Interstellar communication using microbial data storage: implications for SETI (part 2)by Robert Zubrin
|
| One could search for genetic material being carried by bacteria that can be inserted into animals or plants and result in the production of striking adaptations that have not yet manifested themselves in living species on Earth. |
That said, there is, as previously noted, grounds for skepticism that this is the form of communication that intelligent species would find interesting. If there is a field of life throughout the galaxy, initiated on innumerable worlds by natural panspermia, it could be expected to evolve in multitudes of new and unpredictable directions through natural processes, including mutation and natural selection, driven by chance and diverse environmental conditions. It seems to me that the most portentous form of communication that intelligent extraterrestrials could undertake would be to try to propagate themselves by sending out genetic information to influence this chaotic process in their own direction. Genomes can contain dormant plans for complex traits, as evidenced by recent work in which scientists activated what had been considered junk DNA in chickens to produce long-lost dinosaur features, like teeth (Hoggenboom 2015, Bhuler 2015).
Well, chickens are descended from dinosaurs, so perhaps it’s not too astonishing to discover that they still keep some of the old body plans on file. Such plans could come in handy if new conditions require a radical evolutionary leap. But could it be possible that some such genetic plans were sent here intentionally by bacterial conveyance? Could some have been used, and others be still awaiting their chance? There are a variety of physiological features, such as the complex eye, or bird wings, whose origin is hard to explain in terms of incremental natural selection, as they appear to be completely non-functional in partially-developed form. The raising of such paradoxes has long been a stock in trade of theists arguing the case for supernatural “intelligent design.” But while there are, by definition, no supernatural phenomenon, there really are many things—including not only buildings and ships, but also domesticated animals and plants—that are the product of intelligent design. Bacteria can transfer genes among themselves, and to and from macrofauna and macroflora (Yong, 2016, Margulis and Sagan, 2008). Instead of sending us greetings and saluting us with the value of pi, could extraterrestrial be sending us microbial messages for the purpose of guiding the evolution of our biosphere?
As fantastical as it sounds, I believe this is a testable hypothesis. Specifically, one could search for genetic material being carried by bacteria that can be inserted into animals or plants and result in the production of striking adaptations that have not yet manifested themselves in living species on Earth. Birds once had teeth, but they never had radar. No species is known which communicates telepathically using bioradio. There are any number of useful adaptations that are physically possible but which never have manifested themselves in terrestrial biology. Furthermore, there are traits that we do see in some species, but not in their ancestors. Mice were once fish, but fish were never mice. The fact that mice still carry fish traits has been clear since the 19th century, when it was observed that mammal embryos exhibit fish traits such as gills. But do fish carry in their inactive DNA advance plans for mammalian traits? It is possible that such traits could be induced by the transference of bacterial genes. But from what source? If such were found, could they have been sent by extraterrestrials in the distant past, to either fish or their ancestors? Or did fish get them from mammals by natural local bacterial transference much more recently? How could these two possibilities be sorted out?
We could also consider the past. There were certain periods when a massive amount of evolutionary innovation occurred within an extremely brief period of time, with the Cambrian Explosion and the early Eocene immediately following the KT extinction immediately coming to mind. Can the sudden appearance of so many radically new traits be best explained by the prior existence of readily available genetic plans? Or perhaps could it be the result of the opening of expanded channels of communication between genetic plans? (Hoyle, 1981)
Current evolutionary neo-Darwinian “fundamental dogma” holds that all traits of a species are only passed on genetically, so that traits acquired in life are not inherited. Furthermore, new genetic traits are only created through mutation and passed down within a species own line of descent, and that only if they pass the test of natural selection within that species line. While useful in understanding biological evolution, this theory is clearly false when applied to human social evolution. This is so because, in contrast to what seems to be the case with other animal species, humans can inherit acquired traits, such as technologies, and furthermore, can inherit such traits from unrelated persons or groups. As a result, such theories as national social Darwinism (Bernhardi, 1912, Hitler, 1941), which postulate history as a battle of nations for limited resources with advances occurring via the resulting elimination of the less militarily proficient, are not only morally reprehensible, but scientifically wrong, since inventions made in any nation can (and generally do) ultimately benefit all nations. Indeed, were it the case that technological innovations were not laterally transferable from person to person, tribe to tribe, and nation to nation, it is doubtful that humanity would ever have advanced beyond the old stone age.
