 100 million year subgiant phase. evolution and the approximate duration of these stages. Also, a brief discussion of probable destinations for interstellar mi- 5 A MIGRATION SCENARIO AND APPROACH TO FERMI’S grants will be presented. PARADOX

4 DEPARTURE AND DESTINATION STAR An interstellar migration scenario from a subgiant star is next CHARACTERISTICS considered. The ark or worldship is assumed to have a mass of 109 kg. Scaling from the ~1.4 X 107 kg ark with an interstellar The assumption is made that an advanced technological civili- population of 20-50 considered in Ref. 20, the population of zation conducts interstellar migrations when its star leaves the this craft will be 1,000-3,000. If the ark is cylindrical, it might main sequence. To reduce the duration of interstellar journeys, approximate the size of a large skyscraper. such migrations might be timed to occur when suitable desti- nation stars approach within a few light years. Such close ap- As discussed above, the 1,500-km radius sail has a mass of 7 proaches occur at intervals of ~100,000 years [16-18]. These about 3.5 x 10 kg. The areal mass density of the spacecraft s/c(σ ) data with Table 20.1 and Fig. 20.4 on pp. 532-533 of Ref. 14 are is therefore about = 1.47 x 10-4 kg/m2. For a sail with reflectivity presented in Table 1. The best stage for an interstellar migration to sunlight (R) of 0.05 and fractional absorption of sunlight (A) is the subgiant phase, with a duration of about 108 years and L ≈ of 0.4 departing a subgiant star with L = 10, Eq. (1) can be used 10 . About 1,000 close stellar encounters occur during this stage. to estimate the lightness factor (η) as 26.1.

Because of the increased luminosity of the host star, the sail If the spacecraft departs from perihelion distance (Dau) of craft’s interstellar cruise velocity will be greater than if it were 0.1 AU, Eq. (2) can be used to estimate the interstellar cruise launched from a star like the present-day Sun. Unless some velocity as 676 km/s. The ship will reach a destination star sys- form of electromagnetic interaction with the interstellar me- tem at a 2 light year distance after a 900-year flight, about 1,300 dium were utilized, the best destination from a deceleration years is required to reach a stellar destination at a distance of point of view will be a subgiant or giant star. But since the 3 light years. lifetime of such a giant is limited, the best strategy might be to direct the migrants towards a widely separated giant-red dwarf At perihelion, the radiation-pressured acceleration on the binary. The sail would be used to decelerate at the destination space craft will be 26.1 x the solar gravitational acceleration. system’s giant star. The migrants might land on habitable plan- Since the solar gravitational acceleration at 1 AU is 6.04 x 10-4 ets circling the long-lived red dwarf companion or establish g and the spacecraft departs from a 0.1-AU perihelion, maxi- expanded in-space communities near that star. To avoid issues mum acceleration is a very tolerable 1.6 g. with the evolving giant star, the system selected should have a wide separation. Sail emissivity ε is equal to 0.44. Therefore, Eq. (3) can be used to estimate the perihelion sail temperature as 1818 K, well To investigate the astronomical frequency of such binary below the melting point of graphene. systems, one can first consult an on-line source solstation.( com) to ascertain that the nearest giants are Pollux and Arc- 6 CONCLUSIONS turus, K0III and K2III stars at 34 and 37 light years respec- tively.. There are 34 giants within 100 lightyears according to The resolution to Fermi’s Paradox might be as follows. Al- that source. though a technologically advanced extraterrestrial civilization might explore its interstellar environment using robot probes, From I. Neill Reid’s “The Solar Neighborhood” (astro.phys. it might delay interstellar migration until such migration is ethz.ch) there are 139 stellar systems within 8 parsecs, includ- necessary. The enhanced radiant output of their subgiant star ing 38 multiples. This amounts to about 0.114 stars per cubic might prompt sail-launched migrations to near giant-dwarf parsec. Within 100 light years (about 30 parsecs), there will be stars. If they colonize planets of a K-M dwarf, further migra- about 3,000 stars—about 30 of these systems contain giant stars. tion might not be necessary for another 50 billion—1 trillion years. ET is not in our solar system because the period of inter- According to The Observer’s Handbook, of the 56 stars with- stellar migration is far in our future. in 16.6 light years, 36 are very-long lived M dwarfs and 8 are long-lived K dwarfs [19]. We estimate that about one of every It is recognized, of course, that there are many possible an- 400 stellar systems will be a giant-dwarf binary. From Matthew swers to Fermi’s Paradox. Examples include self-destruction of Francis, “Binary Stars Make for Unstable Planets” (arstechni- advanced galactic civilizations, cosmic catastrophes, societal ca.com), about half of all binaries have large ~1,000 AU sep- evolution in unforeseen directions, etc. But the concept of de- arations. There is therefore a good chance that a suitable gi- layed migration until future eras of galactic history deserves ant-dwarf binary will closely approach a star during that star’s further study.

184 Vol 72 No.6 June 2019 JBIS THE MOTIVATION AND FREQUENCY OF INTERSTELLAR MIGRATIONS: a Possible Answer to Fermi’s Paradox

REFERENCES 1. P. Lubin, “A Roadmap to Interstellar Flight”, JBIS, 69, pp 40-72 (2016). Forum—2001, American Institute of Physics, New York, NY (2001). 2. G. K. O’Neill, The High Frontier: Human Colonies in Space, Morrow, NY 12. D. G. Korycansky, G. Laughlin, F. C. Adams, “Astronomical (1977). Engineering: A Strategy for Modifying Planetary Orbits”, Astrophysics 3. R. D. Johnson and C. Holbrow eds., Space Settlements: A Design Study, and Space Science, 275, 349-366 (2001). On-line version: arXiv:astro- NASA SP-413, Scientific and Technical Information Office, NASA, ph/0102126v1 7 Feb 2001. Washington, D.C. (1977). 13. F. J. Dyson, “Search for Artificial Stellar Sources of Infra-Red Radiation”, 4. A. M. Hein, M. Pak, D. Putz, C. Buhler, and P. Reiss, “World Ships— Science, 31, pp 1667-1668 (1960) and F. Dyson, Disturbing the , Architectures and Feasibility Revisited”, JBIS, 65, pp 119-133 (2012). Harper & Row, NY, (1979), p. 211. 5. G. L. Matloff, “World Ships: The Solar-Photon Sail Option”, JBIS, 65, pp 14. E. Chaisson and S. McMillan, Astronomy Today, 6th ed., Pearson/ 114-118 (2012). Addison-Wesley, San Francisco, CA (2008), p. 532. 6. P. Davies, The Eerie Silence: Searching for Ourselves in the Universe, 15. G. L. Matloff, “Graphene: The Ultimate Interstellar Solar Sail Material?” Penguin, NY (2010). JBIS, 65, pp 378-381 (2012). 7. S. P. Worden, J. Drew, A. Siemion, D. Werthimer, D. DeBoer, S. Croft, 16. E. Mallove and G. Matloff, The Starlight Handbook, Wiley, NY (1989), D.MacMahon, M. Lebofsky, H. Isaacson, J. Hickish, D. Price, V. Gajjar, Table 2.1. This table is derived from R. J. Cesarone, A. B. Sergeyevsky J. T. Wright, “—A New Search For Life in the and S. J. Kerridge, “Prospects for the Voyager Extra-Planetary and Universe”, Acta Astronautica, 139, pp 98-101, (2017). Interstellar Mission,” JBIS, 37, pp 99-116 (1984). 8. G. Anglada-Escude, P. J. Amado, J. Barnes, et al., “A 17. J. Garcia-Sanchez, R. A. Preston, D. L. Jones, P. Weissman, J.-F. Lestrade, Candidate in a Temperate Orbit Around Proxima Centauri,” Nature, D. W. Latham, and R. P. Stephanie, “Stellar Encounters with the Oort 536, pp 437- 440 (2016). Cloud Based on Hipparcos Data”, Astronomical Journal, 117, pp 1042- 1055 (1999). 9. F. J. Tipler, The Physics of Immortality: Modern Cosmology, God and the Resurrection of the Dead, Doubleday, NY (1994). 18. C. A. I. Bailer-Jones, “Close Encounters of the Stellar Kind”, Astronomy and Astrophysics, 375, A35 (2015). 10. D.J. Wright, R. A. Wittenmyer, C. G. Tinney, J. S. Bentley, and J. Zhao, “Three Planets Orbiting Wolf 1061”, arXiv:1512.05154v1 {astro-ph.EP] 19. R. M. Petrie and J. K. McDonald, “The Nearest Stars”, The Observer’s 16 Dec. 2015. Handbook 1968, R. M. Northcott ed., Royal Astronomical Society of Canada, Toronto, Canada (1968), pp. 86-87. 11. K. I. Roy, “Solar Sails – An Answer to Global Warming?”, Paper CP552, in M. S. Genk ed., Space Technology and Applications International 20. G. L. Matloff, “Graphene Solar Photon Sails and Interstellar Arks”, JBIS, 67, pp 237-248 (2014).

Received 12 July 2018 Approved 10 March 2019

JBIS Vol 72 No.6 June 2019 185 JBIS VOLUME 72 2019 PAGES 186–189

TOWARDS A COMPREHENSIVE BIBLIOGRAPHY FOR SETI

ALAN REYES & JASON WRIGHT Department of Astronomy & Astrophysics and Center for and Habitable Worlds, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA

Email [email protected] / [email protected]

In this work, the authors motivate, describe, and announce a living bibliography for academic papers and other works published in the Search for Extraterrestrial Intelligence (SETI). The bibliography makes use of bibliographic groups (bibgroups) in the NASA Astrophysics Data System (ADS), allowing it to be accessed and searched by any interested party, and is composed only of works which have a presence on the ADS. We establish criteria that describe the scope of our bibliography, which we define as any academic work which broadly: 1) advances knowledge within SETI, 2) deals with topics that are fundamentally related to or about SETI, or 3) is useful for the better understanding of SETI, and which has a presence on ADS. We discuss the future work needed to continue the development of the bibliography. The bibliography can be found ADS website (see References) or by using the bibgroup field (bibgroup: SETI) in the ADS search engine.

Keywords: SETI – Bibliography, Extraterrestrial Intelligence

1 INTRODUCTION raphy include:

The Search for Extraterrestrial Intelligence (SETI) has been a i) SETI has historically had a difficult struggle with public formal pursuit of generations of scientists stretching back to the perception, especially at the legislative level. When politi- mid-20th century. Although thousands of works have been pub- cians - with the ability to set budgets and allocate funding lished on the subject, it has been difficult to manipulate or peruse for scientific endeavors - have a gross misunderstanding the corpus of published SETI work due to the absence of a public of what SETI is, and even more so of the level of academ- bibliography. The goal of this work is to deliver the first step -to ic rigor adhered to by its practitioners, the field suffers. wards such a bibliography, by starting with those papers already This perception has resulted in the wholesale defunding of cataloged on the NASA Astrophysics Data System (ADS) [1]. SETI-related research. Having a comprehensive bibliogra- phy would help to formalize the field and improve public There are several reasons for why a SETI bibliography is perception, and reintroduce the possibility of funding. warranted and would be an important asset for the field. ii) For a field as small as SETI, it ought not to be difficult to The first is that it has been somewhat difficult to trace the keep tabs on all publications in an organized manner. The field's historical development or natural progression of ideas, low publication rate and small overall literature volume especially in the case of those ideas that were either not pub- makes a comprehensive bibliography a realistic goal. lished in an academic journal or even at all. One example of this is the confusion around the proper citation for the Drake iii) A bibliography would enable projects to quantify the Equation - often given an improper citation or no citation at amount of searched parameter spaces (e.g., in the con- all. The earliest edited and citable reference to a work by Drake text of carrier waves) [3, 4] by providing a complete re- containing the equation (as identified by Drake himself in pri- cord of past observations. vate communication) is an excerpt from the 1965 textbook published by the Oxford University Press, The Radio Search for iv) Concrete criteria for what constitutes a “SETI paper” Intelligent . For a long time, there was no helps users understand which works are and are not ex- ADS entry for this reference, and hence no citation available pected to be cataloged. from the ADS BibTeX generator. (Upon communication with the ADS administration, an entry has since been created [2].) v) A bibliography would enable the generation of BibTeX citations for the SETI community. Since attribution is a There is also a lack of uniformity in the jargon used by SETI fundamental aspect of the scientific process, having an practitioners. Having a definitive bibliography would help au- easy way to identify relevant citations and extract their thors to trace the emergence of specific terms and understand bibliographic records promotes engagement with the lit- their context and precise meanings. This in turn would foster erature and facilitates the connection and progression of the consolidation of present and future use of SETI jargon. ideas.

More broadly, some other arguments in favor of a bibliog- Prior to this work, no well-maintained compilation of SETI

186 Vol 72 No.6 June 2019 JBIS TOWARDS A COMPREHENSIVE BIBLIOGRAPHY FOR SETI works universally accessible by any interested party and with around SETI, which would give the interested reader a healthy the explicit goal of completeness has existed. Several partial introduction to the field. This resource could then be used as compilations have been made, but none were easily available the basis for the curriculum of a graduate course on SETI, such to the public. In many instances, access was held privately and as the new graduate course at Penn State. (https://sites.psu.edu/ only available as, for instance, a PDF file upon request from the seticourse/) maintainer. The lack of uniform formatting between compila- tions is also an impediment to their usability. In spite of these 2.1 Criteria shortcomings, they served as a healthy place for us to begin the development of the ADS bibgroup. An important question to tackle when attempting to construct a comprehensive bibliography is: ``What constitutes a SETI In Section 2, we discuss the specific goals of this work, with paper or work?’’ There must be a rigorous selection criteria emphasis on its scale and intended outcomes. In Section 2.1, we which allows for the decisive categorization of works. Towards set the vetting criteria for papers or other citations to be con- this end, Table 1 lists some broad areas or classes of papers that sidered viable additions to the bibliography. Then in Section 3, were identified while processing the bibliography, separated we discuss the exact procedure that was undertaken to produce into what is considered in-scope and out-of-scope by this bib- the bibliography. Lastly, in Section 4, we reflect on what has liography. been accomplished and what work still needs to be done. Most obviously, those papers which discuss formal obser- 2 GOALS vational campaigns, apparati, and results are included. Impor- tantly, null results are included, as they often demonstrate the Since a bibliography which is both cross-disciplinary and com- level of scientific rigor involved in SETI, as well as contribute prehensive across all media was too ambitious for our efforts, to the overall searched parameter space. Papers which attempt we restricted our scale to focus on only those academic works to formalize or sharpen the Fermi Paradox, as well as those which are directly relevant to the astronomical practice of which offer solutions to it (many of which go by the descrip- SETI, according to a general criterion established below. The tion “hypothesis/scenario”), are also included. Artifact SETI is end product is a SETI bibliographic group (bibgroup) hosted a branch of SETI which takes the approach of searching for ev- on the NASA ADS. Bibgroups are search filters that can restrict idence of the activity of extraterrestrial intelligent civilizations search results to query only those records attached to a particu- as opposed to their transmissions [5]. As it is its own sub-field, lar ADS collection. They are also a powerful way to discover publications from artifact SETI necessarily merit inclusion. thematic groupings in databases and generate impact metrics, Also to be included are those citations which deal with SETI among other functions. We choose to host the bibliography on on a “meta” level, namely those papers or other media about ADS because of the close relationship between SETI and tradi- SETI as a field rather than on a topic within SETI. Those cita- tional astronomy, which means that many SETI works already tions that made it to our collection of foundational papers but have entries in the ADS. Additionally, ADS automatically in- which were not already circumscribed by the other categories gests much of the arXiv, so that any publications there—even are added by default due to their pedagogical relevance (e.g. those SETI papers which are not directly connected to astron- [6]). Based on these categories, the criterion for a citation to be omy—can be easily added. included in the bibliography can be summarized as: “Any work which either: 1) advances knowledge within While in the process of collating citations, we encountered SETI, 2) deals with topics that are fundamentally related to and took note of relevant citations in formats other than tra- or about SETI, or 3) is useful for the better understanding ditional academic and peer-reviewed publications. Alongside of SETI.” these were citations to SETI works outside of the physical sciences (e.g., the social ramifications of a first contact, etc.) Excluded types of citations include papers on what is While a comprehensive bibliography would include these en- dubbed “pure” - although SETI is fundamentally a tries, for practical reasons we have restricted ourselves to those part of astrobiology, the formal study of life in the universe, it is with a presence on the ADS and leave their inclusion for a fu- mostly concerned with the search for evidence of technological ture project. life, and hence . (Generally speaking, SETI is concerned with the search for any form of intelligent life, but An important auxiliary goal of this work is to develop a list a non-technological intelligent species may not leave detecta- from which an informal SETI “canon” can be derived. The in- ble traces.) Therefore, by this distinction, “pure” astrobiology tention is that this list covers a broad range of topics in and encompasses all other works on life in the universe, on such

