the search for extraterrestrial intelligence

SETI is the Search for Extraterrestrial Intelligence. The first practical proposals in SETI were made in the nineteenth and early 20th centuries, but were aimed only at interplanetary communication (see communication, with the Moon and planets). The roots of SETI in its modern form, as a search for artificial signals over interstellar and even intergalactic distances, can be traced back to the 1930s with the discovery by Karl Jansky of radio waves coming from a source outside the Solar System. This led, in time, to the birth of radio astronomy and to the construction, in the late 1950s, of the first large radio telescopes such as those at Jodrell Bank and Green Bank. Only with such powerful instruments was it feasible to start looking for Earth-type signals from nearby stars.




Dawn of SETI

As a serious scientific enterprise, SETI began in 1959 with two independent events. The first was the publication of a paper in which Cornell researchers Philip Morrison and Guiseppe Cocconi discussed the suitability of radio waves as a communication medium, man's newly-acquired ability to eavesdrop on interstellar radio conversations, and the optimum frequency at which to conduct a search. They argued that:


If signals are present, the means of detecting them is now at hand. Few will deny the profound importance, practical and philosophical, which the detection of interstellar communications will have. We therefore feel that a discriminating search for signals deserves a considerable effort. The probability of success is difficult to estimate; but if we never search, the chance of success is zero.


Meanwhile, at the National Radio Astronomy Observatory, Green Bank, Frank Drake had independently arrived at the same conclusion. Indeed, Drake had already begun to assemble the equipment he would use in Project Ozma, the first observational SETI program, carried out in 1960. Despite Ozma's failure to detect any extraterrestrial transmissions from its two target stars, it served to stimulate widespread public interest and lively (often critical) scientific debate. In 1961, an informal conference, sponsored by the Space Science Board, was held at Green Bank (see Green Bank conference). A dozen scientists and engineers deeply interested in the possibility of extraterrestrial life were invited to attend, including Drake himself, Morrison, Cocconi, Carl Sagan, Su-Shu Huang, Melvin Calvin, John Lilly, Bernard Oliver, Dana Atchley, and J. P. T. Pearman. At the conference, the now famous Drake Equation was discussed for the first time.


SETI strategies

The dawn of the SETI era stimulated scientists to consider the pros and cons of searching at all, the best strategy involving radio wave searches, and alternative approaches to finding evidence of extraterrestrial intelligence.1 Into the last category came the suggestions of Dyson spheres and Bracewell probes. The feasibility of interstellar travel was also considered around this time by Robert Bussard, Freeman Dyson, Sebastian von Hoerner, John Pierce, Edward Purcell, and Carl Sagan.


In the Soviet Union, SETI was seen as a natural outcome of communist philosophy and, partly for this reason, became quickly established as a respectable field of research. Interest there was initially sparked by the writings of Iosef Shklovskii and tended to focus on the possibility of civilizations far in advance of our own. In 1963, a radio wave search was carried out for Kardashev civilizations, though at a different frequency to that employed by Project Ozma.


Within the radio search paradigm, which by the mid-1960s was well-established as the standard approach to SETI, much debate focused on methodology. An artificial signal, unlike one of natural origin, would be expected to have a narrow bandwidth so that it could only be detected by an antenna tuned to one particular frequency. The question was which frequency offered the best chance of success. Morrison and Cocconi had argued for 1,420 MHz, corresponding to the ubiquitous 21-centimeter line of neutral hydrogen, but the subsequent discovery of other common natural radio frequencies (see waterhole) complicated the decision. Moreover, the merits of targeted searches had to be weighed against those of all-sky surveys. In the decades following Project Ozma, many different approaches were tried.


In 1970, NASA,s involvement with SETI began through some preliminary work carried out by John Billingham. This led, in 1973, to an ambitious proposal by Billingham and Bernard Oliver, known as Project Cyclops, to detect extraterrestrial civilizations by eavesdropping on their stray electromagnetic signals. Although the proposal failed, it served to stimulate interest in a NASA SETI program of more modest scale. This eventually led to the High Resolution Microwave Survey and, when this had to be abandoned because of budget cuts, its privately-funded successor, Project Phoenix.


In 1973, what was to become the longest-running SETI program to date, began at the Ohio State University Radio Observatory. Its most dramatic moment came in 1977 with the detection of the mysterious Wow! signal. Meanwhile, Drake and Sagan employed the giant Arecibo radio telescope to search for emissions from advanced civilizations in other galaxies. The mid 1970s also saw the very foundations of SETI challenged by a revival of interest in the Fermi Paradox.


