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The technology for even the most advanced missions in the solar system doesn't need advances in basic science. Traveling through the solar system can be described through what is called ‵hard science fiction', i.e. science fiction strictly based on scientific knowledge. Interstellar exploration is a completely different matter. Robotic flyby missions to the nearest stars using nanoprobes can be performed using technologies based on known science, while anything beyond this requires advances which we don't know how to implement, or even we are not sure whether they are possible at all. The point applies not only to the technological aspects but even the scientific bases on which the relevant technologies may rest. The missions requiring less scientific-technological advances, are slow missions, like space arks (generation ships) or missions based on hibernation with travel times up to hundred years. To implement both, the uncertainties are more related to the advances in space medicine and biology than in propulsion and physics. The fastest travels allowed by the current interpretations of the relativity theory are relativistic missions in which the time contraction at speeds closing the speed of light is exploited to decrease the travel time for the astronauts, although the travel time seen by those who remain on Earth is close, in years, to the distance traveled expressed in light years. However, the energy required for this type of missions is large and grows drastically with the increase of time contraction. Faster than light travel, which seems to be possible following some interpretations of relativity involving either wormholes or some sort of warp drive, requires substantial advances in fundamental physics. A symptom of this is that the novels dealing with interstellar travels belong more to the space opera – which doesn't follow strict scientific credibility – than to the hard science fiction subgenre. No novel of this kind explains details about how the relevant machinery works, and even less scientifically realistic are the movies, TV series, and videogames of this kind. Moreover, to achieve a travel time allowing to reach distant star systems in reasonable times using warp drives, the authors of Star Trek had to resort to the Warp Factor which is essentially a nonlinear scale. This makes the requirements for FTL travel even more difficult to achieve. • Interstellar travel requires advances in technology and, in some cases, in science. • Generation ships, which travel for centuries, require only technological advances. • Slow travel with hibernated crew is simpler but requires to develop long term hibernation. • Faster than light (FTL) travel might be not incompatible with present day science. • Interstellar travel as fast as in science fiction, requires speeds hundreds times FTL. [ABSTRACT FROM AUTHOR]

• Interstellar exploration is defined as bi-objective multi-vehicle problem with profits. • Mission return increases approximately linearly with mission duration. • Lower probe numbers enable more efficient routing through shorter transfers. In the past decade, the discovery of exoplanets has sparked new interests in the idea of interstellar travel and exploration. Despite various proposals for probe concepts and relevant technologies, there is a lack of extensive literature on viable exploration strategies for journeys beyond a single star system. Such exploration strategies might not only have implications on technology development strategies for achieving interstellar exploration but could also enrich existing models for galactic exploration feeding into solutions of the Fermi Paradox. This article presents optimal strategies for the exploration of a large number of near-by stars, using a dedicated, novel methodology, which sets it apart from existing literature: For the first time, the mission design problem of interstellar exploration is redefined as a bi-objective multi-vehicle open routing problem with profits. It is tackled by an adapted hybrid multi-objective genetic algorithm, which is further improved and modified according to the problem characteristics (e. g. large search space). The overall mission model assumes probes traveling on straight trajectories, utilizing flybys, and maintaining an average velocity of 10% of the speed of light. Surpassing prior research that typically relies on statistical models or restricted star data, the star models are founded on the second Gaia data release (Gaia DR2), which represents the most extensive star catalogue to the date of this study and is employed for the first time in the context of interstellar exploration. The resulting star model contains a maximum of 10,000 stars within a spherical region around Sol, covering a distance of 110 light years. It is found, that the number of explored stars J 1 scales with mission duration J 2 and probe number m according to J 1 ∼ J 2 m 0.66 , which provides an initial guidance for future interstellar mission design. Furthermore, the routes and selection of stars vary depending on the number of probes used: When conducting missions with a large number of probes, stars in close proximity to the Solar System are given more focus. On the other hand, missions with a small number of probes include more distant stars to facilitate shorter transfers along the route. Based on these findings, the following recommendations for interstellar exploration strategies can be drawn: When energy resources such as fuel reserves are scarce and the exploration mission is not limited to nearby stars, low probe numbers are more efficient. In contrast, high probe numbers enable faster exploration of nearby stars but involve less resource-efficient transfers, making them a suitable option for small, remotely propelled probe concepts. To address crowding effects in high probe number missions, swarm-based probe concepts are recommended based on the scaling law characteristics derived. [ABSTRACT FROM AUTHOR]

