Centauri Dreams
Imagining and Planning Interstellar Exploration
Small Payloads to the Stars
Making things smaller seems more and more to be a key to feasibility for long-haul spaceflight. Recently I went through solar sail ideas from the 1950s as the concept made its way into the scientific journals after an interesting debut to the public in Astounding Science Fiction. We also discussed Sundiver missions taking advantage of a huge ‘slingshot’ effect as a sail skims the photosphere. These could yield high speeds if we can solve the materials problem, but the other issue is making the payload light enough to get maximum benefit from the maneuver.
It puzzles me that in an age of rapid miniaturization and increasing interest in the technologies of the very small, we tend to be locked into an older paradigm for starships, that they must be enormous structures to maintain a crew and carry out their scientific mission. Alan Mole’s recent paper reminds us of an alternative flow of work beginning in the 1980s that suggests a far more creative approach. If we’re going to extrapolate, as we must when talking about actual starships, let’s see where nanotech takes us in the next fifty years and start thinking about propulsion in terms of moving what could be a very small payload instead of a behemoth.
I think sails connect beautifully with this kind of thinking. Mole envisions a sail driven by a particle beam, with the beam generator in Earth orbit fed by ground-based power installations, but we continue to look at other sail concepts as well, including laser and microwave beaming to ultralight sails made of beryllium or extremely light metamaterials. Payload-inefficient rockets don’t scale nearly as well to the kind of interstellar missions we are thinking about, but sails leave the propellant behind to enable fast missions delivering extremely small payloads.
This kind of thinking was already becoming apparent as early sail work emerged in the hands of Konstantin Tsiolkovsky, Fridrickh Tsander and others, and I’ll point you back to From Cosmism to the Znamya Experiments for more on that. For now, though, have a look at the marvelous Frank Tinsley illustration below. Here’s a startlingly early version (1959!) of sails in action, painted before Cordwainer Smith’s “The Lady Who Sailed the Soul” and Arthur C. Clarke’s “Sunjammer” ever hit the magazines. When Robert Forward began working on laser-pushed lightsails, he would have had images like this from popular culture to entice him.
Image: An early look at the solar sail from a 1959 advertising image by Frank Tinsley. Credit & copyright: GraphicaArtis/Corbis.
Tinsley’s career is worth lingering on. A freelance illustrator known for his cover paintings for pulp magazines, he covered a wide range of subjects in magazines like Action Stories, Air Trails, Sky Birds and Western Story, with a stint in the early silent film industry in New York City in the 1920s, where he served as a scenic artist and became friends with William Randolph Hearst. By the 1950’s, he was illustrating articles for Mechanix Illustrated. A representative sample of the latter work can be seen here, packed with speculations about futuristic technologies.
But back to sails carrying small and innovative payloads. In a 1998 paper in the Journal of the British Interplanetary Society, Anders Hansson, who had two years earlier described what he called ‘living spacecraft’ in the same journal, reported on NASA Ames work into spacecraft consisting of only a few million atoms each. The study speculated that craft of this size would travel not as single probes but as a swarm that could, upon arrival at a destination system, link together to form a larger spacecraft for exploration and investigation.
Gregory Matloff, who along with Eugene Mallove wrote the seminal paper “Solar Sail Starships: The Clipper Ships of the Galaxy” for JBIS in 1981, has recently discussed the design advantages of solar sail nano-cables that would be much stronger than diamond. Nanotechnology in one form or another could thus influence the design even of the large sail structures themselves, not to mention the advantages of shrinking the instruments they deliver to the target. We may one day test out these ideas through nanotech deployed to asteroids to harvest resources there, teaching us lessons we’ll later apply to payloads that assemble research stations or even colonies upon arrival.
The Hansson paper is “From Microsystems to Nanosystems,” JBIS 51 (1998), 123-126. Greg Matloff’s 1981 paper with Eugene Mallove is “Solar Sail Starships: The Clipper Ships of the Galaxy,” JBIS 34 (1981), 371-380.