TABLE 1 Defining the Scope of the Bibliography (see text for details)

In-scope Out-of-scope SETI Observation papers and null results “Pure” Astrobiology

SETI-specific instrumentation papers Physics or General Radio Instrumentation Fermi Paradox and attempted solutions Crank/Pseudoscientific Hypotheses Sociology/Anthropology/Humanities Papers (for the purposes of the Artifact SETI theory and searches current effort) Meta-SETI Papers that made it to our collection of foundational papers

JBIS Vol 72 No.6 June 2019 187 ALAN REYES & JASON WRIGHT topics as the , , or the search for alien was for citations which contained any combination of the body , to name a few examples, and is excluded from a keywords “extraterrestrial intelligence civilization” using the “pure’’ SETI bibliography. OR logic of the ADS search system. Another search we per- formed was for titular keywords “Fermi paradox” using AND Another class of paper frequently cited by SETI papers are logic. (The aforementioned are just two examples of a variety papers from physics journals. SETI searches are made possible of combinations of commonly used SETI words.) We purpose- by contributions from fundamental physics. However, while fully cast a wide net so as to be as inclusive as possible, and to they are certainly relevant for understanding the context of any catch those missed by the citation tree. given SETI approach, a general physics paper is not necessarily appropriate for inclusion to the bibliography. Similarly, papers Next, we folded in partial compilations generously provid- describing general radio facilities or instruments which do not ed by Robert Gray, Shelley Wright, , and others by primarily serve SETI are likewise excluded, even if they have searching for entries in their lists on the ADS. The largest of SETI time allotments on them. these compilations was a multi-hundred page file maintained by Stephane Dumas which contained records on papers, popu- As with many fields, SETI is not immune to an inflow of lar books, proceedings, reports, IAC conferences, special jour- crank or quack papers. Indeed, due to the level of popular nal issues, a popular magazine, and miscellaneous meetings. interest on the topic, SETI is especially susceptible to them. Another large compilation was a one hundred page NASA JPL While it is important to be inclusive of a variety of unconven- reference publication consisting of a mixed variety of papers, tional ideas, some carefully considered limitations ought to be popular articles, and government documents. Lastly, all entries set on how deviant a paper can be from the traditional scientif- maintained by the SETI.news website were added. We were un- ic approach. We identified a handful of such citations that were able to complete a search for every entry on all of these lists, but deemed inappropriate for inclusion in what is supposed to be believe that we have found the vast majority with a presence a collection of scientific SETI papers. Moving forward, careful on ADS. vigilance will be observed to guarantee the scientific authentic- ity and integrity of included works. The next task was to vet and condense the library. The first step in this process was to export the ADS citations to a spread- Lastly, as previously discussed, we have not incorporated pa- sheet where they could be assigned a variety of disposition pers outside the physical sciences. Some excluded papers were flags. We allocated a total of eight distinct disposition catego- those on post-biological evolution, animal communication, ries. Going individually through the list of candidates and vet- and speculative linguistics or reply construction. While these ting them based on their title, several hundred citations were are important and relevant works, they have poor representa- deemed suspect and flagged for a further manual inspection tion on the ADS and are outside the scope of this primary of their abstract. On the second pass, we perused the abstracts effort. Finding ways to incorporate them, whether here or in and eliminated entries whose content were deemed outside our some other repository, would be an interesting future exten- scope, as well as many entries with missing abstracts, duplicate sion of this project. titles, and/or broken links.

We also encountered a variety of media types, including but Lastly, we created a new SETI bibliography of ADS entries not limited to papers, popular articles, books, book reviews, matching our criteria, and requested that ADS create a bib- poster abstracts, talk or presentation abstracts, conference pro- group based on it. A static image of this library (as opposed ceedings, private letters or correspondences, videos and vid- to the dynamic and living bibgroup) can be found on the eo books, lecture slides, and government documents. Many of ADS website (https://ui.adsabs.harvard.edu/public-libraries/ the aforementioned media formats do not constitute academ- ODGXxvcxTu68ytUsazrGSg). ic works and thus are outside the scope of this compilation. Therefore, a second layer of criteria needs to be established 4 DISCUSSION AND FUTURE WORK to distinguish between academic and non-academic media, which can often times have blurred boundaries, as in the case In general, we pursue completeness as an aspirational goal. The of non-peer-reviewed works. For the purposes of this bibliog- effort strives to be as high fidelity a representation of the body raphy, only papers, books, and abstracts (which also obeyed of academic works published in SETI as is possible given the the first criterion) were admitted without further considera- tools and resources available as well as the accepted selection tion. While it may be beneficial to have repositories of SETI in criteria. However, in the case of more obscure or the newest other media, it may be difficult to track down every instance of works, the bibliography will not have perfect coverage, and it a SETI-related work in those media, and therefore should be can be expected that some percentage slip through and are yet considered another ambitious task for a future work. to have a presence. That is why the bibliography will be con- tinually updated as more citations are discovered, and we wel- 3 PROCEDURE come suggestions from the community for new entries in the form of a link to the appropriate ADS entry sent via email to Our foundation was an ADS library we constructed from a [email protected]. We will hold the editorial privileges citation tree of SETI papers, which originally had ~1900 en- for the library and properly vet all suggested additions to main- tries in it, many of which did not fit the inclusion criterion. tain the integrity of the master list. The point of this cursory assemblage was to put together an unrefined library of all potential SETI citation candidates, and Other future work for this project includes completing only then later refine its constituents via a robust vetting pro- the folding-in of the previous compilations, the continued cedure. To build it up further, we conducted a manual search tab-keeping on SETI works from other non-ADS-covered of citations based on SETI-themed keywords appearing in ti- disciplines as well as media formats, and the continued inte- tle or body, allowing for a further several hundred candidates gration of SETI.news [7] publications as they are released. For to be added. An example of one kind of search we performed popular articles, an online spreadsheet shall be a starting point

188 Vol 72 No.6 June 2019 JBIS TOWARDS A COMPREHENSIVE BIBLIOGRAPHY FOR SETI for documenting publications of that format. Importantly, we Acknowledgements intend to collaborate in the future with librarians to create bib- This paper grew out of a final project (https://sites.psu.edu/ liographic entries using professional software for non-ADS en- seticourse/a-comprehensive-bibliography-for-seti/) in the tries. Similarly, the creation of ADS entries for relevant papers 2018 graduate course on SETI at Penn State. The authors are lacking an ADS presence is warranted and is currently under- extremely grateful to Stephane Dumas, Jill Tarter, and Robert way by the ADS administrators. Gray, whose partial compilations set the foundation of this work. We also thank , Alberto Accomazzi, Carolyn We hope that this bibliography will be a useful resource for Grant, Shelley Wright, , Robert Gray, James current and future researchers in the field that fosters the utili- Guillochon, and Michael Oman-Reagan for their personal cor- zation of the rich SETI literature. respondences, advice, and information which aided this work.

REFERENCES

The bibliography can be found at the ADS website: https://ui.adsabs.harvard.edu/search/q=bibgroup%3ASETI&sort=date%20desc%2C%20bibcode%20desc

1. M.J. Kurtz, G. Eichhorn, A. Accomazzi, C.S. Grant, S.S. Murray, and Done? Finding Needles in the n-dimensional Cosmic Haystack’’, The J.M. Watson, “The NASA Astrophysics Data System: Overview’’, Astronomical Journal, Vol 156, pp 260, 2018. Astronomy and Astrophysics Supplement, Vol 143, pp 41-59, 2000. 5. J.T. Wright, S. Sheikh, I. Almár, K. Denning, S. Dick, and J. Tarter, 2. F.D. Drake, The Radio Search for Intelligent Extraterrestrial Life, Oxford ``Recommendations from the Ad Hoc Committee on SETI University Press, pp 323-345, 1965. Nomenclature’’, arXiv e-prints, arXiv:1809.06857, 2018. 3. J.C. Tarter et al., “SETI turns 50: five decades of progress in the search 6. J. M. Cordes et al., “Theory of Parabolic Arcs in Interstellar Scintillation for extraterrestrial intelligence’’, Proceedings of the SPIE, Vol 7819, Spectra’’, The Astrophysical Journal, Vol 637, pp 346-365, 2006. id:781902, 2010. 7. J. Davenport and D. LaCourse, http://SETI.news, (Last Accessed 10th 4. J.T. Wright, S. Kanodia, and E. Lubar, “How Much SETI Has Been December 2018)

Received 8 May 2018 Approved 5 June 2019

JBIS Vol 72 No.6 June 2019 189 JBIS VOLUME 72 2019 PAGES 190–197

ULTRA-HIGH ACCELERATION NEUTRAL PARTICLE BEAM-DRIVEN SAILS

JAMES BENFORD1 & ALAN MOLE2, 1Microwave Sciences, 1041 Los Arabis Lane, Lafayette, CA 94549, USA; 21441 Mariposa Ave, Boulder CO 80302, USA

Email [email protected] / [email protected]

The authors advance the concept for a 1 kg probe that can be sent to a nearby star in about seventy years using neutral beam propulsion and a magnetic sail. The concept has been challenged because the beam diameter was too large, due to inherent divergence, so that most of the beam would miss the sail. Increasing the acceleration from 1000 g to 100,000 g along with reducing the final speed from 0.1 c to 0.06 c redeems the idea. Such changes greatly reduce the acceleration distance so that the mission can be done with realistic beam spread. Magsail-beam interaction remains an aspect of this concept that needs further study, probably by simulations. We describe key elements of neutral particle beam generators, their engineering issues, cost structure and practical realities. Comparison with the Starshot laser beam-driven concept gives roughly similar costs.

Keywords: Neutral Particle Beam, Interstellar, Interstellar Precursor, Directed , Beam-driven Sail

1 CENTRAL FEATURES OF NEUTRAL PARTICLE BEAM The thrust per beam is maximized when the PROPULSION particle velocity is twice the spacecraft velocity. The Magsail, with a hoop force from the magnetic field, is an ideal structure Use of a neutral particle beam to drive a Magsail was proposed because it is under tension. High-strength low-density fibers by Geoffrey Landis as an alternative to photon beam-driv- make this lightweight system capable of handling large forc- en sails [1]. Compared to beam-driven propulsion, such as es from high accelerations. The rapidly moving magnetic field Starshot, particle beam propelled magnetic sails, Magsails, of the Magsail, seen in the frame of the beam as an electric substitute a neutral particle beam (NPB) for the laser and a field, ionizes the incoming neutral beam particles. Nordley and Magsail for the ‘lightsail’, or sailship. The particle beam in- Crowl discuss on-board lasers to ionize the incoming beam, tercepts the spacecraft: payload and structure encircled by a although this adds additional on-board mass and power [2]. magnetic loop. The loop magnetic field deflects the particle When the dipole field of the Magsail is inclined to or translat- beam around it, imparting momentum to the sail. The gen- ed away from the beam vector the Magsail experiences a force eral ‘mass beam’ approach has been reviewed by Nordley and perpendicular to the beam vector, which centers it on the par- Crowl [2]. ticle beam, perhaps providing beam-riding stability.

At low relativistic velocities, < 0.5 c, particle beam pro- 2 ULTRAHIGH ACCELERATION pelled Magsails require far less power for acceleration of a given mass. There's also ~ 103 increase in force on the sail for Benford showed that the beam divergence is fundamental- a given beam power. Deceleration at the target star is possible ly limited by the requirement, at the end of the acceleration with the Magsail but not with a laser driven sail. process, to strip electrons from a beam of negative hydrogen ions to produce a neutral beam [4,5]. The minimum divergence The neutral particle beam approach is conceptually similar produced by that is: to photon beams such as the laser-driven Starshot project. A disadvantage of reflecting photons from the sail will be that they carry away much of the energy because they exchange (1) only momentum with the sail. The doppler shift on reflection at lower velocities is small – the reflected photons are almost as energetic as the incident photons. Neutral particle beams where mi and me are the ion and electron masses, Ei and Ee the transfer energy, which is much more efficient. The reflected ion energy and electron affinity, the energy required to detach particles may in principle be left at zero relative velocity, and an electron. thus zero kinetic energy, with respect to their origin, their ki- netic energy having been deposited in the vehicle. In practice, We see that the divergence is due to two ratios, both of energy efficiency will be less than 100%; there would be some them small: a ratio of particle masses (~10-3) and a ratio of thermal radiation and imperfect reflection losses. neutralization energy to beam particle energy (about 10 eV/10

190 Vol 72 No.6 June 2019 JBIS ULTRA-HIGH ACCELERATION NEUTRAL PARTICLE BEAM-DRIVEN SAILS

MeV~10-6). Therefore neutral beam divergence is typically a the beam driver can be at a fixed field gradient. Lighter-particle few microradians. shorter beam drivers may cost less but they would require a larger sail due to the higher divergence of the beam (Eq. 1). In previous work, the neutral hydrogen beam at 18.8 MeV per particle accelerated to a two-tenths of the speed of light (0.1 Note that at constant acceleration the ratio of sail momen- c) had acceleration of 103 gs for 50 minutes [3]. A nanoradian tum to energy is equal to the ratio of the force on the sail to the divergence was assumed. But from (1), the inherent beam di- power of the beam accelerating the sail, momentum/energy = vergence would actually be 4.5 µradians, far larger. This result- force/power. This results in the following relation for the peak ed in a 411 km diameter beam spot, far larger than the Magsail beamer power Pb: diameter, which was 0.27 km. So most of the beam misses the (7) sail.

But if we use much higher acceleration, the sail will stay where ms is the sailcraft mass. Obviously the power ramps up as 2 within the beam until it reaches the desired final velocity, even the square of sail velocity, P~ vb . with microradian divergence. We choose 105 gs, 106 m/s2 to ac- celerate to 0.06 c, 1.8 x 107 m/s. The peak beam current is:

The velocity vs the sail achieves is given by: (8)

(2) which is within the regime of existing beam generators. Conse- quently power required from the beam is 60 times larger than where a is the acceleration provided by the particle beam, S the the 0.3 TW by Mole [3], (for different specifications: 1000 g, distance over which acceleration occurs. If the sail of acceler- 0.1c and 42 minutes acceleration time), a far more powerful ates to where the beam diameter is the sail diameter, beam generator to produce higher acceleration.

