Search for Extraterrestrial Intelligence

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Search for Extraterrestrial Intelligence ( English for search for extraterrestrial intelligence , also called SETI forshort) describes the search for extraterrestrial civilizations . Since 1960, various scientific projects are operated, among other things, the radio range of the electromagnetic spectrum for possible signs and signals technical civilizations in the universe investigate.

Basics and estimates

The SETI research is based on the assumption that extraterrestrial civilizations exist in the universe and similar communication systems and news technologies use as on Earth. So far it is not known whether extraterrestrial life exists or whether there are other technical civilizations that are able to send and receive interstellar signals. The astronomer Frank Drake attempted to estimate this with the Drake equation . With an optimistic assessment of the factors of this equation, there is a possible number of over 300 such civilizations in the Milky Way . In 1964, Stephen Dole carried out a study for the RAND Corporation for the first time to estimate the number of possible habitable worlds in the galaxy. The Kardaschow scale is used to estimate possible technical possibilities of extraterrestrial civilizations. The galaxy in which the earth is located, the Milky Way, has a diameter of around 200,000 light years and contains between 200 and 400 billion stars and - according to the Kepler mission - 50 billion planets , of which an estimated 500 million planets are in habitable zones . Further analyzes of the Kepler data and investigations with the Keck telescope (status: 2013) suggest that there is an even higher number of planets in habitable zones within the Milky Way.

Search area restrictions

The assumption that alien life forms mostly on carbon - chemistry could be based - like all forms of life on earth - is in the exobiology polemical than carbon chauvinism referred. Although also hypothetically an alternative biochemistry such as B. is discussed on the basis of silicon , carbon offers a greater variety for the formation of semi-stable molecules .

Another assumption is that life requires liquid water . The water molecule is a simple molecule and an optimal environment for the development of complex carbon-based molecules that could lead to the development of life.

A third caveat is to focus on sun-like stars. Very large stars have relatively short lifetimes of only a few million years to a few tens of thousands of years, so that intelligent life on the surrounding planets would have very little time to develop. On the other hand, the released is energy very small stars so low, only that planet on a close orbit as candidates for life would qualify. However, the lifetime of such a star is up to 20 billion years. Due to the narrow orbit and the effect of the strong tidal forces associated with it , the self-rotation of such planets has usually changed very slowly or into a bound rotation . The result is an unfavorable, very strong temperature gradient between the day and night side, as can be observed in Mercury .

Properties of a hypothetical signal

In order to receive a radio transmission from an alien civilization, one has to search the most common electromagnetic frequencies , as one does not know which frequency the aliens might be using. Since the signal should be stronger than the home star's radiation for easier detection, it does not make sense to transmit a strong signal over a wide range of wavelengths , and therefore it is likely that such a signal will be transmitted over a very narrow frequency band (channel). is sent. This means that a large number of very narrow channels must be searched.

The modulation and coding of an alien signal is also unknown. Very narrow-band signals that are stronger than the background noise and constant in strength could be of interest. A regular and complex pulse pattern would indicate that the signals are artificial. Studies have been done on how to send a signal that can be easily found and deciphered . However, it is of course not known whether the assumptions made in these studies are actually valid.

Cosmic radiation and also terrestrial radiation sources form a certain threshold value for signals that we can still recognize as such. In order to be able to locate an alien civilization that is broadcasting its signals in all directions, it would have to use a very powerful transmitter. Its output should at least be comparable to the total electrical output that is available on earth today. The beam of an extraterrestrial civilization can be obstructed: it could be blocked by interstellar nebula, or it could be overlaid by interference and thus become illegible. A very similar effect occurs with television sets with terrestrial antenna reception : if the television signals are reflected from a mountain or a large object and thus reach the antenna on two different distances, a time-shifted overlap occurs.

In the same way, the bundled communication beam of a distant civilization could be deflected or shifted by interstellar clouds and thus come under the influence of interference that could weaken the signal or even make it illegible. When interstellar messages are broadcast via bundled broadcast beams and encounter such problems, there is nothing we can do on our part to deal with these problems - except to be aware of the problem and to anticipate possible interference. The time required to receive and find a broadcast increases considerably. Searching through just one million reception channels takes a considerable amount of time, even when using very fast programs and when only about one second is expected for each channel with no interesting information content.

