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Our closest neighboring star system, Alpha Centauri, is located at “mere” 275,000 astronomical units (au) from Earth. Because of its “close proximity” to Earth, Alpha Centauri is often considered as the prime target for a future interstellar exploration mission [1]. It holds significant scientific importance for better understanding our sun, and stars in general, and for advancing our knowledge on the formation and evolution of the solar system [2]. Moreover, by visiting Alpha Centauri, Earth-like exoplanets may be discovered (in addition to the discovery of Proxima Centauri b in 2016 [3]). Voyager 1, our fastest and farthest spacecraft, 146 au from the sun at the time of writing, would take approximately 75,000 years to reach Alpha Centauri. Photon-sail propulsion could substantially reduce this travel time [4] even though it exclusively makes use of the radiation pressure from a star to drive the spacecraft forward [5]. This novel propulsion technology has made significant technological progress in recent years through JAXA’s IKAROS mission, NASA’s NanoSail-D2 mission, and the LightSail-1 and LightSail-2 missions by the Planetary Society. Continued technological advancement will be achieved through other proposed and scheduled missions such as NASA’s NEA Scout mission [6]. Proposals for using photon sails to reach targets far beyond our solar system are not new (see, e.g., Refs. [4,7]), and initiatives with clear goals of reaching Alpha Centauri within a generation are underway, e.g., the Breakthrough Starshot project.§ However, little is known about the dynamics of the photon sail once it arrives in the other star system, especially in a multistar system like Alpha Centauri. To date, the only works investigating these dynamics include Refs. [8,9]. Both articles focused on the dynamics in the binary-star system composed of the stars Alpha Centauri A and B, and investigated the possibility of decelerating the spacecraft after arrival, assuming a graphene-based sail covered with a highly reflective coating. The MIRA Collaboration focused on the computation of artificial equilibria (AE) in the same binary-star system, but for a photon-balloon spacecraft, before investigating capture and transfer trajectories for a photon-sail propelled spacecraft in the elliptical restricted three-body problem (ER3BP) [2,10].

Exploration of the heliosphere and the far reaches of our solar system by the Voyagers and New Horizons and near-Earth observations of the Kuiper Belt and exoplanetary systems have all profoundly changed how we view our own home in space. An Interstellar Probe escaping beyond the solar-system boundaries with new observational techniques would be a bold move in space exploration, enabling detailed, new understanding of the global heliosphere in the context of other astrospheres, further discoveries of unexplored Kuiper Belt Objects, and the first observations of our circumsolar dust disk. These would offer insight into the evolution of the solar system and our understanding of exoplanetary systems. With the power supplies on the Voyagers nearing their end of life, the rapid pace of discovery of exoplanets in other stellar systems, and the evolution in our own understanding brought about by results from the Voyager, Cassini, and Interstellar Boundary Explorer (IBEX) spacecraft, the time is right for looking at the next steps we can make into our stellar neighborhood. The question facing us today is what the appropriate next step - a true first step - is in negotiating the transition from science fiction to engineering reality. The scientific imperatives associated with reaching the near interstellar medium in order to understand our origins and our own current locale in the Milky Way trace to the beginnings of the Space Age, and they have been debated and refined since that time and into the current epoch. The subjects of interstellar travel, interstellar probes, and interstellar “precursor” missions are not new but have lacked traction with policy makers and the scientific community at large because of the states of both scientific knowledge and engineering realities. The next step in reaching to the stars will require the recognition of engineering limits, scientific trades, and scientific compromises, but this is new neither in science nor exploration. Such a step would be an “Interstellar Probe.” The time for that step has come.

