Once Upon a Time

The best prophet of the future is the past.
George Gordon, Lord Byron

The year is 2033 and humans have finally made a landing on the planet Mars. The planet appears, for all intents and purposes, to be devoid of life. However, during a field excursion by our intrepid explorers, they find a skeletonized humanoid body. Who was this being? How did it get here? How long has it been here? Is it even a human? Such a discovery would have at least the same impact on our civilization as the actual discovery of intelligent extraterrestrial life itself.

The questions raised by such a discovery on another world are little different from those asked when modern archaeologists make a discovery here on Earth. Archaeology is not treasure-hunting, as is often depicted in movies; rather it is a highly meticulous science that draws heavily on many different hard sciences including geology, physics, chemistry, physical anthropology and biology as well as some of the soft sciences such as cultural anthropology, sociology and psychology to achieve its goal of understanding past human cultures. To achieve an understanding of past cultures, it sifts through the material evidence of the past to extrapolate what a vanished people were like, who they were, how they lived from day to day and even possibly what was their fate.

Xenoarchaeology is a form of archaeology that lives mostly in the annals of science fiction works. Since it is a study of remains that allow scientists to knit together past cultures of alien civilizations, it is really a science without a subject. At least for now. Or is it?

The field had dubious beginnings in the theory espoused by American astronomer Percival Lowell, who “found” canals on a dying Martian world in 1894. The canals were seen by Lowell as a network of long straight lines around the equator of Mars. In all likelihood he saw them; today, with improved astronomical instruments, the “canals” are known to have been an optical illusion. Though the canals are known not to exist, the appetite for discovering evidence of past alien civilizations was whetted. A new science was born.

In 2004, at the annual meeting of the American Anthropological Association, there was featured a session entitled, “Anthropology, Archaeology and Interstellar Communication,” thus placing xenoarchaeology into the mainstream.

Science fiction also looked at xenoarchaeology, and unfortunately alien artifacts are often referred to in the trade as Big Dumb Objects, especially the megastructures that may be found in space and on other worlds. Lowell’s “canals,” if they existed, could be considered just one of these Big Dumb Objects. It is actually an unfortunate term, since if we were to discover an alien artifact, no matter where, it would have a truly profound influence on our everyday lives, hardly worthy of the adjective of “dumb.” The term came from an essay by Roz Kaveney, “Science Fiction in the 1970s” in Foundation, a semi-academic journal published by the Science Fiction Foundation of North East London Polytechnic. The term was specifically a reference to the ringworld megastructure of Larry Niven’s Ringworld.

The canals of Mars were thrown into reality by the pictures taken by Mariner 4 in 1965, so what are we left with? What about some of the claims that have been made regarding other “artifacts” found on worlds that humans and their technology have visited such as Mars and the Moon?

During NASA’s Viking I mission in 1976, the Cydonia region of Mars’ northern hemisphere was photographed. One frame appeared to feature a human face etched into the surface of the planet. When NASA explained it away as a trick of the light, the explanation was met with skepticism in some quarters when a second photograph featured the face again, this photograph taken at a different angle and in different lighting. Did we have a face on Mars, our first alien artifact? It appeared to be the case. However, when the higher resolution imaging equipment of future Martian probes such as NASA’s Mars Global Surveyor (1997-2006) and Mars Reconnaissance Orbiter (2006 to present) and the European Space Agency’s Mars Express (2003 to present) took pictures in the Cydonia region of Mars, no “face” was seen. Our first “artifact” was not a reality.

Another possible Martian artifact is the moons of Mars themselves. In 1959, Walter Houston, an amateur astronomy writer, perpetrated an April Fool’s hoax in an amateur astronomy publication which claimed that Dr. Arthur Hayall of the University of the Sierras, both fictitious, reported that the moons of Mars are artificial. The hoax gained worldwide attention when the claim was repeated by Soviet scientist Iosif Shklovsky, the coauthor with Dr. Carl Sagan of the groundbreaking book Intelligent Life in the Universe. He had done some calculations on the moons and made the determination that they could very well be hollow, leading to speculation that they might be spacecraft.

