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S.E.T.I. is the Search for ExtraTerrestrial Intelligence or the search for life from other planets other than Earth. We can not do this, at this time by checking out the planets by traveling to them in a space ship. This would take too long because the distances to other planets that are not in our solar system could take millions of years to get to.
The current project is the S.E.T.I. Microwave
Observing Project. In this project sensitive receivers and unique
S.E.T.I. signal processors will be combined with existing large
radio telescopes to explore large portions of the microwave
spectrum for signals that would be generated by extraterrestrial
technology.
Natural radiation is broad band, that is, it occurs over a wide range of frequencies. Such radiation is far broader in frequency than the kinds of signals our technology can generate. Even the narrowest natural emission spreads its energy over a band of frequencies hundreds of times wider than artificial signals. Narrow band radiation occurring at one or a few frequencies would clearly be a technological artifact. This is why NASA's S.E.T.I. program only covers narrowband signals. These signals may be present at all times or they may be pulses. In either case the frequency of the signal can be expected to change or drift over time because of the possible relative acceleration between the transmitter and the receiver. Only in the microwave region of the electromagnetic spectrum(a spectrum is a specific range of frequencies of light)(1000 to 100000 MHz, where 1 MHz corresponds to a frequency of 1 million vibrations per second) is the Galaxy fairly quiet. Here there is only a faint whisper from the remains of the Big Bang explosion. A transmitter at microwave frequencies requires only modest power to produce a signal detectable above the natural noise background. This is the most logical frequencies to transmit at.
When looking for a narrowband signal, it is required that the detector covers no more range than the signal itself occupies. If the detector covers more range than the signal itself, it will pick up additional noise. If the detector covers less range than the signal, it will miss some of the energy in the signal. If we were to use a detector that uses a signal narrow band channel it would take billions of years to find the correct channel to receive anything from the correct direction.
The only way to find the correct channel would be to use a detector that is sensitive to many narrowband channels at the same time. Some radio detectors have thousands of individual channels but these can be scanned to cover only a small range of frequencies at any instant, so these tools are inadequate for S.E.T.I.
Almost perfect radio telescopes and
receivers now exist for the frequency range 1000 to 10000 MHz,
the quietest part of the microwave spectrum for any telescopes
that must work from within the Earth's atmosphere. Some advances
in microelectronics have allowed S.E.T.I. engineers to test there
equipment with tens of millions of simultaneous, narrow band
channels. The same electronics technology also permits computers
to find narrowband signals even if their frequency changes in
time or if they pulse on and off.
NASA has had two different searches being conducted. One is the Targeted Search and the other is the Sky Survey.
The objective is to see if any civilizations in the vicinity of
the target stars are transmitting signals that our present day
radio telescopes, outfitted with S.E.T.I.-specific
instrumentation, are sensitive enough to detect.
Because so many frequencies and so many directions will be
searched, the radio telescope can not spend very much time on any
one area and the survey will be less sensitive. The sky survey
will complement the targeted search by sacrificing some
sensitivity in order to search the 99% of the sky not covered by
the targeted search.
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