The Probe and the Particle Beam
For those wanting to dig deeper into Alan Mole’s 1 kilogram interstellar colony probe idea, the author has offered to email copies of the JBIS paper — write him at RAMole@aol.com. For my part, writing about miniaturized probes with hybrid technologies inevitably calls to mind Freeman Dyson, who in his 1985 title Infinite in All Directions (Harper & Row) discussed a 1 kilogram spacecraft that would be grown rather than built. Here’s Greg Matloff’s description of what Dyson whimsically called ‘Astrochicken’:
Genetically engineered plant and animal components would be required in Astrochicken. Solar energy would power the craft in a manner analogous (or identical) to photosynthesis in plants. Sensors would connect to Astrochicken’s 1-gm computer brain with nerves like those in an animal’s nervous system. This space beast might have the agility of a hummingbird, with ‘wings’ that could serve as solar sails, sunlight collectors and planetary-atmosphere aerobrakes. A chemical rocket system for landing and ascending from a planetary surface would be based upon that of the bombardier beetle, which sprays its enemies with a scalding hot liquid jet.
The passage is from Matloff’s Deep Space Probes (2nd ed., Springer, 2006), which goes on to discuss the need to master nanotechnology so that we can manipulate objects on the scale of atoms. He even speculates, citing Alan Tough, that nanotechnology could produce a hyperthin communication antenna for relaying information to Earth from an interstellar probe, constructing it from resources in the destination star system. And he cites Anders Hansson’s 1996 paper in JBIS that sketches an interstellar Astrochicken, one with
…miniaturised propulsion subsystems, autonomous computerised navigation via pulsar signals, and a laser communication link with Earth. The craft would be a bioengineered organism. After an interstellar crossing, such a living Astrochicken would establish orbit around a habitable planet. The ship (or being) could grow an incubator/nursery using resources of the target solar system, and breed the first generation of human colonists using human eggs and sperm in cryogenic storage.
Infrastructure for the Probe
Just as Project Icarus is an attempt to update the Daedalus design of the 1970s, Alan Mole’s work re-examines ideas like these in light of recent work. Yesterday we looked at the trends he thinks make probes with this degree of miniaturization possible. For getting the probe to destination at 0.1 c, he relies on a magnetic sail which draws on studies performed by Dana Andrews in the 1990s. Andrews was talking about a 2000 kg payload, but Mole’s 1 kg payload could be accelerated at 1000 g to cruising speed, with an acceleration distance he calculates at only 0.3 AU, using 1/9th the kinetic energy of the Andrews probe.
To push the magnetic sail a particle beam produced by an accelerator is demanded. Here is Mole’s description:
The probe interacts with the beam by having a magnetic sail, a loop of superconducting wire. This is ejected from the probe and a large current introduced. The magnetic field repels all parts of the wire, so it naturally inflates to a circular loop. For one kg and 0.1 c the acceleration distance is only 0.3 AU and the loop diameter…is just 270 m. After cruise, it is possible to slow the probe by reintroducing a current into the sail and using “friction” from interstellar magnetic fields. This method seems to scale successfully.
The beam generator in Earth orbit would be powered by beamed power from the Earth’s surface. Mole describes the installation as ‘an orbital solar power farm in reverse,’ with the advantage of tapping directly into Earthbound power resources. He points out that NASA drew up studies of a Solar Power Satellite system in 1981, coming in at a cost of about $4 billion to beam power to Earth by microwaves. The Mole plan reverses the process to power an orbital beam generator that would accelerate the magsail, with an estimated beam generator cost of $17 billion.
Image: We’ve long imagined beaming power down to Earth to tap the Sun’s abundant energies. But is there a case for beaming power up to drive an interstellar beam technology? Credit: Mafic Studios/National Space Society.
Mole believes the cost of such a beam generator might actually come in considerably lower than $17 billion, but the appendix to his paper shows a wide range of possible costs. In a recent email, Mole wrote me that he is offering a stipend of $5000 (negotiable) for a suitable expert to design and produce cost estimates for the beam generator. Those interested should apply to Mole directly at the email address given at the top of this article. The cost estimates are significant, to say the least, and getting them right should illustrate the value of moving to a smaller probe.
In terms of energy use, by Mole’s figures, a 2000 kg probe of the sort Dana Andrews discussed in his 1994 paper, would require 560 times the US capacity for power generation today. A 1 kg probe would require 28 percent of the US generating capacity, making it far more feasible for a civilization at our stage of development over the next fifty years. The author adds:
Large probes and worldships inspire readers to imagine the vast civilizations that could afford them, but not to start work in hopes of seeing them launched in the readers’ lifetimes. In contrast, a 1 kg probe is plausible for the present civilization. If discussion begins now such a probe could be launched in fifty years.
