The Fabric of the Universe: Exploring the cosmic web in 3D prints and woven textiles [CL]

http://arxiv.org/abs/1702.03897


We introduce The Fabric of the Universe, an art and science collaboration focused on exploring the cosmic web of dark matter with unconventional techniques and materials. We discuss two of our projects in detail. First, we describe a pipeline for translating three-dimensional density structures from N-body simulations into solid surfaces suitable for 3D printing, and present prints of a cosmological volume and of the infall region around a massive cluster halo. In these models, we discover wall-like features that are invisible in two-dimensional projections. Going beyond the sheer visualization of simulation data, we undertake an exploration of the cosmic web as a three-dimensional woven textile. To this end, we develop experimental 3D weaving techniques to create sphere-like and filamentary shapes and radically simplify a region of the cosmic web into a set of filaments and halos. We translate the resulting tree structure into a series of commands that can be executed by a digital weaving machine, and describe the resulting large-scale textile installation.

Read this paper on arXiv…

B. Diemer and I. Facio
Wed, 15 Feb 17
23/47

Comments: 9 pages, 10 figures. Updated information about our art and science collaboration can be found at this http URL

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Post-Detection SETI Protocols & METI: The Time Has Come To Regulate Them Both [CL]

http://arxiv.org/abs/1701.08422


Regulations governing METI are weak or non-existent. Post-detection SETI protocols are non-binding and too general. Vastly increased SETI capabilities, Chinese involvement in the field, and an intensified effort by METI-ists to initiate radio transmissions to the stars are among reasons cited for urgency in addressing the question of appropriate regulations. Recommendations include regulations at the agency level and laws at the national level as well as international treaties and oversight.

Read this paper on arXiv…

J. Gertz
Tue, 31 Jan 17
3/58

Comments: 20 pages, 0 figures, Accepted for publication in the Journal of the British Interplanetary Society (JBIS)

Affordable, Rapid Bootstrapping of the Space Industry and Solar System Civilization [CL]

http://arxiv.org/abs/1612.03238


Advances in robotics and additive manufacturing have become game-changing for the prospects of space industry. It has become feasible to bootstrap a self-sustaining, self-expanding industry at reasonably low cost. Simple modeling was developed to identify the main parameters of successful bootstrapping. This indicates that bootstrapping can be achieved with as little as 12 metric tons (MT) landed on the Moon during a period of about 20 years. The equipment will be teleoperated and then transitioned to full autonomy so the industry can spread to the asteroid belt and beyond. The strategy begins with a sub-replicating system and evolves it toward full self-sustainability (full closure) via an in situ technology spiral. The industry grows exponentially due to the free real estate, energy, and material resources of space. The mass of industrial assets at the end of bootstrapping will be 156 MT with 60 humanoid robots, or as high as 40,000 MT with as many as 100,000 humanoid robots if faster manufacturing is supported by launching a total of 41 MT to the Moon. Within another few decades with no further investment, it can have millions of times the industrial capacity of the United States. Modeling over wide parameter ranges indicates this is reasonable, but further analysis is needed. This industry promises to revolutionize the human condition.

Read this paper on arXiv…

P. Metzger, A. Muscatello, R. Mueller, et. al.
Tue, 13 Dec 16
22/77

Comments: 24 pages, 14 figures

The Log Log Prior for the Frequency of Extraterrestrial Intelligences [EPA]

http://arxiv.org/abs/1609.05931


It is unclear how frequently life and intelligence arise on planets. I consider a Bayesian prior for the probability P(ETI) that intelligence evolves at a suitable site, with weight distributed evenly over ln(1 – ln P(ETI)). This log log prior can handle a very wide range of P(ETI) values, from 1 to 10^(-10^122), while remaining responsive to evidence about extraterrestrial societies. It is motivated by our uncertainty in the number of conditions that must be fulfilled for intelligence to arise, and it is related to considerations of information, entropy, and state space dimensionality. After setting a lower limit to P(ETI) from the number of possible genome sequences, I calculate a Bayesian confidence of 18% that aliens exist within the observable Universe. With different assumptions about the minimum P(ETI) and the number of times intelligence can appear on a planet, this value falls between 1.4% and 47%. Overall, the prior leans towards our being isolated from extraterrestrial intelligences, but indicates that we should not be confident of this conclusion. I discuss the implications of the prior for the Search for Extraterrestrial Intelligence, concluding that searches for interstellar probes from nearby societies seem relatively effective. I also discuss the possibility of very small probabilities allowed by the prior for the origin of life and the Fermi Paradox, and note that similar priors might be constructed for interesting complex phenomena in general.

Read this paper on arXiv…

B. Lacki
Wed, 21 Sep 16
16/53

Comments: 32 pages (aastex6), 5 figures, 4 tables

ET Probes: Looking Here as Well as There [CL]

http://arxiv.org/abs/1609.04635


Almost all SETI searches to date have explicitly targeted stars in the hope of detecting artificial radio or optical transmissions. It is argued that extra-terrestrials (ET) might regard sending physical probes to our own Solar System as a more efficient means for sending large amounts of information to Earth. Probes are more efficient in terms of energy and time expenditures; may solve for the vexing problem of Drake’s L factor term, namely, that the civilization wishing to send information may not coexist temporally with the intended recipient; and they alleviate ET’s reasonable fear that the intended recipient might prove hostile. It is argued that probes may be numerous and easier to find than interstellar beacons.

Read this paper on arXiv…

J. Gertz
Fri, 16 Sep 16
8/63

Comments: 4 pages, no figures, JBIS Accepted

Space Development and Space Science Together, an Historic Opportunity [IMA]

http://arxiv.org/abs/1609.00737


The national space programs have an historic opportunity to help solve the global-scale economic and environmental problems of Earth while becoming more effective at science through the use of space resources. Space programs will be more cost-effective when they work to establish a supply chain in space, mining and manufacturing then replicating the assets of the supply chain itself so it grows to larger capacity. This has become achievable because of advances in robotics and artificial intelligence. It is roughly estimated that developing a lunar outpost that relies upon and also develops the supply chain will cost about 1/3 or less of the existing annual budgets of the national space programs. It will require a sustained commitment of several decades to complete, during which time science and exploration become increasingly effective. At the end, this space industry will capable of addressing global-scale challenges including limited resources, clean energy, economic development, and preservation of the environment. Other potential solutions, including nuclear fusion and terrestrial renewable energy sources, do not address the root problem of our limited globe and there are real questions that they may be inadequate or too late. While industry in space likewise cannot provide perfect assurance, it is uniquely able to solve the root problem, and it gives us an important chance that we should grasp. What makes this such an historic opportunity is that the space-based solution is obtainable for free, because it comes as a side-benefit of doing space science and exploration within their existing budgets. Thinking pragmatically, it may take some time for policymakers to agree that setting up a complete supply chain is an achievable goal, so this paper describes a strategy of incremental progress.