Could it be true? In 2010, the European Space Agency, with data from its Mars Express mission, stated that the larger of Mars’ two moons, Phobos, is not a captured asteroid as was previously thought. In fact, some of the data seems to indicate that as per Shklovsky’s calculations, at least Phobos may have large porous chambers in it. Some will pounce on this evidence as an indication of an alien spacecraft, but similar porous bodies have been detected in the asteroid belt.

Another possible artifact on another world is a “bridge” that was observed in 1953 by amateur astronomer John O’Neill using a four-inch refractor. The “bridge” was found on the western “shore” of Mare Crisum, a flat relatively smooth basaltic basin in the northern hemisphere of the Moon close to the dark-side border; it is also the site of the crash of the Soviet Luna 15 in 1969. O’Neill did the right thing and sought verification of his finding from professionals. British astronomer Hugh Percy Wilkins actually validated the “bridge.” Other astronomers disputed the finding, but not before the story hit a UFO-crazed media of the 1950s. Wilkins was forced to resign his membership from the British Astronomical Society for his role in the media hype.

In 1966, photographs taken by NASA’s Lunar Orbiter 2 revealed objects that cast elongated shadows. Some scientists stated that the shadows could not be explained by boulders or other geological structures on the Moon. One of the most vocal scientists was William Blair of the Boeing Institute of Biotechnology, who went further to state that the shadows were arranged regularly, suggesting artificial origins; the objects became known as “Blair bridges.” NASA was able to prove that the shadows were not the result of artificial spires but of an extremely low solar angle when the photograph was taken.

Though we may have been fooled by these anomalies on the surface of Mars and the Moon, at least one scientist, astrobiologist Ian Crawford of the Birkbeck College of the University of London, has not given up hope, and neither should we, that something will be found. Crawford feels that due to the lack of any active geology or atmosphere on the Moon, it is ideal for the preservation of artifacts should they exist. Christopher Rose, an electrical engineer at Brown University in Rhode Island (he was working at Rutgers University in New Jersey at the time of his comments) and Gregory Wright, a physicist who works at Antiope Associates in New Jersey, feel that finding an alien artifact is more likely than intercepting an interstellar radio message.

Even NASA’s Chris McKay, a planetary scientist, speaking at a Search for Extraterrestrial Intelligence (SETI) conference in 2008, made an even bolder statement that the best place to look for dinosaur artifacts would be the Moon. Relics of a dinosaur civilization on Earth would have simply disappeared from the geological record; it is arrogant to think that remnants of human civilization will be found 65 million years in our future.

How would we know if we have discovered an alien artifact on a world within our solar system though? The solar system is a big place. Where do we even start looking? One method used by archaeology on Earth is through fractal geometry. Fractal geometry, whose main developer was Benoit Mandelbrot, was born in the 1970s.

The geometry that most of us learned in high school is known as classical geometry and seeks to define shape via formulae. In the case of fractal geometry, iteration (repetition) is used instead to describe shape. The process by which shapes are made in fractal geometry is amazingly simple yet completely different to classical geometry.

As a result, shapes in fractal geometry look like what is observed in nature: no perfect circles, squares or rectangles. Instead a tree or natural surface feature can be modeled with fractal geometry by using formulae iterated multiple times, thus allowing us to model natural phenomenon with accuracy. Fractal shapes exist in nature, but without fractal geometry, there was never a way to describe them in mathematical terms.

A simple example of fractal geometry is the drawing of a fractal tree. Draw a vertical line. Now draw two more lines at a given angle in the middle of that initial line. Now repeat this process over and over and you will find that you will end up with something that appears very much like a tree.

Fractals have the property of looking the same over a range of resolutions, and have been used to model a number of natural phenomena including surface features. Computer algorithms can detect a variance in the landscape through anomaly detection techniques. Objects made by intelligence (human or alien) would not be similar in structure to the background of landforms, and therefore, they would stand out against the background.

Other remote sensing imaging, such as satellite imaging, can also be used to detect potential archaeological sites. With satellites orbiting a planet, xenoarchaeologists can employ a number of methods including light detection and ranging (LIDAR). LIDAR is a surveying method that illuminates a target with a pulse of laser light. Differences in return times of the laser light are measured, allowing for the creation of a three-dimensional representation of a target. LIDAR has been used to create high-resolution maps on Earth.