Sail technologies, whether beamed by microwave, laser or particle beam, come to the fore in discussions like these because we’re already beginning to build experience with solar sails in space. Laboratory experiments have shown that sail beaming works, with the added benefit — shown in Greg and Jim Benford’s experiments — that materials can undergo ‘desorption,’ providing additional acceleration. Now we need to learn how well particle beams can drive a magnetic sail because, like all these sail concepts, this one requires no propellant aboard the spacecraft, a huge plus given the challenge of pushing even a 1 kilogram payload to a tenth of lightspeed.
The paper we’ve been discussing is Mole, “One Kilogram Interstellar Colony Mission,” Journal of the British Interplanetary Society Vol. 66, No. 12, 381-387 (available at the site). The Dana Andrews paper referred to above is Andrews, “Cost considerations for interstellar missions,” Acta Astronautica 34, pp. 357-365,1994. The Hansson paper is “Towards Living Spacecraft,” JBIS 49 (1996), 387-390.
Small Probes, Hybrid Technologies
Reducing the size of a starship makes eminent sense, and as we saw yesterday, Alan Mole has been suggesting in the pages of JBIS that we do just that. A 1 kilogram interstellar probe sounds like it could be nothing more than a flyby mission, and with scant resources for reporting back to Earth at that. But by Mole’s calculation, a tiny probe can take advantage of numerous advances in any number of relevant technologies to make itself viable upon arrival.
Just how far can nanotech and the biological sciences take us in creating such a probe? For what Mole proposes isn’t just an automated mission that uses nano-scale ‘assemblers’ to create a research outpost on some distant world. He’s talking instead about an actual human colony, one whose supporting environment is first guaranteed by nanobots and, in turn, the robots they build, and whose population is delivered through the hatching of human embryos or perhaps even more exotic methods, such as building humans from DNA formulae stored in memory.
Let’s look at some of the factors the author lists, and bear in mind that we are trying to sketch out the shape of technologies that will have advanced in ways we can’t predict by the time such a probe is ready to fly, even if we allow it the relatively short time-frame (by interstellar scales) of fifty or sixty years of development before launch. From the “One Kilogram Interstellar Colony Mission” paper, here are the key points:
1) Increases in memory density show no sign of slowing. Mole cites small media memory chips that will soon carry two terabytes, but I’d point to Charles Stross’ fascinating discussion of ‘memory diamond,’ which sets theoretical limits on memory density by manipulating carbon atoms. If we need to pack vast amounts of memory into tiny spaces, the future is increasingly bright.
2) Within fifty years, nanotechnology may be able to produce tiny machines — nanobots — capable of complex tasks including self reproduction. The key question then becomes, can such technologies build humans? Mole recognizes the size of the challenge:
“Whether nanomachines can build full humans is unknown. It is physically possible — nature does it when a single fertilized egg cell grows into a human or animal. The DNA of a bacterium has been produced from stored ones and zeros in a computer. Granted, this required a full laboratory of equipment, but in five decades nanobots may be able to do it.”
3) I would feel better about the nanotechnology cited above if we took that fifty year restriction out of the equation, but even without humans ‘built’ by nanotech, we still have the option of sending embryos. Here the relevant citation is a 1989 Japanese project to incubate a goat fetus in an artificial womb, where the fetus grew to birth size but did not survive. Using vast numbers of human embryos on a colony ship, to be raised by robots at destination (robots that have themselves been built by nanobots), allows humanity to spread without large ships and without the need for hibernation (the large ships may be less feasible than the hibernation).
4) That Mole’s proposal is audacious is underlined by the fact that artificial intelligence may be its least controversial feature. Not everyone agrees with Ray Kurzweil that within three decades we’ll be able to essentially duplicate a human mind and run it as a program on a computer. But watching the trends in memory and recent work in brain architecture, including the Blue Brain Project, makes the prospect of uploaded minds at least possible. In any event, we are talking about running some kind of artificial intelligence on tiny CPUs that can manage the activities of nanobots as they build androids that go on to create a human colony. We’re in Singularity territory now, and in the nature of things, that makes predictions tricky indeed.