A propulsion concept in which a spacecraft interacts with both a stream of high-velocity macroscopic pellets and the mass of the interplanetary or interstellar medium is proposed. Unlike previous pellet-stream propulsion concepts, the pellets are slower than the spacecraft and are accelerated backwards as they are overtaken by it, imparting a forward acceleration on the spacecraft. This maneuver is possible due to the interaction with a fixed medium (e.g., interstellar medium); as the spacecraft travels through the medium, it is able to extract power from the relative wind blowing over the spacecraft. As viewed from the rest frame, the kinetic energy of the pellets is transferred to the spacecraft; the source of energy for the propulsive maneuver (i.e., whether the source is the pellets or the wind) is thus reference-frame dependent. This concept relies upon the relative velocities (or shear) between the pellet stream and the fixed medium in order to concentrate the energy of the pellets into the spacecraft and is therefore termed wind–pellet shear sailing. The equations governing the mass ratio of pellets to the spacecraft and its dependence on the final spacecraft velocity are derived, analogous to the classical rocket equation; the critical role of the efficiency of the power extraction and transfer process is identified. Natural sources of energy (photon and particle fluxes from the sun) are considered as a means to accelerate the pellets, produced from in-situ sources of material in the solar system, to velocities of 1000 to 6000 km/s. Techniques for onboard generation of power via electromagnetic interaction with the interstellar medium (ISM) are reviewed, with a repetitively stroked plasma magnet being identified as a promising approach. The necessity of the spacecraft to detect and track the pellets as they are overtaken dictates the desired properties of the pellets. Pellet pushers (i.e., accelerators) on board the spacecraft are also preliminarily explored in their engineering considerations, with electric field accelerators of charged nanometric particles, Lorentz-force accelerated ionized pellets, or expansion of vaporized pellets via a nozzle being highlighted as potential approaches. A preliminary mission profile is defined in which a 500-kg scientific payload is delivered to orbit about α -Centauri, using wind–pellet shear sailing as the intermediate stage to bring the spacecraft from 2% to 5.5% of c , with a total mission duration (launch to arrival) of 27 years. The concept design illustrates the potential for synergy at the low-velocity end with advanced solar photon or solar-wind sailing and at the high-velocity end with the q -drive concept. • A new concept is proposed: a spacecraft overtakes pellets, transferring their energy to the spacecraft. • The concept exploits the use of free energy sources available in space. • Analysis suggests spacecraft may be accelerated from 2% to 5% of the speed of light. • The technique may be combined with other propulsion devices to achieve greater speeds. • A mission to return data from nearby stars within 30 years of launch may be possible. [ABSTRACT FROM AUTHOR]

Demonstrations of photonically-propelled materials for solar and laser sailing have relied on indirect techniques for determining the propulsion efficiency. These methods, utilizing drop towers or cameras to determine differential displacement, are necessarily low-throughput and cannot be easily compared directly. Furthermore, the methods require sample sizes that are prohibitive to the fabrication of low TRL photonic materials (e.g. thin-film metasurfaces) that discourages rapid iterative testing. Herein, a nanocantilever-based test bed is proposed to simplify and standardize the determination of the characteristic acceleration for a given sail swatch. The proposed system is designed to allow for flexible, automated spectral analysis, requiring sample sizes ∼10 μm2. This system is scalable to the micro- and macroscale as material technologies mature and advance toward deployment scale. • Laser sources irradiating cantilever systems lead to measurable force deflection. • Quantify spectral characteristics of light sails for mission qualification. • Sail candidate materials may be directly tested for failure. • Requires swatches on the order of μm2. • Allows for high-throughput iteration. [ABSTRACT FROM AUTHOR]

Civilizations ahead of our own may wish to control their star system's motion for a variety of reasons, ranging from the avoidance of cataclysmic cosmic events to the facilitation of interstellar, and perhaps intergalactic, colonization. Such an endeavor can be achieved via the construction of megastructures known as stellar engines, which utilize a star's own resources to gradually accelerate the star system in the desired direction over extended periods of time. Here, we conceptualize the 'Star Tug', a stellar engine capable of producing significantly higher accelerations than previous stellar engine concepts. The Star Tug consists of an engine positioned ahead of the star along the desired direction of acceleration. The engine is gravitationally bound to the star and utilizes mass acquired from the star as propellant to generate thrust which counterbalances the force of gravity between it and the star. As long as the propellant exhaust velocity exceeds the escape velocity of the star system, the star system should accelerate. Depending on efficiency and engineering constraints, the Star Tug may be capable of accelerating a sun-like star to relativistic velocities within megayear time scales. • Stellar engines are structures that manipulate the motion of entire star systems. • The 'Star Tug' can produce higher accelerations than prior stellar engine concepts. • The Star Tug may be able to accelerate a sun-like star to relativistic velocities. [ABSTRACT FROM AUTHOR]

