On Kepler’s 1284 New Planets

by Paul Gilster on May 11, 2016

If you look into the software that made possible yesterday’s exoplanet results, you’ll find that VESPA (Validation of Exoplanet Signals using a Probabilistic Algorithm) is freely available online. The work of Princeton’s Timothy Morton, who spoke at the announcement news conference, VESPA is all about calculating the probabilities of false positives for signals that look like transiting planets. Transits, of course, are what the Kepler space telescope has been about, catching the slight stellar dimming as a planet crosses across the face of a star.

The numbers quickly get mind-boggling because while Natalie Batalha (NASA Ames), joined by Morton, NASA’s Paul Hertz and Kepler/K2 mission manager Charlie Sobeck (a colleague of Batalha at Ames) could point to 1284 newly confirmed exoplanets, they represent only a fraction of what must be in the Kepler field of view. Out of its over 150,000 stars, Kepler can only see the planets that transit their host stars, making this a problem of orientation. We now have 2300 confirmed exoplanets in the Kepler catalog, but it’s clear that countless stellar systems are simply unviewable because they’re not lined up so as to make the identifying transit.

But back to VESPA and the technique that made yesterday’s announcement possible. The problems of false positives are legion when you’re looking at a light curve suggestive of a planet. For one thing, a brown dwarf or extremely low mass star may pass between Kepler and the star. For that matter, a larger star in a binary system may just ‘graze’ the limb of the host star, sending a planet-like signal that has to be untangled from the true planetary count. With these and other possibilities, we’ve relied upon verification through follow-ups, usually performed through radial velocity checks or even direct imaging of exoplanets.

All of this takes time and is resource-intensive, serious issues given the number of candidates (4700) found since launch. Morton calls the false positive signals ‘imposters,’ describing the VESPA method, which allows researchers to quantify the probability that any candidate signal is in fact a planet without requiring the lengthy follow-up investigations cited above. Two different kinds of simulation come into play, one involving transit signals and their causes, the other simulating how common the ‘imposter’ signals are likely to be in the Milky Way.

Kepler_fig01

Image: Kepler candidate planets (orange) are smaller and orbit fainter stars than transiting planets detected by ground-based observatories (blue). Credit: NASA Ames/W. Stenzel; Princeton University/T. Morton.

As this Princeton news release explains, the duration, depth and shape of a transiting planet signal are thus weighed against simulated planetary and false positive signals even as VESPA factors in the projected distribution and frequency of star types in the galaxy. Says Morton:

“If you have something that passes all those tests, then it’s likely to be a planet. We know small planets are common, so if Kepler sees a small-looking planet candidate and it passes the strict internal vetting, it’s more likely to be a planet than a false positive because it’s hard to mimic that signal with anything else… It’s easier to mimic something the size of Jupiter, and we know Jupiter-sized planets are less common. So the likelihood of a Jupiter-sized candidate actually being a planet that large is typically relatively low.”

VESPA works with information from both kinds of simulation to produce a reliability score between zero and one for each candidate signal. The candidates with a reliability greater than 99 percent are considered validated. The 1284 exoplanets announced yesterday all fit this standard, meeting what we can consider the minimum requirements for validation, while another 1327 candidates are considered likely to be planets although they do not score as high. 707 candidates turn out to be caused by non-planetary phenomena. It’s worth pointing out, too, that 984 candidates were revalidated — these had previously been verified by other methods.

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Image: Since Kepler launched in 2009, 21 planets less than twice the size of Earth have been discovered in the habitable zones of their stars. The orange spheres represent the nine newly validated planets announcement on May 10, 2016. The blue disks represent the 12 previous known planets. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. The sizes of the exoplanets indicate the sizes relative to one another. The images of Earth, Venus and Mars are placed on this diagram for reference. The light and dark green shaded regions indicate the conservative and optimistic habitable zone. Credit: NASA Ames/N. Batalha and W. Stenzel.

The announcement of 1284 confirmed exoplanets is the largest single announcement of new planets ever made, doubling the number of confirmed planets, with VESPA being used to calculate the reliability values of over 7000 signals from the latest Kepler catalog. This work gains additional weight as we consider the upcoming TESS (Transiting Exoplanet Survey Satellite) mission, which will be performing an all-sky survey of bright, nearby stars that will doubtless reveal tens of thousands of new candidates, all in need of confirmation.

Natalie Batalha recalled early work on transit photometry with a small robotic telescope at Mount Hamilton (CA), where researchers were plagued with false alarms, up to 70 percent of the signals proving to be non-planetary. Clearly, as Kepler became available, our methods increased greatly in accuracy, but as we move toward the final discovery catalog of this mission next year, we’ll be using methods like VESPA to continue sorting through candidate data, looking ahead not just to TESS, due to launch in 2017, but also the European Space Agency’s PLATO (PLAnetary Transits and Oscillations of stars ), designated for launch by 2024.

The paper is Morton et al., “False positive probabilities for all Kepler Objects of Interest: 1284 newly validated planets and 428 likely false positives,” Astrophysical Journal Vol. 822, No. 2 (10 May 2016). Abstract / preprint.

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KIC 8462852: Where Are We After Eight Months?

by Paul Gilster on May 10, 2016

The unusual star designated KIC 8462852, and now widely known as ‘Tabby’s Star,’ continues to be an enigma. As discussed in numerous articles in these pages, KIC 8462852 shows anomalous lightcurves that remain a mystery. Recently Michael Hippke explored a related question: Was the star dimming over time, as postulated by Louisiana State’s Bradley Schaefer? The two sharply disagreed (references below), leading Hippke and co-author Daniel Angerhausen to re-examine their conclusions. Now, with further collaboration from Keivan Stassun and Michael Lund (both at Vanderbilt University) and LeHigh University’s Joshua Pepper, Hippke and Angerhausen have a new paper out, peer-reviewed and accepted for publication by The Astrophysical Journal. What follows are Michael Hippke’s thoughts over the controversy as it stands today, with the dimming of KIC 8462852 again in doubt.

by Michael Hippke

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Tabetha Boyajian et al. released a paper on the preprint platform astro-ph in September 2015, which quickly got the Internet up to speed. Planet Hunters, an open community that searches data from the Kepler space telescope, found this unusual star that is now known as “Tabby’s Star”. It is observed to undergo irregularly shaped, aperiodic dips in flux of up to ~20%, much more than expected for any orbiting planets.

Media interest skyrocketed in October, when Jason Wright et al. released a preprint in which they discussed — among many other possibilities — the idea that the dips could originate from an alien race building a mega-size construction around the star, perhaps in the form of a “Dyson sphere”. Could it really be true that we found the first ever evidence of a powerful extraterrestrial civilization? A controversial discussion quickly ensued.

Further examination in other electromagnetic wavelengths only brought disappointing null results. The star was unremarkable in the infrared, showed no sign of artificial laser pulses, or radio emissions. The only somehow realistic astrophysical explanation was offered by Bodman & Quillan, who suggest the presence of a large family of comets, and which as of today are considered to be the “best” explanation.

comets-converging-on-star

Image: Cascading comets around a distant star (NASA/JPL/Caltech).

In January 2016, Bradley Schaefer released a preprint that examined historical photographic glass plates from the Harvard Observatory taken since 1889. His results seemed to show that “Tabby’s Star” had dimmed by 20% since that time, which was interpreted by many in the media as an indication of a quickly proceeding alien construction.