All of this grows out of a foundation of thinking that combines biology and silicon in interesting ways. Back in June of 1999, then NASA administrator Daniel Goldin spoke before the American Astronomical Society. It had been two years since he announced (in the same year that the Pathfinder probe landed on Mars) that reaching another star would be a new goal for NASA. That was startling enough, but Goldin went on to speak about a combination of lightsail technologies, artificial intelligence advances and hybrid systems tapping advances in biology.
It was an exciting time, even if the interstellar vision was quickly submerged in NASA’s more immediate goals and the ever present challenge of funding work in low Earth orbit. But Goldin’s probe — he described it as a space vehicle about the size of a Coke can — was meant to build itself by scavenging an asteroid, using the abundant supplies of carbon, iron and other materials such an object could provide. Mole’s paper reminded me of Goldin’s quote from that time:
“This reconfigurable hybrid system can adapt form and function to deal with changes and unanticipated problems. Eventually it will leave its host carrier and travel at a good fraction of the speed of light out to the stars and other solar systems… Such a spacecraft sounds like an ambitious dream, but it could be possible if we effectively utilize hybridized technologies.”
With Goldin as with Mole, the intent was to craft a starship without the need to push thousands of tons of payload, using the ability of technology to build and extend itself with local materials. In any case, we’re getting better and better at working with small spacecraft. Consider the Viking landers, each of which massed about 1200 kilograms (the Viking orbiter was 2300 kg). Mars Pathfinder’s lander came in at 100 kilograms, while the Sojourner rover itself massed only 12 kg.
Freeman Dyson laid out a concept for a 1 kilogram probe back in 1985 that set the stage not only for increased miniaturization but the fusion of biology with digital tech. Tomorrow I’ll get into some of Dyson’s ideas as a way of framing what Alan Mole is discussing, and then we need to focus in on the propulsion question. Getting anything — even something as small as a 1 kilogram probe — to another star is an extraordinary undertaking. But finding ways to leave the propellant behind can make it more feasible.
The paper under discussion is Mole, “One Kilogram Interstellar Colony Mission,” Journal of the British Interplanetary Society Vol. 66, No. 12, 381-387.
Interstellar Probe: The 1 KG Mission
Reading Charles Adler’s Wizards, Aliens and Starships over the weekend, I’ve been thinking about starflight and cost. Subtitled ‘Physics and Math in Fantasy and Science Fiction,’ Adler’s book uses the genres as a way into sound science, and his chapters contain numerous references to writers like Poul Anderson, Larry Niven and Robert Heinlein. On the matter of speculative propulsion systems, he lingers over fusion and describes the work of Project Daedalus back in the 1970s, when an ad hoc team of volunteer scientists and engineers put together a serious starship study.
Like the vessels written about in the science fiction of that era and before, Daedalus was simply a mammoth craft — 53 million kilograms! — but that corresponded with what SF had been telling us all along. We would travel to the stars aboard vessels not so different from ocean liners, perhaps big enough to be livable on a daily basis, or at least big enough to pack thousands of humans into cryogenic containers for a trip under suspended animation. It’s a natural enough thought: Long journeys demand big vessels. Scenarios like this burn up plenty of energy, as Adler is quick to note:
…the implication of an interstellar probe [like Daedalus]…is that we possess an extremely energy-rich society. The cost of Project Daedalus was estimated at $10 trillion. Using the rule of thumb that prices for everything double every 20 years, the estimate comes in at about $40 trillion today, dwarfing the U.S. GDP. This amount of money is about equal to the GDP of the entire world. Energetics tell us why this is so: the total energy contained in the payload is about 10% of the total world energy usage for one year. This is too expensive for any current world civilization to undertake, and it may well be too expensive for any civilization to undertake under any circumstances.
Adler, a professor of physics at St. Mary’s College in Maryland, is a lively writer who is well versed in both science fiction and fantasy, making this an entertaining volume indeed. He doesn’t mention the ongoing Project Icarus study, but it will be interesting to see how the ensuing years have modified the original Daedalus concept to produce a less costly, more viable design. Even so, the assumption is that a fusion starship as designed today is going to be a large vehicle because it has to deliver enough of a payload to make the journey to the star worthwhile.