SETI today

Today, interest in SETI has never been greater. A number of programs are active, including the SETI Institute's Project Phoenix, Harvard's BETA, Berkeley's SERENDIP, and the SETI League's Project Argus, this last one unique in that it relies upon data gathered worldwide by a growing army of amateur observers. The public's interest in SETI has been heightened by the suggestion of possible life on Mars and the giant moons of Jupiter, as well as by fictional portrayals, such as Sagan's Contact.2 Finally, there is a sense in which SETI, in addition to being a scientific quest, answers a modern spiritual need.


See also SETA (Search for Extraterrestrial Artifacts) and SETV (Search for Extraterrestrial Visitation) and the list of SETI observing programs after the references below.2, 3, 4



1. Bracewell, R. N. "Communications from Superior Galactic Communities," Nature, 186, 670–671 (1960). Reprinted in A.G. Cameron (ed.), Interstellar Communication, W. A. Benjamin, Inc., New York, pp. 243–248 (1963).


Introductory paragraph: Since Morrison and Cocconi published the suggestion that there might be advanced societies elsewhere in the Galaxy, superior to ourselves in technological development, who are beaming transmissions at us on a frequency of 1,420 MHz/s., Drake has described equipment under construction to look for such transmissions. The confidence necessary to commence actual observations is based on an opinion that planets are a common by-product of the formation of starts. One argument among others is that stars of spectral type later than F5 have low angular momenta, just as the Sun has; and in the case of the Sun we know that it is because the momentum (98 per cent of it) resides in planets. Of the thousands of millions of planets in the Galaxy likely to be situated similarly to the Earth in relation to their star, it is hard to dismiss the possibility that some have more advanced civilizations than ours. In view of the acceleration with which technology develops, advanced societies could be incredibly more advanced.


2. Sagan, Carl. Contact. New York: Simon & Schuster (1985).


3. Bova, Ben, and Preiss, Byron. First Contact: The Search for Extraterrestrial Intelligence. New York: NAL Books (1990).


