SETI is the Search for Extraterrestrial Intelligence. The first practical proposals in SETI were made in the nineteenth and early twentieth 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.
The 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 (see Morrison and Cocconi Conjecture). 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.
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.
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 SETI, religious dimensions of).
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).
|SETI OBSERVING PROGRAMS: 1960–1999|
|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|
|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|
|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 FGK 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||WRST||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 Bracewell probes|
|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