Stellar engines, megastructures used to control the motion of a star system, may be constructible by technologically advanced civilizations and used to avoid dangerous astrophysical events or transport a star system into proximity with another for colonization. This work considers two designs for stellar engines, for both human applications in the solar system and for advanced civilizations around arbitrary stars more generally, and presents analytic calculations of the maximum acceleration and deflection of a star in its galactic orbit. The first is a large 'passive' solar sail, similar to that proposed by Shkadov, which we find produces accelerations of order 10 − 12 m / s 2 for sun-like stars. The second 'active' engine uses a thermonuclear driven jet, as in a Bussard ramjet, which collects matter from the solar wind to drive He fusion. This engine requires additional mass to be lifted from the sun, beyond what is provided by the nascent solar wind, but may achieve accelerations up to 10 − 9 m / s 2 producing deflections of 10 pc in as little as 1 Myr for a sun-like star. While passive engines may be insufficient for catastrophe avoidance on short timescales, they can produce arbitrary deflections of a star in its galactic orbit over a stellar lifetime. Active engines are sufficient for retrograde galactic orbits or galactic escape trajectories, which we argue are useful to expansionist civilizations. These populations of stars may be candidates for observationally detecting megastructures. • Stellar engines may be used to arbitrarily manipulate a star's galactic orbit. • Solar sails, as in Shkadov, produce large orbital deflections on gigayear timescales. • Thermonuclear ramjets produce large orbital deflections on megayear timescales. • Hypervelocity or retrograde stars may be candidates for observing megastructures. [ABSTRACT FROM AUTHOR]

Abstract Propellantless propulsion is believed to be the best option for interstellar travel. However, photon rockets or solar sails have thrusts so low that maybe only nano-scaled spacecraft may reach the next star within our lifetime using very high-power laser beams. Since 2012, a dedicated breakthrough propulsion physics group was founded at the Institute of Aerospace Engineering at TU Dresden to investigate different concepts based on non-classical/revolutionary propulsion ideas that claim to be at least an order of magnitude more efficient in producing thrust compared to photon rockets. Most of these schemes rely on modifying the inertial mass, which in turn could lead to a new propellantless propulsion method. Our intention is to develop an excellent research infrastructure to test new ideas and measure thrusts and/or artefacts with high confidence to determine if a concept works and if it does how to scale it up. At present, we are focusing on two possible revolutionary concepts: The EMDrive and the Mach-Effect Thruster. The first concept uses microwaves in a truncated cone-shaped cavity that is claimed to produce thrust. Although it is not clear on which theoretical basis this can work, several experimental tests have been reported in the literature, which warrants a closer examination. We are building several models of different sizes to understand scaling laws and the interaction with the test environment. The second concept is theoretically much better understood and is believed to generate mass fluctuations in a piezo-crystal stack that creates non-zero time-averaged thrusts. Apart from theoretical models, we are testing and building several such thrusters in novel setups to further investigate their thrust capability. In addition, we are performing side-experiments to investigate other experimental areas that may be promising for revolutionary propulsion. To improve our testing capabilities, several cutting-edge thrust balances are under development to compare thrust measurements in different measurement setups to gain confidence and to identify experimental artefacts. Highlights • Thorough investigation of breakthrough propulsion concepts. • Thrust balance development to test EMDrive and Mach-Effect thrusters. • Complementary experiments to test physics behind claims. • Education project towards interstellar spaceflight. [ABSTRACT FROM AUTHOR]

Abstract In traditional form-finding of a deployable mesh reflector (DMR), the nodes of the DMR mesh are placed on the desired working surface and the surface accuracy of the DMR is measured either by the deviation of the nodes from the desired working surface or by the deviation of the mesh from its best-fit surface. Placement of nodes on working surface and inaccurate measures of surface accuracy cause non-negligible surface errors that cannot be further reduced. To deal with these issues and to further improve surface accuracy of DMRs, a new mesh geometry design method, called the Projecting Surface Method (PSM), is presented in this paper. The highlight of the PSM is that it purposely places the nodes of a DMR off its working surface, to achieve higher surface accuracy. To this end, a direct RMS error measuring the deviation of a DMR mesh from its desired working surface is introduced and a projecting surface for hosting the nodes of the DMR mesh is defined. By the direct RMS error and projecting surface, an optimization process produces a mesh geometry with its best-fit surface closest to the desired working surface, leading to significant surface error reduction. As shown in numerical examples of DMRs with 37, 271 and 817 nodes, the PSM can reduce surface errors by 50% or more. The proposed method is usable with existing form-finding methods for further improvement of surface accuracy of DMRs. Highlights • A newly defined RMS error for precise measurement of surface accuracy. • Establishment of a projecting surface for form-finding of mesh reflectors. • Purposely placing mesh nodes off the working surface of a reflector in design. • Significant reduction of surface errors (by 50–97%). • Usable with any existing form-finding method for further surface error reduction. [ABSTRACT FROM AUTHOR]

Abstract Two technological problems must be solved before daring to interstellar flight: fuel and propulsion. The highest energy-density 'fuel' is antimatter in its solid or liquid state and this fuel is likely to be our primary choice for multi-ton relativistic rockets. High-energy ion thrusters powered by annihilation reactors promise superior performance in comparison with direct propulsion by annihilation products. However the power generator onboard can significantly enlarge the rocket dry mass thus limiting the achievable speed. Two physical factors that stand against our dream of the stars are thermodynamics and radiation hazard. Heat-disposing radiator also increases the rocket dry mass. Interstellar gas turns into oncoming flux of hard ionizing radiation at a relativistic speed of the rocket while the oncoming relativistic interstellar dust grains cause mechanical damage. Economy and psychology will play a decisive role in voting for or against the manned interstellar flights. Highlights • Technical challenges are discussed. • High-energy ion thruster is our only option. • Achievable rocket speed as a function of exhaust mass is estimated. • Limiting physical and social factors are indicated. • Robotic rocket is argued to be preferred. [ABSTRACT FROM AUTHOR]