Read this paper on arXiv…

P. Metzger
Tue, 6 Sep 16
65/74

Comments: 38 pages. Accepted for publication in Space Policy journal

Long-term consequences of observing an expanding cosmological civilization [CEA]

http://arxiv.org/abs/1608.07522


Suppose that advanced civilizations, separated by a cosmological distance and time, wish to maximize their access to cosmic resources by rapidly expanding into the universe. What sort of boundary forms between their expanding domains, and how does the presence of one limit the ambitions of another? We describe a general case for any expansion speed, separation distance, and time. We then specialize to the main question of interest. How are the future prospects for a young and ambitious civilization altered if they can observe the presence of another at a cosmological distance? We treat cases involving the observation of one or two expanding domains. In the single-observation case, we find that almost any plausible detection will be limiting to some extent. Also, practical technological limits to expansion speed (well below the speed of light) play an interesting role. If a domain is visible at the time one embarks on expansion, there exists an optimum value for the “practical speed limit,” and if the speed limit is much higher than optimal, one’s future will be severely limited. In the case of two visible domains, it is possible to be “trapped” by them if the practical speed limit is high enough and their angular separation in the sky is large enough, i.e. one’s expansion in any direction will terminate at a boundary with the two visible civilizations.

Read this paper on arXiv…

S. Olson
Mon, 29 Aug 16
12/41

Comments: 7 pages, 7 figures

Odds for an enlightened rather than barren future [CEA]

http://arxiv.org/abs/1608.05776


We are at a stage in our evolution where we do not yet know if we will ever communicate with intelligent beings that have evolved on other planets, yet we are intelligent and curious enough to wonder about this. We find ourselves wondering about this at the very beginning of a long era in which stellar luminosity warms many planets, and by our best models, continues to provide equally good opportunities for intelligent life to evolve. By simple Bayesian reasoning, if, as we believe, intelligent life forms have the same propensity to evolve later on other planets as we had to evolve on ours, it follows that they will likely not pass through a similar wondering stage in their evolution. This suggests that the future holds some kind of interstellar communication that will serve to inform newly evolved intelligent life forms that they are not alone before they become curious.

Read this paper on arXiv…

D. Haussler
Tue, 23 Aug 16
5/51

Comments: N/A

The Venus Hypothesis [EPA]

http://arxiv.org/abs/1608.03074


Current models indicate that Venus may have been habitable. Complex life may have evolved on the highly irradiated Venus, and transferred to Earth on asteroids. This model fits the pattern of pulses of highly developed life appearing, diversifying and going extinct with astonishing rapidity through the Cambrian and Ordovician periods, and also explains the extraordinary genetic variety which appeared over this period.

Read this paper on arXiv…

A. Cartwright
Thu, 11 Aug 16
42/51

Comments: N/A

The Peaks of Eternal Light: a Near-term Property Issue on the Moon [CL]

http://arxiv.org/abs/1608.01989


The Outer Space Treaty makes it clear that the Moon is the province of all mankind, with the latter ordinarily understood to exclude state or private appropriation of any portion of its surface. However, there are indeterminacies in the Treaty and in space law generally over the issue of appropriation. These indeterminacies might permit a close approximation to a property claim or some manner of quasi-property. The recently revealed highly inhomogeneous distribution of lunar resources changes the context of these issues. We illustrate this altered situation by considering the Peaks of Eternal Light. They occupy about one square kilometer of the lunar surface. We consider a thought experiment in which a Solar telescope is placed on one of the Peaks of Eternal Light at the lunar South pole for scientific research. Its operation would require nondisturbance, and hence that the Peak remain unvisited by others, effectively establishing a claim of protective exclusion and de facto appropriation. Such a telescope would be relatively easy to emplace with todays technology and so poses a near-term property issue on the Moon. While effective appropriation of a Peak might proceed without raising some of the familiar problems associated with commercial development (especially lunar mining), the possibility of such appropriation nonetheless raises some significant issues concerning justice and the safeguarding of scientific practice on the lunar surface. We consider this issue from scientific, technical, ethical and policy viewpoints.

Read this paper on arXiv…

M. Elvis, T. Milligan and A. Krolikowski
Mon, 8 Aug 16
2/61

Comments: 20 pages, 3 figures (color). Space Policy in press

Lessons from Mayan Astronomy [CL]

http://arxiv.org/abs/1608.01731


The Mayan culture collected exquisite astronomical data for over a millennium. However, it failed to come up with the breakthrough ideas of modern astronomy because the data was analyzed within a mythological culture of astrology that rested upon false but mathematically sophisticated theories about the Universe. Have we learned the necessary lessons to prevent our current scientific culture from resembling Mayan Astronomy? Clearly, data collection by itself is not a guarantee for good science as commonly assumed by funding agencies. A vibrant scientific culture should cultivate multiple approaches to analyzing existing data and to collecting new data.

Read this paper on arXiv…

A. Loeb
Mon, 8 Aug 16
26/61

Comments: 4 pages, 1 figure, to be published

The long-term scientific benefits of a space economy [IMA]

http://arxiv.org/abs/1607.04901


Utilisation of the material and energy resources of the Solar System will be essential for the development of a sustainable space economy and associated infrastructure. Science will be a major beneficiary of a space economy, even if its major elements (e.g. space tourism, resource extraction activities on the Moon or asteroids, and large-scale in-space construction capabilities) are not developed with science primarily in mind. Examples of scientific activities that would be facilitated by the development of space infrastructure include the construction of large space telescopes, ambitious space missions (including human missions) to the outer Solar System, and the establishment of scientific research stations on the Moon and Mars (and perhaps elsewhere). In the more distant future, an important scientific application of a well-developed space infrastructure may be the construction of interstellar space probes for the exploration of planets around nearby stars.