Modern SETI research began with the publication “ Searching for Interstellar Communications ” by the two physicists Giuseppe Cocconi and Philip Morrison , which was published in Nature in September 1959 . Cocconi and Morrison came to the conclusion that microwave frequencies between 1 and 10 gigahertz would be best suited for interstellar communication. The so-called synchrotron radiation (caused by electrons wandering through galactic magnetic fields ) begins to drown out other radiation sources below one gigahertz . The radiation from hydrogen and oxygen atoms in our earth's atmosphere has a disruptive effect on possible signals at over 10 gigahertz . Even if extraterrestrial worlds have completely different atmospheric conditions, quantum effects make the construction of conventional (electrotechnical) receivers for signals above 100 gigahertz difficult. The lower limit of this “microwave window” is particularly suitable for communication: it is in principle easier to send and receive signals with low frequencies than those with high frequencies. The lower frequencies are also better suited because of the Doppler effect caused by planetary motions. This effect leads to a change in the signal frequency in the course of a transmission, the more serious the higher the frequency of the transmitted signal. Cocconi and Morrison came to the conclusion that the frequency of 1.42 gigahertz (also called the HI line ), the so-called 21 cm line , would be particularly interesting for interstellar transmission: neutral hydrogen radiates at this frequency. Radio astronomers often search space for this frequency to locate large hydrogen clouds . If you were to send a message close to this “marking frequency”, this would increase the chance of an accidental discovery. Since one is looking for spectrally narrow-band signals, confusion with neutral hydrogen can be ruled out because its radiation has a high Doppler broadening due to the temperature movement (see also spectral line ). Another frequency of interest is 1.720 gigahertz ( 18 cm line ). It's a frequency of OH , an oxygen-hydrogen molecule. The area between 1.420 and 1.720 gigahertz is also referred to by radio astronomers as the cosmic water hole . The frequency range is protected by international agreements. The term water hole was coined in 1971 by Bernard M. Oliver in this context . Two possible search strategies for signals would be targeted search (English Targeted search ) and search the entire sky (English all-sky survey ). Another way to detect signals from possibly existing extraterrestrial civilizations would be to study the radio waves focused by the gravitational lensing effect of a star with space probes . This method is called GL-SETI , which is the abbreviation for gravitational lensing SETI and means SETI with the help of gravity lenses . In 1968 Stanisław Lem mentioned in his novel Głos Pana (Eng. The Voice of the Lord ) the possibility of using neutrinos for SETI, Isaac Asimov also pursued this idea in extraterrestrial civilizations. The search for artificial neutrinos or antineutrinos has also been discussed several times by scientists.

Artificial terrestrial radiation

Through the use of radio waves , TV signals , civil and military radar systems and other sources, our civilization produces an artificial EM signature of the earth (leakage radiation), which extraterrestrial technical civilizations with astronomical research interests within a distance of about 60 to 80 ly can optionally be detected. Estimates (status: 2009) assume around 3000 stars and an unknown number of planetary systems within a distance of 100 ly. Some Seti researchers believe it is possible that military facilities such as B. the long-range phased array radar Don-2N , Cobra Dane , Sea-Based X-Band Radar or HAARP could still be detected at distances of 500 light years and more due to the radiation power used.

History and methods of search

Radio telescope at the Green Bank Observatory

Early attempts to identify radio signals from aliens were made by Guglielmo Marconi , who in the early 1920s claimed to have received signals, but this could not be confirmed. Nikola Tesla dealt with alleged signals from Mars much earlier . As early as 1909, the astronomer David Peck Todd unsuccessfully suggested searching for possible extraterrestrial radio signals with a research balloon and receiving device.

Beginnings

On April 21, 1960, Frank Drake of Cornell University began the first modern SETI experiment, the so-called Project Ozma (named after the Queen of Oz from the fantasy books by Frank L. Baum). Drake used a radio telescope from the Green Bank Observatory with a diameter of 26 meters to study the two stars Tau Ceti and Epsilon Eridani near the 1.42 GHz band. He examined a 400 kHz band around the marker frequency and saved the recording on tape in order to search through it later for abnormal signals. However, the investigation did not reveal any particular abnormalities. The total observation time was around 200 hours, with 2000 USD available for the project.