(3) The impedance of the beam is: where Ds is the sail diameter. Therefore the required accelera- (9) tion scales as: These are the parameters for the peak beam power, which is (4) what we must design the beam generator for. When the beam divergence is in reality roughly 3 orders of magnitude high- er than previous studies have assumed, from a nanoradian to Consequently, since high velocity is essential to the mission microradian, the impedance of the beam generator falls pro- and beam divergence has a lower bound by fundamental phys- portionately. This rapidly moves the beam generator regime ics, if one wants to keep sail diameter and therefore sail mass toward being very large and expensive. low, high acceleration is required. The mission parameters for a hydrogen beam then become Numerical experiments with the model developed by Nord- those shown in Table 1. ley [6], and later replicated by Crowl, showed that the greatest momentum delivery efficiency occurs when: A mercury beam has a smaller minimum divergence of 0.8 µradians, but must use far higher voltage because of the larger (5) mass (Eq. 6). Mercury beam parameters are also given in Table 1. where vb is the velocity of the neutral beam. The physics of this is straightforward: Maximum energy efficiency comes when TABLE 1 Parameters of neutral particle beam-driven sail all of the energy goes to the sail and none of it remains in the probes beam. For a sail that is perfectly reflective, the beam bounces off the sail at the same velocity it impinges the sail. If after re- Beam & Sail Hydrogen Beam Mercury Beam flection it is moving at zero velocity (so none of the energy is Parameters left in the beam), the initial beam velocity must be twice the Beam Divergance θ 4.5 microradian 0.8 microradian sail velocity, so that it impinges on the sail at a relative velocity Acceleration a 105 gs=10 6 m/sec2 105 gs=10 6 m/sec2 equal to the sail velocity. Sail diameter Ds 1.46 km 260 m

7 7 We take the beam velocity at the end of acceleration to be Sail final velocity vs 0.06 c, 1.8 x 10 m/s 0.06 c, 1.8 x 10 m/s 7 2 the twice the final sail velocity, vs 0.06c = 3.6 10 m/s . The en- Acceleration distance S 1.6 x 105 km, 10-3 AU 1.6 x 105 km, 10-3 AU -12 ergy of a hydrogen atom EH striking the sail will be 1.08 10 J. This energy is imparted by accelerating through a voltage V: Acceleration time 18 s 18 s

Magsail mass ms 1 kg 1 kg Kinetic energy 1.6 1014 J 1.6 1014 J (6) 13 13 Beam peak power Pb 1.8 10 W, 18 TW 1.8 10 W, 18 TW Beam voltage V 6.76 MeV 1.35 GeV where Ma is the atomic mass. This is a voltage within the realm Beam current I 2.66 MA 1.33 kA of several types of existing high voltage generators. Equation 6 means that the lighter the particle to be accelerated, the shorter Beam impedance Zb 2.5 ohm 1 M ohm

JBIS Vol 72 No.6 June 2019 191 JAMES BENFORD & ALAN MOLE

Note that the sail diameter given in Table 1 is taken to be Inflation of the magnetic field due to a particle beam pressure simply the diameter of the divergent beam encountering the could occur. However, the effect would be to allow the beam di- Magsail. The diameter of the reflection region produced by the vergence to be only a bit larger. magnetic field of the sail could well be somewhat larger than the superconducting hoop diameter. (Of course, early in the acceler- Note also that in this diagram the sail is shown as dragging ation, the beam will be smaller in diameter, will hit it at the axis the payload behind it as it accelerates. If part of the particle beam where the magnetic field is greatest.) reaches the payload it could create substantial damage. Conse- quently, it might it be better to distribute the payload around the Because in Table 1 the hydrogen beam sail diameter is so superconducting hoop where it would have the most protection large, we will focus the rest of this paper on the mercury beam. against incoming charged particles. Such a distributed payload Even so, the mercury beam Magsail has a 260 m diameter and 1 allows graceful degradation if local damage occurs. Note also the kg mass (sail plus payload)-. If the superconducting hoop has a stability of the superconducting loop on a beam of finite width density of steel, the thickness must be no larger than 0.44 mm, has not been investigated to date. However, the Starshot program if the density of carbon, 0.8 mm. Although heavier atoms can be is looking at this issue extensively. used, the length and cost of the accelerator becomes larger, as it scales with the mass (see Section 6). The Figure shows the field being compressed by solar wind, which is taken to be uniform across a magnetic dipole. There 3 MAGSAIL-BEAM INTERACTION are no simulations in the literature of a beam comparable to or smaller than the sail. One would expect the loop generated field When a Magsail driven by a neutral particle beam is at the ear- to be in the direction of motion, and will be expanded radially, ly stages of the acceleration, the beam will have a considerably especially if charged particles are trapped in it for significant pe- smaller spot size on the Magsail than it will later and will hit riods of time. it at the axis where the magnetic field is greatest. Later on, as the Magsail flies away, the beam will reach a size dictated by its Andrews and Zubrin have done single particle numerical cal- divergence. A question is: does the initial beam high intensity of culations that do not include modeling dynamic effects (such as the beam on the magnetic field tend to push the sails magneto- field distortions from magnetic pressure) and do not include any sphere outward radially and make the effective diameter of the such "inflation" of the mirror due to trapped beam ions [7]. Magsail larger? If it does, then the beam divergence can be a bit larger and still strike the Magsail. Or conversely, one could ac- Hydrogen and mercury are paramagnetic, as are many ma- celerate the Magsail for a longer time because some of the beam terials. Paramagnetic materials are attracted to an external would still be captured. applied magnetic field, whereas diamagnetic materials are re- pelled. Therefore as the beam approaches the sail, it is attracted Fig. 1 is taken from the late Jordan Kare’s NIAC report [8]. to the magnetic field of the hoop, which will help to keep the (From his figure, he considered the MagOrion concept, so there beam and the hoop aligned. The force is small, but helps. There is a nuclear propellant, which is not relevant to our discussion.) is as yet no analysis to quantify this effect for neutral particle From the left a uniform solar wind strikes the Magsail, which beams and Magsails. in our case would be a non-uniform neutral particle beam. The beam encounters the peak of magnetic field along the axis of the The assumption made in the literature, that the moving sail. On the right of the figure, the field is distorted, producing a magnetic field of the Magsail, seen in the frame of the beam as plasma interface shock against the magnetic field of the Magsail. an electric field, ionizes the incoming neutral beam particles

Fig.1 Interaction of streaming plasma flow with a Magsail. From Kare NIAC report [8].

192 Vol 72 No.6 June 2019 JBIS ULTRA-HIGH ACCELERATION NEUTRAL PARTICLE BEAM-DRIVEN SAILS

Fig.2 Block diagram of neutral particle beam generator. Drift-Tube Linac is not shown. [11]

Fig.3 Ion beam on left is propagated along converging magnetic field to the linac. must be quantified. A relatively dense plasma will form near space charge forces are strong and conventional mag- the axis; interactions between the beam and that plasma would netic quadrupoles are less effective; also facilitate ionization. It may be the case that once the plas- • adiabatic bunching of the beam: starting from the ma is “ignited” it will sustain itself well beyond the volume in continuous beam produced by the source it creates which field ionization occurs. with minimum beam loss the bunches at the basic RF frequency that are required for acceleration in the The force from the ionizing electric field will be null on axis. subsequent structures; It is maximized where the magnetic field is perpendicular to • acceleration of the beam from the extraction energy the velocity vector and therefore the force is an annulus around of the source to the minimum required for injection the axis of the sailcraft. Therefore one would want to use a hol- into the following linac structure. low neutral particle beam so as not to waste the center of it. It is quite possible to generate such a hollow beam. It's also at- 3. After the ions exit the RFQ at of a few MeV, fur- tractive because the force is then applied out toward the where ther acceleration to increase the particle energy is done the hoop is, rather than on axis, where there is no matter to with a drift-tube linac (DTL), which consists of drift tubes accelerate. separated by acceleration regions, as shown in Fig. 4. Par- ticles arriving at the gaps at the proper phase in the radiof- 4 BEAM GENERATOR CONCEPT requency waves are given acceleration impulses. When the electric field of the wave reverses, the particles are shielded Creation and acceleration of the neutral particle beam begins from being accelerated by passing through the drift tubes. with: The typical accelerating gradient is a few MeV/m.

1. Extraction of a negative ion beam (ion with attached elec- trons) from a plasma source; it then drifts into the first acceleration stage, the RFQ. The first element of the accel- erator will appear much like the geometry shown in Fig. 3. Here ions are extracted from the plasma source on the left by electrostatics and brought by a converging magnet- ic field to the linear accelerator.

2. The ion beam enters a radiofrequency quadrupole (RFQ) accelerator, a vane-like structure where the application of radiofrequency power produces a continuous gentle ac- celeration much like a surfer riding a wave. It also pro- vides strong electrostatic focusing to prevent divergence growth. The structure bunches the particles in phase space. The RFQ fulfils at the same time three different functions: • focusing of the particle beam by an electric quadru- Fig.4 Drift-Tube Linac, which consists of drift tubes separated by pole field, particularly valuable at low energy where acceleration regions.

JBIS Vol 72 No.6 June 2019 193 JAMES BENFORD & ALAN MOLE

4. In order to maintain low emittance and produce the mi- of opportunity occur. croradian divergence we desire, the beam is expanded considerably as it exits the accelerator. Beam handling Neutral particle beam generators so far have been operated elements must have minimal chromatic and spherical ab- in pulsed mode of at most a microsecond with pulse power errations. equipment at high voltage. Going to continuous beams, which would be necessary for the seconds of beam operation that are 5. Beam pointing to be done by bending magnets with large required as a minimum for useful missions, would require re- apertures. thinking the construction and operation of the generator. The average power requirement is quite high, and any adequate 6. Finally, the extra electrons are stripped from the beam, cost estimate would have to include substantial prime power making a neutral particle beam. This can be done with by and pulsed power (voltage multiplication) equipment, the ma- stripping the electrons in a gas neutralization cell or by jor cost element in the system. Of course, it will vastly exceed photodetachment with a laser beam. It may be possible to the cost of the Magsails, which is an economic advantage of achieve 100% neutralization by a combination of meth- beamed propulsion. ods. Thus far this high-efficiency neutralization has not been demonstrated. However, this needs economic analysis to see what the cost optimum would actually be. Such analysis would take into ac- 5 BEAMER ENGINEERING count the economies of scale of a large system as well as the cost to launch into space versus the advantages and disadvan- There are several possible schemes for building the beam gen- tages of beaming from Earth. erator. Both electrostatic and electromagnetic accelerators have been developed to produce high power beams. The most likely 6 COST ESTIMATES approach is to use linear accelerators. In the past, the cost of an electromagnetic accelerator is on the order of a person year per The interstellar neutral particle beam system described here is meter of accelerator (~1 man-year/m) but this could be larger a substantial extrapolation beyond the present state-of-the-art. for the more sophisticated . Nevertheless, estimates can be made of both the capital and op- erating costs. The power system to drive such accelerators could come from nuclear power (fission or fusion) or solar power. Fur- The cost of the Beamer is divided between the cost of the ac- thermore, if it were to be space-based, the heavy mass of the celerator structure (RFQ and DTL) and the power system that TW-level high average power required would mean a substan- drives it. For a cost estimate for the mercury beam system, we tially massive system in orbit. Therefore Mole’s suggestion, that assume that the present day accelerating gradient is maintained the neutral beam be sited on Earth, has its attractions. There for this very high-power system. That gradient is ~2 MeV/m. is also the question of the effects of propagating in the atmos- For the mercury neutral particle beam the length of the 1.35 phere, on both beam attenuation and on divergence. GeV accelerator would be 675 m.

If the beam generator were to be on Earth, it should be at the There is an extensive technology base for drift-tube linacs; highest altitude for practical operations. The Atacama Desert, there are many in operation around the world [10]. We use as a for example, would offer very low humidity and half of sea level model the well-documented Brookhaven National Laboratory pressure. Starshot estimates are that its laser beam would lose 200 MeV ion beam system, which was completed in 1978 at about 30% of its power in transiting through the atmosphere a cost of $47M. It used 22 MW of radiofrequency power and from a high-altitude low-humidity desert [9]. was 145m long. In that era, the cost of microwave equipment was ~$1/W. The cost today is ~$3/W, so the 22 MW would cost In addition, a way to reduce beam losses in the atmosphere 22 M$ then and 66 M$ today. Since the total cost of accelera- would be to launch a hole-boring laser beam in advance just tor was $47 M$, the Accelerator structure would cost 47 M$ before the neutral beam. This laser would heat up a cylinder of -22 M$ = $25 M$. Thus at this level the two cost elements are atmosphere to lower the pressure, allowing the neutral beam to roughly equal. The accelerator structure then costs $25 M$/145 propagate with less loss. Such hole-boring exercises have been m = $0.17 M$ per meter in 1978. We multiply all costs by a conducted in laser weapon studies and does appear to be a via- factor of three to account for inflation to get today’s costs. In ble technique. No estimates have been made for a neutral par- the future, 3D printing technology may have a cost impact in ticle beam preceded by a laser punching a hole for it. Without the other direction. that laser the losses would be prohibitively high. To estimate the capital cost of the mercury in NPB described The final neutral beam can be generated by many small here, we have the following relations: beam drivers or a single large beam driver. If a great number of driver devices and their associated power supplies are required, Caccl= 0.5 M$/m x 675 m = 0.35 B$ increasing the construction and maintenance expense of this portion. Of course, economies of scale will reduce the cost of Cmicrowave= 3$/W x 18 TW = 5.47 B$ individual segments of the Beamer by mass production of the system modules (see Section 6). Making such choices is an ex- Therefore the dominant cost element would be the micro- ercise for future engineers and designers. wave system driving the accelerator.

Calculations by Starshot about the possibility of a beam in- However, high-volume manufacturing will drive costs tercepting show it to be a very low probability event. down. Such economies of scale are accounted for by the learn- No doubt any impacted by these intense beams would ing curve, the decrease in unit cost of hardware with increas- be destroyed. Nevertheless, one can simply fire when windows ing production. This is expressed as the cost reduction for each

194 Vol 72 No.6 June 2019 JBIS ULTRA-HIGH ACCELERATION NEUTRAL PARTICLE BEAM-DRIVEN SAILS doubling of the number of units, the learning curve factor f. en high-velocity systems, we make the following comparison This factor typically varies with differing fractions of labor and between their key parameters and cost elements: automation, 0.7 < f < 1, the latter value being total automation.