Synthetic aperture radar (SAR) is a radar that is used to create both two and three dimensional images of objects, such as surface features. By targeting a series of pulses of radio waves to a site of interest on the ground, the echo of each pulse is recorded, very similarly to classical radar. However, using a single beam-forming moving antenna to receive the echo signals, with wavelengths from a meter down to several millimeters, clearer images than are possible with conventional radar are created.

Satellite imagery is one way of surveying a world for archaeological sites, but not the only one. With our probes on a planet, we can do several more things to assist an archaeological survey, drawing from Earth-bound archaeology methodologies. A geophysical survey of a landscape, which on the surface may reveal nothing, is used to create a map of subsurface archaeological features. It can be noninvasive or destructive in nature, but site preservation is a key goal of any archaeological survey.

One method of geophysical surveying is with ground-penetrating radar. It uses radar pulses to image the subsurface, and the return pulses can be used to detect and map subsurface artifacts. Though a powerful tool here on Earth, it may not be ideal on all alien worlds. For example, if a soil being surveyed is fine-grained such as a clay or silt, it will have a high electrical conductivity, which would scatter any ground-penetrating radar signal.

Another geophysical surveying method used in archaeology is with a magnetometer, which measures both the magnetism of a magnetic material or a change in the magnetic field at a particular location. It works because each object has a unique magnetic signature, even objects we do not think of as magnetic; different subsurface materials can cause local disturbances of a planet’s magnetic field.

In particular, magnetometers react strongly, as would be expected, to metals such as iron and steel, but also to burned soil and many types of rocks. Even the absence of a magnetic signal provides valuable information such as the presence of disturbed soil, due to agriculture or construction, for example, as well as decayed materials, a possible indication of the waste of a society. Magnetometers have been used in archaeology to detect sites ranging from a tiny temporary campsite to something as large as an ancient city. For this methodology to work, though, an understanding of the planet or moon’s magnetic field is essential; Earth’s magnetic field is unique to Earth and Earth-based models cannot be used on other planets or moons.

Science fiction is certainly no stranger to planetary archaeology. One of the earliest and perhaps one of the best of such tales is a novella by H. Beam Piper, “Omnilingual.” In the story, archaeologists explore some ruins found on Mars. What appears to be writing is also discovered; the majority of the story follows the painstaking attempts at translation of the alien writing to the discovery of a Martian Rosetta Stone.

Another of the science fiction classics deals with planetary artifacts, Arthur C. Clarke’s 2001: A Space Odyssey, which was made into an equally classic movie directed by Stanley Kubrick. In the story, an ancient crystalline monolith is discovered on the Moon. A similar monolith in the story is also found by ancient hominids over three million years ago on Earth. Its purpose is at first unknown, but as the story unfolds, it appears to play a role in the evolution of humankind.

Clarke’s story is hardly the only xenoarchaeological tale that takes place on the Moon. In James Hogan’s Giants series, the discovery of a humanoid skeleton in a spacesuit on the Moon sets off a tale of the discovery of a technologically advanced alien race that inhabited the solar system a long time before humans ever entered the scene. Another classic is Rogue Moon by Algis Budrys, which describes the discovery of an alien artifact on the Moon that kills explorers who enter its labyrinth.

Other xenoarchaeological science fiction stories take place on Earth’s nearest planetary neighbor. In Richard Morgan’s Broken Angels, an alien artifact is found on Mars that provides evidence that the ancient Martians were not originally from the red planet but from somewhere else in the universe. In Ian Douglas’ Heritage trilogy, evidence of an alien civilization is found on Mars in the form of underground ruins. Ben Bova’s Return to Mars looks at the exploration of an ancient Martian cliffside ruin first mentioned in the preceding novel, Mars. The Martian Chronicles by Ray Bradbury is another time-spanning tale that takes place on Mars and deals with both long-lost civilization and the eventual colonization of the planet.

Further out in our solar system, science fiction has also touched on the idea of xenoarchaeology. Icehenge by Kim Stanley Robinson takes place at Pluto’s north pole, where a mysterious monument was found. The story is told through three different narratives. One is from the point of view of an archaeologist who lives three centuries in the future of the protagonist in the first narrative.

