Here are abstracts of the papers that have been accepted for presentation at the 6th Interstellar Symposium and Advanced Interstellar Propulsion Workshop. As more papers are accepted, they will be added here. Check often for additions!

Calculation and Analysis of the Curvature Invariants for Traversable Lorentzian Wormholes and for Warp Metrics

Author: Gerald Cleaver, Ph.D. Early Universe Cosmology & String Theory, Professor and Graduate Program Director, Dept. of Physics & Center for Astrophysics, Baylor University

Abstract Background: In their classic papers, Morris and Thorne and Morris, Thorne, and Yurtsever were the first to analyze traversability of a classical wormhole. They studied the question of what the properties of a classical wormhole would have to be in order for the wormhole to be traversable by a human without fatal effects on the traveler.

Abstract Objectives: We present a process for applying the full set of spacetime curvature invariants as a new means to evaluate the traversability of Lorentzian wormholes and also of warped spacetime manifolds. This approach was formulated by Henry, Overduin, and Wilcomb for black holes.

Abstract Methods: Curvature invariants are independent of coordinate basis, so the process is free of coordinate mapping distortions and the same regardless of your chosen coordinates. The thirteen independent G’eh’eniau and Debever (GD) invariants are calculated for a given metric and the non-zero, independent curvature invariant functions are plotted and displayed. Four example traversable wormhole metrics are investigated: (i) thin shell at-face, (ii) spherically symmetric Morris and Thorne, (iii) thin-shell Schwarzschild, and (iv) Levi-Civita. We similarly calculate and display the curvature invariants for Alcubierre and Natario Warp Drive metrics. Both the constant velocity and accelerating Alcubierre and Natario metrics are presented.

Abstract Results: The wormholes are shown not to contain within physical bounds any internal divergences. The invariants plots demonstrate very non-trivial and non-intuitive time evolution dynamics of the warp bubbles.

Abstract Conclusions: The wormholes investigated are (at least theoretically) traversable.

Power System for Miniature Interstellar Flyby Probe

Author: Geoffrey Landis, PhD, Researcher, NASA John Glenn Research Center

Abstract Background: In the last few years,the concept that an ultra-lightweight probe could be sent to one of the nearby stars pushed by a laser beam reflecting from a lightweight sail has moved from science fiction into conceptual design. The Breakthrough Starshot project envisions a two- to three-gram “starchip” micro probe, flying past a planet of Proxima Centauri after a 20 year voyage.

With the probe moving at 60,000 km/sec, the flyby encounter at the target planet lasts at most a few hours. With current technology, no power system exists that can produce the required power with a mass of less than one gram.

Abstract Objectives: Baseline requirements for the power system are:

Weight: 1 gram or less

Lifetime: 25 year cruise, followed by encounter phase.

The operational lifetime during the encounter phase can be a trade-off against the power level. The baseline requirement is 1 watt peak, 10 mW continuous, with higher power levels desirable.

Abstract Methods: Several power systems were analyzed.

Abstract Results: Radioisotope Power

Analysis shows that radioisotope thermal power system scale poorly to small sizes, and would be four to five orders of magnitude too heavy for such a microprobe.

An alternative proposal would be to use direct energy conversion, rather than thermal conversion. Betavoltaic decives scale well to low power levels. Betavoltaic cells have future specific power anticipated at 1 W/kg. The half life of tritium, 12.3 years, results in decay to about 25% of baseline power during the cruise.

An alternative uses the energy of alpha particles from spontaneous fission. Isotopes include plutonium-238 or curium 244. Semiconductor alphavoltaic converters, however, are subject to alpha-induced degradation, and may not make the lifetimes required.

Advanced Technology

The proposed flyby spacecraft has a kinetic energy of 10 million megajoules per gram. We can use the energy of the spacecraft”™s motion through the interplanetary medium by using the ambient plasma and magnetic environments. The anticipated density of solar-wind generated plasma is ~6E12 protons per cm2 at an energy of ~5 MeV. Thiscould be turned into power with an electrostatic grid. Alternatively, for a target magnetic field of 1 nT, we can create an electric field by the spacecraft”™s motion of about 60 volts per meter to generate power.