The cost of N units is TABLE 2 Physical parameters and cost elements of beam- driven probes (10) Mercury Neutral Starshot Particle Beam System where C1 is the cost of a single unit [11]. Sail mass 1 kg 1 g It is well documented that microwave sources have an 85% Velocity 0.06 c 0.2 c learning curve, f = 0.85 based on large-scale production of an- Beamer capital cost 1.45 billion $ 4.9 billion $ tennas, magnetrons, klystrons, etc [11]. From the above, Energy store cost 2.25 billion $ 3.4 billion $ Total capital cost 3.7 billion $ 8.3 billion $ (11) Energy cost/launch 4.5 million $ 7 million $ Kinetic energy 1.6 x 1014 J 1.8 x 1012 J Today's cost is about $3/W for ~1 MW systems. Note that this includes not only the microwave generating tube, but also Kinetic energy/capital cost 43.2 kJ/$ 0.2 kJ/$ the power system to drive that continuous power. The 18 TW power needed would require 18 million such units. Therefore the cost is: Here we have summed the accelerator and microwave pow- er system costs for the neutral Beamer and the laser and optics (12) cost for Starshot. Our costs assume unrealistically high effi- ciencies. A major caveat is that Parkin’s estimates have realistic efficiencies of some cost elements of Starshot, except for the Adding together the accelerator and microwave power sys- laser for which the cost is extremely optimistic [12]. tem, the cost will be 1.45 B$. Although they differ in detail, the two concepts give the The electrical power to drive this large system cannot pos- same order of magnitude cost. However, the kinetic energy in sibly come from the electrical grid of Earth. Therefore a large the NPB-driven probe is 90 times that of the Starshot probe. cost element will be the system that stores the 162 TJ of energy. This shows the disadvantage of reflecting photons from the (Note that the beam power starts at zero and rises as t2 to 18 sail: they carry away much of the energy because they exchange TW at the end.) From Parkin's estimates of the Starshot system, only momentum with the sail. Neutral particle beams transfer based on Li-ion batteries, we take the storage cost to be $50 per energy, which is much more efficient. The kinetic energy/capi- kilowatt-hour, which is $13,900/TJ. Consequently the cost for tal cost ratio is 200 times greater in the NPB case. the energy store is ($13. 9 M$/TJ) 162 TJ = 2.25 B$ It is instructive that the high-energy requirement of inter- So the energy stores cost is comparable to that of the accel- stellar probes drives the existence of a stand–alone storage sys- erator. The total capital cost is tem, which is a major element in the total cost of both systems. The similarity of costs for these rather different beam- driven Caccl= 0.35 B$ systems gives us some confidence that these rough estimates in this paper are credible. Cmicrowave = 1.1 B$ 8 NEUTRAL PARTICLE BEAM REALITIES Cstore= 2.25 B$ Practical realities are always bad news. Performance of most Total accelerator capital cost is 3.7 B$. systems degrades to below their design points because of inef- ficiencies of processes. Note that the beam systems described The operating cost to launch a single Magsail is of course here are perfectly efficient, as determined from equation 5. That far smaller. It is simply the cost of the spacecraft, the energy to is, the beam reflects from the sailcraft with perfect efficiency so launch it and the labor cost of operating the facility. We will as- as to stop dead, transferring all the energy to the spacecraft. sume that the cost of the spacecraft will be on the order of $10 The realities of neutral particle beams in the present day are million. The cost of the electricity at the current rate of $.10 per substantially poorer. kilowatt-hour is $4.5 million. To see where the problems lie, we consider a daring experi- ment called BEAR, conducted 30 years ago [13, 14]. A neutral Total operating cost for a single launch is ~15M$. particle beam generator was actually deployed and operated in space and its performance was measured. 7 COMPARISON WITH STARSHOT On July 13, 1989 the Beam Experiment Aboard Rocket (BEAR) The Starshot system, a laser beam-driven 1 g sail with the goal linear accelerator was successfully launched and operated in of reaching 0.2c, has been quantified in a detailed system mod- space by Los Alamos National Laborotory. The rocket trajec- el by Kevin Parkin [9]. Since both the high acceleration neutral tory was sub-orbital, reaching altitude of 220 km. The flight particle beam described here and Starshot are both beam-driv- demonstrated that a neutral hydrogen beam could be suc-

JBIS Vol 72 No.6 June 2019 195 JAMES BENFORD & ALAN MOLE

ACCELERATOR SECTION (172 in) SUPPORT PAYLOAD SEGMENT (112 in) Recovery parachute Getter Pump (1 of 3) Injector Vacuum Isolation Gate Valve (1 of 3) Battery Box (1 of 6) RF Amplifier (1 of 2) Xenon Reservoirs

ACS N2 Bottle Neutralizer Assy. (1 of 4) CCIG (1 of 2) Shadow Wire Telemetry Scanner Assy. Equipment High Impedance Voltmeter Boom Radio Frequency HEBT Quadropole (RFQ) Beamline Cryotrap Plasma Wave Receiver Master Controller Boom (1 of 2) Langmuir Probes Space Frame (1 of 4) N2 Reservoir 44 in diameter Visible Video Electron Sweep Magnet Camera Beam Current Monitor Mirror (1 of 2) Intensified Ultraviolet Video Camera Solid-state Detector (1 of 3)

Fig.5 Payload of Beam Experiment Aboard Rocket (BEAR) [13] cessfully propagated in an exoatmospheric environment. The improvements can be made in the efficiency of NPB's, given cross-section of the rocket is shown in Fig.5. substantial research funding.

The accelerator, which was the result of an extensive collab- Therefore the concept in this paper, with its hundred per- oration between Los Alamos National Laboratory and indus- cent efficiency of energy transfer from the electrical system to trial partners, was designed to produce a 10 mA, 1 MeV neutral the sail, is an upper bound on the performance. Consequently hydrogen beam in 50 microsecond pulses at 5 Hz. The major the parameters in Table 1 and the capital and operating cost components were a 30 kev H- injector a 1 MeV radio frequency estimates given here are lower bounds on what would actually quadrupole, two 425 MHz RF amplifiers, a gas cell neutralizer, occur. beam optics, vacuum system and controls. The beam extracted was 1 cm in diameter with a beam divergence of 1 mradian. 9 CONCLUSIONS There was no unexpected behavior such as beam instability in space. Since beam divergence is fundamentally limited, high accel- erations can be used to insure the sail will stay within the The design was strongly constrained by the need for a light- beam until it reaches the desired final velocity, even with weight rugged system that would survive the rigors of launch microradian divergence. This leads to ultrahigh, 105 g, 106 and operate autonomously. The payload was parachuted back m/s2 to accelerate to 0.06 c. The Starshot system, a laser to Earth. Following the flight the accelerator was recovered and beam-driven 1 gram sail with the goal of reaching 0.2c, has successfully operated again in the laboratory. been quantified in a detailed system model by Parkin [9]. It too uses 105-106 g. Magsail-beam interaction remains an From the paper and report describing this experiment we aspect of this concept that needs further study, eventually see substantial inefficiencies, which should guide our future by simulations. The cost and operating estimates given here expectations. show that we can have reduced efficiencies yet still have a lot of leeway in the expense. The input power to the accelerator was 620 kW for 60 µs, a 7.2 J energy input. The beam as extracted was 27 mA at 1 MeV For early testing purposes, use the same sort of strategy we’re for 50 µs, which gives 1.35 J. The efficiency therefore is 19%, following on Starshot: initial experiments would fire vertically so approximately 4/5 of the energy supplied was lost in the to study stability of the sail on the beam. Once that is shown, beamline shown in Fig. 2. The major loss was in the neutralizer one goes to horizontal high acceleration experiments in a long which was a xenon gas injected into the beamline. The efficien- evacuated tunnel. Only later would experiments move to firing cy of the neutralizer was changed by varying the amount of gas through the atmosphere at high-altitude. injected. They obtained 50% neutral hydrogen and 25% each of negative and positive hydrogen. These charged particles rapidly Certainly the Magsail itself can be tested separately from repelled each other and got out of the beam. Therefore the neu- the neutral particle beam. Such independent development of tralization process was only 50% efficient in producing a neu- the beam generation and the sailcraft is quite possible because tral beam. This accounts for most of the energy loss. The other there are many ways to produce a force on the magnetic field losses can be accounted for by inefficiencies in the optics of the of the hoop. In fact, such experiments have been done over the low-energy beam region and the high-energy beam region. past decades.

In the 30 years since the flight, little work on particle beams We can foresee a development path: a System starts with has occurred at high power levels, because of the termina- lower speed, lower mass Magsails for faster missions in the tion of the Strategic Defense Initiative. Doubtless substantial inner solar system. As the system grows, the neutral beam

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System grows and technology improves. Economies of scale superconducting hoop. On the latter, the Magsail supercon- lead to faster missions with larger payloads. As interplanetary ducting materials required, including the superconducting commerce begins to develop, making commerce operate effi- current, in light of the maximum critical self-field the super- ciently, outcompeting the long transit times of rockets between conductor can sustain, have not been included in the opera- the planets and asteroids, the System evolves [4]. Nordley and tional design constraints. Crowl describe a development scenario [2]. Acknowledgements This promising method for interstellar travel should receive further attention. Future work on this concept that need fur- Alan Mole sponsored this research. We thank Adam Crowl, ther study: beam launch, propagation, magsail-beam interac- John Swegle, Gregory Benford and Gerald Nordley for com- tion and the forces involved and the current required in the ments.

REFERENCES 1. G.A. Landis, “Optics and Materials Considerations for Laser-Propelled Propulsion," Final Report NIAC RG#07600-070, 2002. www.niac.usra. Lightsail,” IAA-89-664, 1989. edu/files/studies/final_report/597Kare.pdf. Accessed 03 Dec 2018. 2. G. Nordley and A. J. Crowl, “Mass Beam Propulsion, An Overview”, 9. K. L. G. Parkin, “The System Model”, Acta JBIS 68, pp. 153-166, 2015. Astronautica 152, 370-384, 2018. 3. Alan Mole, “One Interstellar Colony Mission”, JBIS, 66, 10. H. B. Knowles, “Thirty-Five Years of Drift-Tube Linac Experience” Los pp.381-387, 2013. Alamos Scientific Laboratory Report, LA-10138-MS, 1984. See also 4. J, Benford, “Beam-Driven Sails and Divergence of Neutral Particle reference 4, pg. 81. Beams” JBIS 70, pg. 449-452, 2017. 11. J. Benford, J. A. Swegle and E. Schamiloglu, High Power Microwaves, 5. Report to the APS of the study on science and technology of directed Third Edition, pg. 77, Taylor and Francis, Boca Raton, FL, 2015. energy weapons, Rev. Mod. Phys 59, number 3, part II, pg. 80,1987. 12. J. Benford and G. Matloff, “Intermediate Beamers for Starshot”, JBIS, 72, 6. G. D. Nordley, "Relativistic Particle Beams for Interstellar Propulsion," pg. 51, 2019. JBIS 46 number 4, pp 145-150,1993 13. P. G. Oshey, T. A. Butler, M. T. Lynch, K. F. McKenna, M. B. Pongratz, T. 7. Andrews, D. G. and R. M. Zubrin, "Magnetic Sails and Interstellar J. Zaugg, “A Linear Accelerator In Space-The Beam Experiment Aboard Travel", JBIS, 43, pp. 265-272, 1990 Rocket”, Proceedings of the Linear Accelerator Conference 1990. 8. J. T. Kare, "High-acceleration Micro-scale Laser Sails for Interstellar 14. G. J. Nutz, “Beam Experiments Aboard a Rocket (BEAR) Project Summary” LA-11737, 1990.

Received 11 May 2018 Approved 9 July 2019

JBIS Vol 72 No.6 June 2019 197 JBIS VOLUME 72 2019 PAGES 198–201

THE IMPLICATIONS OF NON-FASTER-THAN-LIGHT TYPE-3 KARDASHEV CIVILIZATIONS

AGUSTÍN BESTEIRO Instituto de Matemática Luis Santaló, CONICET–UBA, Intendente Güiraldes 2160 - Ciudad Universitaria, Pabellón I (C1428EGA) Buenos Aires, ARGENTINA

Email [email protected]

The authors analyse the implications of non-faster-than-light (non-FTL) communications in galactic civilizations. We assume "human-like" radio communications and the possibility of using space probes that achieve relativistic speeds. We study the implications and likelihood of centralized authorities and groupings of sub-states in the galaxy forming a Galactic Club.

Keywords: Astrobiology; SETI, Relativity; Astrosociology; Kardashev’s scale

1 INTRODUCTION Additionally, we can follow the "slow track" Galactic Club, described by Harrison [17]. This means that we expect the ga- The search for extra-terrestrial civilizations is a currently active lactic civilization to be stable over time. No major disaster, such topic. One aspect of interest is how large-scale civilizations, in- as a supernova or star collision, would affect most of the civili- cluding Kardashev’s type-3 civilizations, could exist despite the zations and their connections. Similarly, we assume slow-paced bounds imposed by the physical laws [1, 2, 3, 4]. Galactic colo- political, cultural, and social change. nization has been analyzed in terms of non-FTL travel or com- munications, only implying a longer colonization timescale With these assumptions in mind, we aimed to examine the than that of FTL travel (see [5]). We can analyze this scenario obstacles that non-FTL galactic civilizations could have, and without assuming more advanced technologies and knowledge to a lesser extent, to identify the difficulties in detecting a non- of physical laws than those of our current status. How can a ga- FTL galactic civilization. These objectives are important in the lactic human-like civilization have a centralized government or context of the Fermi Paradox and SETI’s impossibility to con- culture? These ideas, which we explore qualitatively, were men- tact an advanced extra-terrestrial civilization. They also imply tioned by Hair [6] and Forgan [7] and extensively described by an argument against the [18] and the Interdict Shklovsky and Sagan [8] (see Chapter 35), where the impos- Hypothesis [19], which are based on some of the characteristics sibility of cultural homogenization is discussed, especially in mentioned above. a non-FTL communications scenario. Assuming there is only one human-like galactic civilization capable of electromagnetic 2 ELECTROMAGNETIC COMMUNICATIONS communications (radio, laser, or any kind of communication at the speed of light) and relativistic space probe transporta- Assuming a network of interstellar societies with electromag- tion, we can describe and analyze the characteristics of these netic communications, we know that the speed of light limit restrictions and the difficulties in maintaining a centralized imposes a bound on the connectivity of the region in terms of civilization. communication and even warfare situations. We define a max- imum tolerance time, TM, which can be converted to a maxi- Forgan [9] inquired into the possibility of an emerging con- mum tolerance distance DM = c × TM, with c the speed of light. nected galactic civilization and obtained time constraints using We will not explore the reasons for this limit, but only assume an interesting model. The relationship between grand-scale that human-like societies would not interact with sufficiently civilizations and communication lag was also studied by Lan- distant neighbours, as time-response would become an issue dis [10]. We follow the general assumptions used in [11, 12, 13]. to connectivity between city-states. We assume that this limit We can then analyze the feasibility of city-states [14] bound by is not due to the fall of the signal intensity that becomes indis- communication and transport limitations. This context could cernible from the background noise. If that limit R0 exists, then give rise to a Galactic Club [15] but we do not assume that R0 >> DM. Additionally, countering the Interdict Hypothesis, these are all individually generated, independent civilizations. Webb [20] (See Chapter 3) argues that galactic rotation and lo- For instance, we can follow the [16], cal star velocities could make electromagnetic communication which posits one rare human-like civilization spread all over impossible to direct when dealing with large distances. A fur- the galaxy although we could also assume that there is one in- ther obstacle would involve the uncertainty of GHZ locations, fluential civilization imposing its norms and culture over the as concluded by [21]. rest. Thus we shall not analyze the implications of spontaneous, random generation of civilizations in the galaxy. These islands or city-states [14] are represented by spheres

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of radius, DM, in the same way as Prantzos [11] and Grimaldi centrality. This central government could be a primordial civ- [12] expressed the different distances that separate these soci- ilization that has spread through the galaxy or simply a civili- eties. Although the city-states are centralized, they may engage zation that has “conquered” the rest. In any case, it is an influ- in long-distance communications, implying, however, that a ence over the other city-states both culturally and socially, even long-term answer would still be limited by R0. For example, the with non-FTL communications. Although we argued about the city-state formed by the sphere centered on the sun with a radi- problems of communication traveling at the speed of light and us of 10 light years would include stars like Proxima Centauri, having a limit DM, as mentioned in Section 3, the central gov- Sirius, and Wolf 359. ernment could have a representative traveling to the city-states carrying the “news”. The news would arrive in a longer ordi- 3 RELATIVISTIC COMMUNICATIONS nary time than that expected at the speed of light.