This was about the time when Daniel Angerhausen (an experienced astrophysicist at the NASA Goddard Space Flight Center) and I got interested. Our basic thought was: “If this were true, it would be the greatest thing in history!” We wanted to see the evidence with our own eyes, because honestly, we had big hopes ourselves. Initially. We think it is very human to be wanting to be part of something big, or at least seeing it happen. So, we had hopes that something big had been found, and we had the chance to see it happen.

So we dug into the Kepler data and found them rock-solid. These strange dips were really there! Also, we downloaded the historic Harvard data and plotted them. Schaefer had binned them in 5-year intervals, but behind these bins, there were actually over a thousand individual data points. When we plotted these on our screen, and overlaid a linear trend, we became very disappointed. The Harvard plates have an uncertainty (over 100 years) of order 0.1 to 0.2mag, and this was also visually evident. We had serious doubts that this dimming trend was valid.

We selected some comparison stars, which had similar scatter and trends, and decided that we would release our findings as a preprint on astro-ph. KIC 8462852 is a very special case not only scientifically but also in the way it was discussed in the community. It became one of the most publicly discussed astronomical objects. Many articles on this fascinating object (including its original detection) were published on astro-ph before peer review and some even called it a revolution in scientific discussion, making it real time and on various social media channels involving the public.

Bradley Schaefer himself gave at least 2 interviews on the day his (at that time not peer-reviewed) manuscript came out. Following his own arguments that “putting up unchecked and false claims is bad all the way around”, we had no other choice than putting our doubts on his results out immediately, so that the community, the involved media and the interested public did not have to wait many months for the formal rebuttal. We also decided to do this to keep the discussion public and have interested laymen follow it; we even gave a recipe to reproduce the data using the publicly available Harvard DASCH [Digital Access to a Sky Century @ Harvard] data for the interested reader [see KIC 8462852: No Dimming After All?].

The immediate reactions to our preprint were overwhelmingly negative, just as most of the other publications on KIC 8462852 have been. In retrospect, we attribute this to two very human factors. The first was that some of our previous assumptions and methods were indeed inconsistent and the choice of some comparison stars questionable. But errors are human, and everybody makes them at times! While we believe that these errors did reduce the clarity of our result, we still believed that the result itself was correct. This view was not shared by Bradley Schaefer, who published his reply on Centauri Dreams [see Bradley Schaefer: A Response to Michael Hippke]. The other issue we see was that most readers wanted to believe that this thing was real. Schaefer was an authority, while Hippke was described as a “novice”, and “proof by authority” seemed to weigh in.

As this time, we decided to do two things. First of all, we cut all media connections. Second, we started collecting all issues and questions raised by Schaefer, and many others in the community and here on Centauri Dreams. “Tabby’s Star” started off as a community project, and it continued to be one! The pure number of emails we received was enormous. Our favourite mail, which was received via hand-written paper mail actually, was by an American who said that he was once captured by these aliens himself and is a first-hand witness!

Most feedback was incredibly helpful, however. We gained several co-authors who contributed several more analyses, statistical tools and better ways to select comparison stars. We also got an invitation to the ASTROPLATE conference in Prague, where Michael Hippke met most of the glass photography community. We spent a whole week discussing calibration techniques, scanners, and fungus (one of the reasons why digitization of these plates is so important!). These fungi are actually called “gold disease” owing to their look on the glass. We enjoyed long evening talks about the millions of plates that still await their scientific use, and after all of this, had the chance to discuss our own findings in a presentation and discussion.

During all this, and afterwards, we continued the peer-review process at the Astrophysical Journal. We have published several papers in this journal before, and, as always, it was a very professional process. Yet, they appointed two referees instead of the usual one, and we went through several rounds of question-and-answer to nail down every detail. This took considerable time. Now the paper has been accepted for publication (and is updated on astro-ph). From the feedback we got, and given the much more detailed than usual review process, the study is probably one of the most solid and waterproof papers we ever published. We believe that it is established beyond any reasonable doubt that no dimming can be found within the uncertainties of 0.2mag per century.

Now, what does that mean for the mystery? Are there no aliens after all? Probably not! Still, the day-long dips found by Kepler are real. Something seems to be transiting in front of this star. And we still have no idea what it is!

The cool message here, however, is that we now for the first time in human history have technology that can at least in theory detect such things, with upcoming missions such as JWST and PLATO. To solve the mystery, there are now several more projects under way. The American Association of Variable Star Observers (AAVSO) has collected thousands of amateur astronomer observations to discover new dips. Others, such as the Las Cumbres Observatory Global Telescope (LCOGT) have joined the effort. Observing further dips in different colors can reveal information about the chemistry of the transiting object, which might confirm or reject a cometary origin. Who knows, perhaps these telescopes have already captured some exciting new data, and any day researchers might publish a paper that solves the mystery!

The paper is Hippke et al., “A statistical analysis of the accuracy of the digitized magnitudes of photometric plates on the time scale of decades with an application to the century-long light curve of KIC 8462852,” accepted at the Astrophysical Journal (preprint). A Vanderbilt news release is also available.

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Beamed Sail Concepts Over Time

by Paul Gilster on May 9, 2016

If you’ve been following the Breakthrough Starshot concept in these pages and elsewhere, you’ll know that it’s small at one end and big on the other. A beamed sail mission, it would use sails four meters to the side — quite small by reference to earlier beamed sail designs — driven by a massive phased laser array on the Earth. The array is projected to be a kilometer to the side, incorporating laser emitters working in perfect synchronization to produce what Pete Worden, formerly of NASA Ames, described in Palo Alto as “a laser wind of 50 gigawatts.” Worden is now executive director of Breakthrough Starshot.

As with the sail, so with the payload. We have no macro-scale spacecraft here but a ‘Starchip’ about the size of a postage stamp, making it a kind of futuristic smartphone containing not just cameras, communication equipment and navigation instruments but tiny thrusters. If you want to imagine something like this, you take trends in digital technology like Moore’s Law and extrapolate them forward, for this is a generation-length project aiming at an actual mission in, at minimum, thirty years, unless the concept studies find an irrevocable showstopper.

Sails and their Origins

As Breakthrough Starshot begins to organize and get its concept study launched, I’ve been looking back at its roots. Several things come to mind, the first of which is the kind of sail Robert Forward once talked about. In an earlier post, I talked about Starshot as “classic Bob Forward thinking rotated according to the symmetries of our new era,” and I think that’s about right. In the sense that Forward was the first to discuss putting a laser beam on a sail, the project draws undeniable inspiration from him. But it also takes his ideas in entirely new directions.

When I talked to Phil Lubin (UC-Santa Barbara) about Starshot, he referenced his Roadmap to Interstellar Flight paper, one that clearly draws on work he has been developing for the NASA Innovative Advanced Concepts (NIAC) office, where he received a 2015 grant. The ‘Roadmap’ paper, submitted to the Journal of the British Interplanetary Society, also makes use of another JBIS paper Lubin wrote called Directed Energy for Relativistic Propulsion and Interstellar Communications. And the Starchip concept itself has roots in Mason Peck’s continuing work on the tiny craft called ‘sprites’ at Cornell University.

