Centauri Dreams
Imagining and Planning Interstellar Exploration
SETI: A New Kind of ‘Water Hole’
Some of you may recall an episode of Star Trek: The Next Generation in which the inhabitants of a planet called Aldea use a planetary defense system that includes a cloaking device. The episode, “When the Bough Breaks,” at one point shows the view from the Enterprise’s screens as the entire planet swims into view. My vague recollection of that show was triggered by the paper we looked at yesterday, in which David Kipping and Alex Teachey discuss transit light curves and the ability of a civilization to alter them.
After all, if an extraterrestrial culture would prefer not to be seen, a natural thought would be to conceal its transits from worlds that should be able to detect them along the plane of the ecliptic. Light curves could be manipulated by lasers, and as we saw yesterday, the method could serve either to enhance a transit, thus creating a form of METI signaling, or to conceal one. In the latter case, the civilization would want to create a change in brightness that would essentially cancel out the transit light curve. It’s not exactly a ‘cloaking device,’ but it ought to work.
Image: The Next-Generation Transit Survey (NGTS) telescopes operating at ESO Paranal, Chile. Transit observations have SETI implications we are only beginning to explore. Credit: ESO/ G. Lambert.
A Galaxy of Xenophobes?
As I said yesterday, I’m not here to reignite the METI debate as much as to acknowledge that what an alien culture might do is unknown. Rather than asking whether any civilization should try to conceal itself, let’s simply ask what it could do if it made the attempt.
The idea has a brief history, with Eric Korpela (UC-Berkeley) and Shauna Sallmen (University of Wisconsin-La Crosse) suggesting in 2015 that ETI could effectively hide a planetary signature through the use of orbiting mirrors. This would, like the geometric masks envisioned by Luc Arnold, require engineering on a huge scale, and would also demand elaborate tuning for each target. Kipping and Teachey argue for a more affordable alternative using a directed laser beam:
In our scheme… the advanced civilization emits a laser directed towards the other planetary system at precisely the instant when the other system would be able to observe a transit. The power profile of the laser would need to be the inverse of the expected transit profile, leading to a nullified flat line eliminating the transit signature.
Image: Top: The unaltered light curve of the Earth transiting the Sun, as viewed by different broadband optical photometers (offset by 5 ppm). Middle: The power profile of a 600 nm laser array designed to cloak the Earth. An array of lasers producing a peak power of ? 30 MW over 13 hours nullifies the transit. Bottom: Residual light curve, as seen by the different photometers. Credit: David Kipping/Alex Teachey.
The Kepler mission has produced the vast majority of recent exoplanet discoveries, and we have upcoming transit surveys in the works including TESS (Transiting Exoplanet Survey Satellite), PLATO (PLAnetary Transits and Oscillations of stars) and NGTS (Next-Generation Transit Survey). If a civilization wanted to shield itself from this kind of broadband optical survey, a monochromatic optical laser should do the trick. The paper estimates that the Earth could be ‘cloaked’ — hidden from view from a particular star system by having its transit nullified — with a 600 nm laser array emitting a peak power of ~30 MW over 13 hours.
The power requirements are interesting because they are relatively low for a specific target, but the paper adds the obvious point that if we are trying to cloak a planet from a large number of targets, we would require larger power production. Nonetheless, we routinely use much larger numbers when talking about laser lightsails in the configurations that could enable interstellar flight. Kipping and Teachey point out that for a culture that develops those kinds of technologies, cloaking could become a secondary function of the laser arrays used primarily for propulsion.
Chromatic cloaking (across all wavelengths) could be achieved by using a large number of beams (although with an order of magnitude higher energy cost), while tunable (‘supercontinuum’) lasers may emerge that can simulate any spectrum. But even with these capabilities, is cloaking an entire planet the most efficient choice for a civilization trying to hide itself? Perhaps a better course from the standpoint of economics and efficiency is to cloak the biosignatures that announce life’s presence. Let me quote from the paper on this:
It is straightforward to use a chromatic laser array to cancel out the absorption features in the planet’s transmission spectrum, assuming laser emission can be produced at any desired wavelength. Indeed, the presence of an atmosphere could be cloaked altogether if the effective height changes of the planet as a function of wavelength are canceled out by lasers. The planet might then resemble a dead world totally devoid of any atmosphere and appear almost certainly hostile to life. Not only would this approach require a significantly smaller power output, it would also have the benefit of producing self-consistent observations insomuch as the presence of the planet might still be inferred by other means (i.e. through radial velocity analysis).