Abstract Conclusions: The power system for a small (Starshot-sized) interstellar probe is a major component that has, to date, not been well analyzed. The low mass requirement makes the problem very difficult. Several approaches to this power system are possible.

Inevitability, Adaptablity, Destiny: Religious and Non-Religious Arguments for a Human Future in Outer Space

Author: Deana Weibel, Ph.D., Professor of Anthropology and Religious Studies, Grand Valley State University

Abstract Background: As an anthropologist of religion I have been studying sacred places and religious travel for more than two decades. Recent work has explored the religious aspects of space exploration. Here I consider space exploration as having religious elements for some, but not all, individuals doing “space work”. My research combines elements of the anthropology of pilgrimage with the anthropology of space exploration.

Abstract Objectives: My objective is to understand the place of religion as humans move forward in their understanding and exploration of space. My premise is that religion is a universal in nearly all studied societies, and that the ubiquity of religion means that religion will likely be a part of humanity’s future in space. Therefore, it is important to understand how religious and scientific ideas about “the heavens” impact each other in individuals. Understanding how humanity’s “destiny” is conceived, both religiously and non-religiously, among contemporary “space workers” will clarify how the concept of destiny may motivate future space travelers.

Abstract Methods: As a cultural anthropologist, my methods are qualitative and ethnographic, based on participant-observation (essentially “embedding” myself) and conducting interviews with people whose work is connected to space, such as astronomers, engineers, astronauts, practitioners of space medicine, etc. Ethnographic research sites include NASA workshops, space centers, public presentations, laboratories, universities, the Mojave Space Port and the Vatican Observatory.

Abstract Results: Preliminary results indicate that people involved in space exploration often draw on ideas of an inescapable “destiny” when discussing the future of humans in space. Many of these ideas draw from religious scripture (including the Bible, the Qur’an and the Vedas), but even in the non-religious, a sense of an “inevitable” human destiny in space prevails. A high level of scientific knowledge seems to make participants more confident in humanity’s ability to adapt to life in space.

Abstract Conclusions: I conclude that a belief that humans are destined to have a future living in outer space is a powerful motivator for both the religious and non-religious alike. This belief encourages problem-solving in many fields, giving “space workers” confidence that any difficulties will be overcome, a belief that may be of great benefit in human space exploration.

Strategies for Mitigation of Dust and Charged Ion Impact on Laser-Driven Lightsails

Author: Andrew Higgins, PhD, Aeronautics and Astronautics, Professor, McGill University

Abstract Background: The impact of interplanetary and interstellar dust grains on lightsails is a significant concern for laser-driven interstellar flight. Over a light year of travel, the mean spacing between impact sites on forward-facing surfaces is estimated to be on the order of 100 microns. The fundamental particles of the interstellar medium (ISM: protons, alpha particles, etc.) are also of concern. A particular source of apprehension are impacts on the sail in near-earth space (estimated mean spacing of impact sites ~ 1 mm) which could degrade the low absorptivity/high reflectivity requirements of the sail. In the worst case, laser energy deposition could couple to the sail material, resulting in near-instantaneous destruction of the sail, similar to the well- known phenomena of “fiber fuze” and “laser-supported detonation”.

Abstract Objectives: This study will critically examine a number of strategies to minimize or eliminate the threat to the sail presented by the dust grain and ion impact problems: (1) Use of the drive laser as a means to displace or vaporize interplanetary dust in advance of the sail during the acceleration phase. (2) Design of a “fault-tolerant” sail that can withstand local catastrophic failure of the sail due to laser-coupling, but does not propagate to adjacent regions of the sail. (3) For the interstellar cruise phase (when the sail flies edge-on to the ISM), the use of graded materials (bilayers) on the leading edge to act as channels to direct ions outward from the main sail.

