Topic of interest and charm considerable that could stimulate a fruitful link between research centers and motivated and trained groups and radio astronomy amateurs.

Radio astronomy techniques are also used for the targeted search of extraterrestrial signals of artificial origin.

Activities supported by academic institutions (poorly financed) and private and amateur organizations.

The idea of evaluating how far the radio waves emitted by the Earth could be detected in interstellar space was of two physicists at Cornell University, Giuseppe Cocconi and Philip Morrison have shown (1959) as the most suitable radio waves to propagate information space cosmic ones are close to the privileged wavelength 21 cm (1420 MHz emission line of neutral hydrogen interstellar),"natural" frequency known to all hypothetical extraterrestrial civilizations.


Types of signals strategies and research

We identify two main types of signals can be received from a SETI: the electromagnetic radiation transmitted intentionally type "radio beacon" (beacon) and unintentional destinedfor internal use, due to loss of radio-energy (leakage).

The design of the receiver and the choice will depend on the strategy of listening, therefore, the type of signal you want to study.

Search strategies adopted by the various SETI projects are essentially of two types: the direct and the "all-sky" research.

The first research focuses on individual targets (usually stars) chasing it for a certain period of time.

This method has several advantages: You can use radio telescopes with very high resolution, given that the aim is generally not resolved and its position known.

To obtain high signal/noise ratios are adopted techniques of integration of the detected signal and, being well known any drift for the Doppler effect (at the site of reception), are expected compensation techniques to optimize research. On the other hand, it is possible to observe only a small portion of the sky at a time, so that, if the extraterrestrial civilization is not included among the objectives of the project, can never be discovered.

The research for "all-sky" allow observation of the entire sky visible from the observatory: it avoid potential bias due to choices made in advance on the objectives to study.

Useful windows radio for SETI research

Due to the absorption and diffusion caused by the interstellar medium and planetary atmospheres not all radio frequencies are used for SETI searches.

As shown by the graphs shown below that illustrate the effects of various causes of attenuation on radio waves in free space and within the planetary terrestrial frequencies below 1 MHz tend to be absorbed by the charged particles of planetary ionospheres, if we assume that in all the planets can accommodate advanced civilizations are present ionospheres of various extent which act as a shield for the radio waves (similar to that of Earth) of lower frequency to 10-20 MHz.

On the other hand, the same atmosphere would limit superiorly the usable frequencies (around 10 GHz) to phenomena of molecular absorption.

Most likely the planets capable of hosting advanced civilizations there is an atmosphere that opens a "spectral window" to the Hertzian radiation with characteristics similar to Earth's, limiting the useful frequency range of about 10-20 MHz up to 10 GHz.

The problem does not arise if the receivers and the transmitters are located in orbit of artificial satellites outside the planetary atmosphere.

The main source of noise is the background radioemission cosmic.

Using existing radio telescopes can detect intelligent signals from distances of hundreds of light-years, that interstellar distances: these distances can be improved by increasing the sensitivity of the receivers and concentrating all the power of the transmitter in a very narrow frequency band, with antenna very compact bundle.

Sensitive issue is the choice of the frequency of communication.

It 's almost impossible for a civilization discover a receiving a radio message, unless it is known a priori the transmission frequency: the possible frequency band is too wide.

The problem would be greatly simplified if all the intelligent civilizations of the universe arrived at the same conclusions about the adoption of certain natural "privileged" frequencies used as a standard for communications.

The first suggested frequency (G. Cocconi, P. Morrison, 1959) was that of the 21-cm line: the researchers say it is very likely that any extraterrestrial civilizations have knowledge of this fundamental line of the spectrum, which would represent a "sample unique frequency and necessarily objective known to any observer in the universe."
"... It's also reasonable to expect that at this frequency, receivers are built from the beginning of the development of radio astronomy..." operating instruments at that frequency should be common in the universe.

Have been proposed other natural frequencies corresponding to the same spectral lines of molecules discovered in interstellar space, such as those issued by the OH molecule. These frequencies, corresponding to the spectral lines of fundamental astrophysical interest, are a good choice for interstellar communications since they have significant probability of being accidentally revealed by hypothetical extraterrestrial civilizations during a scan of the sky.

With modern receivers it is possible simultaneously analyze several channels spectrum gathering information in a wide range of frequencies.

The interval between 1420 MHz and 1720 MHz is indicated by the suggestive name of "waterhole", the "water hole" around which, like animals in the jungle, gather galactic civilizations.

If the water does have a vital role in life, considering the advantages of communications in this band and the fact that the emission comes from hydrogen to 1420 MHz to 1665 MHz and the H from the OH molecule (the two components forming water), it is reasonable to assume that these frequencies are the most likely choice for any cosmic radio.


Extraterrestrial civilizations

It is interesting to illustrate an approach to SETI research was exploratory, originally developed and launched by Russian scientists, which starts from the idea of N. Kardashev to classify the existing technological civilizations into three types according to their degree of use and the ability to manipulate energy.

A Type I civilization is able to manipulate the environment on a global scale: our civilization has reached such a level of technology as a human activity begins to change the state of the Earth raising the issue of a controlled and sensible management so that interventions do not result in disasters.

A civilization of type II would be able to use and manage an amount of energy comparable to that produced from its sun, manifesting a degree of technological development billion times more advanced than ours.

A Type III civilization would even be able to manipulate an entire galaxy placing a level of a hundred billion times greater development.

The purpose of the research "exploratory" is to identify a Type II civilization intercepting its energy products: even if it is not intentionally targeted messages outside, produces a considerable amount of radio energy emitted for internal use well observable radio wavelengths and millimeter wavelengths.

According to the Russian astrophysicist J.S. Shklovsky (1963) and his school of collaborators, the true hallmark of the presence of a technological civilization on a planet would be just the energy loss due to local radio, television, satellite and radar.


Amateur SETI observations

Groups of radio astronomy enthusiasts may be actively involved in amateur SETI observations, possibly coordinated by radio-institutional observers.

The challenge: receiving very weak signals at the frequency 1420 MHz (21-cm line of neutral hydrogen).

We move on "uncertain border" of the following fascinating theoretical possibility...

  • Intelligent ET civilizations at a distance within 100 years / light, equipped with a radio telescope with a diameter equivalent to 300-400 meters, could transmit radio signals sufficiently intense to be detected on Earth with amateur instruments (reflector antennas with diameter of the order of 3-4 meters).
  • Someone might be very lucky ...

It makes sense to ask: "are we alone in the Universe?" ... Today, because ...

  1. Occur continuous discoveries of new planets and solar systems (including terrestrial) outside of our solar system.

  2. Have been detected in interstellar clouds, many chemical elements and many complex organic molecules, such as those required for the chemistry of life.

  3. The current technology provides the necessary tools (telescopes) and strategies for the analysis of signals required to plan the first attempts to search.

For further information on the activities recommended in the amateur SETI consult the following sites:

It is a catalog of extrasolar planets discovered to date.