Forward’s sail ideas were as large as the Breakthrough Starshot sail is small. Consider this: For a 1984 paper, he considered a vehicle massing 80,000 tons that would be brought up to half the speed of light by a sail fully 1000 kilometers across. A power station in the inner Solar System would power up a 75,000 TW laser system, and the beam would rely on a huge Fresnel lens in the outer system to keep it focused on the departing starship. Forward even imagined a ‘staged sail’ concept that would allow deceleration, rendezvous, and Earth return for his crew.

If you’re interested in this era and the work it produced, be aware that JBIS was a major venue for sail concepts in the 1980s. It was here that Gregory Matloff published his classic 1984 paper “Solar Sail Starships – The Clipper Ships of the Galaxy” (JBIS 34, 371-380), one that would be followed by a series of optimization papers in the same venue, and also developed in his book The Starflight Handbook (Wiley, 1989). The latter contains Forward’s endorsement, one I consider a classic in the book blurb business: “Don’t leave the Solar System without it!” Matloff became one of the leading theorists on sail design along with Forward’s explicitly named successor, writer and physicist Geoffrey Landis.

Here I need to remind those who haven’t already seen it that Landis’ new paper “Mission to the Gravitational Focus of the Sun: A Critical Analysis” (preprint) looks at problems at realizing the FOCAL concept, in particular dealing with the question of imaging capabilities. And we’ve looked recently at Claudio Maccone’s ideas on FOCAL for communications in Starshot and the Gravitational Lens.

You can see that what we have at play here is the evolution of technology making new space concepts possible. Indeed, part of the Breakthrough Starshot premise is that further evolution during the thirty year ‘window’ for the system to be developed will solve many of the intractable problems that face it. The list of issues, as we’ve seen in previous posts, is large, ranging from getting a huge phased laser array to perform to keeping the small sails on the beam for the agonizing two minutes of acceleration at 60,000 g’s. Not to mention getting data back to Earth.

mann_fig01

Image: A futuristic beamed sail design as depicted by Adrian Mann (no relation here to Breakthrough Starshot). Thanks to Jim Benford for passing it along.

Space Sailing in Italy

Thinking about using the gravitational lens of the Sun for communications and imaging calls to mind the rich heritage solar sailing draws upon from Italian scientists. The gravitational lens mission that Maccone has come to call FOCAL can trace its origins back to the late 1980s. Here I should mention Quasat, which was conceived as an Earth-orbiting radio telescope based on an inflatable sail technology.

The Quasat notion came from Italian aerospace company Alenia Spazio. Gregory Matloff would thoroughly analyze the Quasat concept in a 1994 paper, discussing a high-reflectivity sail with an area of 10,000 square meters at a thickness of 2 μm, with an aluminum reflective layer with a thickness of 0.1 μm, carrying a 100 kg payload. Adapted for the gravity focus mission, the sail would, with the help of a close Solar gravity assist and perhaps an assist at Jupiter as well, reach the gravity focus at 550 AU in approximately sixty years.

A gravity focus mission is quite a stretch now and it was even more of one in the early 1990s, which is one reason the European team involved in the Quasat and other sail work turned to a project called Aurora. The effort was conceived at the International Astronautical Congress in Graz, Austria in 1993, with a core team led by Giovanni Vulpetti and including both Claudio Maccone and Italian engineer and author Giancarlo Genta. This turned out to be highly influential work, producing fifteen papers and presentations to European space agencies.

What became known as the Aurora Collaboration would produce a thin-film 250-meter square sail design that would venture to the heliopause and beyond at speeds about three times that of the Voyager probes. The issues it addressed were numerous, as explained in Solar Sails: A Novel Approach to Interplanetary Travel (Springer, 2008), written by Vulpetti, Les Johnson and Greg Matloff:

  • Considering SPS propulsion for realistic extra-solar exploration;
  • Investigating mission classes and related technological implications for significantly reducing the flight time, from departure to the target(s);
  • Analyzing flight profiles; and
  • Sizing sailcraft’s main systems for a technology demonstration mission to be proposed to the space agencies.

In a six-year period ending in 2000, the Aurora Collaboration would analyze telecommunications systems, the optical properties of sails, their optimized trajectories all the way out to the gravitational lens, communications options and the means of reducing the thickness of the sail. The group was an entirely self-supporting initiative whose work relied on the voluntary efforts of top scientists in the area of sail design. It would have later reverberations in NASA’s work on precursor interstellar missions and the European ESA/ESTEC heliopause probe.

I’m thinking about Aurora today particularly because I’ve just heard from Giovanni Vulpetti that he has made available a series of files of recent lectures he made at the University of Rome. Have a look at Dr. Vulpetti’s Astrodynamics and Propulsion website and click on Lectures to see over 400 slides dealing with sailcraft trajectories, thrust calculations, solar photon and plasma flow and a great deal more. Those of you interested in delving into the technical aspects of various sail configurations including magnetic sails will have plenty of material to work with here. Digging around in the site you’ll also find Vulpetti’s Problems and Perspectives in Interstellar Exploration paper now available in its entirety online.

What a pleasure it must be for the scientists who developed key sail concepts to see the actual deployment of sails like IKAROS in space. Now their work is being examined anew as we look for ways to continue reducing the size and thickness of sails, and ponder how best to get a propulsive beam onto a sail for acceleration up to a substantial percentage of the speed of light. Breakthrough Starshot continues to take the process forward as it launches its concept study, and in doing so draws upon a rich history of work in the service of space sailing.

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Toward a Space-based Anthropology

by Paul Gilster on May 6, 2016

Cameron Smith is no stranger to these pages, having examined the role of evolution in human expansion into space (see Biological Evolution in Interstellar Human Migration), cultural changes on interstellar journeys (Human Universals and Cultural Evolution on Interstellar Voyages), as well as the composition of worldship crews (Optimal Worldship Populations). An anthropologist and prehistorian at Portland State University, Dr. Smith today offers up his thoughts on the emerging discipline he calls space anthropology. How do we adapt a field that has grown up around the origin and growth of our species to a far future in which humans may take our forms of culture and consciousness deep into the galaxy? What follows is the preface for Dr. Smith’s upcoming book Principles of Space Anthropology: Establishing an Evolutionary Science of Human Space Settlement, to be published by Springer later this year.

By Cameron M. Smith, PhD

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New Realms of Action Require New Domains of Expertise

In 1963, Siegfried J. Gerathewohl, NASA’s biotechnology chief, wrote the following passage early in his foundation text, Principles of Bioastronautics, outlining the need for this new field of study:

“Manned excursions into space require new types of vehicles, machines and hardware which were unknown in conventional flying. They will carry the traveler into such foreign environments as to pose serious problems of health and survival. The new field of medicine, which studies the human factors involved and the protective measures required, has been called space medicine. From its cooperation with modern technology, particularly with electronics, cybernetics, physics, and bionics, space biotechnology has branched out as a novel field of bioengineering.” [1:5-6]

At the time of that publication, less than ten people had been in space; the moon landings were yet vague plans, the robotic reconnaissance of our solar system was in its infancy and virtually nothing was known of human biology in space. Two generations later, space exploration and space sciences are at an historical apex of activity and rapid technical progress. Low Earth Orbit has been continuously occupied by at least one person, continuously, for over a quarter century, yielding thousands of scientific studies on space biology; Mars has been swarmed by robotic explorers seeking traces of life and mapping landscapes for human exploration and settlement; dozens of private companies and even individuals are re-inventing basic space exploration technologies with cheaper materials and methods than those of last century’s space age with the aim of lowering the cost of space access, and astronomy has entered a new age, with space-based technologies identifying multitudes of exoplanets now slated to be examined for traces of life with methods just coming on-line.