What SETI Can Learn
Kipping and Teachey refer to these methods as a ‘biocloak,’ and suggest that cloaking can be selective indeed, perhaps focusing on the absorption features of molecular hydrogen and ozone. In this case we are dealing with peak laser power of just ?160 kW per transit. But the authors are clear about the limitations of these methods. Radial velocity methods can find a planet otherwise hidden by a chromatic transit cloak, and given technologies not so far advanced over what we have today, direct imaging can reveal atmospheric features of a planet even when a ‘biocloak’ is in place. “For these reasons” write the authors, “perhaps the most effective use of laser enabled transit distortion would be for broadcasting rather than cloaking.”
And it was on that note that I began yesterday’s look at these possibilities. If we have based fifty years-plus of SETI on the notion that another civilization may choose to contact us, we have to acknowledge what Kipping and Teachey make clear: There are ways to alter transit signatures that make it obvious we are dealing with an advanced technology. And you can make the argument, as the authors do, that transits offer a different kind of ‘water hole’ for SETI, comparable in its own way to the ‘water hole’ frequencies we monitor in radio SETI.
Thus while the cloaking aspects of this paper have received the most attention, I think the SETI implications are its strongest takeaway. It is a very short step from existing optical SETI to archival searches of transit signatures already in our files. Knowing what these signatures would look like is a step forward as we continue to probe for civilizations around nearby stars.
Addendum: This email from Dr. Kipping, excerpted below, further explains the authors’ thinking about cloaking possibilities:
…we never intended to solve cloaking from all detection methods in one paper (that would be a tall order to demand of any research paper). Rather, we started with the simplest and most successful technique, transits, and showed that it is energetically and technologically quite feasible for even our current level of technology to build an effective cloak. Whilst we acknowledge that there are ways to defeat the proposed cloak (e.g. polarization of laser beams, direct imaging), we see these as problems which are likely to be solved by more advanced civilizations than ourselves, or indeed in future work (by humans!). What we are trying to do on the cloaking side is stimulate a conversation- that it is surprisingly easy to hide planets. Given that many notable scientists are opposed to METI, it is not unreasonable that other civilizations may choose to do this. The scenario could be that they would have long ago observed the Earth as an inhabited planet, and then turned on a cloak as a insurance policy, buying them time to reveal their presence when they choose to, rather than our increasingly penetrating telescopes finding them before they wish.
The paper is Kipping and Teachey, “A Cloaking Device for Transiting Planets,” accepted at Monthly Notices of the Royal Astronomical Society (preprint), The Korpela and Sallmen paper is “Modeling Indications of Technology in Planetary Transit Light Curves – Dark Side Illumination,” Astrophysical Journal Vol. 809, No. 2 (abstract).
A Transit Signature for SETI?
David Kipping and Alex Teachey have a new paper out on the possibility of ‘cloaking’ a planetary signature. The researchers, both at Columbia University, make the case that any civilization anxious to conceal its existence — for whatever reason — would surely become aware that all stars lying along its ecliptic plane would see transits of the home world, just as its own scientists pursued transit studies of planets around other stars. And it turns out there are ways to make sure this signal is masked by adjusting the shape of the planet’s transit light curves.
Now this is a fascinating scenario as presented by the head of the Hunt for Exomoons with Kepler, whose business it is to know about the slightest of variations in light curves because they may contain information about exomoons or rings. Thus Kipping is a natural to look into the artificial manipulation of light curves, a study with definite SETI implications. Because methods like these work in two directions — a civilization that does want to communicate could also alter its light curve in ways that would be unambiguously artificial. Today I want to focus on the latter idea, reserving cloaking methods for tomorrow’s post.
This veers into the METI debate, but that’s not my purpose. Messaging to Extraterrestrial Intelligence is highly controversial, triggering arguments in these pages for the last decade. But let’s hold METI at one remove. The paper, delightfully titled “A Cloaking Device for Transiting Planets,” allows us to imagine how the manipulation of transit signatures could change a distant planet’s visibility. We may well decide not to brighten the Earth’s visibility through intentional transmissions, while understanding that an extraterrestrial culture might choose differently. Knowing what is possible by way of cloaking or enhancing a planetary signature, then, gives us plenty of food for thought for SETI as we consider what might turn up in our data.
Modifying the Light Curve
Here’s the notion in a nutshell, as drawn from the paper.
