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* L I T E R A R Y F R E E W A R E *
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* F O U N D A T I O N *
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-=ð P R O U D L Y í P R E S E N T S ð=-
THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE
(S E T I)
Our Milky Way Galaxy is only one of 10 billion galaxies in the
presently observable universe. Our Sun is just one of some 300
billion stars in our galaxy alone. Astronomers have confirmed that
the Sun and the galaxy, which make our existence possible, are not
unusual or basically different from other galaxies and stars.
A few generations ago, astronomers believed that planetary systems
were extremely rare--that our solar system and our Earth with its
life-supporting environment might well be unique. Chemists and
biologists knew little if anything about the processes that led to
the origin of life. In the last fifteen years, however, a number
of important discoveries have strongly suggested that there is a
fundamental relationship between the origin and evolution of life
and the origin and evolution of the universe.
Advances in astronomy and physics have given renewed support to the
concept that planets are not rare exceptions, but are a natural
part of the star formation process and may number in the hundreds
of millions in our galaxy alone. [In December 1984, the National
Science Foundation announced that a team of Arizona astronomers had
detected a possible solar system around Beta Pictoris, a star 53
light years from Earth.] Recent biological experiments applying
natural energy sources to molecules have produced some of the
organic building blocks that make up the chemistry of life. Radio
astronomers have discovered that many organic molecules exist even
in the depths of interstellar space. Elements identified in these
molecules include hydrogen, nitrogen, oxygen, carbon, silicon, and
phosphorus. Earth has been without life only a small fraction of
its age, which leads many scientists to look upon the formation of
life on other suitable planets as very likely. Once begun, and
given billions of years of relative stability, life may achieve
intelligence and, in some cases, may evolve into a technological
civilization.
One direct way of testing whether intelligent life exists beyond
our solar system is to search for an artificially generated radio
signal coming from interstellar space. As an example, ultrahigh
frequency and microwave radio signals emanating from Earth are
expanding into space at the speed of light. This radio, radar, and
television "leakage" of ours currently fills a sphere nearly 100
light-years in diameter. The same phenomenon would serve to
announce the presence of other intelligent life. Moreover, advanced
civilizations might be operating radio beacons, possibly to attract
the attention of emerging societies and bring them into contact
with a community of long-established intelligent societies existing
throughout the galaxy.
Either type of signal (leakage or beacon) would be easiest to
detect at frequencies where the background radio noise is minimal.
One of the quietest regions of the electromagnetic spectrum is the
"microwave window" that lies in the frequency band between 1000 and
10,000 megahertz (MHz). It is reasonable to assume that others
wishing to establish interstellar contact by radio might choose
this band.
The search for extraterrestrial intelligence (SETI) is not new,
having first been proposed by U.S. scientists in 1959. Since that
time, numerous scientific and technical studies have been made on
an international scale, and more than 30 radio searches have been
attempted, covering only a minute area of search space. What is new
today is the available technology. Radio telescopes on Earth are
sufficiently sensitive to detect signals no stronger than some
leaving Earth at distances of a thousand light-years or more. The
305 meter (1000-ft) diameter radio telescope at Arecibo, Puerto
Rico, could detect transmissions from nearby stars that are less
powerful but similar to our own television and radars. Advances in
computers and data processing techniques now make it possible to
search automatically through millions of incoming radio signals
each second and, if it is present, to identify a signal transmitted
by an intelligent society.
The NASA SETI Program is nearing the end of a 5-year research and
development phase, using existing radio telescopes and advanced
electronic techniques to develop prototype SETI instrumentation.
The program is being jointly carried out by the Jet Propulsion
Laboratory (JPL) at Pasadena, California, and the NASA Ames
Research Center at Moffet Field, California. Leading radio
scientists from the national laboratories and academic community
have also joined together in the SETI Science Working Group to
assist the JPL-Ames team in developing the instrumentation and the
search strategy.
The proposed plan involves two complementary search modes that are
designed to cover a range of possibilities. One mode is an all-sky
survey that will search the entire celestial sphere over a wide
frequency range (1200 to 10,000 MHz plus spot bands up to 25,000
MHz) to cover the possibility that there may be a few civilizations
transmitting strong signals, possibly as interstellar beacons.
Longer observing times may be allocated to directions that include
a large number of stars, especially the galactic plane. The radio
telescopes employed will be the 34-meter (112-ft) diameter antennas
that are part of NASA's Deep Space Network. The survey will be
conducted by moving the telescope across the sky at a constant
rate. It will cover at least 10,000 times more frequency space than
all previous survey attempts, will be about 300 times more
sensitive, and will take about 5 years to complete.
The second mode is a high-sensitivity targeted search that will
look for weak signals originating near solar-type stars within 80
light-years distance from Earth. The objective is to examine the
possibility that nearby civilizations may have radio transmitters
no more powerful than our own. Some stellar clusters and nearby
galaxies will also be observed. The frequency range covered will
be 1200 to 3000 MHz plus spot bands between 3000 and 10,000 MHz.
To achieve very high sensitivity, the targeted search will use some
of the largest radio telescopes available, including the 305-meter
(1000-ft) diameter antenna at Arecibo, Puerto Rico, and the Deep
Space Network's 64-meter (210-ft) diameter antennas. The number of
targets covered will be much larger than previous searches and the
range of frequencies covered will be thousands of times greater.
The targeted search is expected to take about 3 years to complete.
Current astrophysical knowledge and the available technology make
the SETI observing program both timely and feasible. Timeliness
also relates to the rapidly-increasing sources of radio frequency
interference (RFI) in the microwave band. Portions of the microwave
spectrum that directly concern SETI ar subject to allocation to
numerous users worldwide, emphasizing the need to proceed with SETI
while it remains economically possible with our current technology.
If the use of the microwave spectrum continues to increase at its
present rate, the greatest exploration opportunity in the history
of mankind may be placed economically and technologically beyond
our reach for the foreseeable future.
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S E T I SEARCH SUMMARY
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SEARCH PARTICULARS SKY SURVEY TARGET SEARCH
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Area Coverage All directions 1000 stars,
regions
Signal search Continuous Wave Pulses, drifting
CW
Frequency coverage 1200-10,000 MHz + 1200-3000 MHz +
spot
spot bands bands
Frequency resolution 1000, 32 Hz 1000, 32, 1 Hz
Receiver bandwidth Wide (~250 MHz) Narrow (~10 MHz)
Observing time per
direction at each 0.3 - 3 sec 100-1000 sec
frequency setting
Channels analyzed ~10 million ~10 million
per second
Antenna diameter 34 meters 305 and 64 meters
Search duration ~5 years ~3 years
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SETI, THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE, NASA/JPL
400-265, 9/85