`Oumuamua, the first detected interstellar visitor to the solar system, exhibits non-gravitational acceleration in its trajectory. Ruling out other means of propulsion, such as the evaporation of material via a cometary tail, it has been argued that radiation pressure is responsible for this acceleration. From this, the mass of the object must be approximately 40 tonnes, and given its dimensions, `Oumuamua must have a thickness of ~1 mm if of a similar rock/iron composition as the Earth. This raises the much publicised possibility that `Oumuamua is artificial in origin, sent intentionally across interstellar space by an alien civilisation, This conclusion, however, relies upon the common misapprehension that light (solar) sails can accelerate to a considerable fraction of the speed of light, permitting rapid interstellar travel. We show that such speeds are unattainable for conceptual man-made sails and that, based upon its observed parameters, `Oumuamua would require half a billion years just to travel to our solar system from its closest likely system of origin. These cosmological time-scales make it very unlikely that this is a probe sent by an alien civilisation., Accepted by A&A Letters

Two objects belonging to the interstellar medium have already been registered in the Solar system. For earth science the opportunity to study interstellar wanderers without the implementation of interstellar travel. However, technically it is extremely difficult to detect and intercept bodies flying through the Central part of the Solar system. Therefore, he possibility of sending research probes in pursuit is being considered. After leaving the vicinity of the Sun proper speed their removal from the Sun is 50 km/s or more, so to intercept such bodies at distances, where transmission of information from a research probe is still possible, the speed probe should be even higher. The high rate of approach of the probe and the interstellar wanderer raises the very difficult question of breaking the probe to a safe speed for research equipment. A solution of this task to use is discovered by patent RU 2626792 “Method of delivery of a payload into celestial body soil, maintenance of soil investigations and the celestial bodies”, which allows to ensure the safety and the efficiency of equipment when the speed of the probe up to several kilometers per second. This option would allow the probe’s propulsion resources to be used to homing the probe on a small target in the absence of accurate trajectory data on the interstellar wanderer motion.

In interstellar travel, parallax and stellar aberration have a non-negligible influence on the star identification of star sensors. Through the analysis of the working principle of star sensors, only the guide star catalog generation can solve this problem. The influence mechanism of parallax and stellar aberration is synthetically analyzed by four-dimensional time-space diagrams, and a generation algorithm of the guide star catalog for the interstellar star identification is presented. The update cycle of the catalog is studied, and a block update method based on the distance distribution is given. The Hipparcos catalog is adopted as the fundamental catalog. On the premise of meeting the accuracy requirement of star identification, the maximum travel distance supported by the catalog is about 0.3 light-years. In order to balance the star identification and the influence of parallax, eliminating 30% to 40% of the nearby guide stars is an optimal criterion for the guide star selection. The analysis of the catalog distribution shows that the parallax and the stellar aberration have opposite effects on the distribution of guide stars. The methods and conclusions provide a theoretical basis for interstellar celestial navigation.

Here, we compare Earth to the most common type of exoplanet, the super-Earth, with respect to interplanetary space travel. The typical super-Earth should have higher gravity and atmospheric pressure at its surface. These factors pose significant challenges to rocket launches and to reentering spacecraft. In addition, the Solar System is compared to exoplanetary systems with respect to interstellar travel. It is easier to launch an interstellar spacecraft from a planet in the circumstellar habitable zone of the Sun than from planets in the circumstellar habitable zones of less massive stars. In the larger context of the Milky Way galaxy, our Solar System is in the best location to initiate interstellar missions. In summary, we here confirm and expand upon recent studies that argue that the Earth and the Solar System are rare in the degree to which they facilitate space exploration.