In November 1961 the first SETI conference took place at the Green Bank Observatory . Participants were u. a .: Frank Drake, Otto von Struve , Philip Morrison , Carl Sagan , Melvin Calvin , Bernard M. Oliver and John Lilly . The Soviet Union also began a search program in 1964. 1964 and 1971 organized a. a. Nikolai Kardaschow and Josef Schklowski further SETI conferences, this time at the Byurakan observatory . In 1966, Carl Sagan and Josef Schklowski published Intelligent Life in the Universe, a much-cited book about SETI. In 1971 NASA funded a study on a radio SETI project called the Cyclops. An array of 1,500 36-foot telescopes was proposed, but the cost was too high at around $ 10 billion.

In 1974 the Arecibo Observatory sent a radio message of 1,679 bits in length into space in the direction of the globular cluster M13 (about 25,000 light years away). The number 1,679 has two prime factors, 23 and 73, and the message should be understood as an image of 23 by 73 pixels. The message was sent by frequency modulation at 10 bits per second. The picture is supposed to represent the Arecibo observatory, a human figure, our solar system, the double helix of DNA and the nucleotides necessary for our DNA .

In contrast to passive eavesdropping, the transmission of signals was also known as Active SETI or METI ( Messaging to Extra-Terrestrial Intelligence ) and CETI ( Communication with extraterrestrial intelligence ). Researchers such as astrophysicists Stephen Hawking and David Brin speculate that Active SETI could also be associated with considerable risks. There are plans for a planetary defense . The San Marino scale was created to assess the risk of a transmitted signal . After the ten-point scale the 1,974 sent would Arecibo message level 8. On August 15, 1977, the astrophysicist received Jerry Ehman , the wow! Signal .

In 1979 the University of California at Berkeley (UC Berkeley) started the SETI project SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) with a frequency analyzer with 100 channels. Radio telescopes with mirror diameters of 25 to 65 meters were used. In the summer of 1979, NASA-Ames and other institutions funded the Oasis project . The aim of Oasis was to design a detector for instruments that had been designed as early as 1971 in the Cyclops project .

1980 to 1998

Carl Sagan, Bruce Murray and Louis Friedman founded the Planetary Society in 1980 , which among other things financially supports various SETI projects.

Following suggestions from Paul Horowitz , new portable radio frequency analyzers were developed in 1981. Compared to earlier analog frequency analyzers, they had the advantage that they had many more and narrower channels thanks to their DSPs . From 1982 to 1985, a frequency analyzer with 131,000 channels was used on a 25 m radio telescope at Harvard University (Sentinel project). In 1985, the META (Megachannel Extra-Terrestrial Array) project followed , led by Horowitz and supported by the Planetary Society and director Steven Spielberg , with an analyzer with 8 million channels and a channel width of 0.5 Hz . Another telescope, META II, is searching the southern sky from Argentina .

Also in 1985 Ohio State University started its own SETI program, the Big Ear project , which later received financial support from the Planetary Society. In 1986 UC Berkeley started its second SETI program, SERENDIP II, with 65,536 channels. Mainly a 90-meter radio telescope at the Green Bank Observatory in West Virginia was used. The follow-up project SERENDIP III with around 4 million channels used the Arecibo observatory . Its successor SERENDIP IV also uses the Arecibo observatory and works with around 168 million channels.

In Europe, the Nançay radio telescope was used for a SETI program in the 1980s, and later the 32 m radio telescope in Medicina , Italy.

In 1992 NASA or the US government decided to finance the SETI program MOP (Microwave Observing Program) , which was later referred to as the High Resolution Microwave Survey (HRMS) . HRMS involved a targeted search of 800–1000 Sun-like stars within a distance of 100 light years. The frequency analyzers should have 15 million channels, with each channel being one Hertz wide for the targeted search and 30 Hertz otherwise. The antennas of the Deep Space Network , a 43-meter telescope in West Virginia and the Arecibo Observatory were to be used as radio telescopes . However, the program was ended by the US Congress in 1993, one year after it started.

The privately financed Phoenix project started in 1995 . It was funded by the SETI Institute in Mountain View , California, and began radio searches with the 64-meter Parkes Telescope in Australia. From September 1996 to April 1998 the program used the Green Bank radio telescope and from August 1998 the Arecibo Observatory. Phoenix was discontinued in 2004, 800 stars within a search radius of 200 light years were examined without results.