Conversely, it is easy to see the quantum acceleration of human progress following the establishment of global communications through the development of the printing press and long-distance sailing ships circa 1500, and their further improvement via railroads, steamboats, telegraphs, telephones, radio, TV, and the Internet in the period since. Furthermore, it is unquestionable that the growth of human population has contributed to this trend, since the more people there are, the more inventors there can be, and inventions are cumulative. This is why, contrary to Malthus, as the world’s population has gone up, the standard of living has gone up, not down. For similar reasons, it is in the self-interest of intelligent species to promote the creation of and communication with as many intelligent species elsewhere as possible. The more sources of transferable invention that there are, the more inventions each will receive, and the greater will be the power of each to add further inventions in turn. Furthermore, most intelligent species must be aware that their self-interest lies in increasing, rather than decreasing, the number and effectiveness of the creative agents that they draw upon, because if they were not, they could not survive for long.
| Can bacteria perform the same role from world to world? If they could, they would be a tremendously positive influence on evolution throughout the galaxy, transferring entire encyclopedias of hard-won biological knowledge between planetary biospheres. |
This said, can it really be true that the biosphere is so defective that it is incapable of allowing analogous transfer of useful genes between species? The creation of new useful traits by random mutation is a slow process. Clearly it would benefit all species, and thus the biosphere as a whole, were each to be able to draw to some degree on the genetic innovations of the rest. A community of life that had such ability would have greatly improved odds of survival, and enjoy tremendous adaptive advantage over any that did not. In fact, contrary to the neo-Darwinian fundamental dogma, the biosphere has exactly such a capability. Such transfers from species to species are enabled by bacteria (Hotopp 2011). Such bacterial transfers of entire genes allow the evolution of valuable new inheritable traits to appear in species not over millennia, but within the lifetime of a single individual. Yong (2016) reports many such instances, such as species of woodrats that, upon acquiring certain bacteria, immediately gain the ability to digest various plants like creosote or cactus that they did not have before, thereby becoming able to live and prosper in environments dominated by such species. These valuable bacterial-derived traits are then passed from mother to child during birth, along with samples of the maternal bacteria. But since the rats are constantly broadcasting their bacteria through exhalation, these traits can be transferred to other unrelated individuals and even other species. Other bacteria have been shown to provide various species with inheritable defenses against parasites such as nematodes. Margulis and Sagan (2008) report many additional examples, including cases where the acquired genes eventually move from the bacterial microbiome carried by the animal into the animal cells themselves.
To use an analogy, the genotype of an animal’s cells is its hardware, but an animal’s characteristics are also determined by its bacterial software, or microbiome, which can be rapidly changed in and out. Evolution occurs not by random changes in the hardware circuitry, but by adding or subtracting software programs, which are constantly being developed and exchanged in vast amounts by the extremely prolific bacteria. If found by natural selection to be valuable over the long haul, these programs can end up being written into the hardware.
If the software is found to be useful, the animal’s chances of survival are improved, and the trait is passed on to her descendants. But not only that, she and each of her progeny become agents for spreading the useful trait throughout the biosphere through their exhalations and excretions. As you read this article, you are exhaling half a million of your bacteria every minute, providing every creature around you with samples of what you and your ancestors have found to be useful. Your dog or cat is broadcasting similar service. Thus the web of life on our planet shares its inventions.
Can bacteria perform the same role from world to world? If they could, they would be a tremendously positive influence on evolution throughout the galaxy, transferring entire encyclopedias of hard-won biological knowledge between planetary biospheres. But while benefiting the progress of life, it is hard to see how such capabilities benefit the bacteria themselves. Could they be serving the purposes of others? Perhaps they are, particularly since it is easy to see how others could be readily sending them.
Some may object that ET’s wishing to control evolution elsewhere in order to reproduce themselves on other planets could hardly accomplish such as objective by spreading microbes carrying their genes, as evolutionary processes occurring on destination worlds would certainly carry matters forward in unpredictable directions. This is unquestionably true. However, returning to our example of trying to talk to the children in the neighborhood by leaving books around, we can imagine a more general method of communication. Let’s say the book is Harry Potter and the Sorcerer’s Stone. There are several layers of messaging in the book. These include:
As a (1) method for transforming children into wizards, Harry Potter fails. It can contribute to (2) moral instruction, however, and succeeds brilliantly as (3) a way to get children to become readers. After that, there is no limit to what they might learn or become. By analogy, the purpose of microbial data transmission might not be to direct evolution in particular ways, but simply to encourage evolution to be more creative.