[CASTELLÀ] El objetivo principal del trabajo es elaborar una propuesta de misión espacial para explorar el cinturón de Kuiper, una región del sistema solar situada a 30-55 ua del sol. Para la exploración se plantea el uso de una sonda capaz de llegar e investigar el cinturón. Este trabajo hace un estudio del estado del arte actual y se analizan cuatro temas distintos: el primero es el cinturón de Kuiper y las condiciones a las que nos enfrentamos en el espacio exterior. En segundo lugar, los sistemas de instrumentación de la sonda, donde se habla y se mencionan sistemas existentes y que han sido usados por otras sondas espaciales a lo largo de la historia. De esta manera, se propone un equipo de instrumentación preparado para el estudio de los cuerpos que constituyen el cinturón de Kuiper (objetos transneptunianos, cometas, planetoides). Los tipos de propulsores que puede llegar a equipar, haciendo una comparación y análisis de cada uno de ellos. Por último, se hace un estudio de la mecánica orbital para el cálculo aproximado de la trayectoria de la sonda. Se propone como punto de partida la órbita de la estación espacial internacional (ISS) y, con los cambios orbitales adecuados, la llegada a Júpiter, donde se analiza cómo llegar hasta Kuiper mediante asistencia gravitatoria y sin usarla. Se debe tener en cuenta que el diseño de una misión espacial es una tarea que requiere años y muchas personas especializadas en la materia. Por lo tanto, lo que recoge este trabajo es un humilde acercamiento al tema de viajes interplanetarios con sondas espaciales, teniendo en cuenta las limitaciones de tiempo y recursos para resolver los problemas que surjan durante el desarrollo del proyecto. [ANGLÈS] The main objective of this project is to elaborate a proposal of a space mission that aims to explore the Kuiper Belt. A payload that is able to properly investigate the Kuiper belt is presented for this exploration. This Project studies the recent state of the art and analyses four different aspects: In first place, the Kuiper belt and the conditions to be faced in the outer space, the instrumentation systems of the payload, where mentioned the previous systems employed in other payloads along the history. Thus, an instrumentation equipment towards the Kuiper belt objects is suggested. Lastly, a study of orbital mechanics for the approximate calculation of the payload path. The orbit of the International Space Station is suggested as a starting point, and with the right orbital changes, the arrival in Jupiter where we analyse how to arrive to Kuiper with -or without- the gravity assist. It should be taken into account that the work of designing a space mission involves years and many specialized people. Therefore, what the project accomplishes is a humble approach to the topic of interplanetary travelling with payloads, considering the limitations attached to the timing and resources when solving the problems that appear in the project’s development.

Interstellar travel is the term used for hypothetical piloted or unpiloted travel between stars. Interstellar travel will be much more difficult than interplanetary spaceflight; the distances between the planets in the Solar System are less than 30 astronomical units (AU), whereas the distances between stars are typically hundreds of thousands of AU.

Interstellar missions are generally regarded as something for the relatively far future, and not even the most ardent and optimistic of space advocates envisage seeing them before the end of the 21st century. Not only does there still remain so much to discover, see and understand within our own solar system, but even the most basic technology to send probes, let alone humans, to the stars is at an extremely nascent stage. Starship technology is to us what jet aircraft technology would be to the first steam engine pioneers. Wormholes, science fiction’s most popular means for interstellar travel, are only a remote, very theoretical possibility.

The scientific community has advocated a scientific probe to the interstellar medium for over 30 years. While the Voyager spacecraft have passed through the termination shock of the solar wind, they have limited lifetimes as their radioisotope power supplies decay. It remains unclear whether they can reach the heliopause, the boundary between shocked solar wind and interstellar plasmas, and, in any case, they will not reach the undisturbed interstellar medium. As with most exploratory space missions, their ongoing observations continue to raise even more questions about the nature of the interaction of our heliosphere and the interstellar medium. Scientific questions including: 1. What is the nature of the nearby interstellar medium? 2. How do the Sun and galaxy affect the dynamics of the heliosphere? 3. What is the structure of the heliosphere? 4. How did matter in the solar system and interstellar medium originate and evolve? can only be answered by an “interstellar precursor” probe. Such a mission is required to make in situ measurements in the interaction region and interstellar medium itself at distances far from the Sun, but in a finite mission lifetime. By launching a probe toward the incoming “interstellar wind,” whose direction is known, the distance to be traveled can be minimized but is still large. The current consensus is that a scientifically compelling mission must function to at least a distance of 200 astronomical units (AU) from the Sun and return a reasonable stream of data during the voyage. The central problem is that of providing a means of propulsion to accelerate a probe from the Solar System. Even with a low-mass payload and spacecraft, achieving the high speeds needed, even with gravity assists, have remained problematic. Voyager 1, the fastest object ever to leave the system is now traveling ∼3.6 AU/yr, and a credible probe must reach at least 2–3 times this speed. The use of an Ares V is an approach for enabling a fast interstellar precursor mission. Maximum capability uses the combination of an Ares V, two-engine Centaur upper stage, close fly-by of Jupiter, and radioisotope electric propulsion (REP). Deletion of any of these pieces does not disable the mission, but does increase the flyout time to a given distance. This approach is more robust and provides a faster probe than an earlier alternative, designed for launch by a Delta IV 4050H plus twin Star 48A upper stages.