Read this paper on arXiv…

I. Crawford
Tue, 19 Jul 16
58/68

Comments: Accepted for publication in Space Policy

On The Relativity of Redshifts: Does Space Really "Expand"? [CL]

http://arxiv.org/abs/1605.08634


In classes on cosmology, students are often told that photons stretch as space expands, but just how physical is this picture? Does space really expand? In this article, we explore the notion of the redshift of light with Einstein’s general theory of relativity, showing that the core underpinning principles reveal that redshifts are both simpler and more complex than you might naively think. This has significant implications for the observed redshifting of photons as they travel across the universe, often refereed to as the cosmological redshift, and for the idea of expanding space.

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G. Lewis
Wed, 8 Jun 16
44/45

Comments: 6 pages, 5 figures, appeared in Australian Physics

Reviewing METI: A Critical Analysis of the Arguments [CL]

http://arxiv.org/abs/1605.05663


There is an ongoing debate pertaining to the question of whether Earth should initiate intentional and powerful radio transmissions to putative extra-terrestrial (ET) civilizations in the hope of attracting ET’s attention. This practice is known as METI (Messaging to ET Intelligence) or Active SETI. The debate has recently taken on a sense of urgency, as additional proponents have announced their intention to commence de novo transmissions as soon as they become funded and acquire the needed time on a powerful transmitter such as Arecibo. Arguments in favor of METI are reviewed. It is concluded that METI is unwise, unscientific, potentially catastrophic, and unethical.

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J. Gertz
Thu, 19 May 16
56/57

Comments: 8 pages, 0 figures, JBIS Accepted May 1, 2016

Why is there no von Neumann probe on Ceres? Error catastrophe can explain the Fermi-Hart Paradox [CL]

http://arxiv.org/abs/1605.02169


It has been argued that self-replicating robotic probes could spread to all stars of our galaxy within a timespan that is tiny on cosmological scales, even if they travel well below the speed of light. The apparent absence of such von Neumann probes in our own solar system then needs an explanation that holds for all possible extraterrestrial civilisations. Here I propose such a solution, which is based on a runaway error propagation that can occur in any self-replicating system with finite accuracy of its components. Under universally applicable assumptions (finite resources and finite lifespans) it follows that an optimal probe design always leads to an error catastrophe and breakdown of the probes. Thus, there might be many advanced civilizations in our galaxy, each surrounded by their own small sphere of self-replicating probes. But unless our own solar system has the extraordinary luck to be close enough to one of these civilizations, none of these probes will ever reach us.

Read this paper on arXiv…

A. Kowald
Tue, 10 May 16
77/85

Comments: In press by JBIS

Gravitational Wave for a pedestrian [CL]

http://arxiv.org/abs/1605.00761


The physics of gravitational wave and its detection in the recent experiment by the LIGO collaboration is discussed in simple terms for a general audience. The main article is devoid of any mathematics, but an appendix is included for inquisitive readers where essential mathematics for general theory of relativity and gravitational waves are given.

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A. Chaudhuri
Mon, 9 May 16
14/48

Comments: Few mistakes are corrected. 19 pages, 8 figures

Type III Societies (Apparently) Do Not Exist [IMA]

http://arxiv.org/abs/1604.07844


[Abridged] Whether technological societies remain small and planet-bound like our own, or ultimately span across galaxies is an open question in the Search for Extraterrestrial Intelligence. Societies that engineer on a galactic scale are classified as Type III on Kardashev’s scale. I argue that Type III societies can take the form of blackboxes, entire galaxies veiled in an opaque screen. A blackbox has a temperature that is just above that of the cosmic microwave background. The screen can be made from artificial dust pervading the galaxy. I show that there is enough material in galaxies to build blackboxes if the dust is fashioned into dipole antennas. The thermal emission of a blackbox makes it a bright microwave source. I examine the Planck Catalog of Compact Sources to constrain the abundance of blackboxes. None of the 100 GHz sources has the spectrum expected of a blackbox. The null result rules out shrouded galaxy clusters out to z ~ 1 and shrouded Milky Ways out to (comoving) 700 Mpc. The reach of the results includes 3 million galaxies containing an estimated 300 quadrillion terrestrial planets, as well as tens of thousands of galaxy clusters. Combined with the null results from other searches for Type III societies, I conclude that they are so rare that they basically do not exist within the observable Universe. A hypothesis of “Cosmic Pessimism” is discussed, in which we are alone, our long-term chances for survival are slim, and if we do survive, our future history will be checkered. Our loneliness is suggested by the lack of Type III societies. I discuss the remaining forms of Type III societies not yet well constrained by observation. I argue that the ease of building blackboxes on planetary and Solar System scales may lead, within a few centuries, to environmental catastrophes vastly more devastating than anything we are doing now, boding ill for us.

Read this paper on arXiv…

B. Lacki
Thu, 28 Apr 16
24/57

Comments: 38 pages (emulateapj), 11 figures, 7 tables

Directed Energy Missions for Planetary Defense [EPA]

http://arxiv.org/abs/1604.03511


Directed energy for planetary defense is now a viable option and is superior in many ways to other proposed technologies, being able to defend the Earth against all known threats. This paper presents basic ideas behind a directed energy planetary defense system that utilizes laser ablation of an asteroid to impart a deflecting force on the target. A conceptual philosophy called DE-STAR, which stands for Directed Energy System for Targeting of Asteroids and exploRation, is an orbiting stand-off system, which has been described in other papers. This paper describes a smaller, stand-on system known as DE-STARLITE as a reduced-scale version of DE-STAR. Both share the same basic heritage of a directed energy array that heats the surface of the target to the point of high surface vapor pressure that causes significant mass ejection thus forming an ejection plume of material from the target that acts as a rocket to deflect the object. This is generally classified as laser ablation. DE-STARLITE uses conventional propellant for launch to LEO and then ion engines to propel the spacecraft from LEO to the near-Earth asteroid (NEA). During laser ablation, the asteroid itself provides the propellant source material; thus a very modest spacecraft can deflect an asteroid much larger than would be possible with a system of similar mission mass using ion beam deflection (IBD) or a gravity tractor. DE- STARLITE is capable of deflecting an Apophis-class (325 m diameter) asteroid with a 1- to 15-year targeting time (laser on time) depending on the system design. The mission fits within the rough mission parameters of the Asteroid Redirect Mission (ARM) program in terms of mass and size. DE-STARLITE also has much greater capability for planetary defense than current proposals and is readily scalable to match the threat. It can deflect all known threats with sufficient warning.