One significant outcome of these many efforts to better understand our stellar neighborhood will be the settlement of space by populations of humans and their domesticates. The ancient dream of setting off across space to explore and settle new lands—for freedom, exploration, economic advantage, the safeguarding of humanity by spreading out from the home planet, and a multitude of other motivations—appears more likely than ever, and its earliest steps are being taken now. For example, the SpaceX corporation was founded “…to revolutionize space technology, with the ultimate goal of enabling people to live on other planets” [2] and indeed in October 2016 Elon Musk is set to announce detailed Mars settlement plans. Such proposals involve not just individual people but populations, which have their own biological and behavioral (cultural) properties. In the same way that space exploration required Gerathewohl’s bioastronautics, space settlement planning requires a field of study to ensure that plans are designed and carried out informed by all we know of the adaptive tools and techniques of our species.

A New Branch of Anthropology

Traditionally, the study of humanity’s adaptations has been the domain of anthropology. Over the last century this field has capably documented our species’ remote origins, long and complicated evolution, and myriad manifestations in the present, but it has only occasionally (and then unsystematically) forayed into humanity’s distant future (e.g. see [3,4,5]). It is a premise of this book that that future should include the human settlement of environments beyond Earth, particularly for the purposes of safeguarding humanity’s apparently unique mode of consciousness, its hologenome and many of its domesticates, and the totality of human knowledge—accumulated over about 3,000 generations since the origins of behavioral modernity—by the method of establishing populations of humanity culturally and biologically independent of our home planet. I discuss arguments for space settlement in [5] and [6], but in the present book I focus on how the resources and expertise of anthropology may be deployed to assist in the goals of human space settlement.

While bioastronautics was established during the First Space Age (hereafter FSA) with tight focus on safeguarding the short-term health of individuals or small crews, today, plans include space settlement by communities, which raises many new issues; individual physiology is a different phenomenon than, say, population genetics, and individual psychology as short-term adaptation is different from cultural adaptation by reshaping cultural norms in accordance with new circumstances; I tabulate some other such differences below.

Chief Differences Between Space Exploration and Space Settlement Relating to Adaptation

 Space ExplorationSpace Settlement
Goalsspecific, short-termgeneral, long-term
Group Sizesmall (crews)large (communities)
Social Organizationcommand hierarchycivil community
Essential Social Unitscrewsfamilies and communities
Adaptive Meanstechnological, individual behavior, and some reversible acclimatizationtechnological, cultural and biological adaptation
Adaptive Timescaleshort; weeks to monthslong; multigenerational

For these reasons a new field of study is required. In this book I propose, describe, and outline the scope of space anthropology or exoanthropology, and present some of my own results in this new discipline.

In the same way that Gerathewohl identified the need for his field in the quotation at the opening of this Preface, below I formally outline the need for space anthropology:

Space settlement will require novel biological and cultural adaptations to support populations of humans, on multigenerational timescales, in environments so far unfamiliar to our species even after 100,000 years of human cultural and biological adaptation to myriad Earth environments. The new field of anthropology that studies such adaptive efforts is space anthropology or exoanthropology, exo- referring to beyond Earth, in the same way it is used in the term exobiology.

Specifically, I propose space anthropology to have three main functions:

  • 1. To identify the biological and cultural adaptive suite of humanity globally and to date, resulting in a catalog of our species’ adaptive tools and capacities useful to space settlement planners.
  • 2. To evaluate the capacities of humanity’s various adaptive tools to adapt to reasonably forseeable space settlement plans, bettering the prospect of productive adaptations to new conditions, e.g. on Mars.
  • 3. To make recommendations, some broad and some specific, that would assist in human adaptation to environments beyond Earth, particularly based on evaluations of human adaptive capacities identified in functions 1 and 2.

The scope of exoanthropology, then, will be broad. I propose it as an applied form of anthropology with the specific goal of evaluating the adaptive capacities of our species, both biologically and culturally, so that they may be best deployed to assist in successful permanent space settlement. This will guide space settlement planning in a genuinely adaptive and evolutionarily-informed way, applying the lessons of billions of years of Earth life adaptation to what I consider to be the completely natural and expected dispersal of life throughout the solar system and beyond. This book, then, will thoroughly review the phenomenon of evolutionary adaptation, particularly among our species.

Human ‘adaptive tools’ are biological and cultural (which subsumes technology) [7]; an array of such adaptations so far recognized in the Earth’s cold, high altitude and hot regions are tabulated below as examples—these will be fully explored later in this book.

Some Human Adaptations to Earth Environments

BiomeLimiting FactorsBiological, Cultural and Technological Adaptations
Arctic / Cold*Extremely low temperatures for long periods

*Extreme light / dark seasonal cycles

*Low biological productivity
Biological
* increased Basal Metabolic Rate
* increased shivering, vasoconstriction and cold thermoregulation activity and efficiency
* compact, heat-retaining body stature

Cultural and Technological
* bilateral kinship = demographic flexibility
* clothing insulates but can prevent sweating
* semi-subterranean housing including igloo made of local, free, inexhaustible reosurce (snow)
* high fat diet yielding many calories and vitamins
* low tolerance of self-aggrandizement
* low tolerance of adolescent bravado
* high value of educating young
* social fission
* mobile, field-maintainable, reliable tools
* population control methods including voluntary suicide, infanticide
* high value on apprenticeship
* low tolerance for complaint: 'laugh don't cry'

High Altitude* Low oxygen pressures

* Nighttime cold stress

* Low biological productivity

* High neonatal mortality
Biological
* dense capillary beds shorten distance of oxygen transport
* larger placenta providing fetus with more blood-borne oxygen
* greater lung ventilation (capacity)

Cultural and Technological
* promotion of large families to offset high infertility
* use of coca leaves to promote vasoconstriction and caffeine-like alertness
* woolen clothing retains heat when wet
* trade connections with lowland populations
Arid / Hot* Low and uncertain rainfall

* High evaporation rate

* Low biological productivity
Biological
* tall, lean, heat-dumping body
* lowered body core temperature
* increased sweating efficiency
* lower urination rate
* increased vasodilation efficiency

Cultural & Technological
* flexible kinship & land tenure system = demographic flexibility matching shifting water resources
* intercourse taboo maintain sustainable population
* loose, flowing clothing blocks sunlight
* wide sandals block ground-reflected sunlight
* nakedness socially accepted during physical labor

In fulfilling Function 1, exoanthropology will survey humanity’s adaptations through time and across the globe, identifying patterns pertinent to space settlement planners. In fulfilling Function 2, it will review the adaptive competence of many of our species’ adaptive tools, allowing us to evaluate our readiness for space settlement and, where we find ourselves unready, suggest courses of action; it will also characterize forseeable space settlement conditions and limiting factors as needed. In fulfilling Function 3, recommendations for space settlement planners will be formulated, varying in specificity, based on the lessons identified in the surveys serving Functions 1 and 2. Finally, in fulfilling Function 4, directly actionable engineering and other design recommendations will be made, materially assisting in space settlement planning.

References to Author’s Preface

1. Gerathewohl, S. 1963. Principles of Bioastronautics. Prentice-Hall, New Jersey.

2. SpaceX website (accessed 14 April 2016): http://www.spacex.com/about.

3. Finney, B. and E. Jones (eds). 1985. Interstellar Migration and the Human Experience. Berkeley, University of California Press.