According to special relativity, despite the limitations of elec- The advantage of having a central government at the core tromagnetic communication, relativistic spaceships could of the galaxy is that it is an optimal place for communications reach distant destinations in short “proper” times. This capabil- and transport to other city-states. Like big cities on earth, the ity would allow space travelers somewhere in the galactic civ- central government would be located in an area with high den- ilization to reach any part of it, in as short a time as a lifetime. sity of matter, which implies a considerable number of cities. This is how a dominant culture or central referential govern- Additionally, it would optimize distances, travel times, and ment could be maintained, although the travelers would suffer communications. time dilation effects, as compared with observers in the desti- nation, throughout the round trip. The main issue concerning Conversely, if we assume Galactic Habitable Zones (GHZ) the "messengers" is that they would not arrive back home with- theories [21], then few cities could be located near the galactic out suffering the effects of time dilation. These time differences center, as this galactic core would be a very dangerous place are given by the time dilation formula: for civilizations and transportation. Additionally, this zone is covered by heavy gravitational wells, which, due to gravitation- al time dilation, would increase the difference between proper time and ordinary time, reinforcing the problems mentioned in Section 3. where t' is the dilated time, t the stationary time, v the velocity, Then we should expect the central government to be located and c the speed of light (accelerations are not considered). Local in the middle of the GHZ band. If we use the Milky Way as stations could have a timetable of departures and arrivals, only an example, the GHZ can be approximated by a cylinder, only to keep track of what is expected to happen in this local place. covering 4 ≤ R ≤ 10 Kpc. Then we could assume that the cen- tral government is located around 7 Kpc. This would optimize For instance, if we consider again the local city-state cen- distances, travel times, and communications, but only in one tered around the sun, we can analyze the different options of direction (radial). We could assume a Gaussian population dis- local travel and communication. We can assume that the trav- tribution, with a peak of 7 Kpc: eler can reach velocities of 0.99999999c. Then we consider a list of locations inside and outside the local sphere (Table 1). (1) Local sphere destinations can be reached in extremely short proper times and "ordinary" times are still approximately TM. for r ≥ 0. If we consider distant destinations like Aldebaran or the galac- tic center, then a round-trip could take a considerable amount The distribution mentioned in equation (1) does not de- of time, meaning that the traveler would arrive back home to pend on angular variables because of the galactic symmetry. find a totally different homeland. This example shows us that it This means that approximately 99% of the population would would be very difficult to maintain an updated culture and im- be in the GHZ. Of course, unless the distribution is not sym- plies that SETI could have difficulties detecting civilizations in metric, any location of the central government will not be “cen- the galaxy, in which cities are all disconnected from each other. tral”. In this case, it would be necessary to have at least more than one governmental seat. The optimal location for satellite 4 CENTRALIZED GOVERNMENT governments is symmetrical around the galactic center. These could be located in the center ring of the GHZ. We can take, If there must be a central government or a main "influencer" for instance, between 6 and 8 Kpc and integrate equation (1) on the rest of the city-states, we can analyse the location of this as follows:

TABLE 1 A description of the time dilation for the galactic Traveler for different locations Then the satellite governments are situated on the strip Star Distance Time dilation Local sphere where 68% of the population is located. If there are N satel- (Lys) (Yrs) lite governments symmetrically located on the strip, then the Proxima Centauri 4.243 0.0006 In percentage of the population in GHZ “around” every satellite is 0.68/N, with N << Ncs, where Ncs is the number of city-states. Sirius 8.611 0.0012 In Assuming N << Ncs means that the civilization is more connect- Wolf 359 7.795 0.0011 In ed. Otherwise, the need for many satellites would favor discon- Ross 248 10.3 0.0014 Border nected cultures. Aldebaran 65.23 0.0092 Out We assume, in a first approximation, that the galactic disk is Galactic center 22830.9 3.228 Out a cylinder of radius RG = 12 Kpc and height h = 1 Kpc. If the disk

JBIS Vol 72 No.6 June 2019 199 AGUSTÍN BESTEIRO

is filled with spheres of radius DM, the volume of the galactic axy. In addition to what we mentioned in Section 3 about spe- cylinder is and DM << h. Then the GHZ volume is: cial relativity time dilation, we now consider gravitational time dilation. The travelers that run into a non-rotating, massive, spherically symmetric object on their path would experience gravitational time dilation given by the Schwarzschild formula: where RGHZ = 10-4 = 6 Kpc. (4) Therefore, for each satellite we have a volume: where t0 is the traveler’s proper time, tf is the coordinate time for an observer at an arbitrarily large distance from the mas- sive object, G is the gravitational constant, M is the mass of the We can approximate the number of city-states in the GHZ massive object, r is the radial coordinate of the observer, and c (NGHZ) as spheres of radius DM filling up the disk. Covering the is the speed of light. volume of a cylinder with spheres is not an easy problem (In this case VGHZ and VN), so we approximate the task by using For small to medium-sized black holes, travelers at a short lower bounds in every direction as follows: distance to the Schwarzschild radius would not experience strong gravitational time dilation. For instance, if we use M = (2) 30M , r = 10000 km in equation (4), then for each Traveler’s year, the external stationary observer would experience only where RI = 4 Kpc is the interior radius of GHZ. 1.004 years. Unless the Traveler comes close to the , it is not a major issue. Every term in equation (2) is an approximate lower bound of the number of city-states in every direction. If we now consider a , such as the one inferred to be in the center of the milky way, Sagittarius A*, then is the number of city-states in the radial direction, we have M = 4×106M [22], r = 1.2× 107 km for each Traveler’s year, while the external stationary observer would experience in the “vertical” direction, and eight years. The difference between the radial distance chosen in each case is important, but if we consider the Schwarzschild in the angular direction. radius RS, then for Sagittarius A*, we have r-RS = 190000 Km, i.e. the distance to the event horizon is still considerable. Specifically, for the satellite zone, we have: This is another reason for the problems involved in civili- (3) zations located close to massive objects, particularly near the galactic nucleus, and in traveling through this zone. Similar where NCSN is the ratio of city-states per satellite, i.e. the num- problems can be considered for gravitational lensing and for ber of city-states in each satellite zone. aiming laser pulses in electromagnetic communication.

Note that the city-states outside the GHZ region are not 6 CONCLUSIONS considered, i.e. as f(r) → 0 when r → +∞ or 0, then NGHZ corre- sponds to 68% of NCS. Outside the GHZ, the density of spheres We analyzed the problems that civilizations with non-FTL decreases. communications would have and made a case against “em- pire-like” civilizations such as the ones mentioned in the Zoo If we use RGHZ = 6 Kpc, RI = 4 Kpc, h = 1 Kpc and DM = 10 Hypothesis and the Interdict Hypothesis. Although these hy- Lys for equation (3), we have: potheses consider alternative FTL communications or FTL travel, we argue against civilizations with centralized decision making and culture that could have been populated as a result of long-term expansion, as proposed by theories considering then for the desired number of city-states per satellite gov- long-term robotic probe expansion [23]. We are not interested ernment NCSN, a minimum number N of satellites is needed. in ruling out non-FTL theories, but rather in expressing the This shows us that for a connected galactic civilization (like intrinsic problem that relativity imposes on the likelihood of the ones described in the Zoo Hypothesis [18] or the Inter- galactic civilizations. dict Hypothesis [19]) in a non-FTL universe, the number of satellites N must be large, which challenges the likelihood of We address a problem sometimes neglected in scientific connectivity in the galaxy. articles and used by science fiction writers and popularizers of science. FTL travel is safe in theoretical frameworks, but 5 GRAVITATIONAL TIME PRISONS we build our arguments on experimental physics despite un- known underlying engineering problems. In other words, we In this section, we analysze the problems that gravitational could be even more pessimistic by stating that the technology time dilation could pose to a galactic civilization. As the sec- for relativistic velocities is impossible. tion title suggests, gravitational wells can be useful as time prisons (where prisoners can serve their sentences in a shorter However, on a more optimistic note, we could assume a time than that experienced by external observers), but the con- worst-case scenario in which the galaxy is unpopulated. This sequences of having massive objects around the galaxy could would mean that humans could be the first civilization to ex- be problematic for galactic travelers. pand through the galaxy. In this sense, all the ideas suggested by the scientific community may inspire new projects about A connected civilization needs paths to travel across the gal- future expansions and developments.

200 Vol 72 No.6 June 2019 JBIS THE IMPLICATIONS OF NON-FASTER-THAN-LIGHT TYPE-3 KARDASHEV CIVILIZATIONS

ACKNOWLEDGEMENTS for useful discussions. We thank L. Trombetta and J.M. Fol- gueiras for suggesting interesting ideas. This work was partially We are grateful to the astronomy club CAIFA and its members supported by CONICET–Argentina.

REFERENCES 1. N. S. Kardashev, “Transmission of information by extraterrestrial Publishing Group, Scientific Reports, Vol. 7, 2017. civilization”, Soviet Astronomy, Vol. 8, p.217, 1964. 13. R. D. Smith, “Broadcasting but not receiving: density dependence 2. N. S. Kardashev, “On the Inevitability and the Possible Structures considerations for SETI signals”, International Journal of Astrobiology, of Supercivilizations”, The Search for Extraterrestrial Life: Recent Vol. 8, No.2, pp. 101–105, 2009. Developments, Vol.112, pp 497-504, 1985. 14. M. M. Cirkovic, “Against the Empire”, Journal of the British 3. C. Vidal, “Black Holes: Attractors for Intelligence?”, Kavli Royal Society Interplanetary Society, Vol. 61, pp.246-254, 2008. International Center, arXiv preprint, arXiv:1104.4362, 2011. 15. R. N. Bracewell, The galactic club: Intelligent life in , W.H. 4. S.J. Olson, “Estimates for the number of visible galaxy-spanning Freeman and Co., 1976. civilizations and the cosmological expansion of life”, International 16. P. D. Ward and D. Brownlee, Rare Earth: Why Complex Life Is Journal of Astrobiology, Vol.16, No.2, pp.176–184, 2017. Uncommon in the Universe, Springer, New York, 2000. 5. I. A. Crawford, “Some thoughts on the implications of faster-than-light 17. A. A. Harrison, “Slow track, fast track, and the “Galactic Club””, Futures, interstellar space travel”, Quarterly Journal of the Royal Astronomical Vol. 32, No.6, pp.569–579, 2000. Society, Vol. 36, pp.205-218, 1995. 18. J. A. Ball, “The zoo hypothesis”, Icarus, Vol. 19, No.3, pp.347-349, 1973. 6. T. Hair, “Temporal dispersion of the emergence of intelligence: an inter- arrival time analysis”, International Journal of Astrobiology, Vol. 10, No. 19. M. J. Fogg, “Temporal aspects of the interaction among the first galactic 2, pp.131–135, 2011. civilizations: The “interdict hypothesis””, Icarus, Vol. 69, No.2, pp.370- 384, 1987. 7. D. H. Forgan, “Spatio-temporal constraints on the zoo hypothesis, and the breakdown of total hegemony”, International Journal of Astrobiology, 20. S. Webb, If the universe is teeming with aliens... where is everybody?: fifty Vol. 10, No. 4, pp. 341-347, 2011. solutions to the Fermi paradox and the problem of extraterrestrial life, Springer Science & Business Media, 2002. 8. I. S. Shklovsky, C. Sagan, Intelligent life in the universe, San Francisco, 1966. 21. C. H. Lineweaver, Y. Fenner, and B.K. Gibson “The and the age distribution of complex life in the Milky Way”, Science, 9. D. H. Forgan, “The Galactic Club or Galactic Cliques? Exploring the Vol. 303, No. 5654, pp. 59–62, 2004. limits of interstellar hegemony and the Zoo Hypothesis”, International Journal of Astrobiology, Vol. 16, issue 4 pp. 349-354, 2017. 22. A. M. Ghez, S. Salim, N.N. Weinberg, J.R. Lu, T. Do, J.K. Dunn, K. Matthews, M.R. Morris, S. Yelda, and E.E. Becklin, “Measuring distance 10. G. A. Landis, “The Fermi paradox: an approach based on percolation and properties of the Milky Way’s central supermassive black hole with theory”, Sverdrup Technology, Inc., NASA Lewis Research Center, 1993. stellar orbits”, The Astrophysical Journal, Vol. 689, No. 2, pp. 1044–1062, 11. N. Prantzos, “A joint analysis of the and the Fermi 2008. paradox”, International Journal of Astrobiology, Vol.12, No.3, pp.246– 23. R. Hanson, “Burning the cosmic commons: evolutionary strategies for 253, 2013. interstellar colonization”, preprint available at http://hanson.gmu.edu/ 12. C. Grimaldi, “Signal coverage approach to the detection probability filluniv.pdf, 1998. of hypothetical extraterrestrial emitters in the Milky Way”, Nature

Received 8 May 2019 Approved 21 July 2019

JBIS Vol 72 No.6 June 2019 201 JBIS VOLUME 72 2019 PAGES 202–212

HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER STAR: The Interstellar Probe Mission

P. C. BRANDT1, R. MCNUTT1, M. V. PAUL1, C. M. LISSE1, K. MANDT1, S. R. VERNON1, E. PROVORNIKOVA, K. RUNYON, A. RYMER, G. HALLINAN2, R. MEWALDT2, L. ALKALAI3, N. ARORA3, P. LIEWER3, S. TURYSHEV33, M. DESAI4, M. OPHER5, E. STONE2, G. ZANK6, L. FRIEDMAN7 1The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA; 2California Institute of Technology, Pasadena, CA 91125, USA; 3Jet Propulsion Laboratory, Pasadena, CA 91109, USA; 4Southwest Research Institute, San Antonio, TX 78238, USA; 5Boston University, Boston, MA 02215, USA; 6University of Alabama in Huntsville, Huntsville, AL 35899, USA; 7Emeritus, The Planetary Society, USA.

Email [email protected]

An Interstellar Probe mission concept using today’s technology is presented. Science targets include the physics of the and obtaining the first external image of our own heliosphere, a flyby exploration of Objects (KBO), such as Quaoar, the circum-solar , and astrophysical observations of the Extragalactic Background Light (EBL) enabled by going beyond the Zodiacal Cloud. The example science payload includes a particle and fields suite, dust, optical and IR imaging systems. The goal of the Interstellar Probe is to reach 1000 AU. Assuming a spacecraft, the size of New Horizons with 478 kg launch mass, and a launch on board a Space Launch System (SLS) vehicle, the payload mass and power cannot exceed 50 kg and 50 W (both including 30% margin). Integrated and mass optimized instrument solutions are needed to achieve these targets. It is suggested to baseline the use of General Purpose Heat Source (GPHS) (RTGs) used on Ulysses, Galileo, Cassini and New Horizons and use Ka-band communication accepting relatively low rates and longer downlink times. Using available heat-shield mass estimates from Parker Solar Probe (PSP), a preliminary trajectory analysis of an Oberth Maneuver near the Sun offers higher asymptotic speeds than that of a direct inject to a Jupiter Gravity Assist (JGA). Required trade-off studies are outlined. An immediate Interstellar Probe mission would be the boldest undertaking in space exploration ever done and break new grounds at the space science frontier marking the historic milestone when humanity took the first explicit step on the expansion beyond our solar system.