Current projects: SETI @ home, BETA, ATA, Sazanka

As the successor to the META project, the BETA (Billion-Channel Extraterrestrial Array) project is now being operated by the Planetary Society. Contrary to the name, fewer than a billion, namely only 250 million channels, each 0.5 Hertz wide, are used. The frequency range from 1,400 to 1,720 megahertz is examined, for two seconds (a shorter observation time would not allow this high spectral resolution) a range of 125 megahertz width (corresponding to the product of the width and the number of channels) is examined, then the area is examined shifted, and it is observed again for two seconds. After eight shifts, the original frequency band is reached again.
The efficiency of the search increases noticeably through parasitic search methods , or piggyback methods, which also use conventional radio astronomical observation programs.

The screen saver of the SETI @ home client

In May 1999 the SETI @ home project was started by UC Berkeley, which uses the data from SERENDIP IV. This project uses the computing power of many computers on the Internet that is voluntarily made available by users. You can download the SETI @ home program, which downloads data from the server at UC Berkeley and analyzes it in the background (with the lowest priority) as soon as computing capacity is free on the computer. A special screen saver shows the progress of the work. After a data package has been processed, the results are sent back. The SETI Institute is now working with the University of California, Berkeley to build a new radio telescope, the Allen Telescope Array (ATA) , in northern California . It is said to be dedicated to both radio astronomy and the search for extraterrestrial intelligence. The telescope is supported by Microsoft co-founder Paul Allen and will consist of around 350 6.1 m telescopes. The observable frequency range is between 0.5 and 11.2 gigahertz.

The individual telescopes are relatively cheap, the observatory is said to cost around 25 million US dollars in total. Construction began in 2005. The SETI Institute is primarily providing money for the construction, while UC Berkeley designed and will operate the telescope. It can simultaneously observe many objects in the fields of view of the individual telescopes at different frequencies and as an interferometer . In April 2011, the SETI Institute was forced to interrupt research with the Allen Telescope Array for financial reasons .

Six months later, provisional funding was secured. With the help of private donors and the US Air Force , operations were resumed and the search for extraterrestrial intelligence continued. In addition, the telescope array is now also supposed to search for space junk that could endanger satellites. In line with the discovery of the exoplanet Kepler-22b , the search for extraterrestrial radio signals is to begin again. Over the next few years, all normally silent frequencies from 1–10 GHz will be systematically searched for signs of life on Kepler-22b. The ATA is the only facility in the world that is able to observe all 9 million channels (1 kHz per channel) simultaneously. The data obtained should continue to be evaluated via the distributed computing project SETI @ home .

In 2009 the SAZANKA project was started in Japan . Using 14 radio and 27 optical telescopes one was multi-site -Beobachtungskampagne performed.

The Dorothy project began in November 2010 . On the occasion of the 50th anniversary of the OZMA project , an observation campaign is being carried out in which researchers from 15 countries are participating. In September 2018, NASA and the Lunar and Planetary Institute held the first Technosignatures workshop.

Optical SETI

In addition to the search for radio signals, the search for signals in the visible range and in the near infrared range is also carried out . This is known as Optical SETI or OSETI for short. It is believed that extraterrestrial technical civilizations could use very powerful lasers to communicate over interstellar distances. With light in the visible range, the mirror or lens size required so that the emitted radiation has a certain divergence angle (half the opening angle of an imaginary radiation cone within which the majority of the radiation is located) is smaller than with the longer-wave radio waves. This reduces the probability of detecting a beam that is not intentionally directed towards the earth, but increases the strength near the beam center for a given output power. The search for these optical signals is carried out with high-resolution spectrographs; one tries to find very narrow spectral lines.

In 1961 Robert N. Schwartz and Charles H. Townes published a paper on the possibility of interstellar and interplanetary communication using maser . In 1965, an article was first published on the use of lasers for interstellar communication. In the 1970s, the first search for optical laser pulses was carried out at the Selentschuk Observatory as part of the MANIA (Multichannel Analysis of Nanosecond Intensity Alterations) project.