Read this paper on arXiv…

P. Lubin, G. Hughes, M. Eskenazi, et. al.
Wed, 13 Apr 16
24/60

Comments: 33 pages, 17 figures. Submitted to ASR

Fixing the shadows while moving the gnomon [CL]

http://arxiv.org/abs/1604.02498


It is a common practice to fix a vertical gnomon and study the moving shadow cast by it. This shows our local solar time and gives us a hint regarding the season in which we perform the observation. The moving shadow can also tell us our latitude with high precision. In this paper we propose to exchange the roles and while keeping the shadows fixed on the ground we will move the gnomon. This lets us understand in a simple way the relevance of the tropical lines of latitude and the behavior of shadows in different locations. We then put these ideas into practice using sticks and threads during a solstice on two sites located on opposite sides of the Tropic of Capricorn.

Read this paper on arXiv…

A. Gangui
Tue, 12 Apr 16
29/63

Comments: Published version available at this http URL

The Search for Directed Intelligence [IMA]

http://arxiv.org/abs/1604.02108


We propose a search for sources of directed energy systems such as those now becoming technologically feasible on Earth. Recent advances in our own abilities allow us to foresee our own capability that will radically change our ability to broadcast our presence. We show that systems of this type have the ability to be detected at vast distances and indeed can be detected across the entire horizon. This profoundly changes the possibilities for searches for extra-terrestrial technology advanced civilizations. We show that even modest searches can be extremely effective at detecting or limiting many civilization classes. We propose a search strategy that will observe more than 10 12 stellar and planetary systems with possible extensions to more than 10 20 systems allowing us to test the hypothesis that other similarly or more advanced civilization with this same capability, and are broadcasting, exist.

Read this paper on arXiv…

P. Lubin
Fri, 8 Apr 16
42/54

Comments: 51 pages, 29 figures. Submitted to Reviews in Human Space Exploration

A Roadmap to Interstellar Flight [EPA]

http://arxiv.org/abs/1604.01356


In the nearly 60 years of spaceflight we have accomplished wonderful feats of exploration that have 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 currently 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 (“wafersats”) that reach more than 1/4 c and reach the nearest star in 20 years to spacecraft with masses more than 10^5 kg (100 tons) that can reach speeds of greater than 1000 km/s. These systems can be propelled to speeds currently unimaginable with 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, imaging systems, photon thrusters, power and sensors combined with directed energy propulsion.

Read this paper on arXiv…

P. Lubin
Wed, 6 Apr 16
49/63

Comments: 66 pages, 32 figures. Submitted to JBIS

Astrology in the Era of Exoplanets [CL]

http://arxiv.org/abs/1603.09496


The last two decades have seen the number of known exoplanets increase from a small handful to nearly 2000 known exoplanets, thousands more planet candidates, and several upcoming missions that are expected to further increase the population of known exoplanets. Beyond the strictly scientific questions that this has led to regarding planet formation and frequency, this has also led to broader questions such as the philosophical implications of life elsewhere in the universe and the future of human civilization and space exploration. One additional realm that hasn’t been adequately considered, however, is that this large increase in exoplanets would also impact claims regarding astrology. In this paper we look at the distribution of planets across the sky and along the Ecliptic, as well as the current and future implications of this planet distribution.

Read this paper on arXiv…

M. Lund
Fri, 1 Apr 16
28/57

Comments: 5 pages, 4 figures, submission for Acta Prima Aprilia

A Cloaking Device for Transiting Planets [EPA]

http://arxiv.org/abs/1603.08928


The transit method is presently the most successful planet discovery and characterization tool at our disposal. Other advanced civilizations would surely be aware of this technique and appreciate that their home planet’s existence and habitability is essentially broadcast to all stars lying along their ecliptic plane. We suggest that advanced civilizations could cloak their presence, or deliberately broadcast it, through controlled laser emission. Such emission could distort the apparent shape of their transit light curves with relatively little energy, due to the collimated beam and relatively infrequent nature of transits. We estimate that humanity could cloak the Earth from Kepler-like broadband surveys using an optical monochromatic laser array emitting a peak power of about 30 MW for roughly 10 hours per year. A chromatic cloak, effective at all wavelengths, is more challenging requiring a large array of tunable lasers with a total power of approximately 250 MW. Alternatively, a civilization could cloak only the atmospheric signatures associated with biological activity on their world, such as oxygen, which is achievable with a peak laser power of just around 160 kW per transit. Finally, we suggest that the time of transit for optical SETI is analogous to the water-hole in radio SETI, providing a clear window in which observers may expect to communicate. Accordingly, we propose that a civilization may deliberately broadcast their technological capabilities by distorting their transit to an artificial shape, which serves as both a SETI beacon and a medium for data transmission. Such signatures could be readily searched in the archival data of transit surveys.

Read this paper on arXiv…

D. Kipping and A. Teachey
Thu, 31 Mar 16
46/53

Comments: 10 pages, 6 figures, accepted in MNRAS

Viewpoint: The First Sounds of Merging Black Holes [CL]

http://arxiv.org/abs/1602.04476


Gravitational waves emitted by the merger of two black holes have been detected, setting the course for a new era of observational astrophysics.