`Oumuamua (1I/2017 U1) is the first object of interstellar origin observed in the Solar System. Recently, \citet{Micheli2018} reported that `Oumuamua showed deviations from a Keplerian orbit at a high statistical significance. The observed trajectory is best explained by an excess radial acceleration $\Delta a \propto r^{-2}$, where $r$ is the distance of `Oumuamua from the Sun. Such an acceleration is naturally expected for comets, driven by the evaporating material. However, recent observational and theoretical studies imply that `Oumuamua is not an active comet. We explore the possibility that the excess acceleration results from Solar radiation pressure. The required mass-to-area ratio is $(m/A)\approx 0.1$ g cm$^{-2}$. For a thin sheet this requires a thickness of $\approx 0.3-0.9$ mm. We find that although extremely thin, such an object would survive an interstellar travel over Galactic distances of $\sim 5$ kpc, withstanding collisions with gas and dust-grains as well as stresses from rotation and tidal forces. We discuss the possible origins of such an object. Our general results apply to any light probes designed for interstellar travel., Comment: To be published in "The Astrophysical Journal Letters" on November 12, 2018

In the nearly 60 years of spaceflight we have accomplished wonderful feats of exploration and shown the incredible spirit of the human drive to explore and understand our universe. Yet in those 60 years we have barely left our solar system with the Voyager 1 spacecraft launched in 1977 finally leaving the solar system after 37 years of flight at a speed of 17 km/s or less than 0.006% the speed of light. As remarkable as this is, we will never reach even the nearest stars with our current propulsion technology in even 10 millennium. We have to radically rethink our strategy or give up our dreams of reaching the stars, or wait for technology that does not exist. While we all dream of human spaceflight to the stars in a way romanticized in books and movies, it is not within our power to do so, nor it is clear that this is the path we should choose. We posit a technological path forward, that while not simple; it is within our technological reach. We propose a roadmap to a program that will lead to sending relativistic probes to the nearest stars and will open up a vast array of possibilities of flight both within our solar system and far beyond. Spacecraft from gram level complete spacecraft on a wafer (“wafer sats”) that reach more than ¼ c and reach the nearest star in 15 years to spacecraft with masses more than 105 kg (100 tons) that can reach speeds of near 1000 km/s such systems can be propelled to speeds currently unimaginable with our existing propulsion technologies. To do so requires a fundamental change in our thinking of both propulsion and in many cases what a spacecraft is. In addition to larger spacecraft, some capable of transporting humans, we consider functional spacecraft on a wafer, including integrated optical communications, optical systems and sensors combined with directed energy propulsion. Since “at home” the costs can be amortized over a very large number of missions. The human factor of exploring the nearest stars and exo-planets would be a profound voyage for humanity, one whose non-scientific implications would be enormous. It is time to begin this inevitable journey beyond our home.

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 10 20 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.

The time has arrived for designing, building, and instrumenting a spacecraft for a dedicated foray into the galactic environment surrounding our star, the sun. This region was probed in the past by remote techniques and it will be explored in situ by the NASA’s planned Interstellar Probe mission. The mission will significantly advance our understanding of the nature of the local interstellar medium and explore the distant frontier of the solar system by revealing the details of the interaction between the sun and the Galaxy. This mission will also be an important practical step toward interstellar flight of the future. Reaching interstellar space in reasonable time requires high escape velocities and will likely be enabled by non-chemical propulsion such as nuclear-powered electric propulsion or solar sailing. Unusually high spacecraft velocities, enormous distances from the Sun, and non-chemical propulsion will significantly influence design of the mission, spacecraft, and scientific instrumentation. We will review measurement objectives of the first dedicated mission into interstellar space and outline constraints on the instrumentation. Measurement of particles, fields, and dust in the interstellar medium will be complemented by search for complex organic molecules and remote sensing capabilities in various spectral bands. A ‘‘look’’ back at our solar system will also be a glimpse of what a truly-interstellar mission of the distant future would encounter in approaching a target star. The instrumentation for interstellar exploration presents numerous challenges. Mass, telemetry, and power constraints would place a premium on miniaturization and autonomy. There are, however, physical limits on how small the sensors could be. New instrument concepts may be required to achieve the desired measurement capabilities under the stringent constraints of a realistic interstellar mission.