Image: Figure 1. Illustration (not to scale) of the transit cloaking/broadcasting device. A laser beam (orange) is fired from the night side of inhabited planet (blue) towards a target star whilst the planet appears to transit the star, as seen from the receiver. In the case of the Earth, the planet could be cloaked by generating an inverse transit-like signal of peak power 60 MW. Credit: Kipping and Teachey.
Controlled laser emissions are the key, effectively distorting the shape of a transit light curve. Just how this could be done is something we’ll discuss in the next post. But to begin, let’s think about how visible we are to any advanced civilization studying us. Forget about our radio and television signature, which is infinitesimal, and focus instead on the things we are doing right now to study other stars. I often write about transmission spectroscopy, which is how we search for the constituent molecules in a planetary atmosphere. The transiting planet, as it moves in front of its host star (as seen by our instruments) is bathed in the star’s light, its atmosphere providing a particular ‘overlay’ to the star’s spectrum.
We’ll use the method to look for biosignatures as we refine it for smaller and smaller worlds, but we’ve already seen how successfully we can examine the atmospheres of ‘hot Jupiters’ like HD 189733b. A sufficiently capable civilization able to see our world transiting should be able to pick up the cluster of biosignatures that would identify the Earth as a living world. There have been proposals to look for atmospheric pollutants produced by industrial activity as a part of future SETI practice, and such activity could indeed become visible, though the idea of widespread pollution lasting for millennia seems a stretch. Understanding the damage it caused, surely the culture in question would either solve its pollution problems or else succumb to them.
Let’s not forget the recent flurry of interest in the unusual star KIC 8462852. Here we’re looking at what could well be a natural phenomenon in the form of clouds of comets, but could possibly be evidence of artificial megastructures throwing highly distinctive light curves. We have much work ahead on KIC 8462852, so I only bring it up to suggest that there are many ways an intelligent species might make itself visible whether its intent was to do so or not.
Turning Transits into ‘Broadcasts’
Scientists as diverse as Ronald Bracewell, Richard Carrigan, Michael Papagiannis and Robert Freitas have studied the possibilities of such detections, with Carrigan most prominently identified with the search for Dyson spheres, swarms of energy collectors that surround a star to feed its colossal energies to a growing Type II civilization. And back in 2005, Luc Arnold (now at Aix Marseille Université) suggested that a civilization wanting to be known could resort to deliberate signaling by building a particular kind of geometric megastructure, one that when viewed in a transit would all but shout, through its distinctive light curve, that it was artificial.
Kipping and Teachey are, as you would imagine, well aware of Arnold’s work, and argue that lasers offer far more practical methods. From the paper:
Whilst any number of artificial transit profiles can be created with lasers, one ideally seeks a profile which is both energy efficient and unambiguously artificial. Producing upward spikes in-transit might seem like an obvious suggestion, but star spot crossings produce these forms with complex and information rich signatures (e.g. see Beky et al. 2014). Here, we argue that cloaking the ingress/egress of a transit, but leaving the main transit undistorted, would be a highly effective strategy since no known natural phenomenon is likely to produce such an effect.
Image: Figure 6 from the paper, highlighting broadcasting rather than cloaking. Top: The power profile of a laser array designed to broadcast the Earth. An array of lasers producing a peak power of ? 20 MW for approximately 15 minutes nullifies the transit ingress and egress. Bottom: The resulting light curves as viewed by different broadband optical photometers. The observed impact parameter would be complex infinity, for which a normal light curve fit would be unable to explain and thus indicating the presence of artificial transit manipulation.
A monochromatic laser emission could be spotted in a transit survey (and could be searched for in Kepler data), with follow-up spectroscopy confirming the artificial nature of the transmission. Kipping and Teachey also note what James and Dominic Benford recently pointed out in their paper on SETI efforts at KIC 8462852 — a beamed signal of whatever kind could carry information, so that in addition to identifying the presence of a technological culture, the beam could attempt more detailed communication (see SETI: Power Beaming in Context).
I’ve focused in on broadcasting via transit light curve alteration because, as the paper argues, it is the most efficient use of these techniques as compared to cloaking, which I’ll describe tomorrow. I’m trying to stay out of the METI weeds here — this is not an argument in favor of identifying the Earth to ETI. Rather, it is an argument that other civilizations may use such methods, and thus this paper shows us a signature we should add to our catalog.
And it has this further implication: If a civilization did choose to broadcast its existence through these methods, it would probably choose the shortest period planet in its solar system to carry the message. The choice is obvious, as the paper notes, for this produces “a higher duty cycle of distorted events,” making the detection all the more obvious. “We therefore suggest that any survey in archival data should not be limited to rocky planets in the habitable-zone of their host star.” An excellent reminder not to succumb to easy assumptions!