Realm of the Small
Enter Alan Mole. A retired engineer, Mole is an aerospace stress analyst who has worked at the University of Colorado Laboratory for Atmospheric and Space Physics, and as a contract engineer for Ball Aerospace, McDonnell Douglas, Pratt and Whitney, Thiokol-ATK and other firms. A recent issue of the Journal of the British Interplanetary Society contains his paper “One Kilogram Interstellar Colony Mission,” which reverses the big starship paradigm and looks to deliver a seriously effective payload at a sharply reduced cost. Mole is, he tells me, interested not only in physically possible ways to solve difficult problems, but also in making the solutions economically feasible.
Image: The Milky Way over Ontario. As we ponder a human future in the stars, can nanotech and biology breakthroughs show the way forward? Credit & Copyright: Kerry-Ann Lecky Hepburn.
The difficulty of the problem is hard to overstate. It was not some skeptical bystander but Anthony Martin himself, a major player in the Daedalus design effort, who noted the cost to the society that chose to build Daedalus: “It seems probable that a Solar System wide culture making use of all of its resources would easily be wealthy enough to afford such an undertaking.” But Alan Mole is not the first to point out that we are developing lower cost alternatives. If we can create a smaller payload and find a propulsion method that scales down to meet its requirements, we can start talking about an interstellar effort that would prove economically viable while offering choices for human expansion including interstellar colonization.
If Daedalus totalled 53 million kilograms, Mole thinks we should be looking at a single kilogram as sufficient for our colony probe. Making something like this even imaginable involves advances in artificial intelligence, computer memory, materials science, nanotechnology and biology that we can imagine continuing throughout the century, barring the kind of societal catastrophe that disrupts civilization itself. The kind of probe Mole envisions is a world in itself or, I should say, the seed of a world to come, for it uses technology to raise a human colony at destination:
Consider a one kg colony probe sent to a nearby extrasolar planet at about 0.1 c. It will land and nanobots will emerge to build ever larger robots and greenhouses etc. for colony infrastructure. The nanobots will be powered by batteries and recharged by solar cells, building larger arrays of these as work progresses. They will then hatch human embryos (millions per gram) or build humans directly from DNA formulas stored in memory (as was done for a simple bacterium in the artificial life experiments in 2010.) The probe will transmit no data to Earth but if the colony is successful it will eventually build transmitters and establish contact.
Charles Adler doesn’t suggest science fictional treatments of such ideas, but I know current authors must be working this turf, and I’d appreciate pointers from readers. I’m reminded of Robert Freitas’ ideas about self-reproducing probes, a concept I discussed in Centauri Dreams (the book) in the context of an earlier Freitas idea called REPRO, which involved probes on a Daedalus scale that built replicas of themselves and continued out into the galaxy. By reducing the probe to the size of a sewing needle, Freitas envisions sending just enough nanotechnology to turn assemblers loose at destination to build a station to take scientific measurements, report findings back to Earth and, eventually, move on to another star.
Alan Mole is likewise intrigued by the world of the small, but as the above quote demonstrates, he’s thinking in terms of biology as well. Tomorrow I want to explore the implications of Mole’s thinking, looking first at previous ideas for very small payloads from the likes of Freeman Dyson, Dan Goldin and Gregory Matloff. Then we’ll talk about the propulsion systems that could make such a concept work. For it may not be feasible to carry our propellant with us, opening the door for a variety of beamed energy concepts whose cost is far less onerous than the alternatives.
The paper we’ll be discussing for the next few days is Mole, “One Kilogram Interstellar Colony Mission, Journal of the British Interplanetary Society Vol. 66, No. 12, 381-387.
Rosetta: Target in Sight
The European Space Agency’s Rosetta spacecraft, having traveled for ten years, is on track for its close-up investigation of comet 67P/Churyumov-Gerasimenko to begin later this year. Three years ago we had the first actual image of the comet, a 13-hour exposure taken shortly before the craft entered a lengthy period of hibernation. On the 20th of January, Rosetta was ‘awakened’ and controllers are in the process of commissioning its onboard instruments. As part of the process, we have two ‘first-light’ images taken on March 20 and 21.
Image: Comet 67P/Churymov-Gerasimenko in the constellation Ophiuchus. This image was taken on 21 March by the OSIRIS Narrow Angle Camera. The comet is indicated by the small circle next to the bright globular star cluster M107. The image was taken from a distance of about 5 million kilometres to the comet. A wide-angle image was taken on 20 March. Credit & copyright: ESA © 2014 MPS for OSIRIS-Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA.