Movies are the myths of late-20th century western culture. Because of the power of films likeETto capture our imagination, we are more likely than past generations to accept the possibility that life exists elsewhere in our galaxy. Such a myth can be used to sketch the main themes of this chapter, which concern the origin of life on the Earth.Imagine that 4 billion years ago, intelligent beings evolved on an Earth-like planet in the solar system of a neighboring star. After ten million years of evolution, they have solved the problems of interstellar travel and aging. Virtually immortal family groups set out to explore the galaxy and almost immediately discover a solar system associated with a nearby star only 80 light years away from their home planet. They find that the third planet is rich in the primary elements of life - carbon, hydrogen, oxygen and nitrogen - which are present in the atmosphere in the form of carbon dioxide (CO2), molecular nitrogen (N2) and water vapor (H2O). They decide to spend a few centuries studying this planet, which they consider to be a possible model of their own primordial world as it was four billion years in their past.

I review those properties of the interstellar medium within 15 light-years of the Sun which will be relevant for the planning of future rapid ($v /ge 0.1c$) 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 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., Comment: Accepted for publication in Acta Astronautica (DOI: 10.1016/j.actaastro.2010.10.016)

It is shown that the maximum velocity attainable by a laser-driven light sail is constrained by the back pressure of the relativistically transformed microwave background radiation field with interstellar material having little effect. Using current sail concepts, terminal Lorentz factors much in excess of 10 are not possible, and therefore, limitations are imposed on the benefits of relativistic time dilation and the possible use of such sails for travel over long interstellar distances. This is a fundamental constraint imposed by nature on the ultimate performance of such sails.

Once considered intractable, the problem of interstellar flight is slowly yielding to analysis. Although manned missions to the stars are exceedingly improbable in this century, the possibility of interstellar robotic probes should not be ruled out. Recent laboratory work and theoretical analysis suggest several near-term technologies that could, given the development of an adequate space-based infrastructure, provide the needed propulsion. Laser-driven lightsails offer the key advantage of leaving the fuel behind, with the laser beam focused by a large Fresnel lens in the outer Solar System. Perhaps more efficient is the use of a particle beam to boost a spacecraft by interacting with its magnetic sail, the latter a system already under intense scrutiny. Variations on "pellet" propulsion using macroscopic objects continue to surface, their mass converted to energy as they arrive at the departing starship. Interstellar flight will be both difficult and expensive, although it can no longer be considered an impossibility. This paper examines the above concepts and relates them to older ideas, such as the Bussard ramjet, that are currently out of favor. The vibrancy of interstellar flight studies is its syncretism-it was through analysis of the drag problem in fusion ramjet designs that a practical means of decelerating an interstellar probe by deployment of a magnetic sail emerged. The intermingling of such ideas offers the hope of robust hybrid concepts that may make interstellar flight a reality.

Interstellar flight is in our future, although we cannot know when. In terms of technology—the hardware and software that will take our first probes to Alpha Centauri and other nearby stars—the challenges are immense but achievable, if only by our descendants. Our future exploring other suns seems assured if we take a moderately optimistic view of scientific progress and a something less than pessimistic view of human nature—if we assume, that is, that we will find a way to transcend sheer human cussedness long enough to explore new homes for our species before we manage to destroy the one we have.

Interstellar travel remains an elusive goal. Will we develop the means to take humankind beyond our solar system?

NASA's Interstellar Probe Mission will be the first spacecraft specifically designed to explore the outer solar system, pass through the boundaries of the heliosphere, and sample the nearby interstellar medium. During the spring of 1999, NASA's Interstellar Probe Science and Technology Definition Team* developed a concept for a mission that will travel to 200- 400 AU using solar-sail propulsion. The principal scientific goals would be to explore the outer solar system, explore the structure of the heliosphere and its interaction with the interstellar medium, and explore the nature of the interstellar medium itself. These studies would be carried out by a -150 kg spacecraft carrying a scientific payload designed to make comprehensive measurements of heliospheric and interstellar plasma, fields, energetic particles, neutral gas, and dust. We discuss the scientific goals and strawman payload for this mission.