4. Cameron, A. G. W. Interstellar Communication. New York: W. A. Benjamin (1963).


date investigator(s) location antenna diameter (m) search freq (Hz) resolution (Hz) targets comments
1960 Drake Green Bank 26 1420 M 100 ε Eridani, τ Ceti Project Ozma
1963-64 Kardashev,
Crimea Deep Space Station 16 × 8 ant. 923 M 10 M 2 quasars CTA 102 - initial report of type III civilization
1966 Kellerman Parkes 64 350-5,000 M full bandwidth 1 galaxy (1934-63)  
1968-69 Troitskii, Rakhlin, Gershtejin, Starodubstev Zimenkie 5 926-8, 1421-3 M 13 11 stars and M31  
1968-82 Troitskii Gorky dipole 1, 1.875, 3.75, 10 G   all-sky  
1969-83 Troitskii, Bondar, Starodubstev Gorky, Crimea, Murmansk, Primorskij dipoles 600, 927, 1863 M   all-sky search for pulsed signals
1970-72 Slysh, Pashchenko, Rudnitskii, Lekht Nancay 40 × 240 antennas 1665, 1667 M 4 k 5 OH masers search for unnatural emission characteristics
1970-72 Slysh Nancay 40 × 240 antennas 1665, 1667 M 4 k 10 nearest stars  
1971, 1972 Verschuur Green Bank 91, 43 1420-1, 1410-30 M 490, 6.9 k 9 stars Project Ozpa
1972 Kardashev, Popov, Soglasnov, et al Crimea, RT-22 22 8570 M   galactic center search for statistical anomalies
1972-74 Kardashev, Gindilis, Popov, Soglasnov, Spangenburg, et al Caucasus, Pamir, Kamchatka, Mars 7 38, 60 371, 408, 458, 535 M 5 M omnidirectional "eavesdropping" search for pulses
1972-76 Zuckerman, Palmer Green Bank 91 1413-25, 1420-1 M 6.4 × 104, 4 k 674 stars Project Ozma II
1972-6 Bridle, Feldman Algonquin 46 22,235 M 30 k 70 stars first search at the water wavelength
1973-74 Shvartsman, et al Special Astrophys. Observatory, MANIA 0.6 optical   21 peculiar objects optical search for short pulses and laser lines
1973-1998 Dixon, Ehman, Raub, Kraus Ohio State 53 1420 M 10 k all-sky Wow! signal in 1977
1974 Wishnia Copernicus 1 UV   nearby stars search for UV lasers lines
1975 Drake, Sagan Arecibo 305 1420, 1667, 2380 M 1 k 4 galaxies search for type II civilizations
1975-79 Israel, de Ruiter WRST 1500 1415 M 4×106 50 star fields  
1976 Wielebinski, Seiradakis Max Planck Institute 100 1420 M 20 M various nearby stars search for pulsed signals with periods of 0.3-1.5 s
1976 Black, Cuzzi, Clark, Tarter Green Bank 43 8522-3 M 5 4 stars  
1977 Black, Cuzzi, Clark, Tarter Green Bank 91 1665, 1667 M 5 200 stars  
1977 Drake, Stull Arecibo 305 1664-8 M 0.5 6 stars  
1978 Shvartsman,
et al
Special Astrophys. Observatory, MANIA 6 optical     optical search of 30 radio objects for pulses from Kardashev II or III civiliz.
1978 Horowitz Arecibo 305 1420 M 0.015 185 stars  
1978 Cohen, Malkan, Dickey Arecibo, HRO, Parkes 305, 36, 64 1665-7, 22235, 1612M 9.5 k, 65 k, 4.5 k 25 globular clusters passive search for type II and III civilizations
1978 Knowles, Sullivan Arecibo 305 130-500 M 1 2 stars  
1978 Makovetskij, Gindilis, et al Zelenchukskaya, RATAN-600 7.4×450     Barnard's Star  
1978-80 Harris Pioneer Venus, Venera 11 and 12   20 keV - 1 MeV   54 gamma-ray bursters search for 3 linear events as sign of i/s spacecraft
1979 Cole, Ekers Parkes 64 5000 M 10 M, 1 M nearby F, G, K stars  
1979 Freitas, Valdes Leuschner Observatory 0.76 optical   Lagrangian points Bracewell probes
1979-81 Tarter, Clark, Cuquet, Lesyna Arecibo 305 1420, 1666 M 5, 600 200 stars  
1979-85 Bowyer et al Hat Creek 26 917-937, 1410-30, 1602-5 M, etc 2×500 all-sky SERENDIP piggyback
1980 Witteborn NASA-Univ. of Arizona, Mt Lemon 1.5 infrared   20 stars infrared excess from Dyson spheres
1980-81 Suchkin, Tokarev, et al Nirfi, Gorkii, Gaish, Moscow   9.3 M 1.5 k Lagrangian points Bracewell probes
1981 Lord, O'Dea Univ. of Mass. 14 115 G 20 k, 125 k N. galactic rot. axis  
1981 Israel, Tarter Westerbork 3000 max. baseline 1420 M 4 M, 10 M 85 star fields parasitic search
1981-82 Biraud, Tarter Nancay 40×240 1420 M, 1665-7 M 49 343 stars  
1981 Shostak, Tarter WRST 3000 max. baseline 1420 M 1.2 k galactic center interferometer search for pulsed signals
1981 Talent Kitt Peak 2.1 optical   3 stars search for enhanced stellar lines of rare earth elements as evidence of ET nuclear waste disposal
1981-2 Valdes, Freitas Kitt Peak 0.61 optical   Langrangian points Project SETA. Search for Bracewel lprobes