Given these observations, how can we make use of them to detect ET? A good place to start might be to focus study on those bacteria that show the strongest signs of most recent extraterrestrial origin. These would be those best adapted for spaceflight. In nature, adaptations for particular purposes always come at a cost, and therefore generally disappear over time if they become unnecessary. Some bacteria, such as radiodurans, have excellent astronautical adaptations, including extreme resistance to hard radiation and vacuum. If people exiting an airplane landing in a warm city are observed to be wearing winter coats, the chances are good that they came from someplace cold. Similarly, if certain types of bacteria are adapted to space, there is reason to suspect they came from space. Accordingly, samples containing large varieties of bacteria could be exposed to space conditions prevailing in space. Those that survive the best could then be cultured and identified for further focused study. Evolution since arrival could have erased any past encoded genetic message, but remnants might still be found. There is only one way to find out.
| If people exiting an airplane landing in a warm city are observed to be wearing winter coats, the chances are good that they came from someplace cold. Similarly, if certain types of bacteria are adapted to space, there is reason to suspect they came from space. |
But bacteria can evolve quickly, thereby potentially erasing essential information about the past of those who joined Earth’s melting pot some time ago. Therefore the most convincing place to look for microbial astronauts would be in space itself. Spacecraft carrying aerogels or other suitable capture media could be deployed with the mission of trying to gather spacebugs in flight. In principle, they could be sent anywhere, with perhaps the most promising location being the vicinity of a comet as it outgases volatiles through its trip through the inner solar system. This could be a favorable location for space microbe collecting because it is possible that Oort Cloud objects might collect such interstellar voyagers over time and then, when heated during close solar approach, release them in large numbers along with the vapors of the frozen volatiles that preserved and held them until that time (Hoyle 1981).
However the problem with this approach is that the characteristic relative velocities of objects moving in various orbits in space exceed several kilometers per second, so that microbes on one such trajectory slamming into anything remotely as dense as an aerogel on another trajectory would almost certainly be destroyed on impact. What is needed is an extremely diffuse medium of large expanse that can be used to slow the fast-moving microbes down to nearly zero velocity to so that they can be gathered without harm on a solid collecting surface. Fortunately, there are such media available. They are called planetary atmospheres.
Atmospheric entry from space is ordinarily thought of as a very high temperature affair, and it certainly can be. But the temperature reached will be a function of the object’s mass to surface area ratio, or ballistic coefficient. Objects like the Space Shuttle or Dragon capsule typically hit the atmosphere with ballistic coefficients between 100 and 1000 kilograms per square meter, and these glow incandescent during reentry. However, as design studies for the German Mars Society’s ARCHIMEDES mission showed in considerable detail, a balloon with a ballistic coefficient on the order of 1 kilogram per square meter might well survive atmospheric entry at Mars without ever exceeding the temperature limits of Mylar.
A spherical bacterium with a radius of 1 micron would have a ballistic coefficient three orders of magnitude lower still, and readily be able to avoid incineration after entry at speeds on the order of the Earth’s 11 kilometers per second escape velocity. After being slowed down in such a manner, such a microbe would continue to fall downward, with a terminal velocity of about 3meters per second at 40 kilometers altitude, decreasing to 1.5 meters per second at 30 kilometers altitude, and still slower at yet lower altitudes. Larger bacteria would have faster terminal velocities, with falling speed increasing as the square root of the microbe radius. For similar reasons, any microsailcraft with a low enough ballistic coefficient to escape its solar system using light pressure would have no problem safely entering the atmosphere of practically any planet.
It is possible to fly balloons at 40 kilometers altitude, and use them to try to collect microbes. In the 1960s this was actually done by Jerry Soffen, later the Viking chief scientist, who did indeed discover viable microbes at that altitude, in such numbers that the results were considered so counterintuitive that the experiments were discontinued (Hoyle 1981). While it is clear that any microbes found in the Earth’s upper stratosphere could be the result contamination from the biosphere below, such upward delivery by upwelling is made difficult by the strong temperature inversion in the stratosphere. This inversion places very cold (-50°C at 25 kilometers) air below warm air (30°C at 40 kilometers), and thereby suppresses upward convection. As a result, there is reason to suspect that microbes collected at 40 kilometers altitude might be from space.