Tomorrow I’ll return to the Kipping and Teachey paper with a look at cloaking possibilities. The paper is “A Cloaking Device for Transiting Planets,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).
John Ford Fishback and the Leonora Christine
Like the Marie Celeste, the Leonora Christine is a storied vessel, at least among science fiction readers. In his 1967 story “To Outlive Eternity,” expanded into the novel Tau Zero in 1970, Poul Anderson described the starship Leonora Christine’s stunning journey as, unable to shut down its runaway engines, it moved ever closer to the speed of light. Just how a real Leonora Christine might cope with the stresses of a ramjet’s flight into the interstellar deep is the subject of Al Jackson’s latest, which draws on memories not only of Robert Bussard, who invented the interstellar ramscoop concept, but a young scientist named John Ford Fishback.
by A. A. Jackson
Project Pluto – a program to develop nuclear-powered ramjet engines – must have been on Robert Bussard’s mind one morning at breakfast at Los Alamos. Bussard was a project scientist-engineer on the nuclear thermal rocket program Rover — Bussard and his coauthor DeLauer have the two definitive monographs on nuclear propulsion [1,2]. He said many times that the idea of the hydrogen scooping fusion ramjet came to him that morning. This was sometime in 1958 or 1959 and the SLAM (Supersonic Low Altitude Missile) would have been well known to him. SLAM was an nuclear ramjet, a fearsome thing, sometimes called the Flying Crowbar. Finding a solution to the mass ratio problem for interstellar flight was also something on Bussard’s mind. Thus was born the Interstellar Ramjet, published in 1960 [3].
Image: Al Jackson delivering a plenary talk at the recent Tennessee Valley Interstellar Workshop. Credit: Joey O’Loughlin.
Most here at Centauri Dreams know that the interstellar ramjet scoops hydrogen from the interstellar medium and uses this as both a fuel and energy source by way of fusion reactor. The sun does proton fusion using gravity as the agent of confinement and compressional heating. However, doing fusion in a ‘non-gravitational’ magnetic fusion reactor makes the process very difficult [3,4]. That is, the proton and Deuterium burning is quite severe to realize on a ‘small scale’. Dan Whitmire attacked this problem by proposing the use of a carbon catalyst using the CNO cycle [4]. The CNO cycle is about 9 orders of magnitude faster than proton-proton fusion. It would still require temperatures and number densities way beyond any technology known at this time.
Bussard noted a number of problems such as losses from bremsstrahlung and synchrotron radiation. He also noted scooping with a material scoop would create a problem with erosion, hinting that magnetic fields might be used, and noting that drag would have to be accounted for.
About 8 years after Bussard’s paper, an undergraduate at MIT, John Ford Fishback, took up the problems Bussard had mentioned. He wrote this up for his Bachelor’s thesis under the supervision of Philip Morrison. The thesis was published in Astronautica Acta [5] in 1969.
Image: Physicist Robert W. Bussard.
Fishback did three remarkable things in his only journal paper: finding an expression for the ‘scoop’ magnetic field, computing the stress on the magnetic scoop sources, and working out the equations of motion of the ramjet with radiation losses. These calculations were done using a special relativistic formulation.
Fishback’s most important finding is noticing that when capturing ionized hydrogen to funnel into the fusion reactor, there is a large momentum flow of the interstellar medium which must be balanced by the scooping and confining magnetic fields. Using very general arguments, Fishback showed that sources (magnetic coils and their support) of the magnetic field determine an upper limit on how fast a ramjet can travel. The convenient measure of starship speed is the Lorentz factor
where v is the starship velocity and c the speed of light. It comes from the physical properties of the field sources, in particular the shear stress.
At the time, Fishback modeled the upper limit using diamond, because of its shear stress properties, and found that one could only accelerate until the Lorentz factor reaches about 2000 [5,6]. Tony Martin expanded on Fishback’s study [6, 7] in 1971, correcting some numbers and elaborating on Fishback’s modeling. Since that time, Graphene has been discovered and it has a shear stress that allows a limiting Lorentz factor of about 6000. This in turn implies a range of over 6000 light years when under 1 g acceleration. It does not mean the final range is 6000 light years, but one must travel at a reduced acceleration and then constant speed, which means a longer ship proper time.