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E. Berti
Tue, 16 Feb 16
51/71

Comments: APS Physics Viewpoint article on the LIGO gravitational wave detection paper. I would like to dedicate this Viewpoint to the memory of Steve Detweiler. I wish he could celebrate with us. Animations and a better looking pdf file are available at this http URL

Les composantes obscures de l'Univers [CL]

http://arxiv.org/abs/1602.00711


This article is the detailed version of a paper on dark matter, dark energy, and modified gravity, published in the December 2015-January 2016 special issue of “La Recherche” (in French)

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L. Blanchet and B. Famaey
Wed, 3 Feb 16
34/54

Comments: 10 pages, in French, 1 figure

Kardashev's Classification at 50+: A Fine Vehicle with Room for Improvement [EPA]

http://arxiv.org/abs/1601.05112


We review the history and status of the famous classification of extraterrestrial civilizations given by the great Russian astrophysicist Nikolai Semenovich Kardashev, roughly half a century after it has been proposed. While Kardashev’s classification (or Kardashev’s scale) has often been seen as oversimplified, and multiple improvements, refinements, and alternatives to it have been suggested, it is still one of the major tools for serious theoretical investigation of SETI issues. During these 50+ years, several attempts at modifying or reforming the classification have been made; we review some of them here, together with presenting some of the scenarios which present difficulties to the standard version. Recent results in both theoretical and observational SETI studies, especially the G-hat infrared survey (2014-2015), have persuasively shown that the emphasis on detectability inherent in Kardashev’s classification obtains new significance and freshness. Several new movements and conceptual frameworks, such as the Dysonian SETI, tally extremely well with these developments. So, the apparent simplicity of the classification is highly deceptive: Kardashev’s work offers a wealth of still insufficiently studied methodological and epistemological ramifications and it remains, in both letter and spirit, perhaps the worthiest legacy of the SETI “founding fathers”.

Read this paper on arXiv…

M. Cirkovic
Thu, 21 Jan 16
37/52

Comments: 15 pages, 2 figures, 2 tables

Life under a black sun [CL]

http://arxiv.org/abs/1601.02897


Life is dependent on the income of energy with low entropy and the disposal of energy with high entropy. On Earth, the low-entropy energy is provided by solar radiation and the high-entropy energy is disposed as infrared radiation emitted into the cold space. Here we turn the situation around and assume cosmic background radiation as the low-entropy source of energy for a planet orbiting a black hole into which the high-entropy energy is disposed. We estimate the power that can be produced by thermodynamic processes on such a planet, with a particular interest in planets orbiting a fast rotating Kerr black hole as in the science fiction movie {\em Interstellar}. We also briefly discuss a reverse Dyson sphere absorbing cosmic background radiation from the outside and dumping waste energy to a black hole inside.

Read this paper on arXiv…

T. Opatrny, L. Richterek and P. Bakala
Wed, 13 Jan 16
49/81

Comments: 8 pages, 6 figures

Tentative planetary orbital constraints of some scenarios for the possible new Solar System object recently discovered with ALMA [CL]

http://arxiv.org/abs/1512.05288


Some of the scenarios envisaged for the possible new Solar System object, whose discovery with the ALMA facility has been recently claimed in the literature, are preliminarily put to the test by means of the orbital motions of some planets of the Solar System. It turns out that the current ranges of admissible values for any anomalous secular precession of the perihelion of Saturn, determined in the recent past with either the EPM2011 and the INPOP10a planetary ephemerides without modeling the action of such a potential new member of the Solar System, do not rule out the existence of a putative Neptune-like pointlike perturber at about 2500 au. Instead, both a super-Earth at some hundreds of au and a Jovian-type planet up to 4000 au are strongly disfavored. An Earth-sized body at 100 au would have a density as little as $\sim 0.1-0.01~\textrm{g}~\textrm{cm}^{-3}$, while an unusually large Centaur or (Extreme) Trans Neptunian Object with linear size of $220-880~\textrm{km}$ at $12-25~\textrm{au}$ would have density much larger than $\sim 1~\textrm{g}~\textrm{cm}^{-3}$.

Read this paper on arXiv…

L. Iorio
Thu, 17 Dec 15
1/55

Comments: LaTex2e, 1 table, 3 figures, 4 pages

The Size Distribution of Inhabited Planets [CL]

http://arxiv.org/abs/1503.07804


Earth-like planets are expected to provide the greatest opportunity for the detection of life beyond the Solar System. However our planet cannot be considered a fair sample, especially if intelligent life exists elsewhere. Just as a person’s country of origin is a biased sample among countries, so too their planet of origin may be a biased sample among planets. The magnitude of this effect can be substantial: over 98% of the world’s population live in a country larger than the median. In the context of a simple model where the mean population density is invariant to planet size, we infer that a given inhabited planet (such as our nearest neighbour) has a radius $r<1.2 r_\oplus$ (95% confidence bound). We show that this result is likely to hold not only for planets hosting advanced life, but also for those which harbour primitive life forms. Further inferences may be drawn for any variable which influences population size. For example, since population density is widely observed to decline with increasing body mass, we conclude that most intelligent species are expected to exceed 300kg.

Read this paper on arXiv…

F. Simpson
Tue, 8 Dec 15
2/71

Comments: 5 pages, 3 figures; changes reflect version published in MNRAS

Early Telescopes and Ancient Scientific Instruments in the Paintings of Jan Brueghel the Elder [IMA]

http://arxiv.org/abs/1512.01347


Ancient instruments of high interest for research on the origin and diffusion of early scientific devices in the late XVI – early XVII centuries are reproduced in three paintings by Jan Brueghel the Elder. We investigated the nature and the origin of these instruments, in particular the spyglass depicted in a painting dated 1609-1612 that represents the most ancient reproduction of an early spyglass, and the two sophisticated spyglasses with draw tubes that are reproduced in two paintings, dated 1617-1618. We suggest that these two instruments may represent early examples of keplerian telescopes. Concerning the other scientific instruments, namely an astrolabe, an armillary sphere, a nocturnal, a proportional compass, surveying instruments, a Mordente’s compass, a theodolite, etc., we point out that most of them may be associated with Michiel Coignet, cosmographer and instrument maker at the Court of the Archduke Albert VII of Hapsburg in Brussels.

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P. Selvelli and P. Molaro
Mon, 7 Dec 15
26/46

Comments: Published in Astronomy and its instruments before and after Galileo, Edited by Luisa Pigatto and Valeria Zanini 2009, p193-208

Debris Engine: A Potential Thruster for Space Debris Removal [IMA]

http://arxiv.org/abs/1511.07246


We present a design concept for a space engine that can continuously remove the orbit debris by using the debris as a propellant. Space robotic cleaner is adopted to capture the targeting debris and to transfer them into the engine. Debris with larger size is first disintegrated into small pieces by using a mechanical method. The planetary ball mill is then adopted to grind the pieces into micrometer or smaller powder. The energy needed in this process is get from the nuclear and solar power. By the effect of gamma-ray photoelectric or the behavior of tangently rub of tungsten needles, the debris powered is charged. This behavior can be used to speed up the movement of powder in a tandem electrostatic particle accelerator. By ejecting the high-temperture and high-pressure charged powered from the nozzle of the engine,the continuously thrust is obtained. This thrust can be used to perform orbital maneuver and debris rendezvous for the spacecraft and robotic cleaner. The ejected charged particle will be blown away from the circumterrestrial orbit by the solar wind. By digesting the space debris, we obtain not only the previous thrust but also the clean space. In the near future, start trek will not just a dream, human exploration will extend to deep universe. The analysis shown, the magnitude of the specific impulse for debris engine is determined by the accelerating electrostatic potential and the charge-to-mass ratio of the powder.