Other authors have taken the idea of xenoarchaeology beyond the solar system. In Alastair Reynolds’ Revelation Space, an archaeologist is studying the ruins of a long-dead alien civilization found on the planet Yellowstone. J. G. Ballard’s “The Waiting Grounds” is based on the discovery of alien megalithic structures on an alien world. Across a Billion Years by Robert Silverberg is about a futuristic archaeological dig that takes place on an alien world. This alien world was home to a billion-year-old star-faring race; evidence of their existence has been discovered on many other planets. John Brunner’s Total Eclipse is about a team of space explorers who find evidence of another highly advanced civilization, the Draconians, who have been extinct for nearly one hundred thousand years. Some of Jack McDevitt’s stories have an archaeological basis. In his The Engines of God, ancient monument makers leave behind beautiful statues inscribed with some form of writing. Anne McCaffery in her Pern series writes of an excavation of a site of the first settlers of Pern. McCaffery and Mercedes Lackey’s The Ship Who Searched is about an extraterrestrial dig where one of the daughters of an excavator becomes mysteriously ill. Robert Silverberg’s Valentine Pontifex of his Lord Valentine series contains a narrative about an excavation of an ancient city of shapeshifters.

Other alien artifacts that we may find in our immediate vicinity are alien probes in our solar system. Such a probe is often identified as a Bracewell probe, named after the late Robert Bracewell, an electrical engineer at Stanford University. In a 1960 paper he determined that an interstellar probe equipped with artificial intelligence and sent out with the mission to communicate with alien life is an alternative to the attempts at interstellar radio communication through the various SETI programs. Bracewell hypothesized that once such a probe found evidence of a civilization, it could begin to communicate with that civilization over short distances rather than the gaping voids between stars; radio communication from Earth, should it ever happen with an alien sun, would be a very long conversation taking centuries to just get past a greeting. In a sense, the probe would be our sentinel of contact between two civilizations by sending translated information back over the void to its homeworld. One could argue that though unable to communicate directly with alien civilizations, Pioneer 10 and Pioneer 11 are primitive Bracewell probes, with the message in the form of a gold plaque, they are an attempt at one-way communication with aliens. The plaques placed on the spacecraft at the late Carl Sagan’s insistence, display a nude male and female human along with symbols that indicate where the craft came from. They also contain recordings of some of the sounds of human civilization. Perhaps one day an alien spacecraft will find the craft and attempt to look at the probe’s makers. We may be long gone from Earth, but then again, maybe not… It is not inconceivable to consider that an advanced alien civilization also has sent a probe to explore our own solar system. There may be a probe sitting right on our cosmic doorstep.

A science fiction masterpiece, Rendezvous with Rama by Arthur C. Clarke, describes a mysterious enormous spacecraft that enters our solar system. In his short story “Jupiter Five,” perhaps the precursor to his landmark novel, Clarke describes an archaeological expedition to explore an alien vessel orbiting Jupiter. David Brin, in his novel Existence, describes a person responsible for cleaning up the garbage found in Earth’s orbit (mostly satellites and spacecraft parts) who finds something that turns out to be an alien artifact that wants to communicate with Earth, a true Bracewell probe.

In 1991, a faint near-earth object was discovered and dubbed “1991 VG.” Its orbit is eerily similar to that of the Earth, which is difficult to explain naturally, but several alternative theories have been laid out. One theory in 1995 that gained traction was that it was just such an alien Bracewell probe. Another idea is that it might merely be an accumulation of space junk from previous space missions. More recent study of the object, however, indicates that it is natural in origin.

We see that it is now possible to detect potential alien artifacts on other worlds of our solar system and even to find alien probes within our solar system, should they indeed exist, but what about the stars? Is there any way to find artifacts that are evidence of interstellar civilizations?

Currently astronomers have been discovering alien worlds by what is known as the transit method. The transit method is rather simple in principle but does require highly sensitive optical equipment. The transit method is able to detect alien planets by measuring the dimming of a star as the orbiting planet moves into the space between the Earth and the alien sun. Armed with this information, astronomers are able to determine the size, mass and even the density of the object.