This is bad news for the Leonora Christine of Poul Anderson’s Tau Zero [8].The range can probably be pushed to 10,000 light years, but accelerating at 1 g for 50 years would bust the Lenora Christine’s coils! That is, unless some magic material is found to take the stress loading at a Lorentz factor 1019, there is no way to circumnavigate the universe. And with the new accelerating universe, the story of Tau Zero becomes still more complicated.
Image: The interstellar ramscoop as envisioned by artist Adrian Mann.
What became of John Ford Fishback? I went to a lecture in California at Stanford in 1979 by Phillip Morrison. After the lecture I asked Morrison what had happened to Fishback. Morrison sadly told me that Fishback had gone to the University of California at Berkeley to work on his doctorate, but had committed suicide.
1. Bussard, R. W.; DeLauer, R.D. (1958). Nuclear Rocket Propulsion. McGraw-Hill.
2. Bussard, R.W.; DeLauer, R. D. (1965). Fundamentals of Nuclear Flight. McGraw-Hill.
3. Bussard, R.W., “Galactic Matter and Interstellar Flight”, Acta Astronatica, VI, pp. 179-195, 1960.
4. Whitmire, Daniel P. , “Relativistic Spaceflight and the Catalytic Nuclear Ramjet”, Acta Astronautica, 2 (5-6): 497-509, 1975.
5. Fishback, J. F., “Relativistic interstellar spaceflight,” Astronautica Acta, 15 25-35, 1969.
6. Anthony R. Martin; “Structural limitations on interstellar spaceflight,” Astronautica Acta, 16, 353-357 , 1971.
7. Anthony R. Martin, “Magnetic intake limitations on interstellar ramjets,” Astronautica Acta, 18, 1-10 , 1973
8. Anderson, Poul (2006), Tau Zero, Gollancz. ISBN 1407239139.
Table 1. Cut-Offs and Range for Ramjet accelerating at 1g. Interstellar medium 1/cm-3 using the p-p fusion reaction.
| Structural Material | 𝛔/𝛒 dyn cm-2/gcm-3 1010 | 𝛄𝛃c Proton | Range LY |
|---|---|---|---|
| Aluminum | .062 | 8.6 | 12.6 |
| Stainless Steel | .261 | 36.2 | 7.5 |
| Silica | 3.31 | 73.6 | 120 |
| Copper | 4.36 | 605 | 1000 |
| Diamond | 15.2 | 2110 | 3550 |
| Graphene | 600.0 | 6628.0 | 6418.0 |
Addendum: While he was working on this article and corresponding with me, Al shared the story with Greg Benford, who had further thoughts on John Ford Fishback, as below:
Good article. I can add a touch: I was interested in this, after Edward Teller pointed out to me Fishback’s 1969 paper in Astronaut. Acta. I discussed it with Teller and did some calculations (just exploratory, never published). The idea seemed extreme but enlivened my discussions of the paper with Poul Anderson, who lived in Orinda near my Walnut Creek home and whom I saw often.
Someone told me Fishback was at Berkeley and I called him, agreed to meet. I had a one-day-per-week agreement with the Livermore Lab, where I was just turning from being a postdoc for Teller into a staff physicist — I spent Wednesdays at the Lab in Berkeley. So I met him at an Indian restaurant–a rail-thin smoker, nervous, ascerbic. “I wanted to show that we could reach the stars, really do it, with the right engineering,” he said, approximately. His anxiety was clear, but not its cause.
I found him an odd duck but was shocked when a bit later I heard he had killed himself.
When I mentioned it to Poul, he found it contrasting that a man who wanted the stars would cut off his own personal hopes. We often discussed Tau Zero, Poul once remarking that he wished he had taken more time to polish and expand the novel, since it already looked as though it might be the most remembered of his works–and indeed, seems so. He said he had written it in a few months and needed the money–its 1967 serialization in Galaxy helped, but it was tough going as a full-time pro writer then. Plus he had a word limit on the hardcover.
Poul used his Nordic background in the novel, as he liked to do. From Wikipedia:
“Incidental to the main themes is the political situation on the Earth from which the protagonists set out: a future where the nations of the world entrusted Sweden with overseeing disarmament and found themselves living under the rule of the Swedish Empire. This sub-theme reflects the great interest which Anderson, an American of Danish origin, took in Scandinavian history and culture. In later parts of the book, characters compare their desperate situation to that of semi-mythical characters of Scandinavian legend, with the relevant poetry occasionally quoted.”