We’re seeing Rosetta from a distance of 5 million kilometers, from which vantage we see its light in less than a pixel through a series of 60 to 300 second exposures. Even so, the sense of exhilaration in the words of OSIRIS principal investigator Holger Sierks (Max-Planck-Institut für Sonnensystemforschung, Göttingen) is palpable:
“Finally seeing our target after a 10 year journey through space is an incredible feeling. These first images taken from such a huge distance show us that OSIRIS is ready for the upcoming adventure.”
Keep in mind the relevance of Rosetta’s mission not only to the evolution of the Solar System but also to future propulsion ideas. One area of interest is the interaction between the solar wind and cometary gases, needed information as we deepen our knowledge not only of the solar wind itself but how its stream of charged particles might be used in electric and magnetic sail concepts. The solar wind’s variability is one key issue about which we have much to learn.
Rosetta’s studies will be wide-ranging. The spacecraft flies with eleven science instruments onboard, fine-tuned to study everything from the comet’s surface geology to its internal structure and the dust and plasma that surround it. OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) has both a wide-angle and a narrow-angle camera involved in the capture of the early images, all part of six weeks of activity as all eleven instruments are checked out for arrival in August.
This ESA news release offers more, noting that on its current trajectory, the spacecraft would pass approximately 50,000 kilometers from the comet at a speed of 800 meters per second. It will be in May that a series of maneuvers are begun to reduce Rosetta’s velocity relative to the comet to 1 meter per second, with the aim of bringing it within 100 kilometers by the first week of August. The re-activation of OSIRIS now gives way to checks on the other instruments as we prepare for what ought to be a memorable encounter. The Philae lander is scheduled to attempt its landing in November.
Habitability: The Case for F-Class Stars
When it comes to habitable planets, we focus naturally enough on stars like our own. But increasing attention has been paid to stars smaller and cooler than the Sun. M-class dwarfs have small but interesting habitable zones of their own and certain advantages when it comes to detecting terrestrial planets. K-class stars are also interesting, with a prominent candidate, Alpha Centauri B, existing in our stellar back yard. What we haven’t examined with the same intensity, though, are stars a bit more massive and hotter than the Sun, and new work suggests that this is a mistake.
Manfred Cuntz (University of Texas at Arlington), working with grad student Satoko Sato, has been leading work on F-class stars of the kind normally thought problematic for life because of their high levels of ultraviolet radiation. Along with researchers from the University of Guanajuato (Mexico), Cuntz and Sato suggest that we take a closer look at F stars, particularly considering that they offer a wider habitable zone where life-sustaining planets might flourish.
Cuntz thinks the case is a strong one:
“F-type stars are not hopeless. There is a gap in attention from the scientific community when it comes to knowledge about F-type stars and that is what our research is working to fill. It appears they may indeed be a good place to look for habitable planets.”
Image: The habitable zone as visualized around different types of star. Credit: NASA.
The team’s paper in the International Journal of Astrobiology makes this argument based on its studies of the damage that ultraviolet radiation can cause to the carbon-based macro-molecules necessary for life. Its estimates of the damage that would accrue to DNA on planets in F-class star systems covered calculations for F-type stars at various points in their evolution. Planets in the outermost regions of the habitable zone experience much lower levels of radiation. This UT-Arlington news release quotes the paper:
“Our study is a further contribution toward the exploration of the exobiological suitability of stars hotter and, by implication, more massive than the Sun…at least in the outer portions of F-star habitable zones, UV radiation should not be viewed as an insurmountable hindrance to the existence and evolution of life.”
F-type stars represent 3 percent of the stars in the Milky Way, as compared with G-class at about 7 percent and K-class at approximately 12. And then there are M-dwarfs, which may account for over 75 percent of all main sequence stars. In any event, the more we widen the prospects for astrobiology beyond stars like the Sun, the more we address the possibility of a galaxy suffused with life, even if we still have no direct evidence. Just as intriguing: If it turns out life is abundant, is intelligence abundant as well?
The paper is Sato et al., “Habitability around F-type Stars,” International Journal of Astrobiology, published online 25 March 2014 (abstract).
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