As the major part of space life supporting systems, artificial gravity requires further study before it becomes mature. Spinning tether system is a good alternative solution to provide artificial gravity for the whole spacecraft other than additional devices, and its longer tether length could significantly reduce spinning velocity and thus enhance comfortability. An approximated overload-based feedback method is proposed to provide estimated spinning velocity signals for controller, so that gravity level could be accurately controlled without complicated GPS modules. System behavior in high eccentricity transfer orbits is also studied to give a complete knowledge of the spinning stabilities. The application range of the proposed method is studied in various orbit cases and spinning velocities, indicating that it is accurate and reliable for most of the mission phases especially for the final constant gravity level phase. In order to provide stable gravity level for transfer orbit missions, a sliding mode controller based on estimated angular signals is designed for closed-loop control. Numerical results indicate that the combination of overload-based feedback and sliding mode controller could satisfy most of the long-term artificial gravity missions. It is capable of forming flexible gravity environment in relatively good accuracy even in the lowest possible orbital radiuses and high eccentricity orbits of crewed space missions. The proposed scheme provides an effective tether solution for the artificial gravity construction in interstellar travel. [ABSTRACT FROM AUTHOR]

While special relativity imposes an absolute speed limit at the speed of light, our Universe is not empty Minkowski spacetime. The constituents that fill the interstellar/intergalactic vacuum, including the cosmic microwave background photons, impose a lower speed limit on any object travelling at relativistic velocities. Scattering of cosmic microwave photons from an ultra-relativistic object may create radiation with a characteristic signature allowing the detection of such objects at large distances. [ABSTRACT FROM AUTHOR]

Nanoscale spacecrafts have shed light on an audacious but promising vision for interstellar travel to neighboring planets beyond the solar system. The nanocrafts can be accelerated to a significant fraction of light speed, which leads to the possibility of completing the otherwise millennia space missions using the current rocket propulsion technology in several decades. However, electrical energy is a severe challenge for long-term interstellar travel. Mechanical energy metamaterials have been proposed as a promising technology integrated multiscale architected structures with energy materials, which have opened an exciting venue for energy harvesting under extreme conditions such as the cosmic environment. Mechanical energy metamaterials are envisioned as an alternative energy solution to address the power issue of nanocrafts in interstellar travel. Here, this article comprehensively overviews the debut and development of mechanical energy metamaterials and discusses the potential and application paradigm for energy harvesting in the cosmic environment. We first clarify the main feature and functionality of mechanical metamaterials and summarize the recent findings on typical mechanical energy metamaterials. Next, we identify the characteristics and challenges of the cosmic environment, deliberate the fundamental energy issue of interstellar travel, and examine the stringent criteria to design mechanical energy metamaterial nanosails for electrical power. Discussion indicates that kinetic energy resulted in cosmic dust grains collision and photovoltaic energy from starlight can be the main energy sources for mechanical energy metamaterials. In the end, we propose the applications of mechanical energy metamaterial nanosails in the nano space energy systems (nano-SES) to address the energy challenges in interstellar travel by generating electrical power from the kinetic and photovoltaic energies. [ABSTRACT FROM AUTHOR]

The possibilities of interstellar flights for extraterrestrial civilizations have been considered. A superluminal motion (hypermotion) via M. Alcubierre's warp drive is considered. Parameters of the warp drive have been estimated. The equations of starship geodesics have been solved. The starship velocity has been shown to exceed the speed of light, with the local velocity relative to the deformed space-time being subluminal. Hawking's radiation does not prove to affect the ship interior considerably. Difficulties related to a practical realization of the hypermotion are indicated. [ABSTRACT FROM AUTHOR]

Light sail-based propulsion systems are a candidate technology for interplanetary and interstellar missions due to their flexibility and the fact that no fuel has to be carried along. In 2014, the Initiative for Interstellar Studies (i4is) hosted the Project Dragonfly Design Competition, which aimed at assessing the feasibility of sending an interstellar probe propelled by a laser-powered light sail to another star system. We analyzed and designed a mission to the Alpha Centauri system, with the objective to carry out science operations at the destination. Based on a comprehensive evaluation of currently available technologies and possible locations, we selected a lunar architecture for the laser system. It combines the advantages of surface- and space-based systems, as it requires no station keeping and suffers no atmospheric losses. We chose a graphene-based sandwich material for the light sail because of its low density. Deceleration of the spacecraft sufficient for science operations at the target system is achieved using both magnetic and electric sails. Applying these assumptions in a simulation leads to the conclusion that 250 kg of scientific payload can be sent to Alpha Centauri within the Project Dragonfly Design Competition's constraints of 100 year travel duration and 100 GW laser beam power. This is only sufficient to fulfill parts of the identified scientific objectives, and therefore renders the usefulness of such a mission questionable. A better sail material or higher laser power would improve the acceleration behavior, an increase in the mission time would allow for larger spacecraft masses. [ABSTRACT FROM AUTHOR]