SETI Looks at Red Dwarfs
When it comes to astrobiology, what we don’t know dwarfs what we do. After all, despite all conjecture, we have yet to find proof that life exists anywhere else in the universe. SETI offers its own imponderables, adding on to the question of life’s emergence. How often does intelligence arise, and if it does, how often does it produce civilizations capable of using technology? Even more to the point, how long do such civilizations last if they do appear?
We keep asking the questions out of the conviction that one day we’ll start retrieving data, perhaps in the form of a signal from another star. It’s because of the lifetime-of-a-civilization question that I’m interested in a SETI search focused on red dwarf stars. True, M-dwarfs have a lot going against them, as Centauri Dreams readers know. A habitable planet around an M-dwarf may be tidally locked, which could be a showstopper except that some scientists believe global weather patterns may make at least part of such planets habitable.
Flare activity is always an issue on younger M-dwarfs, though it’s possible to conceive of this as an evolutionary spur, and we can’t rule out life’s ability to adapt to extreme circumstances. But despite all these unanswered issues, my interest in these stars draws primarily from two main points. First, they are the most common stars in the galaxy, comprising perhaps as much as 80 percent of the total. That gives us a huge number of candidates for life and potential civilization.
And while we can’t say how long civilizations live, not being sure if we ourselves will survive, we can take heart from the idea that if enough of them come into being, at least a few may get past whatever culture-shredding ‘filter’ they encounter to move into a serene maturity. Here red dwarfs truly stand out, because they live so much longer than any other stars. Every red dwarf that has formed in the universe is still there, and we can expect such stars to live for trillions — not billions — of years.
I like the odds, but I’m also trying to imagine what a civilization would look like a billion years after the emergence of tool-making. Or five billion. If a culture can survive for aeons, it will have mastered issues of conflict that plague us daily and much else besides. Surely a mature species long past emotional and technological infancy would want to know about its neighbors. Would such a culture reach out to others, if only to exchange notes? Or would it have moved into realms of philosophy and thought that make all this irrelevant?
We’re deep in imponderables here, but all we can do is look and listen. Thus I was pleased to see that the SETI Institute is initiating a search using the Allen Telescope Array that targets red dwarf stars. As the Institute’s news release explains, we now believe that somewhere from one-sixth to one-half of red dwarfs have planets in their habitable zones, which is a percentage that may be comparable to stars like the Sun, and for all we know at this point, may exceed it.
“Significantly, three-fourths of all stars are red dwarfs,” notes SETI Institute astronomer Seth Shostak. “That means that if you observe a finite set of them – say the nearest twenty thousand – then on average they will be at only half the distance of the nearest twenty thousand Sun-like stars.”
That, of course, means that we have a larger population of stars whose potential signal to us would be stronger. The SETI Institute is drawing on a target list of 20,000 M-dwarfs compiled by Boston University astronomer Andrew West, one that will incorporate new data as it is collected by missions like TESS, the Transiting Exoplanet Survey Satellite, slated for launch next year. Using the ATA’s 42 antennae, the red dwarf survey will take two years to complete, working in several frequency bands between 1 and 10 GHz. Says Institute scientist Gerry Harp:
“Roughly half of those bands will be at so-called ‘magic frequencies’ – places on the radio dial that are directly related to basic mathematical constants. It’s reasonable to speculate that extraterrestrials trying to attract attention might generate signals at such special frequencies.”
My assumption is that as resources become available (never an easy matter), SETI will search broadly through the various stellar types — we can’t know what we’ll find until we look. But it’s heartening to find a SETI attempt specifically turning to a category of star that has generally received little attention. It may well be that a race that is deep into philosophical maturity will have moved beyond beaming signals to other stars. It may, for all we know, have moved beyond biology! But let’s keep up the search and learn as much as we can about the small red stars that pepper the cosmos and may, if in any way habitable, hold clues about life’s emergence.
TVIW 2016: Worldship Track
Our second report from the recent Tennessee Valley Interstellar Workshop is the work of Cassidy Cobbs and Michel Lamontagne, with an emphasis on the worldship track. Cassidy has an MS from Vanderbilt, where she studied ecology and evolution. She currently works at Memorial Sloan Kettering Cancer Center, doing traditional and next-generation gene and genome sequencing. Her interest in space travel/engineering was enhanced by attending Advanced Space Academy in Huntsville at age 14. Michel Lamontagne is a French-Canadian mechanical engineer, practicing in the fields of heat transfer and ventilation, with a passion for space. An active member of Icarus Interstellar, he tells me he has “been designing spaceships since he was 12 years old, and waiting for reality to catch up!” Photos throughout are from New York photojournalist Joey O’Loughlin, and are used with permission.