NASA's Interstellar Probe Mission, which will travel to 200-400 AU using solar sail propulsion, will be the first spacecraft specifically designed to explore the outer solar system and sample the nearby interstellar medium. During the spring of 1999, NASA's Interstellar Probe Science and Technology Definition Team* developed a concept for a mission to be propelled by a solar sail that would travel to >200 AU in 15 years. A 400-m diameter solar sail accelerates the spacecraft to -15 AU/year, approximately 5 times the speed of the Voyager spacecraft. After jettisoning the sail, the spacecraft coasts to 200-400 AU, exploring the Kuiper Belt, the boundaries of the heliosphere, and the nearby interstellar medium. We discuss the scientific goals, mission concept, strawman payload, and technology requirements for this very challenging mission.

NASA's Interstellar Probe will be the first spacecraft designed to explore the nearby interstellar medium and its interaction with our solar system. As envisioned by NASA's Interstellar Probe Science and Technology Definition Team, the spacecraft will be propelled by a solar sail to reach >200 AU in 15 years. Interstellar Probe will investigate how the Sun interacts with its environment and will directly measure the properties and composition of the dust, neutrals and plasma of the local interstellar material which surrounds the solar system. In the mission concept developed in the spring of 1999, a 400-m diameter solar sail accelerates the spacecraft to ~15 AU/year, roughly 5 times the speed of Voyager 1&2. The sail is used to first bring the spacecraft to ~0.25 AU to increase the radiation pressure before heading out in the interstellar upwind direction. After jettisoning the sail at ~5 AU, the spacecraft coasts to 200–400 AU, exploring the Kuiper Belt, the boundaries of the heliosphere, and the nearby interstellar medium.

Although the current space climate is focused on developing cheaper access to space and performing smaller, well-defined science payloads, the ultimate future calls for a permanent human presence in space, crowned with the achievement of interstellar travel. Whether that expansion comes from practical drives — the taxation of the limits of the carrying capacity of Earth — or more romantic, exploratory impulses, prudence dictates that we begin preparations far in advance of our departure date. There are numerous technological feats to master, in the guise of physical systems such as propulsion and shielding, and biological systems such as life support. But serious issues of design and systems integration also emerge, and their scope and difficulties may need several testing iterations for even their definition. A Solar System Cruiser that cycles between inner and outer planets, improving and evolving along each voyage, represents a phased mission architecture that both utilizes existing infrastructure while validating future technologies and testing human factors limits. As such, it might provide a valuable exercise in design learning that will serve humankind well in future grand undertakings such as interstellar travel. In addition, it may provide valuable infrastructure, material resource and science development for its useful lifetime.

This paper discusses the use of solar system-based lasers to push large lightsail spacecraft over interstellar distances. The laser power system uses a 1000-km-diam. lightweight Fresnel zone lens that is capable of focusing laser light over interstellar distances. A one-way interstellar flyby probe mission uses a 1000 kg (1-metric-ton), 3.6-km-diam. lightsail accelerated at 0.36 m/s2 by a 65-GW laser system to 11% of the speed of light (0.11 c), flying by a Centauri after 40 years of travel. A rendezvous mission uses a 71-metric-ton, 30-km diam. payload sail surrounded by a 710-metric-ton, ring-shaped decelerator sail with a 100-km outer diam. The two are launched together at an acceleration of 0.05 m/s2 by a 7.2-TW laser system until they reach a coast velocity of 0.21 c. As they approach a Centauri, the inner payload sail detaches from the ring sail and turns its reflective surface to face the ring sail. A 26-TW laser beam from the solar system, focused by the Fresnel lens, strikes the heavier ring sail, accelerating it past a Centauri. The curved surface of the ring sail focuses the laser light back onto the payload sail, slowing it to a halt in the a Centauri system after a mission time of 41 years. The third mission uses a three-stage sail for a roundtrip manned exploration of e Eridani at 10.8 light years distance.