Interplanet and interstellar travels require long-term propulsion of spacecrafts, whereas the conventional schemes of propulsion are limited by the velocity of the ejected mass. In this study, alpha particles released by nuclear decay are considered as a potential solution for long-time acceleration. The principle of near-light-speed particle propulsion (NcPP) was elucidated and the stopping and range of ions in matter (SRIM) was used to predict theoretical accelerations. The results show that NcPP by means of alpha decay is feasible for long-term spacecraft propulsion and posture adjustment in space. A practical NcPP sail can achieve a speed >150 km/s and reach the brink of the solar system faster than a mass equivalent solar sail. Finally, to significantly improve the NcPP sail, the hypothesis of stimulated acceleration of nuclear decay (SAND) was proposed, which may shorten the travel time to Mars to within 20 days. [ABSTRACT FROM AUTHOR]

Abstract: Designing interstellar starships for human migration to exoplanets requires establishing the starship population, which factors into many variables including closed-ecosystem design, architecture, mass and propulsion. I review the central issues of population genetics (effects of mutation, migration, selection and drift) for human populations on such voyages, specifically referencing a roughly 5-generation (c. 150-year) voyage currently in the realm of thought among Icarus Interstellar's Project Hyperion research group. I present several formulae as well as concrete numbers that can be used to help determine populations that could survive such journeys in good health. I find that previously proposed such populations, on the order of a few hundred individuals, are significantly too low to consider based on current understanding of vertebrate (including human) genetics and population dynamics. Population genetics theory, calculations and computer modeling determine that a properly screened and age- and sex-structured total founding population (N c ) of anywhere from roughly 14,000 to 44,000 people would be sufficient to survive such journeys in good health. A safe and well-considered N c figure is 40,000, an Interstellar Migrant Population (IMP) composed of an Effective Population [N e ] of 23,400 reproductive males and females, the rest being pre- or post-reproductive individuals. This number would maintain good health over five generations despite (a) increased inbreeding resulting from a relatively small human population, (b) depressed genetic diversity due to the founder effect, (c) demographic change through time and (d) expectation of at least one severe population catastrophe over the 5-generation voyage. [Copyright &y& Elsevier]

Abstract: The Fermi paradox is the discrepancy between the strong likelihood of alien intelligent life emerging (under a wide variety of assumptions) and the absence of any visible evidence for such emergence. In this paper, we extend the Fermi paradox to not only life in this galaxy, but to other galaxies as well. We do this by demonstrating that travelling between galaxies – indeed even launching a colonisation project for the entire reachable universe – is a relatively simple task for a star-spanning civilisation, requiring modest amounts of energy and resources. We start by demonstrating that humanity itself could likely accomplish such a colonisation project in the foreseeable future, should we want to. Given certain technological assumptions, such as improved automation, the task of constructing Dyson spheres, designing replicating probes, and launching them at distant galaxies, become quite feasible. We extensively analyse the dynamics of such a project, including issues of deceleration and collision with particles in space. Using similar methods, there are millions of galaxies that could have reached us by now. This results in a considerable sharpening of the Fermi paradox. [Copyright &y& Elsevier]

Abstract: Advanced civilizations capable of interstellar travel, if they exist, are likely to have advanced propulsion methods. Spaceships moving at high speeds would leave a particular signature which could be detected from Earth. We propose a search based on the properties of light reflecting from objects travelling at relativistic speeds. Based on the same principles, we also propose a simple interstellar beacon with a solar sail. [Copyright &y& Elsevier]

Abstract: The detection of nucleic acid bases in carbonaceous meteorites suggests that their formation and survival is possible outside of the Earth. Small N-heterocycles, including pyrimidine, purines and nucleobases, have been extensively sought in the interstellar medium. It has been suggested theoretically that reactions between some interstellar molecules may lead to the formation of cytosine, uracil and thymine though these processes involve significantly high potential barriers. We attempted therefore to use quantum chemical techniques to explore if cytosine can possibly form in the interstellar space by radical–radical and radical-molecule interaction schemes, both in the gas phase and in the grains, through barrier-less or low barrier pathways. Results of DFT calculations for the formation of cytosine starting from some of the simple molecules and radicals detected in the interstellar space are being reported. Global and local descriptors such as molecular hardness, softness and electrophilicity, and condensed Fukui functions and local philicity indices were used to understand the mechanistic aspects of chemical reaction. The presence and nature of weak bonds in the molecules and transition states formed during the reaction process have been ascertained using Bader’s quantum theory of atoms in molecules (QTAIMs). Two exothermic reaction pathways starting from propynylidyne (CCCH) and cyanoacetylene (HCCCN), respectively, have been identified. While the first reaction path is found to be totally exothermic, it involves a barrier of 12.5kcal/mol in the gas phase against the lowest value of about 32kcal/mol reported in the literature. The second path is both exothermic and barrier-less. The later has, therefore, a greater probability of occurrence in the cold interstellar clouds (10–50K). [Copyright &y& Elsevier]