By Cassidy Cobbs and Michel Lamontagne
This year’s Tennessee Valley Interstellar Workshop (TVIW-2016) was held in Chattanooga, Tennessee from February 28 to March 2. Attendance was good, reaching the limits determined by the organization committee. Everything seemed to run smoothly, although one can imagine the usual frantic behind the scenes activity required to create that illusion!
Image: Co-author Michel Lamontagne.
The Life Systems Engineering for the Worldship track was very productive, engaging in active work sessions and managing to start interesting lines of inquiry into some the questions of the biological, social, and heat transfer facets of the worldship concept.
In our first working track session, we split into two groups, designated “Biotic” and “Abiotic” to brainstorm on some of the unanswered questions of Worldship theory and design.
Image: Abigail Sheriff (left), a graduate of the International Space University, and Cassidy Cobbs, co-leader of the Worldship track.
We began populating a whimsical list of included and excluded species, sure to generate heated debate — for example, the entire Australian continent was excluded on account of being too deadly!
We also came up with a number of unexplored questions, including three concerns that we would explore in depth in our Day 2 session: The agricultural framework of a Worldship; how to establish and maintain indefinitely carbon, nitrogen, phosphorus, and oxygen cycling; and how to adapt Earth-normal light, water, and heat cycles to a (much smaller) Worldship.
Cameron Smith (Portland State University) added his ongoing reflections about the human societal aspects of the worldship to the discussions, and provided fascinating parallels with early villages and paleolithic societies, where proto-cities housed small stable communities for periods similar to those expected for a worldship trip.
Image: Biomedical engineer Leigh Boros in the Worldship track. Credit: Joey O’Loughlin.
In our second session, the track split into three groups to look at a few of the questions generated the day before.
Our first group decided to explore some of the changes in heat transfer regimes from living on a sphere with the heat from the outside to living in a cylinder with heat from the inside. We didn’t have the time to work out if we could make it rain in the worldship using only convection cycles, but we agreed that rain would be needed and decided to address the problem in follow-up work sessions on the Internet.
Group two looked at resource cycling, and began to develop the calculations necessary to determine how much of elements such as nitrogen, phosphorus, and oxygen would be needed on board the ship at launch to maintain those natural cycles.
Image: Oz Monroe (left) and Miles Gilster (right), framing Greg Matloff in the background. Credit: Joey O’Loughlin.
The final group explored a potential framework for agri- and aquaculture, creating a list of diverse livestock and crops that would fulfill the nutritional and cultural needs of the humans on board. They also began to think about issues of crop rotation, soil health, and water requirements and to calculate what percentage of land would need to be allocated to agriculture.
The Worldship track was proud to host a new generation of designers, with Hannah Sparkes (age 15) and Ashleigh Hughes (age 17) joining with researchers Anton Smirnov (28) and Andrew Kirkpatrick (26) to ensure that analysis of interstellar worldship engineering has a future.
Image: A poster for the worldship track, as prepared by Michel Lamontagne.
For the plenary events, the subjects covered in the papers and talks ranged widely, as usual for TVIW, from starwisps to space wars. Philip Lubin (UC-Santa Barbara) invited the crowd to do the math for his incarnation of the laser-powered sail, one that recently garnered a lot of media attention with a ’30 minutes to Mars’ thought experiment, although the Mars journey is actually only one element of what Lubin sees as a complete Roadmap to the Stars.
Image: A scene from the Space Mining track. Edwin Etheridge (left) discusses specifics with Matt Ernst. Credit: Joey O’Loughlin.
The Moon vs asteroid mining debate politely raged on, with proponents on both sides and an entire track devoted to exploring detailed mineral processing methods. Melting Lunar basalts to create large caverns for rotating habitats, both in system and at interstellar destinations, was also the subject of an interesting talk by Ken Roy. Meanwhile, the sheer immensity of asteroid resources was highlighted by John Lewis in his keynote address.
Image: Keynote speaker John S. Lewis (author of Mining the Sky). Credit: Joey O’Loughlin.
Jim Benford proposed beam leakage from propulsion systems as a new SETI venue, inspired in part by the KIC 8462852 light anomalies uncovered in the Kepler planet finder data.
Image: Jim Benford discussing beamed propulsion issues in a SETI context. Credit: Joey O’Loughlin.