Abstract Methods: Given the embryonic nature of the techniques considered here, the modelling is done via first-order analytic models, including accepted models for laser ablation, laser-supported detonation propagation, and charge particle penetration.

Abstract Results: Dust removal or vaporization in the volume the light sail will traverse during the acceleration phase does not appear feasible due to the large volume that would need to be cleared. Displacement of dust via the laser light transmitted through the sail, as would be the case with thin dielectric sails, may be feasible. A fault-tolerant sail that prevents laser-supported destruction from propagating across the sail appears possible but may necessitate large gaps in the sail, resulting in wasting much of the laser illumination. Charged particle re-direction via graded materials is an established technology that has been demonstrated experimentally in the particle accelerator community.

TRL Assessment: The present level of the technologies considered in this study are Level 1. Completion of the analyses presented will contribute to elevating the TRL to 2-3.

Abstract Development: The talk will conclude with a proposed roadmap of a progressive hierarchy of models and laboratory validation to evolve the more promising of the proposed approaches to TRL 4.

Abstract Near-Term Technical Milestones: Bench-top demonstration of CW-laser-driven ablation of dust should be feasible with 1 W lasers. Demonstration of the ability to prevent propagation of laser-supported sail destruction would necessitate use of 1kW-class lasers. Charged particle re-direct

Abstract Conclusions: The first-order analysis proposed here suggests that the concepts have sufficient potential to warrant additional analysis. It is hoped that this preliminary analysis will stimulate further thinking about nonconventional solutions to the dust grain impact problem.

Antimatter-Based Interstellar Propulsion

Author: Gerald Jackson, Ph.D. Physics, Co-Founder and President, Hbar Technologies, LLC

Abstract Background: While antimatter-based propulsion concepts have been proposed for several decades, the limited production of antimatter and its storage difficulties have retarded their development. Dr. Steven Howe identified an antimatter niche for the acceleration of small unmanned interstellar probes, a concept funded by NIAC starting in 2002, wherein the antimatter was used to initiate fission events whose daughters provided thrust.

Abstract Objectives: The purpose of this research is to improve the Howe concept by focusing all fission daughters into a coherent exhaust stream, thereby reducing the amount of antimatter needed and enabling spacecraft velocities as high as 0.1c.

Abstract Methods: The first step was to critically evaluate antimatter-based propulsion in light of the rocket equation, which pointed to the induction of fission as the most efficient use of antimatter. The second step was to identify a particle accelerator architecture coupled with a focusing system that mixed antimatter with depleted uranium while simultaneously allowing both fission daughters to escape into the focused exhaust stream. The third step was to generate an unmanned scientific Proxima Centauri mission profile that decelerates and orbits Proxima b, returning data for decades. The fourth step was to generate a plan to synthesis antimatter at the rate needed to enable such a mission.

Abstract Results: Given that the maximum exhaust velocity of fission daughters is only 0.046c, a spacecraft velocity of 0.1c requires 33g of antimatter for every kilogram of spacecraft dry mass. If the spacecraft velocity were reduced to 0.05c the amount of needed antimatter drops to 8g. A plan for producing antimatter at a rate of 10g/year with accompanying cost estimate has been developed.

TRL Assessment: The propulsion concept is based on experimentally validated accelerator and particle physics experience: TRL 3. Enhanced antimatter production consistent with a Proxima Centauri mission is a large extrapolation of experimental work performed at several laboratories: TRL 4.

Abstract Development: The critical path is demonstrating enhanced antimatter production rates. An experimental program has been developed.

Abstract Near-Term Technical Milestones: Generate a technical design report for the first enhanced antimatter production experiment validating technology and production costs.

Abstract Conclusions: Interstellar antimatter-based propulsion at 0.1c and kilogram-scale is feasible and experimentally validated. Demonstration of economic feasibility is required.