The transit information goes even further. When the light of the star passes through the alien planet’s atmosphere, the light will be absorbed at different wavelengths; this is known as the absorption spectrum. Since all materials have a signature absorption spectrum unique to them, astronomers are able to deduce the atmospheres that make up the alien world.

NASA, in 2009, launched the Kepler Space Telescope with the mission to discover extrasolar planets. Already the telescope has exceeded all expectations, having discovered thousands of potential extrasolar planets. NASA launched another mission in April 2018, the Transiting Exoplanet Survey Satellite (TESS), which will survey 2.5 million target stars looking for alien worlds.

It is speculated that if there are artificial planet-sized artifacts circling an alien star, we will also be able to detect them using the transit method. Multiple artificial objects will produce light dimming with a signature that is distinguishable from natural transiting objects. If there is only one artificial object, the artificial signature will be more difficult to detect, but there are ways.

What do these alien megastructures look like? There are a number of speculations, most originating with a thought experiment by English-born American theoretical physicist Freeman Dyson. Dyson felt that as a society advances, its demand for energy will increase. We only have to look to our own civilization to see this reality. Since 1820, world energy demand has increased from approximately ten exajoules to over five hundred today. One exajoule is equal to 1018 joules; a joule is equal to approximately 4.2 calories of energy. To extrapolate such an increase in energy demand, the energy sources that we find on Earth will not be enough to power our civilization in the future, and we could one day need all of the energy from the Sun. To obtain it, he proposed an orbiting spherical structure, the Dyson sphere, designed to intercept and collect all of the energy output of the Sun. Extrapolation of this idea can be applied to other alien civilizations.

How could these megastructures be considered archaeological? Due to the vast distance between the stars, if we were to launch a mission to a world suspected of being surrounded by a Dyson sphere, once we arrive, it will no longer be there. What we are viewing in interstellar space is in truth a step back in time; it takes light sometimes thousands of years to reach us.

For a Dyson sphere that captures a star’s energy, the absorption and emission by the energy collectors making up the megastructure will alter the light that otherwise would reach Earth from the star. Wavelengths of radiation emitted by the orbiting energy collectors would be determined by the emission spectra of the materials making up the collectors—all materials have very specific emission spectra (the frequency of electromagnetic energy emitted by an atom or molecule). Since energy collectors will most likely be made up of certain elements (our own satellites are made up of metals), the emission spectra will be different from what is normally emitted by a star, which is normally made up of lighter materials such as hydrogen and helium. It is this difference in the emission spectra that can be detected over interstellar distances.

In 2005, the Infrared Astronomical Satellite, a joint space telescope venture by NASA, the Dutch Space Agency (NIVR until 2009, to be replaced by NSO) and the United Kingdom’s Science and Engineering Research Council (SERC) has been actively looking at the infrared spectra in space, with Fermilab, a research facility near Chicago, analyzing the data. Infrared radiation is what would be expected from a Dyson sphere-like object. To date seventeen potential candidates have been found. To know for certain if an infrared source is a definite megastructure will require further refining of techniques to distinguish natural from artificial sources.

On October 14, 2015, there was a strange light pattern from Star KIC 8462852 (1,280 light years from the Earth), nicknamed Tabby’s Star after American astronomer Tabetha Boyajian, who discovered the light fluctuation with the Kepler Space Telescope. Though a Dyson sphere-like object was suspected by some, Boyajian was careful to state that the alien intelligence explanation is only a last resort, not a first choice. To date no consensus on the strange light pattern has been accepted.

On August 25, 2016, EPIC 204278916, about four hundred light years from the Earth, also showed regular hard-to-explain dimming. The current explanation is that the dimming is caused by transiting cometary objects. Again no consensus.

Several science fiction authors have created great tales of alien megastructures built by civilizations long extinct. One is Frederik Pohl’s Gateway, which describes an alien technology discovered in a hollowed-out asteroid. In the asteroid is an alien technology that allows instant travel to various corners of the universe in one of many spacecraft. The catch is that nobody, without trial and error, knows the length of the mission or its destination. Many destinations are lethal and others lead to more alien technology which is exploited by the story’s hero. The Ringworld series by Larry Niven follows the exploration of an alien artificial ring encircling a sun, the original Big Dumb Object, by a group of intrepid explorers both human and alien.