1982 Horowitz, Teague,Linscott, Chen, Backus Arecibo 305 2841 M / 1420 M 0.03 250 stars / 150 stars Suitcase SETI
1982 Vallee, Simard-Normandin Algonquin 46 10522 M 185 M galactic center meridian search for strongly polarized signals
1983-85 Horowitz Oak Ridge 26 1420 M, 1667 M 0.03 sky survey Project Sentinel
1983 Damashek Green Bank 92 390 M 2×106 sky survey search for single pulses and telemetry
1983 Valdes, Freitas Hat Creek 26 1516 M 4.9 k / 76 k 80 stars / 12 stars search for radioactive tritium from ET fusion technology
1983 Gulkis DSS 43 64 8 G, 2380 M 40 k southern sky survey  
1983-88 Gray Small SETI Observatory 4 1419-1421 M 1-40 sky survey amateur meridian transit search
1983-4 Cullers AMSETI 2 1420, <1000 M     amateur SETI based on satellite TV dishes
1983-8 Stephens Hay River, NWT two 18×18 1415-25 M 30 k northern sky survey amateur SETI observatory using two 64-ft square dishes
1984 Slysh satellite radiometer   37 G 400 M all-sky Dyson spheres > 1 solar lumunosity within 100 pc
1985-95 Horowitz Oak Ridge 26 1420, 1665, 1667, 2841 M 0.05 sky survey META
1986-88 Bowyer, Wertheimer, Lampton Green Bank 92 400-3,500 M 1 various sky areas SERENDIP II
1986 Mirabel Green Bank 43 4,830 M 76 galactic center, 33 stars  
1986 Betz Mount Wilson 1.65 infrared 3.5 M 100 Sun-like stars search for IR beacons at carbon dioxide laser freq.
1986- Colomb, Martin, Lemarchand Argentina 30 1415, 1425, 1667 M 2.5 k 78 Sun-like stars  
1987 Tarter, Kardashev, Slysh VLA 26 × 9 ant. 1,612 M 6.1 k G357.3-1.3 obs. of IR source near galactic center to check if Dyson sphere
1987 Gray Oak Ridge 26 1,420 M 0.05 sky positions of Wow! signal used META system
1988 Bania, Rood Green Bank 43 8,665 M 305 24 Vega-like stars with IR excess search for narrow-band signal at freq. of spin-flip of 3He+
1990 Colomb et al. Argentina 30 1,420 M 0.05 all-sky META II (southern hemis.)
1990 Blair et al. Parkes 64 4,462 M 100 100 Sun-like stars search at pi times 21-cm hydrogen line frequency
1990 Gray Oak Ridge 26 1,420 M 0.05 M31, M33  
1990- Kingsley Bexley, Ohio 0.25 optical   nearby stars COSETI
1992 NASA Ames Arecibo 305 1.3-2.4 G 1, 7, 28 78 Sun-like stars HRMS targeted search
1992 JPL Goldstone 26, 34 1.7 G, 8.3-8.7 G 30 all-sky HRMS sky survey
1992-96 Bowyer, Wertheimer, Donnelly Arecibo 305 423-435 M 0.6 all-sky SERENDIP III
1992-98 Childers Ohio State 53 1400-1700 M 100 k all-sky LOBES (LOw Buget ETI Search)
1993 Steffes, Deboer NRAO/Tucson 12 203 G 32 40 stars and 3 locations near galactic center search near positronium line
1995- SETI Institute Parkes-Mopra / Arecibo / Jodrell Bank 64 / 305 / 76 1.2-3 G 1 206 stars / 192 stars / 600 stars Project Phoenix
1995 Gray VLA 26 1,420 M 381, 6.1 k "Wow!" signal region  
1995 Beskin Special Astrophysical Observatory 6 optical   several objects  
1995 Mauersberger, Wilson, Rood, Bania, Hein, Linhart IRAM 30 203 G 9.7 k 17 stars and positions search at spin-flip frequency of positronium
1995-99 Horowitz, et al Oak Ridge 26 1.4-1.7 G 0.5 northern all-sky BETA; covered full waterhole (damaged by wind storm)
1996 Blair, Zadnik Perth, Australia 0.64 optical   nearby stars  
1997- Wertheimer, et al. Arecibo 305 1.32-1.52 G 0.6 all-sky SERENDIP IV
1998-99 Gray, Ellingsen Hobart 26 1.42 G 5 "Wow!" signal region Search for signals with periods up to 14 hours
1998- SETI League amateur dishes (up to 5000) 3-5 1.42 G 10 all directions in real time Project Argus
1998- Stootman, et al. Parkes 64 0.6 1,418-1,420 M southern random all-sky Southern SERENDIP; uses 13 beams on sky from focal plane array
1999- Wertheimer Leuschner 0.76 optical   nearby stars search for pulsed signals
1999- Marcy Lick and Keck 4 and 10 optical   nearby stars search for narrowband continuous wave signals in data archive of extrasolar planet search
1999- Horowitz Harvard Smithsonian 1.55 optical   nearby stars search for nanoseccond pulsed signals
1999- Montebugnoli Medicina 32 1,415-1,425 M; 4,255-4,256 M 0.6 northern sky survey SETI Italia; SERENDIP-type ransom sky survey at 21 cm and 7 cm
1999- Wertheimer, Anderson Arecibo 305 1.419-1.421 G 0.1 all sky SETI@home (2.5 MHz of SERENDIP IV data analyzed by screen-savers on home PCs

Based on data compiled by Jill Tarter of the SETI Institute
Thanks to Robert Gray for additional information