An estimate of the number of advanced technical civilizations on planets of other stars depends on our knowledge of the rate of star formation; the frequency of favorably situated planets ; the probabilities of the origins of life, of intelligence and of technical civilization ; and the lifetimes of technical civilizations. These parameters are poorly known. The estimates of the present paper lead to ∼106 extant advanced technical civilizations in our Galaxy. The most probable distance to the nearest such community is then several hundred light years. Interstellar spaceflight at relativistic velocities has several obvious advantages over electromagnetic communication among these civilizations. One striking feature is that with uniform acceleration of 1 g to the midpoint of the journey, and uniform deceleration thereafter, all points in the Galaxy are accessible within the lifetime of a human crew, due to relativistic time dilation. Some of the technical problems in the construction of starships capable of relativistic velocities are discussed. It is concluded that with nuclear staging, fusion reactors, and the Bussard interstellar ramjet, no fundamental energetic problems exist for relativistic interstellar spaceflight. We assume that there exists in the Galaxy a loosely integrated community of diverse civilizations, cooperating in the exploration and sampling of astronomical objects and their inhabitants. If each such advanced civilization launches one interstellar vehicle per year, the mean time interval between samplings of an average star would be 105 years, that between samplings of a planetary system with intelligent life would be 104 years, and that between sampling of another advanced civilization would be 103 years. It follows that there is the statistical likelihood that Earth was visited by an advanced extraterrestrial civilization at least once during historical times. There are serious difficulties in demonstrating such a contact by ancient writings and iconography alone. Nevertheless, there are legends which might profitably be studied in this context. Bases or other artifacts of interstellar spacefaring civilizations might also exist elsewhere in the solar system. The conclusions of the present paper are clearly provisional.

Arguments are presented which show that motives for interstellar travel by advanced technological civilizations based on an extrapolation of Earth’s history may be quite invalid. In addition, it is proposed that interstellar travel is so enormously expensive and perhaps so hazardous, that advanced civilizations do not engage in such practices because of the ease of information transfer via interstellar communication.

The subject discussed by most authors in this section is the potential of our technology to detect planets around other stars. The subject is of fundamental importance to the question of life, because we assume that life may originate and evolve only on planets. If planets were found near other stars, then with the development of proper instruments we could go one step further and search for a spectroscopic evidence of early life on these planets as discussed by Dr. Owen. Similar spectroscopic studies are currently carried out for the other bodies of our solar system. As was pointed out by Dr. Soffen in Montreal, in spite of the generally negative results of the Viking experiments on Mars, and the rather pessimistic prospects for other planets and satellites, the possibility that life might exist in our solar system, outside the earth, lias not yet been completely eliminated. Continued exploration of our solar system in the coming decades could yield useful insights into planetary evolution, the conditions necessary for the origin of life, and possibly on the question of interstellar travel and galactic colonization as discussed by Dr. Papagiannis. Space exploration, therefore, will remain an important companion to astronomical searches for signs of life in the Cosmos.

Studies carried out within the last ten years on the nature and distribution of extraterrestrial intelligent life are reviewed. Arguments for the absence of intelligent life in the Galaxy based on the assumption that at least some of these would have engaged in colonization and for the presence of colonies of extraterrestrials in some undiscovered location in the solar system are presented, and it is noted that both these views rest on the notion that interstellar travel can be achieved at high velocities in very large vehicles, which has been questioned. Alternative suggestions concerning interstellar exploration by automated probes and the possible extended time scale and motivation for galactic colonization are pointed out. Attention is then given to arguments for the extreme smallness of one of the factors in the Drake equation used to estimate the number of communicative extraterrestrial civilizations in the Galaxy, including the frequency of single stars, the likelihood that planets with the correct initial composition and conditions for life are at the proper distance from their stars, the probability of the formation of DNA and the origin of life, and the time for the evolution of intelligence. It is concluded that it seems likely that other civilizations exist in the Galaxy, although the number and distribution of such civilizations may only be determined by the detection of one or more examples.

Terrestrial scientific — technica1 civi1ization (STC) produces the power of about 1013 watt, Dyson’s — type STC energy output — about 4 × 1027 watt. The probability of the existence of the Dyson — type STC is negligible. There are well known the difficu1ties of the rea1ization of the manned interstellar flight. No terrestria1 — 1ike STC could fulfill the energy, power, financial and man — power demands to realize the interstellar flight. We have not observed astroengineering phenomena and any other signals that could be rea1ized by Dyson — type STC. This could constitute the indirect proof that Dyson — type STC controlling the energy of amount 4 × 1026 watt does not exist, at least in the vicinity of Solar System up to 200 light years apart.