Some scientists have commented that applying the rigours of an archaeological dig to alleged spacecraft crashes such as Roswell would reveal some truths. Roswell, New Mexico was where the UFO phenomenon all began. In 1947, an alien spacecraft is said to have crashed there. The heavy subsequent military presence in the area and the official statement that the crashed craft was nothing but a weather balloon led to many conspiracy theories that still persist. An archaeological dig at the site could settle the issue once and for all.

Unfortunately, this type of rigour is not applied to a form of pseudoscience in which many ancient ruins are said to have an extraterrestrial origin — and this is believed by many. Chariot of the Gods, by a major ancient astronaut proponent, Erich von Daniken, was a bestseller published in 1968.

If ancient astronaut theory used the scientific method, it would have some credibility. Instead, it looks at the evidence and makes it fit the premise that ancient astronauts interacted extensively with humans of the ancient past. For example, if an ancient astronaut theorist cannot explain how a structure was built, they immediately leap to the extraterrestrial explanation. This is not to say that alien astronauts have never visited the Earth, however.

Some science fiction authors have looked at the possibility of alien visitors to Earth in our past. Jack London, in his short story “The Red One,” writes of an expedition into the Solomon Islands jungle to collect butterflies. The indigenous race in the jungle are in possession of a large red sphere of apparently alien origin.

Arthur C. Clarke’s short stories often have surprise endings, and “Rescue Party” is no exception. In the story, a group of intrepid aliens explores a dead world with evidence of a past civilization; that civilization just happens to be that of the human race, long vanished. His “The Star” is another such tale. In the story, there is an expedition to an alien solar system only to discover the ruins of an advanced alien civilization.

David Brin, in his short story “Lungfish,” follows the exploits of a space-junk cleaning crew who come across a crystal alien artifact which just happens to have the uploaded personalities of nearly one hundred aliens. Dean McLaughlin’s short story “For Those Who Follow After” traces the discovery of an alien cultural artifact that happens to warn others of their inevitable fate. Horror writer H. P. Lovecraft’s “At the Mountains of Madness” follows an Antarctic expedition to visit an ancient and alien city. In Jack McDevitt’s Ancient Shores, an alien artifact made of an unknown material is discovered on a North Dakota farm. His Eternity Road is set in a future Earth one thousand years from now, when humanity rediscovers the technology of their past. Gregory Benford’s Artifact is about the discovery of an alien artifact in a Mycenean tomb.

With the enormous distances in time and space in our universe, there is always the possibility that we will never come into contact with living aliens; however, all technologies that we have had on Earth have left a footprint, a footprint that we can discover. It is no different for alien civilizations. With improved techniques, finding those alien footprints will improve. Perhaps our “first contact” will not be the photogenic opportunity of “shaking hands” with an alien but something as mundane as a midden on an alien world. Nonetheless, even the discovery of an alien midden would change our world forever.

Additional reading

Aldenderfer, M. and Maschner, H. (eds). 1996. Anthropology, Space, and Geographic Information Systems. Oxford University Press.

Arnold, L. 2005. Transit Lightcurve Signatures of Artificial Objects. Astrophysical Journal. 627:534-539.

Badescu, Viorel (ed.) 2013. Asteroids: Prospective Energy and Material Resources. Springer.

Bracewell, R. N. 1960. Communications from Superior Galactic Communities. Nature. 186:670-671.

Bracewell, R. N. 1973. The Opening Message from an Extraterrestrial Probe. Astronautics & Aeronautics. 11:58-60.

Briggs, John. 1992. Fractals: The Patterns of Chaos. Thames and Hudson.

Brown, C. et al. 2005. The Broken Past: Fractals in Archaeology. Journal of Archaeological Method and Theory. 12(1):37-78.

Carlotto, M. J. 1988. Digital Imagery Analysis of Unusual Martian Surface Features. Applied Optics. 27(10):1926-1933.

Carlotto, M. J. 1997. Evidence in Support of the Hypothesis that Certain Objects on Mars Are Artificial in Origin. Journal of Scientific Exploration. 11(2).

Carlotto, M. J. and Stein, M. C. 1990. A Method for Searching for Artificial Objects on Planetary Surfaces. Journal of the British Interplanetary Society. 43:209-216.