Abstract: Mastering photon propulsion is proposed to be the key to overcoming the limit of the current propulsion technology based on conventional rocketry and potentially opening a new space era. A perspective on photon propulsion is presented here to elucidate that interstellar manned roundtrip flight could be achievable in a century within a frame of exiting scientific principles, once the required existing technologies are further developed. It is shown that the developmental pathway towards the interstellar flight demands not only technological breakthroughs, but consistent long-term world-scale economic interest and investment. Such interest and investment will result from positive financial returns from routine interstellar commutes that can transport highly valuable commodities in a profitable manner. The Photonic Railway, a permanent energy-efficient transportation structure based on the Beamed-Laser Propulsion (BLP) by Forward and the Photonic Laser Thruster (PLT) by the author, is proposed to enable such routine interstellar commutes via Spacetrains. A four-phased evolutionary developmental pathway towards the Interstellar Photonic Railway is proposed. Each phase poses evolutionary, yet daunting, technological and financial challenges that need to be overcome within each time frame of 20 _ 30 years, and is projected to generate multitudes of applications that would lead to sustainable reinvestment into its development. If successfully developed, the Photonic Railway would bring about a quantum leap in the human economic and social interests in space from explorations to terraforming, mining, colonization, and permanent habitation in exoplanets. [Copyright &y& Elsevier]

Abstract: In this paper, a complete system for interstellar navigation using quasars is described and analyzed. Planning interstellar voyages at velocities comparable to the velocity of light forces us to consider relativistic aberrational effects. Moreover, great distances do not agree to control the ship''s trajectory by Earth. Measurements of the aberrated angular distances between two quasars belonging to a set of three quasars, at least, can be used to rectify automatically the spacecraft trajectory by means of an algorithm reported in this paper. The best result of the simulation was obtained using apical longitudinal angular distances between the quasars around 90° and 180°. [Copyright &y& Elsevier]

Abstract: I review those properties of the interstellar medium within 15light-years of the Sun, which will be relevant for the planning of future rapid (v≥0.1c) interstellar space missions to the nearest stars. As the detailed properties of the local interstellar medium (LISM) may only become apparent after interstellar probes have been able to make in situ measurements, the first such probes will have to be designed conservatively with respect to what can be learned about the LISM from the immediate environment of the Solar System. It follows that studies of interstellar vehicles should assume the lowest plausible density when considering braking devices, which rely on transferring momentum from the vehicle to the surrounding medium, but the highest plausible densities when considering possible damage caused by the impact of the vehicle with interstellar material. Some suggestions for working values of these parameters are provided. This paper is a submission of the Project Icarus Study Group. [Copyright &y& Elsevier]

Abstract: In the context of special relativity, we discuss the specific impulse of a rocket whose exhaust jet consists of massive and/or massless particles. This work generalizes previous results and corrects some errors of a recently published paper by U. Walter. (The errors stem from the omission of a Lorentz factor.) [ABSTRACT FROM AUTHOR]

Hyperthin solar sails deployed as close to the Sun as possible are the only currently feasible approach to extrasolar solar exploration and interstellar travel. This paper quantifies and investigates the effects of timing errors in the unfurlment (or inflation) of solar sails at the perihelion of parabolic solar orbits upon the spacecraft's trajectory direction. Methods of correcting such aim errors include on-board solar-, radioisotope-, or nuclear-electric thrusters, electromagnetic thrustless turning, application of electric or magnetic sails, and a new application of toroidal magnetic ion scoops. [ABSTRACT FROM AUTHOR]

Abstract: The SETI Permanent Study Group (abbreviated SETI PSG) of the International Academy of Astronautics (IAA), with web site: http://www.setileague.org/iaaseti/index.html, is one of the few international venues where, as of 2006, scientists with different backgrounds and from all over the world can meet and discuss recent advances in the scientific, technical and societal aspects of SETI. In particular, the Pešek Lecture that traditionally opens in October every year the SETI 1 Session of the International Astronautical Congress (IAC), is intended to describe the updated state-of-the-art in SETI and related fields with the accent on science and technology rather than on the societal consequences of a contact with ET. We have thus come to the conclusion that a Pešek Lecture devoted to the interplay between SETI and the rapidly evolving field of the Search for Extrasolar Planets (or Exoplanets) would be quite up-to-date, especially in view of the over 200 exoplanets rapidly discovered in the 11 years between 1995 and 2006. Moreover, besides SETI and Exoplanets, there is a third field of scientific investigation that, although not as mature as the former two fields in terms of experimental research, is striving ahead among many theoretical difficulties but might really change the course of human history when becoming reality: this is the theory (so far) of Interstellar Flight, that would one day enable us to travel across the vast interstellar distances initially by virtues of probes only, and later “in person”. The present Pešek Lecture is trying to compare the different grow rate and the (now small) overlap in between these three apparently “unrelated” fields. And even if we can hardly find any answer in these “dark ages” we live, let us at least raise the question: “When are SETI, Exoplanet Searches and Interstellar Flight going to merge in the future of Humankind?”. [Copyright &y& Elsevier]