But how could we know? One way is by looking at the nature of the organisms collected themselves. The earliest microbes of Earth were anaerobic archaea. These can no longer live on the surface of the Earth, because they cannot tolerate the presence of oxygen. However, if life on Earth were begun via panspermia, such organisms would have been right stuff to get the ball rolling, because the surface of the prebiotic Earth was a suitable habitat for them. Furthermore, if anyone—be it ET or Nature—is still trying to seed life on prebiotic worlds today, it is these organisms, rather than the Earth’s surface biosphere currently dominant types, who would be the required pioneers. So, in short, what we should look for with our balloon-borne microbe collecting systems are anaerobes, who may exist within the Earth, but not on it, and who have no business flying around the stratosphere unless they just got off the boat.
It is also possible to fly balloons in the atmospheres of Venus and Mars. Both of these planets have sterile surfaces, effectively ruling out contamination via upwelling from below. Venus is especially attractive in this regard, as its thick atmosphere readily facilitates ballooning—two Soviet Vega balloons were successfully flown in the Venusian atmosphere in 1985—and its hot surface precludes indigenous life entirely. Any microbes collected by balloon-borne platforms floating in the atmosphere of Venus—and arguably Mars—clearly would have to come there from space, although the ultimate source of many of them might still well be the Earth.
Perhaps a particularly interesting time to conduct such experiments, whether in Earth’s atmosphere or elsewhere, might be during cometary events. If collecting balloons were flown at various times, including both normal conditions and during or shortly after cometary encounters, the difference in results could be instructive.
| Any genetic information of a forward-looking nature carried by microbes strongly suspected of recent arrival should be a “Wow” signal for the ETI search. |
Using such techniques, suspects for either natural panspermia or ETI-influenced microbial data transmission could be identified. The genome of these top suspects should be sequenced first, identifying which genes play a role in the functioning of the microorganism itself, and which appear to be simply along for the ride. The first set pertains to the messenger, but the second could contain the message. Perhaps it can be decoded, or if not, at least examined for a format suggestive of an artificial design.
Of course, if we captured not merely microbes, but actual microsailcraft, the case for ETI-initiated microbial data transmission would be immediately settled. In that case, the key question would move directly to decryption.
If the transmitter is biological in nature, it would appear to be most logical that the receiver should be as well. If messages are being sent using genes, perhaps their meaning can best be found by inserting them in samples of terrestrial life to see what comes forth. The best terrestrial organisms to use as receivers might be bacteria themselves as, of all life here, they are most adept at adopting and putting to use new genetic information. These could be used to both receive and amplify the signal and then deliver it to more complex organisms. Perhaps novel traits could be made to appear. While radar-wielding birds are not to be expected, there are large numbers of potential animal body plans that are not currently in use of Earth. Many such plans, representing whole phyla of animal life, were briefly exhibited on our planet during the period of first flourishing of multicellular life known as the Cambrian Explosion, some 550 million years ago, only to go extinct shortly thereafter. If genes carried by suspected astrobacteria were found to induce the appearance of traits representative of such extinct phyla or other unknown animal or plant types in current life, that would be very exciting.
Of course, we might not be so lucky. A mother seeking to promote the intellectual development of her child might leave works of literature for a 16 year old, chapter books for an 8 year old, picture books for a 4 year old, and letter blocks for a 2 year old about the house in places where her child might find them. By the same logic, if ET wanted to promote evolution, he might send types of microbes adapted to successive stages of biospheric evolution containing only the information needed for the next steps, rather than the whole library of potential plans right from the start. After all, it would be futile to leave a copy of War and Peace in the nursery of a two-year-old. But even letter blocks are a dead giveaway for developmental intent. So perhaps rather than finding the genes for creating fish, trilobites, edicaria, or even eukaria in anaerobes arriving from space, we might hope to find plans for just the next step, for example chlorophyll. Any microbes carrying plans for further steps might be designed to make their way after arrival in more developed (i.e. oxygenated) phases of the biosphere, and thus be harder to identify as extraterrestrial messengers.
Still, any genetic information of a forward-looking nature carried by microbes strongly suspected of recent arrival should be a “Wow” signal for the ETI search. If such organisms were found to induce the appearance of such traits either uniquely or with markedly greater effectiveness than more mundane microbes, the case for evolutionary influence from space would be proven.