Chennamangalam, J. et al. 2015. Jumping the Energetics Queen: Modulation of Pulsar Signals by Extraterrestrial Civilizations. New Astronomy. 34:245-249.

Daniken, Erich von. 1968. Chariots of the Gods. Putnam.

de la Fuente Marcos, Carlos and de la Fuente Marcos, Raul. 2018. Dynamical Evolution of Near-Earth Asteroid 1991 VG. Monthly Notices of the Royal Astronomical Society. 473(3):2939-2948.

Foster, G. 1972. Non-Human Artifacts in the Solar System. Spaceflight. 14:447-453.

Freeman, Dyson. 1955. The Search for Extraterrestrial Technology. In: Marshak, R. (ed): Perspectives in Modern Physics. John Wiley.

Freeman, J. and Lampton, M. 1975. Interstellar Archaeology and the Prevalence of Intelligence. Icarus. 25:368-369.

Freitas Jr., R. July-August 1983. Debunking the Myths of Interstellar Probes. AstroSearch. 1:8-9.

Freitas, Jr., R. 1983. If They Are Here, Where Are They? Observational and Search Considerations. Icarus. 55:337-343.

Freitas Jr., R. 1980. Interstellar Probes: A New Approach to SETI. Journal of the British Interplanetary Society. 33:95-100.

Freitas Jr., R. 1983. The Case for Interstellar Probes. Journal of the British Interplanetary Society. 36:490-495.

Freitas Jr., R. 1983. Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox. Journal of the British Interplanetary Society. 36:496-500.

Freitas Jr., R. 1983. The Search for Extraterrestrial Artifacts (SETA). Journal of the British Interplanetary Society. 36:501-506.

Freitas Jr. R. and Valdes, F. 1985. The Search for Extraterrestrial Artifacts. Acta Astronautica. 12(12):1027-1034.

Freitas Jr., R. and Valdes, F. 1980. A Search for Natural or Artificial Objects Located at the Earth-Moon Libration Points. Icarus. 42:442-447.

Gardner, Martin (Winter 1985-1986). The Great Stone Face and Other Nonmysteries. Skeptical Inquirer. 10(2):14-18.

Groshong, Kimm. May 16, 2006. Looking for Aliens on the Moon. New Scientist.

Grossinger, Richard (ed). 1986. Planetary Mysteries: Megaliths, Glaciers, the Face on Mars and Aboriginal Dreamtime. Berkeley.

Hoagland, Richard. 1996. The Monuments of Mars: A City on the Edge of Forever. Berkeley.

Kardashev, N. 1984. On the Inevitability and the Possible Structures of Supercivilizations. In: The Search for Extraterrestrial Life: Recent Developments; Proceedings of the Symposium. Reidel Publishing.

Kaveney, Roz. June 1981. Science Fiction in the 1970s: Some Dominant Themes and Personalities. Foundation.

Mandelbrot, Benoit. 1982. The Fractal Geometry of Nature. W. H. Freeman.

McDaniel, Stanley and Paxson, Monica (eds). 1998. The Case for the Face: Scientists Examine the Evidence for Alien Artifacts on Mars. Adventure Unlimited Press.

McGee, Ben. 2010. A Call for Proactive Xenoarchaeological Guidelines: Scientific, International Policy, and Socio-Political Considerations. Space Policy. 26(26):209-213.

Posner, G. 2000. The Face Behind the “Face” on Mars: A Skeptical Look at Robert C. Hoagland. Skeptical Inquirer. 24(6):20-26.

Sagan, Carl. 1995. The Demon-Haunted World: Science As a Candle in the Dark. Random House.

Steel, D. 1995. SETA and 1991 VG. The Observatory. 115:78-83.

Thomas, David and Kelly, Robert. 2009. Archaeology. Wadsworth Publishing.

Valdes, F. and Freitas Jr., R. 1983. A Search for Objects near the Earth-Moon Lagrangian Points. Icarus. 53:453-457.

Wynn, Charles and Wiggins, Arthur. 2001. Quantum Leaps in the Wrong Direction: Where Real Science Ends…and Pseudoscience Begins. John Henry Press.


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