4. Finney, B. 1992. Space Migrations: Anthropology and the Humanization of Space. NASA SP-509: Space Resources, Volume 4: Social Concerns. Washington, D.C.

5. Smith, C.M. and E.T. Davies. 2012. Emigrating Beyond Earth: Human Adaptation and: Space Colonization. Springer, Berlin.

6. Smith, C.M. and E.T. Davies. 2005. The Extraterrestrial Adaptation. Spaceflight 47(12):46.

7. Morphy, H. and G. Harrison (eds). 1998. Human Adaptation. Oxford: Oxford University Press.

© 2016 by Cameron M. Smith, PhD

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Pluto: Unusual Interactions with the Solar Wind

by Paul Gilster on May 5, 2016

David McComas (Princeton University) calls what his team of researchers have learned about the solar wind at Pluto ‘astonishing,’ adding “This is a type of interaction we’ve never seen before anywhere in our Solar System.” The reference is to data from the Solar Wind Around Pluto (SWAP) instrument that flew aboard New Horizons. McComas knows the instrument inside out, having led its design and development at the Southwest Research Institute.

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Image: The first analysis of Pluto’s interaction with the ubiquitous space plasma known as the solar wind found that Pluto has some unique and unexpected characteristics that are less like a comet and more like larger planets. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

What startled McComas was that Pluto’s interactions with the solar wind are nowhere near what had been predicted. This stream of charged particles flowing outbound from the Sun can reach speeds of 500 kilometers per second and above, a ragged, bursting outrush that we may one day be able to capitalize on as a way to drive spacecraft. While planets act to divert the solar wind, comets slow it far more gently, an effect that researchers assumed they would find at Pluto.

Instead, Pluto’s complexities place it somewhere between planet and comet as its atmosphere copes with the solar wind. We learn in the paper from McComas and team that despite expectations, Pluto’s gravity is capable of holding electrically charged ions in its extended atmosphere. The dwarf planet has an ion tail extending ‘downwind’ to a distance of about 100 Pluto radii (120,000 kilometers). Earth has a similar ion tail, and Pluto’s is said to show ‘considerable structure.’

From the paper:

Initial studies of the solar wind interaction with Pluto’s atmosphere…, all assuming the absence of an intrinsic magnetic field, suggested that it would depend on whether the atmospheric escape flux is strong — producing a ‘comet-like’ interaction where the interaction region is dominated by ion pick-up and many times larger than the object — or weak — producing a ‘Mars-like’ interaction dominated by ionospheric currents with limited upstream pick-up and where the scale size is comparable to the object.

And Pluto, we learn, behaves more like its larger planetary cousins, as the image below suggests.

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Image: This figure shows the size scale of the interaction of Pluto (at lower left) with the solar wind. Scientists thought Pluto’s gravity would not be strong enough to hold heavy ions in its extended atmosphere, but Pluto, like Earth, has a long ion tail (red) loaded with heavy ions from the atmosphere. Pluto has a very thin “Plutopause” (purple), or the boundary of Pluto’s tail of heavy ion tail and the sheath of the shocked solar wind (blue) that presents an obstacle to its flow. Credit: American Geophysical Union.

The SWAP instrument was able to separate heavy ions of methane, which is the primary gas escaping the atmosphere, from the light hydrogen ions coming from the Sun. From it we also learn that Pluto has a thin ‘Plutopause,’ a boundary region where the heavy ion tail meets the solar wind along a sheath of particles. Moreover, the solar wind is not blocked until it reaches within about about 3000 kilometers of the dwarf planet — the paper calls this the ‘upstream standoff distance’ — a bit less than three Pluto radii.

As the paper notes, we’ll have a good deal of data from different parts of the Solar System to examine as we try to put Pluto into context:

While the small size of the interaction region relative to Pluto is reminiscent of Mars and Venus, we note that recent observations of the solar wind interaction with the relatively weakly out-gassing Comet 67P Churyumov-Gerasimenko by instruments on ESA’s Rosetta spacecraft show deflection of the solar wind with relatively modest decrease in speed. We anticipate interesting scientific discussions of the relative roles of atmospheric escape rate, solar wind flux, and IMF strength at Mars, Comet 67P and Pluto as the data from the MAVEN, Rosetta and New Horizons spacecraft are further analyzed.

And as McComas notes in this Princeton news release, “The range of interaction with the solar wind is quite diverse, and this gives some comparison to help us better understand the connections in and beyond our solar system. The SWAP data will … be reanalyzed … for many years to come as the community collectively grapples with Pluto’s unique solar wind interaction — one that is unlike that at any other body in the solar system.”

The paper is McComas et al., “Pluto’s Interaction with the Solar Wind,” accepted at the Journal of Geophysical Research: Space Physics (abstract / preprint).

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C/2014 S3: ‘Manx Object’ from the Oort Cloud

by Paul Gilster on May 4, 2016

When you don’t have the technology to get to an interesting place like the Oort Cloud, it’s more than a little helpful when nature brings an Oort Cloud object to you. At least we think that the object known as C/2014 S3 (Pan-STARRS) has moved into the warmer regions of the Solar System from the Oort. A gravitational nudge in that distant region would be all it took to send the object, with an orbital period now estimated to be 860 years, closer to the Sun.

And here things get interesting, because C/2014 S3 is the first object discovered on a long-period cometary orbit that shows all the spectral characteristics of an inner system asteroid. The level of activity on the object, apparently the result of sublimation of water ice, is five to six orders of magnitude lower than what we would expect from an active long-period comet at a similar distance from the Sun. Karen Meech (University of Hawaii) and colleagues believe that the object formed in the inner system at about the same time as the Earth, after which it was ejected before it could be bathed by long-term solar radiation.

Meech calls C/2014 S3 “…the first uncooked asteroid we have found,” which means we’re not only dealing with a building block of the early Solar System, but one that has been preserved for billions of years in a pristine environment. Further underlining its unusual status is the fact that C/2014 S3 is not developing the tail we would expect from a long-period comet as it approaches the Sun. Hence the moniker ‘Manx object’ given by Meech and team, a reference to the tail-less breed of cat from the Isle of Man.

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Image: Observations with ESO’s Very Large Telescope, and the Canada France Hawaii Telescope, show that C/2014 S3 (Pan-STARRS) is the first object to be discovered that is on a long-period cometary orbit, but that has the characteristics of a pristine inner Solar System asteroid. It may provide important clues about how the Solar System formed. This image of the comet was acquired using the Canada France Hawaii Telescope. Credit: K. Meech (IfA/UH) / CFHT/ESO.

A population of objects of the C/2014 S3 class would be useful indeed, and as this University of Hawaii Institute for Astronomy news release suggests, would help us differentiate between differing models of Solar System development. The problem is that while several of these models can duplicate the Solar System’s current configuration, some require the migration of the gas giants while others do not. Moreover, the models yield different predictions on the amount of rocky material expelled early on into the Oort Cloud.