A working group led by Paul Horowitz developed a detector in the 1990s and installed it on a 1.55 m telescope at the Oak Ridge Observatory of Harvard University. The detector worked parasitically , i. H. parallel to other astronomical investigations. Between October 1998 and November 1999, around 2500 stars were examined with the detector. The researchers worked with Princeton University to install a nanosecond detection system on the FitzRandolph Observatory's 0.91 m telescope, too. Both telescopes then observed simultaneously in the same direction, so that the detection of a signal from the other telescope could be confirmed or rejected as a false alarm. From December 2000 a 1.8 m telescope was built for an OSETI observatory, which has been online since April 2006 and is primarily used for all-sky searches for extraterrestrial laser pulses.

UC Berkeley has two optical SETI programs. Geoffrey Marcy , an astronomer who mainly searches for exoplanets , carried out investigations on the spectra at the Keck Observatory , but could not search for pulses because the temporal resolution of the images was too low. The other program uses a 0.76 m telescope; a search similar to the one carried out by the group at Harvard University is conducted. OSETI research was also carried out at the Lick Observatory . In the southern hemisphere there was an OSETI program at the Campbelltown Rotary Observatory of the University of Western Sydney in Australia, which was carried out by the astronomer Ragbir Bhathal from 2000.

SETA and SETV

Kitt Peak Observatory

Indications of extraterrestrial technological activities could not only provide electromagnetic signals . SETI researchers are also looking for extraterrestrial artifacts, spacecraft, space probes in the solar system or their energy and propulsion signatures (such as tritium or possible annihilation processes of antimatter drives ), traces of mining activities on the Earth's moon , Mars , asteroids , comets , At the beginning of the 1980s, among other things, search programs were carried out at the Kitt Peak National Observatory in which Lagrange points of the earth-moon and earth-sun systems were examined for objects. 1980 to 1981 the radar astronomers Suchkin and Tokarev examined the Lagrange points L4 and L5 for artifacts in Park orbits of the earth-moon, earth-sun systems, without success.

Dyson spheres have also been searched for several times . a. with IRAS and WISE . Astronomical instruments such as the Colossus telescope , which is still in the planning stage, could also be used in the future to search for infrared signatures of possibly existing mega-constructs such as Dyson spheres. In 2015, short, non-periodic brightness reductions from KIC 8462852 lead to speculation.

In the asteroid belt , Kuiper belt and in the Oort cloud , relics of extraterrestrial technologies, such as B. inactive or damaged communication and reconnaissance probes or self-replicating spacecraft . These methods are known as SETA (Search for Extraterrestrial Artifacts) or SETV (Search for Extraterrestrial Visitation) or also as Xenoarchäologie or Exoarchäologie and Dysonian SETI .

The theories of Paleo-SETI or Ufology currently have no scientific reception.

Others

Andrew G. Haley and Ernst Fasan , pioneers of space law , dealt early on with possible legal questions of a first contact with non-terrestrial species and developed a concept known as meta law.

Since 1999, the Institute for Astronomy at the University of California, Berkeley has held the Watson and Marilyn Alberts Chair in the Search for Extraterrestrial Intelligence (SETI) , an endowed professorship that was taken over by Geoffrey Marcy in 2012 .

The Exosoziologie tries potential sociological consequences and hypothetical first-contact scenarios between humans and intelligent extraterrestrial species to explore and different: long-distance contact scenario (such as in a technical way through radio waves.) Artefact scenario and direct contact.

The effects of a contact would be versatile for e.g. B. Natural sciences , philosophy , politics , religion and are the subject of current interdisciplinary research and discussion. Some researchers, u. a. Paul Davies , see the effects of initial contact for the established religious communities as potentially problematic. NASA researched possible consequences as early as the 1960s and published this in the NASA Brookings Report ( Proposed Studies on the Implications of Peaceful Space Activities for Human Affairs ). The Global Risks Report 2013 of the World Economic Forum indicates a future discovery of extraterrestrial life as a possible X-Factor, the profound effect might have.

As part of the IYA 2009 , the Vatican organized a week of astrobiology studies with around 30 specialists from astronomy, physics, biology, geology, chemistry and Seti researchers such as Jill Tarter and representatives of the Pontifical Academy of Sciences and the Vatican Observatory such as B. Guy Consolmagno and José Gabriel Funes lectured and discussed.

In 2009, the subject also dealt with the scientific services of the German Bundestag .