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L. Lan, J. Li and H. Baoyin
Tue, 24 Nov 15
32/85

Comments: N/A

A New Empirical Constraint on the Prevalence of Technological Species in the Universe [EPA]

http://arxiv.org/abs/1510.08837


In this paper we address the cosmic frequency of technological species. Recent advances in exoplanet studies provide strong constraints on all astrophysical terms in the Drake Equation. Using these and modifying the form and intent of the Drake equation we show that we can set a firm lower bound on the probability that one or more additional technological species have evolved anywhere and at any time in the history of the observable Universe. We find that as long as the probability that a habitable zone planet develops a technological species is larger than ~$10^{-24}$, then humanity is not the only time technological intelligence has evolved. This constraint has important scientific and philosophical consequences.

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A. Frank and W. Sullivan
Fri, 30 Oct 15
23/54

Comments: 12 pages. 1 Figure. Submitted to AstroBiology

A direct communication proposal to test the Zoo Hypothesis [CL]

http://arxiv.org/abs/1509.03652


Whether we are alone in the universe is one of the greatest mysteries facing humankind. Given the >100 billion stars in our galaxy, many have argued that it is statistically unlikely that life, including intelligent life, has not emerged anywhere else. The lack of any sign of extraterrestrial intelligence, even though on a cosmic timescale extraterrestrial civilizations would have enough time to cross the galaxy, is known as Fermi’s Paradox. One possible explanation for Fermi’s Paradox is the Zoo Hypothesis which states that one or more extraterrestrial civilizations know of our existence and can reach us, but have chosen not to disturb us or even make their existence known to us. I propose here a proactive test of the Zoo Hypothesis. Specifically, I propose to send a message using television and radio channels to any extraterrestrial civilization(s) that might be listening and inviting them to respond. Even though I accept this is unlikely to be successful in the sense of resulting in a response from extraterrestrial intelligences, the possibility that extraterrestrial civilizations are monitoring us cannot be dismissed and my proposal is consistent with current scientific knowledge. Besides, issuing an invitation is technically feasible, cheap and safe, and few would deny the profound importance of establishing contact with one or more extraterrestrial intelligences. A website has been set up (this http URL) to encourage discussion of this proposal and for drafting the invitation message.

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J. Magalhaes
Tue, 15 Sep 15
81/83

Comments: 16 pages

Estimates for the number of visible galaxy-spanning civilizations and the cosmological expansion of life [CEA]

http://arxiv.org/abs/1507.05969


If advanced civilizations appear in the universe with a desire to expand, the entire universe can become saturated with life on a short timescale, even if such expanders appear but rarely. Our presence in an untouched Milky Way thus constrains the appearance rate of galaxy-spanning Kardashev type III (K3) civilizations, if it is assumed that some fraction of K3 civilizations will continue their expansion at intergalactic distances. We use this constraint to estimate the appearance rate of K3 civilizations for 81 cosmological scenarios by specifying the extent to which humanity could be a statistical outlier. We find that in nearly all plausible scenarios, the distance to the nearest visible K3 is cosmological. In searches where the observable range is limited, we also find that the most likely detections tend to be expanding civilizations who have entered the observable range from farther away. An observation of K3 clusters is thus more likely than isolated K3 galaxies.

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S. Olson
Thu, 23 Jul 15
15/39

Comments: 11 pages, 3 figures

At What Distance Can the Human Eye Detect a Candle Flame? [IMA]

http://arxiv.org/abs/1507.06270


Using CCD observations of a candle flame situated at a distance of 338 m and calibrated with observations of Vega, we show that a candle flame situated at ~2.6 km (1.6 miles) is comparable in brightness to a 6th magnitude star with the spectral energy distribution of Vega. The human eye cannot detect a candle flame at 10 miles or further, as some statements on the web suggest.

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K. Krisciunas and D. Carona
Thu, 23 Jul 15
19/39

Comments: 9 pages, 6 figures

Black Holes and the Scientific Process [CL]

http://arxiv.org/abs/1506.01957


Arguably, black hole is perhaps the most popular scientific term among the lay person. Perhaps it is the phrasing of the term ‘black hole’ which appeals to the popular imagination, offering some exotic visual of a cosmic object to the mind’s eye.

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M. Choudhury
Mon, 8 Jun 15
6/37

Comments: This is an article published in Planex Newsletter aimed at undergraduate and postgradute students and teachers. (The issue is available at this url: this http URL,%20Issue-1.pdf)

On the search for artificial Dyson-like structures around pulsars [CL]

http://arxiv.org/abs/1505.05131


Assuming the possibility of existence of a supercivilization we extend the idea of Freeman Dyson considering pulsars instead of stars. It is shown that instead of a spherical shell the supercivilization must use ring-like constructions. We have found that a size of the “ring” should be of the order of $(10^{-4}-10^{-1})$AU with temperature interval $(300-600)$K for relatively slowly rotating pulsars and $(10-350)$AU with temperature interval $(300-700)$K for rapidly spinning neutron stars, respectively. Although for the latter the Dyson construction is unrealistically massive and cannot be considered seriously. Analyzing the stresses in terms of the radiation and wind flows it has been argued that they cannot significantly affect the ring construction, indicating that the search for infrared ring-like sources close to slowly rotating pulsars seems to be quite promising.