Just how widely these models vary is explained in the paper. Here it describes the gas giant migration model:

The “Grand Tack” model starts the simulation of solar system formation at an early phase, when the giant planets grew and migrated in a gas-rich protoplanetary disk. During their inward migration, the giant planets scattered inner solar system material outward; during their outward migration, they implanted a significant amount of icy planetesimals [from 3.5 to 13 astronomical units (AU)] into the inner solar system. The Grand Tack model predicts the presence of rocky objects in the Oort cloud at an icy comets/rocky asteroids ratio of 500:1 to 1000:1…

But how we view the Oort Cloud varies depending upon which model we accept:

Other dynamical models, which assume nonmigrating giant planets, make different predictions about the fraction of the Oort cloud population comprising planetesimals initially within the asteroid belt or the terrestrial planet region. These predictions range from 200:1 to 2000:1. Other models do not explicitly estimate the mass of rocky planetesimals eventually implanted in the Oort cloud, but from the amount initially available, it is reasonable to expect that the ratio of icy planetesimals to rocky planetesimals in the final Oort cloud is between 200:1 to 400:1. Instead, a recent radically different model of terrestrial planet formation predicts that the planetesimals in the inner solar system always had a negligible total mass; in this case, there would be virtually no rocky Oort cloud population.

Be aware that C/2014 S3 is not the first inactive object discovered on a long-period comet orbit, with 1996 PW, found in 1996, being characterized as an extinct comet or asteroid ejected into the Oort Cloud. Five other ‘Manx candidates’ have been observed by Meech’s team, all of them showing colors similar to 1996 PW. C/2014 S3 is the only candidate to date that shows the spectrum of an S-type asteroid, a type usually found in the main asteroid belt.

So it’s an intriguing find, and according to the paper, learning how many S-type objects like it exist in the Oort Cloud will be a useful test of the various models. The paper argues that making a selection between the models will require characterization of up to 100 such ‘Manx objects,’ with the number of S-types helping us determine the ratio of icy to rocky objects in the Oort. As we are now discovering Manx objects on the order of about 15 per year as the Pan-STARRS project continues, scientists will soon have a sufficient number to work with.

The paper is Meech et al., “Inner solar system material discovered in the Oort cloud,” Science Advances Vol. 2, No. 4 (29 April 2016). Full text.

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Perspectives on Cosmic Archaeology

by Paul Gilster on May 3, 2016

I’ve always found the final factor in the Drake Equation to be the most telling. Trying to get a rough idea of how many other civilizations there might be in the galaxy, Drake looked at factors ranging from the rate of star formation to the fraction of planets suitable for life on which life actually appears. Some of these items, like the fraction of stars with planets, are being clarified almost by the day with continuing work. But the big one at the end — the lifetime of a technological civilization — remains a mystery.

By ‘technological,’ Drake was referring to those civilizations that were capable of producing detectable signals; i.e., releasing electromagnetic radiation into space. And when we have but one civilization to work with as example, we’re hard pressed to know what this factor is. This is where Adam Frank (University of Rochester) and Woodruff Sullivan (University of Washington, Seattle) come into the picture. In a new paper in Astrobiology, the researchers argue that there are other ways of addressing the ‘lifetime’ question.

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Image credit: http://www.ForestWander.com [CC BY-SA 3.0 us], via Wikimedia Commons.

What Came Before Us

The idea is to calculate how unlikely our advanced civilization would be if none has ever arisen before us. In other words, Frank and Sullivan want to put a lower limit on the probability that technological species have, at any time in the past, evolved elsewhere than on Earth. Here’s how their paper describes this quest:

Standard astrobiological discussions of intelligent life focus on how many technological species currently exist with which we might communicate (Vakoch and Dowd, 2015). But rather than asking whether we are now alone, we ask whether we are the only technological species that has ever arisen. Such an approach allows us to set limits on what might be called the ‘‘cosmic archaeological question’’: How often in the history of the Universe has evolution ever led to a technological species, whether short- or long-lived? As we shall show, providing constraints on an answer to this question has profound philosophical and practical implications.

To do this, the authors produce their own equation drawing on Drake’s. Consider A the number of technological civilizations that have formed over the history of the observable universe. Rather than dealing with Drake’s factor L — the lifetime of a technological civilization — Frank and Sullivan propose what they call an ‘archaeological form’ of Drake’s equation. The need for the L factor disappears. The new equation appears in this form:

A = Nast * fbt

Where A = The number of technological species that have evolved at any time in the universe’s past

Nast = The number of habitable planets in a given volume of the universe

fbt = The likelihood of a technological species arising on one of these planets.

You can see that what Frank and Sullivan rely on are recent advances in the detection and characterization of exoplanets. We’re learning a great deal more about how common planets are and how many are likely to orbit in the habitable zone around their star, where liquid water could exist. Their term Nast relies on this work and draws together various terms from the original Drake equation including the total number of stars, the fraction of those stars that form planets, and the average number of planets in the habitable zone of their stars.

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From the paper:

With our approach we have, for the first time, provided a quantitative and empirically constrained limit on what it means to be pessimistic about the likelihood of another technological species ever having arisen in the history of the Universe. We have done so by segregating newly measured astrophysical factors from the fully unconstrained biotechnical ones, and by shifting the focus toward a question of ‘‘cosmic archaeology’’ and away from technological species lifetimes. Our constraint addresses an issue that is of particular scientific and philosophical consequence: the question ‘‘Have they ever existed?’’ rather than the usual narrower concern of the Drake equation, ‘‘Do they exist now?’’

The paper is short and interesting; I commend it to you. The result it produces is that human civilization can be considered unique in the cosmos only if the odds of a civilization developing are less than one part in 10 to the 22nd power. Frank and Sullivan call this the ‘pessimism’ line. If the probability of a technological civilization developing is greater than this standard, then we can assume civilizations have formed before us at some time in the universe’s history.

And yes, this is a tiny number — one in ten billion trillion. Frank says in this University of Rochester news release that he believes it implies technology-producing species have evolved before us. Even if the chances of civilization arising were one in a trillion, there would be about ten billion civilizations in the observable universe since the first one arose. As for our own galaxy, another civilization is likely to have appeared at some point in its history if the odds against it evolving on any one habitable planet are better than one in 60 billion.

We fall back on cosmic archaeology in suggesting that given the size and age of the universe, Drake’s factor L may still play havoc with our chances of ever contacting another civilization. Sullivan puts it this way:

“The universe is more than 13 billion years old. That means that even if there have been a thousand civilizations in our own galaxy, if they live only as long as we have been around—roughly ten thousand years—then all of them are likely already extinct. And others won’t evolve until we are long gone. For us to have much chance of success in finding another “contemporary” active technological civilization, on average they must last much longer than our present lifetime.”

We can play with this a bit. Taking the Milky Way and choosing a probability of 3 X 10-9, we are likely to be one of hundreds of civilizations that have arisen. But drop that probability to 10-18 (one in a billion billion) and we are likely the first advanced civilization in the galaxy. Yet even with the latter constraint, that would still mean we are one of thousands of civilizations that have developed at some time in the visible universe.

I always appreciate work that frames an issue in a new perspective, which is what Frank and Sullivan’s paper does. We can’t know whether there are other civilizations currently active in our galaxy, but it appears that the odds favor their having arisen at some time in the past. In fact, these numbers show us that we are almost certainly not the first technological civilization to have emerged. Is the galaxy filled with the ruins of civilizations that were unable to survive, or is it a place where some cultures have mastered the art of keeping themselves alive?

The paper is Frank and Sullivan, “A New Empirical Constraint on the Prevalence
of Technological Species in the Universe,” Astrobiology Vol. 16, No. 5 (2016). Preprint available.