Keywords: Interstellar Exploration, Energetic Neutral Atoms, Kuiper Belt Objects, Quaoar, Solar Gravity Focus Lens, Planet X, Planet Nine

1 INTRODUCTION The desire to explore beyond our solar system has also re- cently been fueled by language from the Committee on Appro- As the Voyagers are crossing into the Local Interstellar Medium priations encouraging “NASA to study and develop propulsion (LISM) and the Kepler Mission has unveiled an abundance of concepts that could enable an interstellar scientific probe” and Earth-like planets around other , inevitably, we are faced further tasks NASA to “submit an interstellar propulsion tech- with the question of how humanity will venture out through nology assessment report with a draft conceptual roadmap” the vast space between our star and other potentially habitable [1]. Although the report states ambitious technological goals, planetary systems (Fig. 1). Such a venture requires new lines of it nevertheless offers support and engagement for interstellar voyages that each would expand our frontier of exploration of exploration. undiscovered worlds beyond our planetary neighborhood and unveil the workings and habitability of exoplanetary systems The discussions of an Interstellar Probe to the immediate by bringing unseen views of our own astrosphere that harbors interstellar medium possibly dates back to 1958 and is closely our solar system. linked to the first concepts of a Solar Probe [2]. The Interstellar Probe mission concept has been studied and presented under Following the Voyagers, New Horizon, and Parker Solar several different names such as the Innovative Interstellar Ex- Probe, NASA is now in a position to take the first explicit step plorer (IIE) [3], the Interstellar Heliopause (IHP) mission [4], scientifically, technologically and programmatically on that and The Interstellar Probe [5]. path through the development of the first Interstellar Probe. Furthermore, remote sensing of the heliospheric boundary by Past studies and workshops include the NASA-funded the NASA IBEX and upcoming IMAP missions will provide Sun-Earth-connection Roadmap study for an Interstellar Probe important information on where to exit the heliosphere to al- mission in 1999-2000 [6,7,8], the Keck Institute for Space Stud- low for the maximum science return. ies Workshop series conducted in 2014 and 2015 on the topic

202 Vol 72 No.6 June 2019 JBIS HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER STAR: The Interstellar Probe Mission JOHNS HOPKINSJOHNS UNIVERSITY

Fig.1 An Interstellar Probe Mission to the Local Interstellar Medium would be daring, challenging and inspirational to the public and will be a rationale first step before attempting to reach another star.

“Science and Enabling Technologies for the Exploration of the In- have emerged in various forms in studies and reports [2]. In terstellar Medium” [9]. this recent study, a set of overarching goals were identified by the science community to better emphasize the cross-discipli- These workshops addressed also several scientific targets that nary nature of the mission. would be within reasonable reach of an Interstellar Probe, in- cluding flybys of Kuiper Belt Objects (KBOs), the global dust • Goal 1: Understand the Heliosphere as a Habitable distribution within and beyond the solar system, observations Astrosphere: enabled by the solar gravity lens, and more. Below we describe – What is the global nature of the heliosphere? some of the most immediate and compelling science targets that – How do the Sun and galaxy affect the dynamics of the will guide further detailed trade-off studies and mission designs. heliosphere? – What is the nature of the nearby interstellar medium? Here, we discuss the compelling science and a pragmatic • Goal 2: Understand the Origin and Evolution of mission design under study that NASA would have to consider Planetary Systems to embark on a path that continually expand humanity’s fron- –How did matter in the solar system originate and tier of exploration. The Phase-I of this Interstellar Probe study evolve? has been completed at The Johns Hopkins University Applied • Goal 3: Explore the Universe Beyond the Circum-Solar Physics Laboratory (JHU/APL), and will shortly be going in to Dust Cloud its Phase II. The study revolves around critical trade-offs in sci- – How did form and evolve in the universe? ence and realistic mission design that achieves high asymptotic speeds with mature technologies for launch in the 2030s with a 3 THE HELIOSPHERE AS A HABITABLE ASTROSPHERE goal to reach 1000 AU. Planetary systems are encased in a magnetic bubble created by 2 SCIENTIFIC JUSTIFICATION AND TARGETS the outward stellar wind of its parent star. The global shape and nature of this astrosphere as it plows through its surrounding Venturing on to an escape trajectory will offer science discov- LISM, is directly constrained by the properties of the stellar eries of different proportions that will naturally bridge helio- wind and the LISM, and can indirectly reveal stellar mass loss physics, planetary sciences and astrophysics by putting our rates and the habitable conditions governed by stellar-wind own solar system and magnetic bubble (“the heliosphere”) in interactions responsible for the loss of planetary atmospheres. the context of the increasing number of other exoplanetary Despite the numerous images in UV and visible observed by systems and astrospheres detected and characterized. Such an the Hubble Space Telescope (Fig. 2 overleaf), our type of helio- external vantage point will also open the window to global ob- sphere is likely too weak to be directly imaged by conventional servations of the circum-solar dust disk and the extragalactic techniques [10]. Therefore, the global shape of our heliosphere background light, whose wavelengths are otherwise and the new astrophysical plasma conditions governing its obscured by the Zodiacal dust cloud. boundary interactions belongs to the most outstanding science The compelling science questions for an Interstellar Probe questions of space physics today [11].

JBIS Vol 72 No.6 June 2019 203 PONTUS BRANDT ET AL

ing the forces that stand off the flow of the interstellar medium. The elusive source of Anomalous Cosmic Rays was expected to be found in the outer boundaries of the heliosphere, but the Voyager measurements have not yet provided any definitive evidence of their source. Furthermore, the lack of direct meas- urements of the low-energy plasma, pick-up ions and neutral densities and large-scale flows has presented a stumbling block in determining how the plasma and neutrals of the LISM inter- acts with the heliosphere, which is one of the critical pieces of information to understand its global structure.

Although UV imaging provides valuable information on the hydrogen densities and flows surrounding and permeating the heliosphere, it is hard to draw conclusions about the 3D struc- ture of the heliosphere. Energetic Neutral Atom (ENA) imag- ing has proven to be so far, the best tool to remotely image the structure of the boundary towards the LISM and its particle populations that uphold the force balance in this region. ENAs P-D ART are formed when the relatively high intensities of charged par- Fig.2 Planetary systems are encased in a magnetic bubble spanned ticles and plasma in the outer heliospheric boundary region by the outward stellar wind of its parent star. The global shape and and possibly beyond, charge exchange with the local neutral nature of this astrosphere as it plows through its surrounding LISM, atoms such as hydrogen and helium. Therefore, it not only pro- is directly constrained by the properties of the stellar wind and vides images of the boundary region, but also carries with it therefore reveals the habitable conditions governed by stellar-wind important spectral information about the parent plasma here interactions responsible for the loss of planetary atmospheres. that can be used to uncover the important physics.

The Interstellar Boundary Explorer (IBEX) mission have In August 2012, Voyager 1 crossed the outer boundary of revealed a completely unpredicted pattern in ENAs of a thin the heliosphere, the heliopause, at 122 AU from the Sun as in- “ribbon” across the sky (Fig. 3 - left) that is believed to be or- dicated by charged particle measurements [12,13]. Voyager 2 ganized by the interstellar magnetic field and many other dis- arrived at the heliopause around November 2018 at a distance coveries that can be used to provide constraints on the global of about 119 AU from the Sun. structure and physics of the heliosphere [17]. In addition, the Ion and Neutral Camera (INCA) on board the Cassini mission Both these missions have revealed surprising gaps in our [18] have provided ENA images at higher energies that have understanding of the exotic plasma-physical processes at work led to conclusions about the global structure (Fig. 3- right) in the boundary region. The implied shape of the boundary re- that are seemingly contradictory to the IBEX results [19]. This gion and the extent of the Sun’s influence on the LISM cannot apparent contradiction only illuminates the difficulty to draw be fully explained by current theoretical models [14,15,16]. In- conclusions about a 3D global structure by imaging it from direct measurements pointed to an unexpected heated charged within. Recently NASA selected the Interstellar Mapping and particle population, the so-called pick-up ions (PUI), dominat- Acceleration Probe (IMAP) mission which carries a payload P-D ART P-D ART

Fig.3 As Interstellar Probe traverses in to the pristine Interstellar Medium it will lay claim to the first historical view of our own astrospheres from the outside, allowing us to extrapolate and understand other astrospheres and the habitability of the planetary systems they harbor. Figure depicts our current understanding derived from Energetic Neutral Atom (ENA) images looking out from the inside our bubble from the IBEX Mission [17] and Cassini mission [19].

204 Vol 72 No.6 June 2019 JBIS HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER STAR: The Interstellar Probe Mission of higher-resolution ENA imagers that will greatly improve the characterization of the ENA emissions from the interstellar boundary region [20].

Together with comprehensive plasma, particle and magnetic field measurements, ENA imaging generates critical measure- ments for an Interstellar Probe mission. As it traverses through the heliospheric boundary into the LISM, the Interstellar Probe will probe the plasma physics governing this unique astrophys- ical region and conduct remote ENA imaging of the enormous three-dimensional boundary from progressively different vantage points to pinpoint the sources of ENA emissions. As the Probe eventually leaves our heliosphere behind, it will lay claim to the first historical views of the heliosphere from the outside allowing us to extrapolate and understand other astrospheres and the hab- P-D ART itability of the planetary systems they harbor, and ultimately place Fig.4 Keys to the evolution of our own and exoplanetary systems our own heliosphere within the family other astrospheres. lie in the distribution of dust in the solar system out to the . Dust is thought to be produced by Comets originating in the 4 THE ORIGIN AND EVOLUTION OF PLANETARY SYSTEMS Oort cloud, asteroid collisions and also from collisions of KBOs. On its way out an Interstellar Probe will for the first time unveil the belts and debris disks full of dust are known as previously invisible dust distributions of the Outer Solar System to the "signposts of planet formation" in exosystems. The over- enable a first 3D map of our own circum-stellar dust disk that are all brightness of a disk provides information on the amount common around other stars. of sourcing planetesimal material, while asymmetries in the shape of the disk can be used to search for perturbing planets. unconstrained. Therefore, understanding the 3D structure and The solar system is known to host two such belts, the Asteroid distribution of our own circum-solar disk will enable a direct belt and the Kuiper Belt (Fig. 4), and at least one debris cloud, constraint between large-scale structure and dust formation the Zodiacal Cloud. The zodiacal cloud of dust in our system is mechanisms. formed from dust created by grinding in our asteroid and Kui- per Belts and by dust shed from comets as they pass through 5 THE COMPELLING WORLDS OF KBOS the inner solar system close to the Sun. Although even dark dust emits heat radiation at infrared wavelengths, from a van- As the New Horizons Pluto flyby has shown, this extended part tage point inside the solar system’s dust cloud it is intrinsical- of our solar system holds a diversity of undiscovered worlds ly difficult to determine its large-scale distribution. While the (Fig. 6), which should unlock many of the secrets of the evolu- distribution of the asteroid- and Kuiper belts are relatively well known, it remains unclear how much, if any, dust is produced from the Kuiper belt since the near-Sun comet contributions dominate near-Earth space. Understanding how much dust is produced in the Kuiper belt provides important clues on the total number of bodies in the belt, especially the smallest ones, and their dynamical collisional state. Even for the close in Zo- diacal cloud, questions remain concerning its overall shape and orientation with respect to the ecliptic and invariable planes of the solar system - they cannot be explained from the perturba- tions caused by the known planets alone.

An Interstellar Probe would provide a historically unique opportunity to measure the entire extent of the inner, near- earth zodiacal cloud; whether it connects smoothly into an outer cloud, or if there is a second outer cloud sourced by the Kuiper belt and isolated by the outer planets, as predicted by Kuchner and Stark [21]; Stark and Kuchner [22] and Poppe [23]. Visible/Near Infrared (VISNIR) imagery will inform about the dust cloud density, while Mid-Infrared (MIR) cam- eras will provide thermal imaging photometry related to dust particle size and composition. Deep searches for the presence of rings and dust clouds around discrete sources would be en- abled by observing at high phase angles towards the Sun from 200 AU or beyond. For example, searches for debris clouds as- P-D ART sociated the Haumea family collisional fragments, or the rings Fig.5 Circumstellar dust disks are commonly observed around of the Centaur Chariklo, or dust emitted from spallation off the other young stars as tracers of planetary birth and formation, six known bodies of the Pluto system. that often cannot be directly discerned otherwise due to the vast distances between us. The two images at top reveal debris disks Multiple other circum-stellar dust disks have been observed around young stars uncovered in archival images taken by HST. (Fig. 5) and while it is clear that they are signatures of planetary The illustration beneath each image depicts the orientation of system formation, many of the fundamental problems remain the debris disks.

JBIS Vol 72 No.6 June 2019 205 PONTUS BRANDT ET AL tion of our solar system, but would, more importantly put the evolution of other exoplanetary systems in context. At 40-50 AU, conveniently lining up with the nose direction of the helio- sphere and the mysterious ribbon, lies the Quaoar (Fig. 7) that is in the last stages of losing its methane atmos- phere. This KBO represent a missing data point in that it lies in between the volatile-dominated and volatile-poor objects. Sur- Sedna prisingly, crystalline ice has been detected on the surface imply- ing cryo-volcanism active in the immediate past or even still ac- tive. Quaoar therefore represents one of the possible targets that could unveil yet another unexpectedly exotic world of a KBO with critical implications for planetary formation. Quaoar also harbors its own moonlet. Investigating this pair would provide further data on the birth and evolution of KBOs.

The New Horizons spacecraft flew by Pluto at approximately Quaoar Pluto Moon Earth 14 km/s, which imposed strict requirements on the speed of its surface imaging systems. Naturally, maximizing the asymptotic speed of an Interstellar Probe is a key mission goal that needs to P-D ART be carefully traded off with the flyby speed of a KBO. Although Fig.6 At 40-50 AU, conveniently lining up with the nose direction of imaging at high flyby speeds is challenging, the Juno mission the heliosphere lies the dwarf planet Quaoar that is in the last stages has accomplished high-quality imaging at speeds significantly of losing its methane atmosphere. Surprisingly, crystalline ice has exceeding that of New Horizons. been detected on the surface implying cryo-volcanism active in the immediate past or even still active (Jewitt et al., 2004). 6 OBSERVING THE UNIVERSE BEYOND THE ZODIACAL CLOUD solar system, and, thus obscures the EBL. Therefore, a vantage point beyond the Zodiacal cloud offered by the Interstellar The combined diffuse light from galaxies at redshifts between Probe would open a very important window in IR, enabling a z~2-5 is called the Extragalactic Background Light (EBL) and leap in our understanding of early galaxy and . provides important insights in to early galaxy and star forma- Groundbreaking astrophysical observations could be accom- tion. Redshifted starlight from unresolved galaxies appear at plished even with relatively modest IR instrumentation. 1-10 µm, while starlight reprocessed by dust grains appears in the mid to far infrared. Less well understood is the diffuse back- 7 OTHER TARGETS ground from the reionization epoch, which is the earliest period of star and galaxy formation. Together these backgrounds trace 7.1 Planet X the history of nucleosynthesis in the universe. The Zodiacal light produced by scattered sun light from the Zodiacal cloud Although highly speculative, a possible clustering of KBOs in dominates the IR spectrum from a vantage point in the inner their respective arguments of perihelion could be explained by

Fig.7 The first explicit Interstellar Probe Mission could potentially flyby the KBO Quaoar (left inset), on its way towards the nose direction of the heliosphere and through the mysterious ENA Ribbon (right inset)