Abstract: From the point of view of radiation safety, interstellar space is not an empty void. Interstellar gas and cosmic rays, which consist of hydrogen and helium nucleons, present a severe radiation hazard to crew and electronics aboard a relativistic interstellar ship. Of the two, the oncoming relativistic flow of interstellar gas produces the most intense radiation. A protection shield will be needed to block relativistic interstellar gas that can also absorb most of the cosmic rays which, as a result of relativistic aberration, forms into a beamed flow propagating toward the front of the spaceship. [Copyright &y& Elsevier]

Abstract: Space technology and activities have changed markedly from the early days of the space era and are continuing to develop rapidly. The TechCast Project—set up to pool the knowledge of experts in the scientific fields—includes space in its predictions. Here its president reports on its space forecasts for the 21st century. The importance of the shift towards the privatization of space is noted and the near-to-medium-term impossibility of interstellar travel conceded. Contact with ETI is deemed much more likely, however. [Copyright &y& Elsevier]

Abstract: Interstellar transportation to nearby star systems over periods shorter than the human lifetime requires speeds in the range of 0.1–0.15c and relatively high accelerations. These speeds are not attainable using rockets, even with advanced fusion engines because at these velocities, the energy density of the spacecraft approaches the energy density of the fuel. Anti-matter engines are theoretically possible but current physical limitations would have to be suspended to get the mass densities required. Interstellar ramjets have not proven practicable, so this leaves beamed momentum propulsion or a continuously fueled Mag-Orion system as the remaining candidates. However, deceleration is also a major issue, but part of the Mini-Mag Orion approach assists in solving this problem. This paper reviews the state of the art from a Phases I and II SBIT between Sandia National Laboratories and Andrews Space, applying our results to near-term interstellar travel. A 1000T crewed spacecraft and propulsion system dry mass at contains . The author has generated technology requirements elsewhere for use of fission power reactors and conventional Brayton cycle machinery to propel a spacecraft using electric propulsion. Here we replace the electric power conversion, radiators, power generators and electric thrusters with a Mini-Mag Orion fission–fusion hybrid. Only a small fraction of fission fuel is actually carried with the spacecraft, the remainder of the propellant (macro-particles of fissionable material with a D-T core) is beamed to the spacecraft, and the total beam energy requirement for an interstellar probe mission is roughly , which would require the complete fissioning of 1000ton of Uranium assuming 35% power plant efficiency. This is roughly equivalent to a recurring cost per flight of 3.0 billion dollars in reactor grade enriched uranium using today''s prices. Therefore, interstellar flight is an expensive proposition, but not unaffordable, if the nonrecurring costs of building the power plant can be minimized. [Copyright &y& Elsevier]

Abstract: Considering the first interstellar mission to Alpha Centauri we will necessarily obey the physics of special relativity, rather than of classical Newtonian mechanics. From this viewpoint, several issues were recently discussed by one of the two authors (L.D.) in his Master''s thesis written under the supervision of the senior author (C.M.). The relevant conclusions were that a “relativistic” mission to Alpha Centauri might be feasible with the following requirements: [(1)] The speed profile should be the so-called “relativistic hyperbolic motion”. [(2)] The propulsion system should be one of the followings: [(a)] Antimatter (specific impulses of the order of the speed of light). [(b)] Carlo Rubbia''s “nuclear propulsion” based on taking aboard a black body source (“take half the Sun with you!”). [(3)] An adequate protection system against impacts from space debris (a critical problem not adequately solved at the moment). [(4)] The telecommunication system should be based on the Karhunen–Loève Transform (KLT) for data compression and filtering, as theory of the optimal relativistic telecommunications outlined long ago by one of us (C.M.). With all the above remarks, the authors think they outlined a basic, but technically sound, theory for the first interstellar special-relativistic flight to Alpha Centauri. [Copyright &y& Elsevier]