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TRAPPIST-1: Three Nearby Worlds

by Paul Gilster on May 2, 2016

About forty light years from Earth in the constellation Aquarius is the star designated 2MASS J23062928-0502285, which as of today qualifies as perhaps the most interesting ultracool dwarf we’ve yet found. What we learn in a new paper in Nature is that the star, also known as TRAPPIST-1 after the European Southern Observatory’s TRAPPIST telescope at La Silla, is orbited by three planets that are roughly the size of the Earth. We may have a world of astrobiological interest — and conceivably several — orbiting this tiny, faint star.

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Image: Comparison between the Sun and the ultracool dwarf star TRAPPIST-1. Credit: ESO.

If we untangle the TRAPPIST acronym, we find that it refers not to an order of monks (famous for their beers) but to the TRAnsiting Planets and PlanetesImals Small Telescope, a 60 cm robotic instrument that is operated from a control room in Liège, Belgium. TRAPPIST homes in on sixty nearby dwarf stars at infrared wavelengths to search for planets. Michaël Gillon, who led the team from the University of Liège that made the recent discovery, nails its significance:

“Why are we trying to detect Earth-like planets around the smallest and coolest stars in the solar neighbourhood? The reason is simple: systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look.”

In other words, life may exist on many worlds, and many of us believe that it does. But at our current level of equipment and expertise, a star like TRAPPIST-1 is significant because the star is small and dim enough for the atmospheres of Earth-sized planets to be studied. We learn from the Nature paper that two of the planets here have orbital periods of 1.5 days and 2.4 days respectively. The third is not as well characterized, with its orbit in a range from 4.5 to 73 days despite follow-up work with ESO’s 8-metre Very Large Telescope in Chile.

Noting how close the planets are to the host star, Gillon likens the scale of the system to that of Jupiter and its larger moons. These are worlds twenty to one hundred times closer to their star than the Earth is to the Sun, but the two inner planets receive only twice and four times respectively the stellar radiation that the Earth receives from the Sun. That puts them too close to the star to be in the habitable zone, while the outer planet may possibly lie within it.

But bear in mind that at least the inner two planets are probably tidally locked, with one side perpetually facing the star, the other turned away from it. Hence there may be regions near the terminator that receive daylight but maintain relatively cool temperatures. Given that the third planet may turn out to be entirely within the habitable zone, we have a fascinating test case for upcoming attempts to characterize the atmospheres of each of these Earth-sized worlds.

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Image: Artist’s impression of the ultracool dwarf star TRAPPIST-1 from close to one of its planets. Credit: ESO.

This ESO news release quotes Julien de Wit, a co-author of the paper on this work, with regard to where we go next:

“Thanks to several giant telescopes currently under construction, including ESO’s E-ELT and the NASA/ESA/CSA James Webb Space Telescope due to launch for 2018, we will soon be able to study the atmospheric composition of these planets and to explore them first for water, then for traces of biological activity. That’s a giant step in the search for life in the Universe.”

The paper is Gillon et al., “Temperate Earth-sized Planets Transiting a Nearby Ultracool Dwarf Star,” published online in Nature 2 May 2016. I have to leave the office before the link to the abstract goes live, but I’ll get it up within a few hours. I’ll note too the continuing interest Centauri Dreams reader Harry R. Ray has shown in TRAPPIST 1, and thank him for bringing it to my attention before these first news reports surfaced.

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Spacecoaches and Beamed Power

by Paul Gilster on April 29, 2016

If you’re planning to make it to the International Space Development Conference in San Juan, Puerto Rico next month, be advised that Brian McConnell will be there with thoughts on a subject we’ve discussed in several earlier posts: A ‘spacecoach’ that uses water as a propellant and offers a practical way to move large payloads (and crews) around the Solar System. Based in San Francisco, Brian is a technology entrepreneur who doubles as a software/electrical engineer. In the essay below, he looks at the spacecoach in relation to the Breakthrough Starshot initiative, where synergies come into play that may benefit both concepts.

by Brian McConnell

McConnell

The spacecoach is a design pattern for a reusable solar electric spacecraft, previously featured on Centauri Dreams here and developed in A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer Verlag), which I wrote with Alex Tolley. It primarily uses water as its propellant. This design has numerous benefits, chief among them the ability to turn consumables, ordinarily deadweight, into working mass.

The recent announcement of the Breakthrough Starshot project, which aims to use beamed power to drive ultra lightweight lightsail probes on interstellar trajectories, is of note. This same infrastructure could be used to augment the capabilities and range of spacecoaches (or any solar electric spacecraft), while providing a near-term use for beamed power infrastructure as it is developed and scaled up.

The spacecoach design pattern combines a medium sized solar array (sized to generate between 500 kilowatts and 2 megawatts of peak power at 1AU) with electric propulsion units that use water as propellant (and possibly also waste streams such as carbon dioxide, ammonia, etc). We found that, even when constrained to these power levels, they could fly approximately Hohmann trajectories to and from destinations in the inner solar system. Because consumables are converted into propellant, this reduces mass budgets by an order of magnitude, and effectively eliminates the need for an external interplanetary stage, all while greatly simplifying the logistics of supporting a sizeable crew for long duration missions (more consumables = more propellant).

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The primary constraint for space coaches, especially if you want to travel to the outer solar system, is available power. This is an issue for two reasons. First, solar flux drops off by 1/r2, so at Jupiter, a solar array will generate roughly 1/25th the power as it does at Earth distance. Second, trips to more distant locations will typically require a greater delta V (and thus higher exhaust velocity to achieve this with a given amount of propellant). The amount of energy required to generate a unit of impulse scales linearly with exhaust velocity, so the net result is the ship’s power requirements are increased, all while the powerplant’s power density (watts per kilogram of solar array) is decreased.

Testing Beamed Power

Beamed power infrastructure would enable space coaches and solar electric spacecraft in general to operate at higher power levels for a given array size, which would enable them to operate at higher thrust levels, and to utilize higher exhaust velocities to maximize delta V and propellant efficiency. This means they would be able to accelerate faster, achieve higher delta-v, while using less propellant. In effect beamed power to SEP spacecraft will give their operators the equivalent of a nuclear electric power plant (without the nukes).

A spacecoach built for solar only operation would be able to serve as a testbed for beamed power. For example, a space coach departing Earth orbit could be illuminated with a beam that increases its power output by a small amount, say 10% (large enough to make a measurable difference in performance, yet small enough that major modifications are not required to the ship as it just experiences slightly brighter illumination while in beam). At higher light levels, this technique could also be used to simulate lighting and heat loading conditions expected at the inner planets while remaining in near Earth space. Note also that lasers can be tuned to the absorption wavelength(s) of the photovoltaic material, greatly improving conversion efficiency (and reducing heat gain per unit of power delivered). An even cheaper way to build out and test power beaming infrastructure will be with satellites and probes that utilize solar electric propulsion.

The pathway to a system based primarily on beamed power then becomes one based on incremental improvements, both for the ground based facilities and for the ships. This would result in near term applications for the beamed power facilities while the much more technically challenging aspects of the starshot project are sorted out. Meanwhile, satellite and space coach operators could test ships with ever higher levels of beamed power until they hit a limit (heat rejection is probably the main limit to how much power can be concentrated per unit of sail area, as this is similar to concentrated photovoltaics).