206 Vol 72 No.6 June 2019 JBIS HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER STAR: The Interstellar Probe Mission a massive trans-Neptunian planet (“Planet X” or “Planet Nine”) cannot exceed 50 kg and 50 W, both including 30% margin. of ten times the mass of Earth in an orbit with eccentricity of This puts a strict limitation on the scope of the scientific pay- 0.6 and a semi-major axis of 700 AU, corresponding to a peri- load and thus interconnects the mission capabilities with the apsis of 280 AU and apoapsis of 1120 AU [23, 24, 25, 26, 27, 28, science requirements more strongly for an interstellar probe, 29]. While other observations support a uniform distribution possibly more than any other past missions. Therefore, signif- of KBOs [30], other recent observations appear to support the icant effort must be put in to developing a highly integrated Planet-X hypothesis [31]. Should a Planet X exist, it would be- mass- and power-optimized payload, while ensuring a high come an extremely intriguing flyby target within reach of the science return. Previous model payloads for an Interstellar Interstellar Probe. Probe mission concept has focused on the exploration of the interstellar medium and its interaction with the heliosphere 7.2 Exoplanetary Imaging [37]. To address all three science goals discussed above, signif- icant development is needed to optimize mass and power. An In theory, the Sun starts acting as a gravitational lens beyond example payload addressing these science goals is outlined in 542 AU and therefore using it for high-resolution observations Table 1. of distant objects has received significant attention. The idea was advocated by [32] Eshleman and [33] Maccone, and, re- 9 IN-SITU AND REMOTE-IMAGING PARTICLE, PLASMA cently, further analyses have been carried out by Turyshev et al. AND FIELDS SUITE [34]. The remarkable properties of a Solar Gravity Lens could open breathtaking possibilities to directly observe distant and The interaction between our heliosphere and the local inter- faint objects in historic details, such as exoplanets. At least the stellar medium involves particle energies and intensities span- theoretical performance would go far beyond the projected ca- ning orders of magnitude and therefore requires several differ- pabilities of future mission concepts like WFIRST and Large ent instruments integrated in to one suite. UV/Optical/IR Surveyor (LUVOIR) that would target exoplan- et surveys using micro lensing, and direct spectral imaging to Measurements of the low (“plasma”) energies are critical to precisely determine the atmospheric composition of the target. understand the expanding solar wind and its propagation as it Observations from the SGLF would use strong gravitational encounters the local interstellar medium. Furthermore, meas- lensing to obtain highly resolved images of exoplanetary targets. urements of the plasma flow direction, density and temperature of the interstellar plasma is critical for understanding its behav- The feasibility of the SGLF concept is attached with serious ior as it interacts with the heliosphere. Measurements of the questions on the practical aspects including image acquisi- neutral gas is equally important since the neutral component tion, signal strength, and the extreme challenges of operating a of the solar wind and interstellar medium can freely penetrate constellation of a large number of spacecraft required at those plasma boundaries and dramatically alter the system, such as distances [35, 36]. The associated requirements are currently the formation of the hydrogen wall in front of the heliosphere. unattainable with available or near-term technology and there- The intermediate, so-called supra-thermal energy range ad- fore would have to be the focus of future concepts. However, dresses how interstellar neutrals that permeates the heliosphere the return of even one detailed image of a habitable exoplanet are ionized and “picked up” by the solar wind to form pick-up would likely have an impact that would only be second to that ions (PUIs) that appears to be the major component that up- of direct contact with an alien species. holds the force balance against the local interstellar medium. ENA imaging in the range from <1 keV to 200 keV, is currently 8 PAYLOAD the only available technique that can obtain a global image of our heliosphere and thus represents an unchallenged core in- Previous studies have concluded that payload mass and power strument of a model payload. Higher-energy ions above tens of

TABLE 1 Science targets and measurement requirements of a model payload for exploring the interstellar medium, KBOs and the dust distribution

Instrument Science Target In-situ and Remote Imaging Particle, Plasma and Fields Suite Remote imaging to achieve the first outside view of the global heliosphere, and in-situ ENA and Ion Camera measurements of energetic ions and electrons. Measure the high-energy particles including PUIs in the solar wind and during the transition through the termination shock, heliosheath and beyond the heliopause Energetic Particle Detectors to investigate the new astrophysical plasma conditions, where energetic particles appears to play a major role. Determination of the elusive source of Anomolous Cosmic Rays (ACR) would also be a major objective. Plasma Instrument Solar-wind and interstellar plasma composition, density and flow directions. Neutral Gas Detector Solar-wind and interstellar neutral gas density, composition and flow direction. Vector Helium Magnetometer Magnetic field magnitude and direction. Dust Detector Interplanetary and circum-solar dust distribution Imaging Systems Primarily for flyby imaging of Quaoar and its moon Weywot. Other objectives would Optical include mapping of new KBOs and precision . Imaging the distribution of dust beyond our planetary neighborhood. Flyby surface IR Camera imaging.

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Fig.8 The second generation combined ENA and ion camera provides ENA imaging and energetic ion (and electron) measurements in one instrument. By applying a voltage to the deflection plates (red and gray blades) the camera operates in ENA-imaging mode. With no voltage applied the camera operates in ion mode for in- situ measurements. The shutter employs a variable-aperture system for covering the many orders of magnitude in intensity from the relatively weak ENA intensities to the high ion intensities [38]. keV are critical for understanding the new astrophysical plasma For an Interstellar Probe mission, one would have to ensure conditions that exist in the boundary region to the LISM, and that these instruments have a sufficient field of view (FOV) to also for how interplanetary shocks and processes in the outer obtain 3D measurements, which is particularly important for heliosphere accelerate those seed particles to extreme energies obtaining plasma flow directions, something which took Voy- including the elusive Anomalous Cosmic Rays (ACRs). ager deep-space maneuvers to do.

Current ENA cameras are capable of imaging ENAs in the Imaging systems would primarily target optical and IR <1 – 200 keV range but can also be configured to measure ener- surface imaging during flybys of KBO’s and long-range, deep- getic ions up to several MeVs. Fig. 8 shows the Jovian Energetic space imaging to map locations of new KBOs and astrometry. Neutrals and Ions (JENI) camera developed for flight on JUICE Typically, these science observations require two cameras, such [38]. In ENA-imaging mode, a set of deflection plates, each po- as Ralph [39] and Long-Range Reconnaissance Imager [40] on sitioned over the two aperture slits, sweep out charged particles board the New Horizons mission. when a voltage is applied to the plates. The remaining ENAs en- ters through one of the aperture slits in to a chamber, where the As examples of integrated solutions that optimizes mass, time-of-flight (TOF) of the particle is measured along with its power and functionality, are the Multipurpose Imaging Sci- arrival direction. In ion mode, the deflection voltage is turned ence and Communication Instrument (MISCI) outlined in the off, which allows charged particles to enter the TOF chamber. KISS Technology Development Proposal [9], or the Zodiacal A variable-aperture system in front of the slits ensures a suffi- dust, Extragalactic Background, and Reionization Apparatus ciently large geometry factor when imaging the relatively weak (ZEBRA) [39]. MISCI would combine deep-space and fast ENA intensities, as well as preventing count rates of the more flyby imaging with IR imaging and a laser subsystem for op- intense charged-particles to overwhelm the detector system tical communication and ranging. The ZEBRA system (Fig. when in ion mode. 9) integrates optical and IR imaging system consisting of the High-resolution Absolute Module (HAM) with a 15-cm tel- Energetic particle and plasma instruments have flown on numerous missions including Voyager, Galileo, Cassini, Juno and are being developed for flight for the JUICE mission. A total energy range of about 10 eV to 10s of MeV is required due to the nature of the plasma interactions in this region. The plasma regime from 10 eV up to a few keV is critical to un- derstand the plasma flows in the boundary region of the outer heliosphere to the interstellar medium, but also to understand the flow of the interstellar plasma as it encounters the helio- sphere. The energetic particle regime, usually counted from a few keV and up to several MeV covers the dominating part of the plasma pressure, which upholds the force balance against Fig.9 The ZEBRA instrument consists of the High-resolution the flow of the interstellar medium. And lastly, the very high Absolute Module (HAM) with a 15 cm telescope, and the Wide-field energy regime up to 10s of MeV is required to understand the Absolute Module (WAM) with a 3 cm telescope. The instrument elusive ACRs. These three energy ranges usually require three mass is 16.4kg and power dissipation is 12.4W, both including 30% types of instrumentation. margin.

208 Vol 72 No.6 June 2019 JBIS HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER STAR: The Interstellar Probe Mission escope, and the Wide-field Absolute Module (WAM) with a use accepted ‘‘adequate’’ margin philosophy. 3-cm telescope. Important trade-offs include imaging at high flyby speeds and the need for very fast detectors. 11 RESOURCE REQUIREMENTS AND CAPABILITIES

10 MISSION REQUIREMENTS/STUDY GOALS 11.1 Payload Mass and Power

The first explicit Interstellar Probe discussed here would be Payload mass to spacecraft dry mass fractions historically range based on technology and launch vehicles that are available to- from 7% (New Horizons) to 14% (Voyager). Studies of the IHP day or that are under development and have reached signif- and IIE mission concepts have assumed fractions in the range icant maturity. Therefore, a key-enabling component is the 5-7%. Based on a New Horizons spacecraft dry mass of 385 kg, availability of a heavy launch vehicle such as the Space Launch this leaves about a not-to-exceed payload mass of about 40 kg Systems (SLS). The mission concepts under study includes a (including 30% mass margin). In order to achieve this payload direct inject to Jupiter, followed by a passive Jupiter Gravity As- mass limit, substantial optimization is required for the instru- sist (JGA), and one concept based on a reverse passive JGA, mentation. As reference, the payload of New Horizons is 31 followed by an Oberth maneuver near the Sun. kg as flown, but was different than the one discussed above. Integrated configurations have been suggested for particles and While historically, the mission requirements of a first explic- plasma such as, for example the Compact Dual Ion Composi- it Interstellar Probe mission have been focused on reaching the tion Experiment [42], but, in general more work is needed to LISM at 200 AU, a study goal is 1000 AU in 50 years, which optimize mass and power. corresponds to an asymptotic speed more than 15 AU/year. In comparison, Voyager 1 holds the record in asymptotic speed The MISCI concept combines optical and IR imaging with of about 3.6 AU/year. The direction of the asymptotic trajec- laser communication to achieve flyby surface imaging, deep- tory should be with a 20˚ cone angle of the heliospheric nose space exploration, precision astrometry, spectrometry and if the heliosphere has significant asymmetry. However, if the high-rate communication. The concept shares the same 50-cm heliosphere is symmetric, there is more flexibility in choosing telescope optics using detectors with different spectral respons- the trajectory. The results from the upcoming NASA/IMAP es, which would make the system mass efficient, but the laser mission will guide this goal. The Interstellar Probe considers communication capability would require significant power. launches between now and 2050 and assumes no assembly in The ZEBRA instrument mass is 16.4 kg and power dissipation space. To achieve lifetime reliability within reasonable techni- is 12.4 W, both including 30% margin, but does not include la- cal requirements and implementation, Interstellar Probe must ser communication capabilities. Table 2 summarizes the flown

TABLE 2 Summary of masses of relevant instruments of past, current and selected missions

Spacecraft New Instruments Helios Pioneer Voyager Ulysses IBEX STEREO Cassini JUICE* Horizons Vector Helium Magnetometer 4.4 2.7 5.6 2.3 3.0 N/A Fluxgate Magnetometer 4.8 0.3 2.4 0.3 N/A Plasma Wave Sensor N/A 9.1 7.4 13.2 N/A Plasma 15.7 5.5 9.9 3.3 6.7 2.4 12.5 N/A Plasma Composition 5.6 11.4 N/A Energetic Particles 3.5 3.3 7.5 1.5 5.8 1.6 6.7 6.5 6.7 Cosmic Rays: (ACR, GCR) 3.2 7.5 14.6 1.9 Cosmic Rays: 7.2 1.7 2.0 (electrons/positrons, protons, He) Geiger Tube 1.6 Meteoroid Detector 8.9 3.2 1.6 1.6 3.8 16.4 X-rays and gamma burst detector 2.0 Neutral Atoms 4.3 12.1 9.3 N/A ENA 7.7 6.9 6.5 UV 0.7 4.5 4.4 14.5 N/A IR 2.0 19.5 39.2 N/A Imaging Photopolarimeter 4.3 2.6 8.6 Imaging Systems 8.9 38.2 10.5 48.1 37.1 N/A 57.9 Common Elements 5.4 19.1 7.8 N/A (electronics, booms, harnesses) TOTALS 72.2 30.1 104.4 29.9 54.9 25.2 100.0 N/A N/A

JBIS Vol 72 No.6 June 2019 209 PONTUS BRANDT ET AL masses or Current Best Estimates (CBE) of relevant instru- mentation on past, current or selected missions. The masses are therefore without margin.

11.2 Power Sources

General Purpose Heat Source (GPHS) Radioisotope Thermoe- lectric Generators (RTG) have been used successfully on Ulyss- es, Galileo, Cassini and New Horizons and should be baselined for an Interstellar Probe (Fig. 10). The Multi-Hundred Watt RTGs on the Voyagers still have 170 W after about 48 years en-route (450 W at beginning of life). Use of Multi-Mission Ra- dioisotope Thermoelectric Generators (MMRTGs) will require lifetime extension based on ongoing successful developments of new materials, or reclamation of the Si-Ge technology used by several missions. P-D ART Fig.10 The GPHS-RTG used on the Ulysses spacecraft weighed 11.3 Communication about 56 kg including 11 kg of Pu-238.

Ka-band communications with High-Gain Antenna (HGA) and the Deep Space Network (DSN) upgrade to phase-array dishes offers advantages in pointing requirements, but is re- stricted in bandwidth (Voyager 160 bps at 135 AU). Optical communication as proposed in the MISCI concept, offers ad- vantages in bandwidth (1-10 kbps at 200 AU), but imposes very stringent pointing requirements (<µrad). The MISCI concept implements laser communication and optimizes resources by combining it with deep-space, fast-flyby imaging capabilities and laser ranging. It would be challenge to pick up an optical signal against the solar background, which would impose re- quirements on a certain level of autonomy. However, given the size and increasing reliability of Solid-State Mass Memories, the conventional Ka-band communication offers a low-risk ap- proach where vast amounts of data could be stored on board Fig.11 A 585 kg (including margin) spacecraft launched on an and downlinked at a lower rate without being limited by the SLS with a direct inject to a passive JGA followed by an Oberth relatively short lifetimes of interplanetary missions. Maneuver at 10 RS would result in an asymptotic speed of about 8 AU/year using available kick stages. Maneuvers at closer perihelia 12 MISSION DESIGN TRADE-OFFS would increase the speed, but at the expense of a heavy heat shield. Spacecraft design by Boeing from previous study [2] using Three mission scenarios are currently considered that could be six RTGs, Ka-band High-Gain Antenna. During flyby phase the developed and flown with today’s technology: direct injection platform would be three-axis stabilized. During long cruise and to a powered or passive JGA, and an Oberth maneuver near exploration of the LISM the platform would be spin stabilized. the Sun [2]. The use of in-space propulsion including solar sails, nuclear electric propulsion (NEP), and radioisotope pro- pulsion (REP) have all been problematic. The problems have included predicted masses and structural requirements, assem- bly, autonomy, and lifetime, and are therefore not consistent with these mission requirements and study goals.

Both scenarios assume a launch on the SLS with a Castor 30XL stage and a Star-48 kick stage. The example spacecraft dry mass (Fig. 11) is based on the New Horizons launch mass of 585 kg (including margin). The first two scenarios (JGA) would perform a direct injection to Jupiter with a powered or passive JGA yielding an asymptotic speed of about 8 AU/year.

The second scenario would also use a direct inject to Jupiter but perform a reverse JGA to decrease the of the spacecraft around the Sun, and fall in close to the Sun, Fig.12 Figure from (McNutt et al., 2017) assuming a New Horizons where the Star-48 kick stage would deliver all of its ∆v around spacecraft with a 153 kg heatshield derived from currently available perihelion. This would result in an asymptotic speed of about PSP estimates. An Oberth maneuver at 9.4 solar radii would result 8.3 AU/year, assuming a perihelion distance of 9.4 solar radii in an asymptotic speed of about 8.3 AU/year. An Oberth Maneuver (RS) with a heat-shield mass similar to that of Parker Solar at closer distances would increase the asymptotic speed significantly Probe (PSP) (Fig. 12). This maneuver is nothing new, but was (up to a factor of two), at the expense of stricter thermal first considered by Oberth [43], who realized that a powered requirements and a heavier heat shield, which, in turn affects the maneuver deep in the Sun’s gravity well could enable solar sys- available ∆v.