Al Jackson revisited and augmented his seminal Interstellar Laser Powered Interstellar Ramjet design, applying graphene to increase performance and setting the ultimate physical limits of the technology. Creating antimatter from space vacuum fluctuations using high energy lasers, as a part of an advanced antimatter drive, while respecting classical conservation of energy, was the subject of the exotic physics talk by Gerald Cleaver.
Image: Stefan Zeidler (left), newest member of the board of the Initiative for Interstellar Studies, with i4IS founder Kelvin Long and Bill Cress. Credit: Joey O’Loughlin.
Of a Mountain on Titan
If Saturn’s inner moons are, as we discussed yesterday, as ‘young’ as the Cretaceous, then we have much to think about in terms of possible astrobiology there. But Titan is unaffected by the model put forward by Drs. ?uk, Dones and Nesvorný, being beyond the range of these complex interactions. Huge, possessed of fascinating weather patterns within a dense atmosphere, Titan probably dates back to Saturn’s earliest days, in some ways a frigid ‘early Earth’ analog.
When my son Miles was a boy, we drove through the Appalachians on a journey that eventually took us into Canada. Somewhere in the Shenandoah Valley he commented on how insignificant the mountains seemed compared to what he was used to out west, where the Rockies dominate the sky. True enough, but of course the Smokies and the Cumberlands have their own tale to tell. Once monumental, they’ve fallen prey to wind and rain, ancient relics of once grander peaks.
The latest work on Titan from Cassini data now reveals something about similar erosion on Titan, where we have rain, lakes and seas, not to mention rivers cutting their way through the landscape. But Jani Radebaugh (Brigham Young University, Utah), who works with the Cassini radar team, notes that erosion on Titan is actually a much slower process than on Earth, thanks to Titan’s being ten times Earth’s distance from the Sun. There is just that much less energy to drive these processes in the thick atmosphere. See this JPL news release for more.
With Titan we have to think in terms of analogies. On Earth it’s water that freezes, thaws, vaporizes, providing a hydrological cycle that works its seasonal magic in terms of weather change. On Titan it’s methane that performs a similar function. Meanwhile, Titan’s water ice behaves much more like rock on Earth, an icy crust overlaying what is likely to be an ocean of liquid water — here the analogy is with Earth’s upper mantle. In both cases, these inner layers accommodate slow changes as mountains form and ranges begin to settle.
Radebaugh’s team used Cassini’s radar instrument to study the ridges known as the Mithrim Montes, among which is found the moon’s tallest peak, some 3337 meters high. “It’s not only the highest point we’ve found so far on Titan, but we think it’s the highest point we’re likely to find,” says Stephen Wall (JPL), deputy lead of the Cassini radar team. The results were presented at the 47th Lunar and Planetary Science Conference in Texas.
Image: The trio of ridges on Titan known as Mithrim Montes is home to the hazy Saturnian moon’s tallest peak. The mountain, which has an elevation of 3,337 meters, is located midway along the lower of the three ridges shown in this radar image from NASA’s Cassini spacecraft. Credit: NASA/JPL-Caltech/ASI.
The view above was acquired on the T-43 flyby back on May 12, 2008 at an incidence angle of about 34 degrees. Remember that this is a radar image, which uses reflections scattered off the moon’s surface to see through the thick, opaque atmosphere. Dark areas indicate regions that are relatively smooth or otherwise absorb radar waves, while bright regions are rougher materials that scatter the beam. A ‘speckle’ pattern is an artifact of the technique — in this image, ‘despeckling’ methods were used to reduce the noise and produce clearer views.
Titan’s mountains don’t reach the heights we see in some of Earth’s ranges, but researchers hadn’t expected they would because the water-ice bedrock is softer than Earth’s rock. But it is significant that we find tall mountains here, an indication of active forces shaping the surface that are perhaps Titan’s response to tidal forces from Saturn, or perhaps cooling of the crust. Finding such ‘active zones’ in the crust tells us something about Titan’s history.
“As explorers, we’re motivated to find the highest or deepest places, partly because it’s exciting,” adds Radebaugh. “But Titan’s extremes also tell us important things about forces affecting its evolution. There is lot of value in examining the topography of Titan in a broad, global sense, since it tells us about forces acting on the surface from below as well as above.”
Titan’s highest mountains all seem to be close to the equator, with other peaks of a similar height being found within the Mithrim Montes (for Tolkien cognoscenti, the Mountains of Mithrim ran northwest from the Ered Engrin, dividing Dor-lómin from Mithrim, and that is as far as I go with Tolkien today). Other peaks are known in the Xanadu region. Learning more about the forces that formed them is now a priority for researchers probing Titan’s mysteries.
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