But there might be an even simpler way to search, because the biosphere has been acting as a giant receiver and amplifier for such messages for the past three billion years. That is, the history of such messages may be recorded in the biosphere itself. Consider this: the mustangs of the American West are well-adapted to their current environment, and might appear to a naïve biologist to be a product of local Darwinian evolution. But, while plausible, this conclusion would not be entirely correct. In fact, mustangs are descended from horses that escaped the Spanish Conquistadors, and their ancestors were the products of selective breeding to enable them to carry medieval knights. No doubt the genes for such past incarnations as a breed of heavily-muscled carriers of armored knights are still to be found in the cells of mustangs today, and could be activated to reveal themselves as actual traits in a mustang colt by an appropriate program. This would prove that the mustangs had a previous form that was actually a product of “intelligent design.” Furthermore, it is probably the case that if someone wanted to breed mustangs back into knight-carriers, they could do it much faster than by using horses with no such genetic history in their ancestral line. In short, both their history and some potential forward-looking traits are encoded in their genome.
| Resolving among these alternatives, which have profound implications for the nature of life and the universe, will require drilling to depths on the order of a kilometer to reach groundwater, bringing up samples, culturing them, and subjecting them to biological analysis. From a practical point of view, this can only be done by sending human explorers to Mars. |
This brings us back to the question of the mice and the fish. We know there are fish traits encoded in mice. No surprise there; mice evolved from fish, so naturally they carry the record of their previous evolutionary career. But do fish carry in their genome plans for any of the noteworthy traits of mice? They very well might, but perhaps only because bacteria can move genetic material around from one species to another. But if this were all there were to it, they might very well carry similar amounts of genetic material transmitted to them from species, such as insects, that are not fish descendants. However, if fish were found to be carrying genes for not only mammalian traits, but the whole roadmap of amphibians, reptiles, and mammal-like reptiles leading from fish to mice, or even just the first essential steps on that path, that would show that somebody had been doing some serious advance planning, at least taking the trouble to shout some useful advice into the arena.
Could potential draft plans for future biospheric evolution be preprogrammed into the genes of space-traveling microbial messengers? Could the history of such past messages be recorded in the genomes of species all around us? Let’s have a look and see.
To be useful, any scientific theory needs to be testable and falsifiable. While the balloon-borne microbe or microsailcraft collection experiments described above could potentially offer strong evidence in favor of panspermia, they cannot disprove it because a negative result could be explained away by the argument that the flux of microbes from space is simply too low to be detected by such means. Similarly, the genomic search for forward-looking traits in terrestrial organisms could reveal directed panspermia, but not disprove it. Such experiments should be done regardless because they are cheap and might produce profound results. But if they are unsuccessful in producing useful data, a more muscular approach to settling the matter will be necessary. This can be done through the exploration of Mars.
Mars was once a warm and wet planet, which could have hosted life on its surface, and there is strong evidence that there is still liquid water to be found underground on Mars that could serve as a habitable environment for microbial life today. If there ever was life on the surface of Mars, it is reasonable to assume that it retreated into the groundwater when conditions on the surface of the planet deteriorated, much as the anaerobic archaea did on Earth after the oxygenation of the atmosphere made the surface here inhospitable for their kind. If we could go to Mars and sample the groundwater, what we find, or fail to find, therein would be very informative. There are four primary possibilities.
It should be noted that while operation of robotic rovers on the surface of Mars might serve to falsify alternative 1 (which is fantastical in any case as it requires accepting the conceit that not only is Earth uniquely capable of originating life, but that life from Earth cannot spread to other habitable places nearby) by discovering fossils, it cannot affirm it. More importantly, such robotic exploration techniques would be incapable of distinguishing among alternatives 2, 3, or 4, which is the most critical scientific question. Resolving among these alternatives, which have profound implications for the nature of life and the universe, will require drilling to depths on the order of a kilometer to reach groundwater, bringing up samples, culturing them, and subjecting them to biological analysis. From a practical point of view, this can only be done by sending human explorers to Mars.
But which of these alternatives is most likely? I predict that number 4 is what we will find. The reason is simply this: The Milky Way galaxy predates the Earth by eight billion years. The early Earth was not exceptional in any significant way, so that if life could evolve here, it could have evolved first on hundreds of billions of other possible locations. Furthermore, the first life that did so which developed adaptations allowing it to survive interstellar flight would necessarily spread throughout the galaxy in less than a billion years by natural collisional processes. This would result in the appearance of life on any planet as soon as conditions there were suitable, which is exactly what we observe in the fossil record on Earth.