The chart below illustrates the power/performance curve by showing the amount of impulse that can theoretically be generated per megawatt hour using electric propulsion, as a function of exhaust velocity. Real world performance will be somewhat lower due to efficiency losses, but this shows the relationship between thrust, ve and power. We see that impulse per MWh varies from 72,000 kg-m/s (ion drive, ve ~ 100,000 m/s) to 1,400,000 kg-m/s (RF arcjet, ve ~ 5000 m/s). A Hall Effect thruster, a flight proven technology, would yield about 300,000 kg-m/s per MWh. Compare this to pure photonic propulsion, which would yield only 12 to 24 kg-m/s per MWh. Clearly photonic propulsion will be necessary to achieve a delta v of 0.2c, but for more pedestrian applications such as satellite orbit raising, launching interplanetary probes or cargo ships from LEO to BEO (beyond earth orbit), electric propulsion will work well at power levels many orders of magnitude lower than what’s required for a starshot.

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Driver for an Interplanetary Infrastructure?

Closer to home there could be lots of opportunities to sell beamed power to space operators. It’s costly to launch large payloads beyond low earth orbit (which isn’t cheap in the first place). Meanwhile, payload fairings limit the size of self-deploying solar arrays, which limits the use of electric propulsion for satellites and probes. If one could launch spacecraft with small solar arrays to LEO, and then use beamed power to amplify their power budget they could use electric propulsion to boost themselves to their desired orbits or interplanetary trajectories within a reasonable time frame. The beamed power infrastructure can also be built up incrementally. Early systems would beam 100 kilowatts to 10 megawatts of power to targets measuring meters to tens of meters in diameter. This should be readily achievable, and can be scaled up from there in terms of power output, beam precision, etc. The result: lower costs per kilogram to deliver a payload to its destination or desired orbit compared to all chemical propulsion.

This could make electric propulsion for transit from LEO to GEO and beyond an attractive option. Meanwhile, the power beaming operator would accrue lots of operational experience with beam shaping, tracking objects in orbit, etc, all things that will need to be mastered for the starshot project, while providing an economic foundation for the power beaming facilities during the buildup to their intended purpose.

In fact, one can imagine the starshot project becoming a profitable LEO to BEO (beyond earth orbit) launch operator in its own right. The terrestrial power beaming infrastructure is one component. A standardized “power sail” that can be fitted to many different payloads, from geostationary satellites to interplanetary probes, is another. The power sail would consist of a self-deploying solar array that is sized to work well with beamed power, heat rejection gear, and electric propulsion units. It would use beamed power during its boost phase to rapidly accrue velocity for its planned trajectory, and then as it leaves near Earth space, would transition to use ambient light as its power source from there. Meanwhile these power sails would provide an evolutionary path from conventional spacecraft to solar electric propulsion to the nanocraft envisioned for purely photonic propulsion.

As a starting point, it would be interesting to conduct ground based vacuum chamber tests to see how a variety of PV materials respond to being illuminated with concentrated laser light tuned to their peak absorption wavelengths. What do the conversion efficiencies look like? How much waste heat is generated? How do the materials perform at high temperatures in simulated in-beam conditions? Building on that one can imagine experiments involving cubesats to validate the data from those experiments in real world conditions, and if that all works out, one could scale up from there to build out beamed power infrastructure for use by many types of solar electric vehicles.

Ambitious R&D projects have a way of generating unintended side benefits. It’s possible that the starshot initiative, in addition to being our first step toward the stars, will also make great contributions to travel and exploration within the solar system.

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Light’s Echo: Protoplanetary Disk Examined

by Paul Gilster on April 28, 2016

The star YLW 16B, about 400 light years from the Earth, has roughly the same mass as the Sun. But unlike the Sun, a mature 4.6 billion year old star, YLW 16B is a scant million years old, a variable of the class known as T Tauri stars. Whereas our star is relatively stable in terms of radiation emission, the younger star shows readily detectable changes in radiation, a fact that astronomers have now used in combining data from the Spitzer space telescope with four ground-based instruments to learn more about the dimensions of its protoplanetary disk.

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Image: This illustration shows a star surrounded by a protoplanetary disk. Material from the thick disk flows along the star’s magnetic field lines and is deposited onto the star’s surface. When material hits the star, it lights up brightly. Credit: NASA/JPL-Caltech.

The method is called photo reverberation, and it takes advantage of the fact that when the star brightens as material from the turbulent disk falls onto its surface, some of the emitted light strikes the disk. The result is what is known as a ‘light echo,’ a delayed flash that can be used to measure how far the star is from the inner edge of the surrounding disk.

The time lag between the stellar emissions and their ‘echoes’ is what is in play here. Over the course of a two-night observing period, the researchers found consistent time lags between emissions and echoes. On the ground, the Mayall telescope at Kitt Peak (Arizona), the Harold L. Johnson telescope in Mexico and the SOAR and SMARTS telescopes in Chile could measure shorter-wavelength infrared wavelengths — these emissions came from the star. Spitzer, meanwhile, measured longer-wavelength light from the disk’s echo. From the paper:

Near-simultaneous time-series photometric observations were conducted on April 20, 22, and 24, 2010, with four ground-based telescopes operating in H and K bands and the Spitzer Space Telescope observing at 4.5 µm. Each session of Spitzer staring mode monitoring lasted ∼8 hours. One (YLW 16B) out of twenty-seven sources detected was found to have mutually correlated hourly variations in all three wavebands. Over all three nights, the time series measurements of YLW 16B in H and K bands are consistently synchronized, while the light curve at 4.5 µm lags behind both H and K by 74.5±3.2 seconds over the first two nights when we have usable 4.5 µm data.

From this we learn that the light must have traveled about 0.084 AU between emission and echo (with an uncertainty on the order of 0.01 AU), meaning the inner edge of the protoplanetary disk reaches in as far as one-quarter the diameter of Mercury’s orbit. The paper notes that the structure of the inner region of a protoplanetary disk depends on the mechanism by which material from the disk accretes onto the star. At work at this inner boundary is sublimation, affecting dust, and the forces of the stellar magnetosphere that shape gas distribution.

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Image: The star’s irregular illumination allows astronomers to measure the gap between the disk and the star by using a technique called “photo-reverberation” or “light echoes.” First, astronomers look at how much time it takes for light from the star to arrive at Earth. Then, they compare that with the time it takes for light from the star to bounce off the inner edge of the disk and then arrive at Earth. That time difference is used to measure distance, as the speed of light is constant. Credit: NASA/JPL-Caltech.

We’ve learned much about disk structure by studying disks around larger mass stars, but until now probing into the inner disk regions of pre-main sequence T Tauri stars like YLW 16B has been hampered by the proximity of the inner disk edge to the star, too small to be directly imaged. Measuring the light travel time from the young star to the inner disk wall thus breaks new ground. The authors believe the method is viable for other young stars that show variability in the near-infrared, giving us a way to measure disks where planet formation has yet to begin.

For more on light echoes, see ‘Light Echo’ Reveals Eta Carinae Puzzle, which looks at the technique in terms of supernovae. To my knowledge, this is the first time light echoes have been studied in relation to protoplanetary disks. The paper is Meng et al., “Photo-reverberation Mapping of a Protoplanetary Accretion Disk around a T Tauri Star,” accepted for publication in the Astrophysical Journal (preprint). A JPL news release is also available.

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