210 Vol 72 No.6 June 2019 JBIS HUMANITY’S FIRST EXPLICIT STEP IN REACHING ANOTHER STAR: The Interstellar Probe Mission tem escape. The technical challenge is the amount of delta-v our own astrosphere and circum-solar dust disk. It would ex- required at the burn maneuver, and the fact that the maneuver plore the new astrophysical plasma conditions governing this has to take place less than 10 solar radii in order to be effective. important boundary region to interstellar space, which would With the imminent launch of the PSP, the existing thermal en- connect our habitable heliosphere with other astrospheres har- gineering solutions can be used to estimate the mass of a heat boring other planetary systems. Flybys of undiscovered KBOs shield required at the Oberth Maneuver. would reveal new worlds that would hold invaluable knowl- edge of the evolutionary history of our solar system. The Oberth Maneuver offers, theoretically, the best option for maximizing the solar-system escape velocities, but further The following steps are considered enabling for an Interstel- analysis on heat-shield performance is required. The in-flight lar Probe to the interstellar medium. performance of the thermal protection system of PSP will certainly guide the trade-off studies for an effective Oberth • Conduct a study team with the objective to use current Maneuver for Interstellar Probe. technologies to design a feasible mission to reach the in- terstellar medium by 2050 The second, and tightly coupled trade-off study should ana- • Complete trade-off studies on trajectory analysis includ- lyze the impact on mission design and reliability of a 50-year ing JGA and Oberth Maneuvers using existing launch mission. and propulsion systems; study integrated solutions for mass- and power-optimized science payload and com- A third trade-off study considers a spin- versus three-axis munication subsystems stabilized spacecraft. Spin stabilization may offer a simpler, • Publish a definitive Interstellar Probe Study Report to lower-risk solution and would achieve the requirement on om- feed in to Heliophysics, Planetary and Astrophysics Dec- ni-directional sampling of the particle and plasma instruments adal Surveys but would severely limit duty cycle on imaging during KBO • Baseline the use of GPHS RTGs used on Ulysses, Galileo, flybys and impose significant restrictions on meeting pointing Cassini and New Horizons requirements such as nadir pointing or push-broom modes. • Continue seeking international advocacy through, for On the other hand, three-axis stabilization would necessitate example through the COSPAR Panel on Interstellar Re- larger or more particle and fields instruments to increase their search FOV coverage to cover all directions by increasing the FOVs of each instrument and accommodate duplicate instruments To summarize on a more philosophical note, our drive to on opposite sides of the spacecraft. However, New Horizons span the vastness of space to where “the stars are other suns” operated in a dual mode with a three-axis stabilized spacecraft may be the ultimate and fundamental justification for embark- during the Pluto flyby, and transitioned in to a spin-stabilized ing on interstellar voyages. A near-term Interstellar Probe with mode on other occasions. With the increased maturity and re- a goal to reach 1000 AU would scout the deepening waters of liability of three-axis stabilization, the dual-mode approach is the vast humanity is about to set out on. Not only would becoming a valid option for an Interstellar Probe with KBO it bring back critical data on the interstellar environment, flybys as one of the science targets. KBOs and astrophysical phenomena, but perhaps more impor- tantly, it would necessitate a programmatic transformation to 13 SUMMARY AND NEXT STEPS ensure continual funding that goes beyond changing political priorities and spans the three NASA science divisions (helio- An Interstellar Probe to the interstellar medium would likely physics, planetary sciences and astrophysics), and to solve how be the boldest undertaking in space exploration ever done and reliability and survivability requirements are handled in a rea- would mark the historic milestone when humanity took the sonable fashion that otherwise would have made sure that such first explicit step on the expansion beyond our solar system. a mission never left the ground. An Interstellar Probe to the The Interstellar Probe Mission would break new grounds in interstellar medium is the first explicit step we take today on the frontiers of space science by obtaining the first image of the path to the stars harboring other homes.

REFERENCES 1. J. Culberson, Report from the Committee of Appropriations for FY17, 6. P. C. Liewer, R. A. Mewaldt, J. A. Ayon, & R. A. Wallace, “NASA's 114th Congress, 2nd Session, 2016. interstellar probe mission.” Paper presented at the Space Technology 2. R. L. McNutt, R. F. Wimmer-Schweingruber, M. Gruntman, S. M. and Applications International Forum, 2000. Krimigis, E. C. Roelof, P. C. Brandt, S. R. Vernon, M. V. Paul, B. W. 7. R. McNutt, “A realistic interstellar explorer.” Paper presented at the 34th Lathrop, D. S. Mehoke, D. H. Napolillo, and R. W. Stough, "Near-term COSPAR Scientific Assembly, 2002. interstellar probe: First step," Acta Astronautica 162, pp 284-299, 2019. 8. R. Mewaldt, , & P. Liewer, “Interstellar Probe - A mission to explore our 3. D. I. Fiehler, & R. L. McNutt, “Mission Design for the Innovative local neighborhood.” Paper presented at the 34th COSPAR Scientific Interstellar Explorer Vision Mission.” Journal of Spacecraft and Rockets, Assembly, 2002. 43(6), pp 1239-1247, 2012. 9. E. C. Stone, Science and enabling technologies for the exploration of the 4. R. F. Wimmer-Schweingruber, R. L. McNutt, N. A. Schwadron, P. C. interstellar medium. Retrieved from Keck Institute for Space Studies, Frisch, M. Gruntman, P. Wurz, & E. Valtonen, “Interstellar heliospheric Pasadena, California, 2015 probe/heliospheric boundary explorer mission—a mission to the 10. B. E. Wood, H. -R. Muller, G. P. Zank, V. V. Izmodenov, J. L. Linsky, outermost boundaries of the solar system.” Experimental Astronomy, 24, “The heliospheric hydrogen wall and astrospheres”, Advances in Space pp 9-46, 2009. Research 34, pp 66-73, 2004. 5. Holzer, T. E., Mewaldt, R. A., & Neugebauer, M. (1990). “The Interstellar 11. G. P. Zank, “Faltering Steps into the Galaxy: The Boundary Regions of Probe: Scientific objectives for a Frontier mission to the heliospheric the Heliosphere.” Annual Review of Astronomy and Astrophysics, 53(1), boundary and interstellar space”. Retrieved from Report of the pp1-52, 2015. Interstellar Probe Workshop. 12. S. M. Krimigis, R. B. Decker, E. C. Roelof, M. E. Hill, T. P. Armstrong,

JBIS Vol 72 No.6 June 2019 211 PONTUS BRANDT ET AL

G. Gloeckler, L. J. Lanzerotti, “Search for the Exit: Voyager 1 at Journal, 824(2), 2016. Heliosphere’s Border with the Galaxy.” Science, 341(6142), pp 144-147, 29. T. Khain, K. Batygin & M. E. Brown, “The Generation of the Distant 2013. Kuiper Belt by Planet Nine from an Initially Broad Perihelion 13. E. C. Stone, A. C. Cummings, F. B. McDonald, B. C. Heikkila, N. Lal, & Distribution.” The Astronomical Journal, 155(6), pp 250. 2018. W. R Webber, “Voyager 1 Observes Low-Energy Galactic Cosmic Rays 30. C. Shankman, J. J. Kavelaars, M. T. Bannister, B. J. Gladman, S. M. in a Region Depleted of Heliospheric Ions.” Science, 341(6142), pp 150- Lawler, T. -T. Chen, M. Jakubik, N. Kaib M. Alexandersen, S. D. 153, 2013. J. Gwyn, J. -M. Petit, K. Volk, “OSSOS. VI. Striking Biases in the 14. V. V. Izmodenov, D. B. Alexashov, “Three-dimensional kinetic-MHD Detection of Large Semimajor Axis Trans-Neptunian Objects.”, The model of the global heliosphere with the heliopause-surface fitting.” The Astronomical Journal, Volume 154, Issue 2, article id. 50, 8 pp., 2017. Astrophysical Journal Supplement Series, 220(2), 2015. 31. J. C. Becker, T. Khain, S. J. Hamilton, F. C. Adams, D. W. Gerdes, L. 15. M. Opher, A. Loeb, J. Drake, & G. Toth, “A Predicted Small and Round Zullo, D. E. S. Collaboration, “Discovery and Dynamical Analysis of an Heliosphere.” arXiv e-prints. Retrieved from https://ui.adsabs.harvard. Extreme Trans-Neptunian Object with a High Orbital Inclination.” The edu/\#abs/2018arXiv180806611O, 2018 Astronomical Journal, 156(2), pp 81, 2018. 16. N. V. Pogorelov, H. Fichtner, A. Czechowski, A. Lazarian, B. Lembege, J. 32. V. R. Eshleman, “Gravitational lens of the sun: Its potential for A. le Roux, M. Zhang, “Heliosheath Processes and the Structure of the observations and communications over interstellar distances.” Science, Heliopause: Modeling Energetic Particles, Cosmic Rays, and Magnetic 205(4411), pp 1133-1135, 1979. Fields.” Space Science Reviews, 212(1-2), pp 193-248, 2017. 33. C. Maccone, “Deep Space Flight and Communications: Exploiting 17. D. J. McComas, E. J. Zirnstein, M. Bzowski, M. A. Dayeh, H. O. Funsten, the Sun as a Gravitational Lens”, Springer, Berlin, Heidelberg, doi: S. A. Fuselier, M. Tokumaru, “Seven Years of Imaging the Global 10.1007/978-3-540-72943-3, ISBN: 978-3-540-72943-3, 2009. Heliosphere with IBEX.” The Astrophysical Journal Supplement Series, 34. S. G. Turyshev, , M. Shao, L. Alkalai, N. Aurora, D. Garber, H. Helvajian, 229(2), pp 41, 2017. H. Zhou, (2018). “Direct Multipixel Imaging and Spectroscopy of an 18. S. M. Krimigis, D. G. Mitchell, D. C. Hamilton, S. Livi, J. Dandouras, S. Exoplanet with a Solar Gravity Lens Mission.” arXiv. 2018 Jaskulek,. D. J. Williams, “Magnetosphere Imaging Instrument (MIMI) 35. G. A. Landis, “Mission to the Gravitational Focus of the Sun: A Critical on the Cassini Mission to Saturn/.” Space Science Reviews, pp 233- Analysis.” 55th AIAA Aerospace Sciences Meeting, 2017. 329, 2004. 36. P. A. Willems, “Photometric limits on the high resolution imaging of 19. K. Dialynas, S. M. Krimigis, D. G. Mitchell, R. B. Decker, & E. C. Roelof, exoplanets using the solar gravity lens,” Acta Astronautica, 152, pp 408- “The bubble-like shape of the heliosphere observed by Voyager and 414, doi: 10.1016/j.actaastro.2018.08.0132018, 2018. Cassini.” Nature Astronomy, 1(5), 2017. 37. R. L. McNutt, R. F. Wimmer-Schweingruber & the I. Team, (2011). 20. D. J. McComas, E. R. Christian, N. A. Schwadron, N. Fox, J. Westlake, “Enabling interstellar probe,” Acta Astronautica, 68(7-8), pp 790-801, F. Allegrini,. . . E. J. Zirnstein, “Interstellar Mapping and Acceleration 2011. Probe (IMAP): A New NASA Mission.” Space Science Reviews, 214(8), pp 116, 2018. 38. D. G. Mitchell, P. C. Brandt, J. H. Westlake, S. E. Jaskulek, G. B. Andrews & K. S. Nelson, “Energetic particle imaging: The evolution of techniques 21. M. J. Kuchner, & C. C. Stark, “Collisional Grooming Models of the in imaging high‐energy neutral atom emissions.” Journal of Geophysical Kuiper Belt Dust Cloud.” The Astronomical Journal, 1007, 2010. Research: Space Physics, 121(9), pp 8804-8820, 2016 22. C. C. Stark, & M. J. Kuchner, “A New Algorithm for Self-consistent 39. D. C. Reuter, S. A. Stern, J. Scherrer, D. E. Jennings, J. W. Baer, J. Hanley, Three Dimensional Modelling of Collisions in Dusty Debris Disks.” The L. A. Young, “Ralph: A Visible/Infrared Imager for the New Horizons Astrophysical Journal, 543, 2009. Pluto/Kuiper Belt Mission.” Space Science Reviews, 140, pp 129-154, 23. A. R. Poppe, “An improved model for interplanetary dust fluxes in the 2008. outer Solar System.” Icarus, 264, pp 369-386, 2016. 40. A. F. Cheng, H. A. Weaver, S. J. Conard, M. F. Morgan, O. Barnouin- 24. E. Bailey, M. E. Brown, & K. Batygin, “Feasibility of a Resonance-based Jha, J. D. Boldt, . . . H. W. Taylor, (2008). “Long-Range Reconnaissance Planet Nine Search.” The Astronomical Journal, 156(2), pp 74, 2018. Imager on New Horizons.” Space Science Reviews, 140, pp 189-215, 25. K. Batygin, & M. E. Brown, “Evidence for a Distant in the 2008. Solar System.” The Astronomical Journal, 151(2), pp 22, 2016. 41. J. Bock, C., Beichman, A., Cooray, W., Reach, R.-R., Chary, M., Werner, 26. K. Batygin, & M. E. Brown, “Generation of Highly Inclined Trans- & M. Zemcov, “Astronomical opportunities from the outer solar Neptunian Objects by Planet Nine.” The Astrophysical Journal, 833(1), system.” SPIE Newsroom, 2012. 2016 42. M. I. Desai, K. Ogasawara, R. W. Ebert, F. Allegrini, D. J. McComas, 27. K. Batygin & A. Morbidelli, “Dynamical Evolution Induced by Planet S. Livi & S. E. Weidner, “Compact Dual Ion Composition Experiment Nine.” The Astronomical Journal, 154(6), pp 229, 2017. for space plasmas—CoDICE.” Journal of Geophysical Research: Space Physics, 121(7), pp 6632-6638, 2016. 28. M. E. Brown & K. Batygin, “Observational Constraints on the Orbit and Location of Planet Nine in the Outer Solar System,” The Astrophysical 43. H. Oberth, Wege zur Raumschiffahrt, R. Oldenbourg Verlag, Munich- Berlin, 1929.

Received 18 June 2018 Approved 30 March 2019

212 Vol 72 No.6 June 2019 JBIS CALL FOR PAPERS

Putting Astronauts in Impossible Locations A one-day technical symposium

9:00 a.m–5.30pm, Wednesday 27 November 2019, BIS HQ, 27/29 South Lambeth Road

While the human exploration of the Moon and Mars has been extensively examined, serious technical consideration of the rest of the solar system has been largely ignored. This symposium is designed to explore the limits of where human exploration can go in the solar system and how to overcome the challenges involved. The symposium has a variety papers on transportation requirements, the practicalities of habitation in extreme environments and other aspects of a solar system-wide civilisation.

Programme

Prospects for Human Expansion into the Solar System – Bob Parkinson

Is There a Safe Haven Somewhere in Heaven? – Reinhold Ewald

Conceptual Design of a Manned Platform in the Venusian Atmosphere – Markus Graß, Marius Schwinning and Reinhold Ewald

Manned Mission to Asteroid Ryugu to Set Up Telescope – William Chin and Jitkai Chin

Missions to the Edge of the Visible – Stephen Baxter

Cycler Links between Earth and the Planets – Stephen Ashworth

Human Health on a Mission to Titan – Victoria Lee

Population Capacity of Outer Planetary Orbits – Mark Hempsell Journal of the British Interplanetary Society

VOLUME 72 NO.6 JUNE 2019

www.bis-space.com

ISSN 0007-084X PUBLICATION DATE: 29 AUGUST 2019