Abstract: In this academy transactions note, we study a type of relativistic interstellar flight that would enable an unmanned space probe to reach the nearest stars in a few decades’ time. “Time” here means “proper time”, that is the time aboard the probe, a little shorter than the time elapsed on Earth for the same flight. We consider what in special relativity is called “hyperbolic motion”, that is a rectilinear, accelerated motion with a constant acceleration a in the probe''s reference frame. The overall mission to any nearby star is thus subdivided into two equal halves: [(1)] from the Sun to half way the distance to the target star, the probe undergoes a uniform proper acceleration a; [(2)] at mid-point the probe is turned by an angle of , so that the propulsion system becomes a braking system; [(3)] the probe then undergoes a uniform proper deceleration a, until it reaches the target star with zero speed. The probe''s top speed is thus achieved at mid-point between the Sun and the target star, and it may equal a significant fraction of the speed of light (20% or higher). An equation is then derived expressing the hyperbolic motion''s uniform acceleration a as a function of the overall mission time and of the distance of the target star from the Sun. This is the first key equation for a realistic mission plan. We next prove, by resorting to the Ackeret equation for a relativistic rocket, that, for the hyperbolic motion, the mass of the “propellant” decreases exponentially. The important point then comes: this exponential decrease of the propellant mass formally has just the same equation as the radioactive decay equation. The way thus is paved to exploit the radioactive decay as a propulsion system to achieve the uniformly accelerated mission profile of the hyperbolic motion. Finally, from this “perfect match” between hyperbolic motion and radioactive decay, one more basic equation is derived. This equation expresses the radioactive decay constant as the ratio between the hyperbolic uniform proper acceleration a and the proper speed w of the ejected radioactive fragments “propelling” the probe in the opposite direction. This equation is vital inasmuch as it enables one to select the appropriate radioactive material as the “propellant” capable of achieving the requested constant proper acceleration a for a given target star. In conclusion, the logical sequence of steps to design an interstellar probe based on the radioactive decay of a certain material has now been made clear: [(1)] First determine the uniform proper acceleration a in terms of the assumed overall flight proper time, , and of the distance of the target star, . [(2)] Then select the appropriate radioactive material by selecting its radioactive decay constant , expressed as the ratio of the previously selected constant proper acceleration a to the (constant) proper speed of the ejected radioactive fragments w. [(3)] For different target stars, select different radioactive materials according to the equation . [Copyright &y& Elsevier]

Interstellar transportation over periods shorter than the human lifetime requires speeds in the range of 0.2–0.3c. These speeds are not attainable using rockets, even with advanced fusion engines. Anti-matter engines are theoretically possible but current physical limitations would have to be suspended to get the mass densities required. Interstellar ramjets have not proven practicable, so this leaves beamed momentum propulsion as the remaining candidate. This paper reviews the state of beamed-momentum in-space propulsion and presents a point design system suitable for human exploration of nearby stars using today''s physics and resources known to exist in the solar system.The total beam energy requirement for an interstellar probe mission is roughly 1020 J, which would require the complete fissioning of one thousand tons of uranium assuming 35 percent power plant efficiency. This is roughly equivalent to a recurring cost per flight of 3.0 billion dollars in reactor grade enriched uranium using today''s prices. Therefore, interstellar flight is an expensive proposition, but not unaffordable, if the nonrecurring costs of building the power plant can be minimized. [Copyright &y& Elsevier]

In the paper we formulate and solve two inverse problems interesting the physical structure of interstellar clouds. In the problems the unknowns are the dimension of the cloud and its total cross-section of absorption. We solve numerically an integro-differential equation of photon transport by using, as additional data, measured values of the photon density outside the cloud. Numerical results are provided by an iterative scheme proposed in the paper. [Copyright &y& Elsevier]

In a previous paper, the author described a hypothetical development path of technical civilisations which has the following stages: planet dwellers, asteroid dwellers, interstellar travellers, interstellar space dwellers. In this paper, several scenarios are described which may cause the rise of an asteroid-based technical civilisation. Before such a transition may take place, certain space technologies must be developed fully (now these exist only in very preliminary forms): closed-cycle biological life support systems, space manufacturing systems, electrical propulsion systems. After mastering these technologies, certain events may provide the necessary financial means and social impetus for the foundation of the first asteroid-based colonies. In the first scenario, a rich minority group becomes persecuted and they decide to leave the Earth. In the second scenario, a “cold war”-like situation exists and the leaders of the superpowers order the creation of asteroid-based colonies to show off their empires’ technological (and financial) grandiosity. In the third scenario, the basic situation is similar to the second one, but in this case the asteroids are not just occupied by the colonists. With several decades of hard work, an asteroid can be turned into a kinetic energy weapon which can provide the same (or greater) threat as the nuclear arsenal of a present superpower. In the fourth scenario, some military asteroids are moved to Earth-centred orbits and utilised as “solar power satellites” (SPS). This would be a quite economical solution because a “military asteroid” already contains most of the important components of an SPS (large solar collector arrays, power distribution devices, orbit modifying rocket engine), one should add only a large microwave transmitter. [Copyright &y& Elsevier]

Discusses the impact of the prospect of man spreading to the stars. Factors driving the expansion; Contact with extraterrestrial intelligence; Interstellar travel; Examination of the consequences of the events through lessons of cosmic evolution.