For these reasons I believe that natural panspermia is extremely probable, and that the results of Mars exploration will prove to be consistent with it. But what about directed panspermia? While the history of life on Earth has demonstrated the ability of biospheres to evolve increasingly intelligent species (Morell, 2013), what reason is there to believe that such intelligent extraterrestrials should want to spread their kind around? None, except this: If there were any such species, it would be their kind that would get spread around. If we find any forward-looking traits encoded in the Martian biota, their handiwork would be there for all to see.
I wish to thank Chris McKay of NASA Ames Research Center and Paul Davies of Arizona State University for useful comments on early drafts of this manuscript.
Arrhenius, S. (1908) “Worlds in the Making: The Evolution of the Universe,” Harper and Brothers Publishers, New York and London, 1908
Ayre, J. (2012) “Text of a full book encoded in DNA,” Planetsave August 18, 2012, accessed Feb 25, 2017.
Bernhardi, F. v. (1912) “Germany and the Next War,” 1912, Kindle edition May 2012 Accessed February 26, 2017
Bhuler B.-A. et al (2015) “A Molecular Mechanism for the origin of a key evolutionary innovation, the bird beak and palate, revealed by an integrative approach to major transitions in vertebrate history,” Evolution, June 30, 2015, accessed February 27, 2017
Cocconi, G and P.Morrison (1959) “Searching for Interstellar Communications,” Nature, vol. 184, no 4690, pp. 844-846, September 19, 1959. Accessed February 26, 2017.
Davies, P. (2010) “The Eerie Silence: Renewing Our Search for Alien Intelligence,” Mariner Books, 2010.
Drake, N. (2017) “This May Be the Oldest Known Sign of life on Earth,” National Geographic, March 1, 2017, accessed March 12, 2017.
Griebel, H. et al (2004) “Project Archimedes: A novel approach to balloon deployment on Mars” 55th International Astronautical congress, Vancouver, Canada, 2004 IAC-04-Q.P.02
Herkewitz, W. (2016) “Scientists turn bacteria into living hard drives,” Popular Science, June 9, 2016, Accessed February 25, 2017
Hitler. A. (1941) “The law of existence requires uninterrupted killing, so that the better may live,” private communication to associates made while issuing the order to exterminate the population of Leningrad, September 1941.
Hogenboom, M. (2015) “Chicken Grows Face of Dinosaur,” BBC News, May 13, 2015, Accessed February 27, 2017
Hornek, G., D. Klaus, and R. Mancinelli, (2010) “Space Mircobiology,” Microbiology and Molecular Biology Reviews, 2010 Mar, 74(1): 121-156.
Hotopp, J.D. (2011) “Horizontal Gene Transfer Between Bacteria and Animals,” Trends Genet. Feb. 18, 2011. Accessed February 27, 2017
Hoyle, F. and C. Wickramasinghe, (1981) “Evolution from Space,” Simon and Schuster, New York, 1981
Margulis L. and D. Sagan, (2008) “Acquiring Genomes: A Theory of the Origin of Species,” Basic Books, 2008
Melosh, J (1988). “The rocky road to panspermia.” Nature 332:687-688.
McDowell, N. (2003) “Data Stored in multiplying Bacteria,” New Scientist, January 8, 2003, Accessed February 25, 2017.
Morell, V. (2013) “Animal Wise: How We Know Animals Think and Feel,” Crown, New York, 2013. A very instructive refutation of the thesis that intelligence is qualitative phenomenon singular to humans, as opposed to a quantitative phenomenon that has been developing in terrestrial animals over time.
Service, R. (2017) “DNA Could Store All the World’s Data in One Room” Science, March 2, 2017. Accessed March 12, 2017.
Wilkins, A. (2010) “Bioencryption can store almost a million gigabytes of data inside of bacteria,” io9, November 26, 2010. Accessed February 25, 2017.
Yong, E. (2016) “I contain Multitudes: The Microbes within Us and a Grander view of Life,” Ecco, 2016
Zubrin, R. (2001) “Interstellar Panspermia Reconsidered,” Journal of the British Interplanetary Society Vol 54, pp. 262-269, 2001. Accessed February 26, 2017
Zubrin, R and D. Andrews, (1989) “Magnetic Sails and Interplanetary Travel,” AIAA 89-2441, AIAA-ASME Joint Propulsion Conference, Monterey, CA, July 10-12, 1989. Reprinted in Journal of Rockets and Spaceflight, April, 1991.