In October 2010, the SETI Permanent Committee of the International Academy of Astronautics adopted a declaration (Declaration of Principles Concerning the Conduct of the Search for Extraterrestrial Intelligence) for the search and the event of a detection of a signal at a symposium in Prague. The SETI research group of the IAA has already proposed a collection of behaviors, the so-called SETI protocols . The IAA runs various working groups that deal with various aspects of SETI, such as B. SETI Post-Detection and Communications with Extraterrestrial Intelligence .

In order to be able to classify and estimate the significance and credibility of a possible discovery of an extraterrestrial signal or artifact, the SETI researchers designed the Rio scale . In 2010, at a meeting of the Royal Society, the London scale (0-10) was presented, which makes it possible to assess scientific significance, validity and potential consequences. In May 2014, Dan Werthimer , director of the SETI Research Center at the University of California, Berkeley, and Seth Shostak , astronomer at the SETI Institute, informed the US House of Representatives Science Committee about the state of research and the future of the search for extraterrestrial intelligence in a public hearing and astrobiology. The scientists explained current projects and discussed the possibility that extraterrestrial life could be found in the next 20 years.

On July 20, 2015, the Breakthrough List Private Research Initiative was announced. Breakthrough lists have been integrated into the SETI @ Home database since 2016 and are sent to the BOINC client as work units together with the normal SETI @ home data. A first large data analysis to search for radio signals from extraterrestrial civilizations has so far yielded no hits.

In April 2016, the astrophysicist René Heller from the Max Planck Institute for Solar System Research in Göttingen called for a "SETI Decrypt Challenge" (SETI decryption competition) in which he called for the decryption of an invented binary-coded message. The message was based on the famous Arecibo message .

See also

literature

  • Aleksandar Janjic: Astrobiology - the search for extraterrestrial life. Springer Berlin Heidelberg 2019, ISBN 978-3-662-59491-9 .
  • Frank Drake, Dava Sobel: Signals from other worlds - the scientific search for extraterrestrial intelligence. Droemer, Knaur, Munich 1998, ISBN 3-426-77351-1 .
  • Sebastian v. Hoerner: Are we alone? - SETI and life in space. Beck, Munich 2003, ISBN 3-406-49431-5 .
  • Emmanuel Davoust: Signals without an answer? - the search for extraterrestrial life. Birkhäuser, Basel 1993, ISBN 3-7643-2731-6 .
  • Tobias Wabbel, Stephen Hawking u. a .: SETI - The search for the extraterrestrial. Beust, Munich 2002, ISBN 3-89530-080-2 .
  • Harald Zaun: SETI - The scientific search for extraterrestrial civilizations. Opportunities, prospects, risks. Heise-Verlag, Hannover 2010, ISBN 978-3-936931-57-0 .
  • Walter, Ulrich: Civilizations in space - are we alone in the universe? Spectrum, Akad. Verl., Heidelberg 1999, ISBN 3-8274-0486-X .
  • Thomas Steinegger: The culture of interstellar communication - a study on the democratization and establishment process around SETI. Diploma thesis, Univ. Vienna 2007.
  • Martin Engelbrecht: SETI - The scientific search for extraterrestrial intelligence in the field of tension between diverging concepts of reality. In: M. Schetsche (Ed.): From humans and extraterrestrials. Transcript, Bielefeld 2008, ISBN 978-3-89942-855-1 , pp. 205-226.
  • P. Morrison, J. Billingham, J. Wolfe: The search for extraterrestrial intelligence-SETI. NASA SP 419, Washington 1977. (online)
  • H. Paul Shuch: Searching for extraterrestrial intelligence - SETI past, present, and future. Springer, Berlin 2011, ISBN 978-3-642-13195-0 .
  • Michael AG Michaud: Searching for extraterrestrial intelligence: prepairing for an expected paradigm break. Pp. 286-298, in: Steven J. Dick: The impact of discovering life beyond earth. Cambridge University Press, Cambridge 2015, ISBN 978-1-107-10998-8 .
  • Claudio Maccone: SETI, extrasolar planets search and interstellar flight - When are they going to merge? In: Acta Astronautica. 64, 2009, pp. 724-734. doi: 10.1016 / j.actaastro.2008.11.006

Web links

Commons : Search for Extraterrestrial Intelligence  - collection of images, videos and audio files

External articles

Individual evidence

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