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Z. Osmanov
Wed, 20 May 15
7/47

Comments: 7 pages, 3 figures

From the Scale Model of the Sky to the Armillary Sphere [CL]

http://arxiv.org/abs/1505.03732


It is customary to employ a semi-spherical scale model to describe the apparent path of the Sun across the sky, whether it be its diurnal motion or its variation throughout the year. A flat surface and three bent semi-rigid wires (representing the three solar arcs during solstices and equinoxes) will do the job. On the other hand, since very early times, there have been famous armillary spheres built and employed by the most outstanding astronomers for the description of the celestial movements. In those instruments, many of them now considered true works of art, Earth lies in the center of the cosmos and the observer looks at the whole “from the outside.” Of course, both devices, the scale model of the sky and the armillary sphere, serve to represent the movement of the Sun, and in this paper we propose to show their equivalence by a simple construction. Knowing the basics underlying the operation of the armillary sphere will give us confidence to use it as a teaching resource in school.

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A. Gangui, R. Casazza and C. Paez
Fri, 15 May 15
31/71

Comments: Published version available at this http URL

Animating Fermi – A Collaboration Between Art Students and Astronomers [CL]

http://arxiv.org/abs/1505.03390


Undergraduate animation students at the Maryland Institute College of Art teamed up with scientists from the Fermi Gamma-ray Space Telescope to produce a set of animations on several astronomy topics. We describe the process and discuss the results, including educational benefits and the cross-cultural experience. These animations are freely available online.

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L. Arcadias and R. Corbet
Thu, 14 May 15
10/57

Comments: 2 pages. Accepted for publication in Leonardo (Transactions). Main animations available at this https URL

Simulating the Phases of the Moon Shortly After Its Formation [CL]

http://arxiv.org/abs/1503.03041


The leading theory for the origin of the Moon is the giant impact hypothesis, in which the Moon was formed out of the debris left over from the collision of a Mars-sized body with the Earth. Soon after its formation, the orbit of the Moon may have been very different than it is today. We have simulated the phases of the Moon in a model for its formation wherein the Moon develops a highly elliptical orbit with its major axis tangential to the Earth’s orbit. This note describes these simulations and their pedagogical value.

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E. Noordeh, P. Hall and M. Cuk
Wed, 11 Mar 15
1/63

Comments: 4 pages, 2 figures

SETI at Planck Energy: When Particle Physicists Become Cosmic Engineers [HEAP]

http://arxiv.org/abs/1503.01509


What is the meaning of the Fermi Paradox — are we alone or is starfaring rare? Can general relativity be united with quantum mechanics? The searches for answers to these questions could intersect. It is known that an accelerator capable of energizing particles to the Planck scale requires cosmic proportions. The energy required to run a Planck accelerator is also cosmic, of order 100 M_sun c^2 for a hadron collider, because the natural cross section for Planck physics is so tiny. If aliens are interested in fundamental physics, they could resort to cosmic engineering for their experiments. These colliders are detectable through the vast amount of “pollution” they produce, motivating a YeV SETI program. I investigate what kinds of radiation they would emit in a fireball scenario, and the feasibility of detecting YeV radiation at Earth, particularly YeV neutrinos. Although current limits on YeV neutrinos are weak, Kardashev 3 YeV neutrino sources appear to be at least 30–100 Mpc apart on average, if they are long-lived and emit isotropically. I consider the feasibility of much larger YeV neutrino detectors, including an acoustic detection experiment that spans all of Earth’s oceans, and instrumenting the entire Kuiper Belt. Any detection of YeV neutrinos implies an extraordinary phenomenon at work, whether artificial and natural. Searches for YeV neutrinos from any source are naturally commensal, so a YeV neutrino SETI program has value beyond SETI itself, particularly in limiting topological defects. I note that the Universe is very faint in all kinds of nonthermal radiation, indicating that cosmic engineering is extremely rare.

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B. Lacki
Fri, 6 Mar 15
9/51

Comments: 22 pages, 3 figures

Citizen Science on the Faroe Islands in Advance of an Eclipse [IMA]

http://arxiv.org/abs/1503.00613


On 2015 March 20, a total solar eclipse will occur in the North Atlantic, with the Kingdom of Denmark’s Faroe Islands and Norway’s Svalbard archipelago (formerly Spitzbergen) being the only options for land-based observing. The region is known for wild, unpredictable, and often cloudy conditions, which potentially pose a serious threat for people hoping to view the spectacle.
We report on a citizen-science, weather-monitoring project, based in the Faroe Islands, which was conducted in March 2014 – one year prior to the eclipse. The project aimed to promote awareness of the eclipse among the local communities, with the data collected providing a quantitative overview of typical weather conditions that may be expected in 2015. It also allows us to validate the usefulness of short-term weather forecasts, which may be used to increase the probability of observing the eclipse.

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G. Sims and K. Russo
Tue, 3 Mar 15
61/69

Comments: N/A

Crowdfunding Astronomy with Google Sky [CL]

http://arxiv.org/abs/1502.07393


For nearly four years, NASA’s Kepler space telescope searched for planets like the Earth around more than 150,000 stars similar to the Sun. In 2008 with in-kind support from several technology companies, our non-profit organization established the Pale Blue Dot Project, an adopt-a-star program that supports scientific research on the stars observed by the Kepler mission. I describe how this innovative crowdfunding program has engaged the public over the past seven years to help support an international team in an era of economic austerity.

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T. Metcalfe
Fri, 27 Feb 15
20/60

Comments: 8 pages, 3 figures, submitted to Communicating Astronomy with the Public. Project website is at this http URL

Comparison of Space Propulsion Methods for a Manned Mission to Mars [CL]

http://arxiv.org/abs/1502.06457


We undertake a comparison of the latest developments in propulsion technologies, for a manned mission to Mars. The main objective is to assess the possibility of reducing travel time keeping the mass at departure within bounds. For the sake of comparison we used representative systems of different state of the art or proposed technologies, from the chemical engine to the “Pure Electro-Magnetic Thrust” (PEMT) concept, using a nuclear engine proposed by Rubbia. A mission architecture is suggested, based on existing mission proposals to Mars, to estimate the mass budget that influences the performance of the propulsion system. The trajectory of the spacecraft is determined by a numerical integration of the equations of motion and a partial optimization procedure, for the interplanetary phase with continuous thrust, and by conics and instant manoeuvres in the regions of influence of the departure and arrival planets. Pareto curves of the duration of the mission and time of flight versus mass of mission are drawn. We conclude that the ion engine technology, combined with the classical chemical engine, is the one which yields the shortest mission times with the lowest mass, and that chemical propulsion alone is the best to minimise travel time. The results obtained using the PEMT suggest that it is a more suitable solution for farther destinations than Mars.

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A. Guerra, O. Bertolami and P. Gil
Tue, 24 Feb 15
53/67

Comments: 31 pages, 7 figures and 2 tables

Using the Galileoscope in astronomical observations [IMA]

http://arxiv.org/abs/1501.07222


This project aims to attract school students and teachers from the state education system from Ca\c{c}apava do Sul – RS to Sciences and specially to Astronomy. We made astronomical observations using a Galileoscope choosing the Moon as a primary target. We also observed others objects that present high brightness in the night sky. The selection of targets, and their identification during the observations were carried out by a free software of planetary simulation, Stellarium. The results were in qualitative form and they show the great interest demonstrated by those participating in the project. Furthermore, this project helped to improve the understanding of the physical proprieties of the night sky objects (e.g. color). Finally, the project has showed that using a simple equipment and of relatively low cost it is possible to bring more people, specially the young students, to the Science World and to Astronomy.

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V. Oliveira and M. Silva
Thu, 29 Jan 15
3/49

Comments: 4 pages, text in Portuguese

Avoiding Intellectual Stagnation: The Starship as an Expander of Minds [CL]

http://arxiv.org/abs/1501.04249


Interstellar exploration will advance human knowledge and culture in multiple ways. Scientifically, it will advance our understanding of the interstellar medium, stellar astrophysics, planetary science and astrobiology. In addition, significant societal and cultural benefits will result from a programme of interstellar exploration and colonisation. Most important will be the cultural stimuli resulting from expanding the horizons of human experience, and increased opportunities for the spread and diversification of life and culture through the Galaxy. Ultimately, a programme of interstellar exploration may be the only way for human (and post-human) societies to avoid the intellectual stagnation predicted for the “end of history”.

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I. Crawford
Tue, 20 Jan 15
21/76

Comments: Published in the Journal of the British Interplanetary Society

On Magnetic Activity Band Overlap, Interaction, and the Formation of Complex Solar Active Regions [SSA]

http://arxiv.org/abs/1410.6411


Recent work has revealed an phenomenological picture of the how the $\sim$11-year sunspot cycle of Sun arises. The production and destruction of sunspots is a consequence of the latitudinal-temporal overlap and interaction of the toroidal magnetic flux systems that belong to the 22-year magnetic activity cycle and are rooted deep in the Sun’s convective interior. We present a conceptually simple extension of this work, presenting a hypothesis on how complex active regions can form as a direct consequence of the intra- and extra-hemispheric interaction taking place in the solar interior. Furthermore, during specific portions of the sunspot cycle we anticipate that those complex active regions may be particular susceptible to profoundly catastrophic breakdown—producing flares and coronal mass ejections of most severe magnitude.

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S. McIntosh and R. Leamon
Fri, 24 Oct 14
29/55

Comments: 14 pages, 5 figures, accepted to appear in ApJL

A Distributed Magnetometer Network [CL]

http://arxiv.org/abs/1409.5815


Various possiblities for a distributed magnetometer network are considered. We discuss strategies such as croudsourcing smartphone magnetometer data, the use of trees as magnetometers, and performing interferometry using magnetometer arrays to synthesize the magnetometers into the world’s largest telescope. Geophysical and other applications of such a network are discussed.

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J. Scoville, J. Spritzer and F. Freund
Tue, 23 Sep 14
18/60

Comments: N/A

EMMI – Electric Solar Wind Sail Facilitated Manned Mars Initiative [IMA]

http://arxiv.org/abs/1409.1036


The novel propellantless electric solar wind sail (E-sail) concept promises efficient low thrust transportation in the solar system outside Earth’s magnetosphere. Combined with asteroid mining to provide water and synthetic cryogenic rocket fuel in orbits of Earth and Mars, possibilities for affordable continuous manned presence on Mars open up. Orbital fuel and water eliminate the exponential nature of the rocket equation and also enable reusable bidirectional Earth-Mars vehicles for continuous manned presence on Mars. Water can also be used as radiation shielding of the manned compartment, thus reducing the launch mass further. In addition, the presence of fuel in Mars orbit provides the option for an all-propulsive landing, thus potentially eliminating issues of heavy heat shields and augmenting the capability of pinpoint landing. With this E-sail enabled scheme, the recurrent cost of continuous bidirectional traffic between Earth and Mars might ultimately approach the recurrent cost of running the International Space Station, ISS.

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P. Janhunen, S. Merikallio and M. Paton
Thu, 4 Sep 14
33/58

Comments: 13 pages, 2 figures, submitted to Acta Astronautica

The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification [CL]

http://arxiv.org/abs/1408.1133


We motivate the \^G infrared search for extraterrestrial civilizations with large energy supplies. We discuss some philosophical difficulties of SETI, and how communication SETI circumvents them. We review “Dysonian SETI”, the search for artifacts of alien civilizations, and find that it is highly complementary to traditional communication SETI; the two together might succeed where either one, alone, has not. We discuss the argument of Hart (1975) that spacefaring life in the Milky Way should be either galaxy-spanning or non-existent, and examine a portion of his argument that we dub the “monocultural fallacy”. We discuss some rebuttals to Hart that invoke sustainability and predict long Galaxy colonization timescales. We find that the maximum Galaxy colonization timescale is actually much shorter than previous work has found ($< 10^9$ yr), and that many “sustainability” counter-arguments to Hart’s thesis suffer from the monocultural fallacy. We extend Hart’s argument to alien energy supplies, and argue that detectably large energy supplies can plausibly be expected to exist because life has potential for exponential growth until checked by resource or other limitations, and intelligence implies the ability to overcome such limitations. As such, if Hart’s thesis is correct then searches for large alien civilizations in other galaxies may be fruitful; if it is incorrect, then searches for civilizations within the Milky Way are more likely to succeed than Hart argued. We review some past Dysonian SETI efforts, and discuss the promise of new mid-infrared surveys, such as that of WISE.

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J. Wright, B. Mullan, S. Sigur%7B%5Ceth%7Dsson, et. al.
Thu, 7 Aug 14
8/46

Comments: 18 pages, accepted for publication in the Astrophysical Journal