A brief history of attempts to use radio in the search for extraterrestrial civilizations.

Published in the WANSAC (Western & Northern Suburbs Amateur Radio Club) monthly club magazine  Vol 39 Issue September 2008


COSMIC DX by Peter Miles

In 1894, Percival Lowell observed a network of canals on Mars using his 450mm refractor telescope near Flagstaff, Arizona. He interpreted these canals as evidence of intelligent life on the planet. Additionally, H.G. Wells' publication of "The War of the Worlds" further fuelled public belief in the existence of an intelligent and potentially hostile alien civilization on the nearby red planet.

By the mid-twentieth century, although there remained hopeful expectations for some form of life on Mars due to its similarities to Earth, it became evident that Mars did not host highly developed extraterrestrial civilizations.

Considering the absence of evidence of intelligent life within our Solar System, one can extrapolate that the vast number of stars in the Milky Way—estimated to range from 200 to 400 billion—with a significant percentage likely having planetary systems, and the possibility of a small fraction of these planets supporting life, suggests the potential existence of thousands of highly developed civilizations.

From the outset, it was clear that physically travelling the enormous distances to or from even the nearest stars would be technically near-impossible. For instance, the nearest star to Earth, Proxima Centauri, is a faint star located approximately 4.3 light-years away. Given that a light-year corresponds to about 9,460,000,000,000 kilometres, it is evident that without a fundamental breakthrough in technology, physical traversal of such vast distances will remain unfeasible.

Consequently, the logical approach was not to undertake personal journeys, but rather to send or listen for messages from distant civilizations. The refinement of radio technology, particularly the development of sensitive VHF and microwave technology during World War II for radar purposes, provided a viable method for communication.

How many civilizations could really be out there? (The Drake equation)

Astronomer Frank Drake formulated an equation in 1960 as part of his preparations for the Green Bank meeting, which established the field of SETI (Search for Extraterrestrial Intelligence). The purpose of the equation was to estimate the likelihood of extraterrestrial civilizations existing within our galaxy and to provide a framework for addressing related issues during the conference. Over time, the equation has gained prominence and is frequently referenced in books and websites on the subject. It has become a cornerstone of scientific discussions within the field of SETI.

The equation is as follows:


   is the number of civilizations in our galaxy that may be capable of contact.

  is the average rate of star formation in our galaxy.

  is the fraction of those stars that have planets

  is the average number of planets that can potentially support life per star that has planets

  is the fraction of the above that actually go on to develop life at some point.

  is the fraction of the above that actually go on to develop intelligent life.

  is the fraction of the above that develop a technological civilization that emits detectable signs of their existence into space. i.e. radio

  is the length of time such a civilization emits detectable signals into space. 

There is obviously great debate over what values should be place in the equation as perhaps only  and  can be estimated on the bases of real scientific observational data.

The values used by Drake and fellow scientist in 1960 were:

   = 10/year (ten new stars are formed on average per year over the life of the galaxy)

 = 0.5 (half of all stars will have planets)

 = 2 (stars with planets will have 2 capable of supporting life)

  1 (100% of these planets will develop life)

 = 0.01 (1% of which will develop intelligent life)

  = 0.01 (1% of which will emit detectable signals into space)

  = 10,000 (This civilization will emit signals for 10,000years)             

The galaxy remains a vast expanse, and it is indeed possible that most or all of the values in the Drake Equation may lie beyond our current radio range. It is important to acknowledge that many of the numbers in the equation are speculative in nature, and their values can vary depending on the assumptions made. Consequently, different estimates can yield a wide range of results, from less than one to potentially thousands of civilizations.

However, it is reasonable to assert that the minimum value in the Drake Equation should be at least one. Given the vastness of the universe, with billions of galaxies and countless stars, the existence of at least one extraterrestrial civilization seems plausible.

The most recent guestimation has produced a result of two. Them and us would you believe!

What is the call frequency?

The radio spectrum is indeed vast, offering a wide range of frequencies to detect weak and narrow-band signals. Nature has shown that a frequency range from just above 1000MHz to about 10,000MHz, known as the terrestrial microwave window, is relatively quiet in terms of galactic interference and exhibits low atmospheric absorption. This range provides an advantageous window for radio observations.

However, it is important to note that this portion of the spectrum is also extensively utilized for various human communication purposes, such as mobile phones and wireless broadband. As a result, the available spectrum for scientific research and radio astronomy is being increasingly encroached upon.

To ensure a frequency that would be universally known by technologically advanced civilizations, a specific frequency called the hydrogen line at 1420.40575 MHz has been chosen. This frequency holds significance because it corresponds to a wavelength at which hydrogen, the most abundant element in the universe, emits and absorbs radiation. Consequently, the frequency range from 1400.00MHz to 1427.00MHz has been exclusively assigned to radio astronomy, prohibiting other activities in order to minimize interference.

Terrestrial microwave window

In April 1960, astronomer Frank Drake conducted one of the early modern experiments known as "Project Ozma." Using a 25-meter-diameter radio telescope at Green Bank, West Virginia, Drake examined a small number of stars near the 1.420 gigahertz marker frequency. The experiment involved scanning a 400 kilohertz band pass around the marker frequency using a single-channel receiver with a bandwidth of 100 hertz. The collected data was stored on tape for later analysis. However, no significant findings were discovered during the study.

A few years later, in 1967, a large radio telescope was conducting experimental observations at 81MHz to study the effects of the Sun's solar wind on distant Quasar radio sources. During these observations, an unusual radio signal was detected, displaying a continuous series of pulses precisely spaced at intervals of 1.337 seconds. This signal, originating from deep space, sparked curiosity and raised the possibility of it being a message or an extraterrestrial beacon. It was initially labelled as LGM1, which stood for "Little Green Men," alluding to the potential of first contact.

Further analysis, including the absence of expected Doppler shifts that would be present if the signal came from a planet or satellite orbiting a distant star, revealed that the source was, in fact, a rapidly rotating neutron star. Neutron stars are the remnants of supernovae, massive exploding stars. As a star explodes and material is ejected, the core collapses inward, forming a super dense object only a few tens of kilometres in diameter. Due to the conservation of angular momentum, the object spins rapidly, with some rotating up to 50 times per second. The intense magnetic fields generated by these neutron stars direct light and radio waves in a manner resembling a rotating searchlight. Consequently, these objects turned out to be not extraterrestrial radio beacons but rather the discovery of one of nature's most peculiar objects: pulsars. Since then, over a thousand pulsars have been identified.

The 'WOW' signal

On August 15, 1977, Dr. Jerry Ehman, a project volunteer for the Ohio State University SETI project, observed an exceptionally strong signal from the radio telescope. He marked the characters corresponding to the signal on the printout and added the notation "Wow" in the margin. This singular signal is regarded by some as the most promising detection of an extraterrestrial signal ever received. However, it has not been detected again since that initial observation.

WOW signal 1977

The circled character code on the print, specifically the 6EQUJ5, represents the amplitude variation of the signal. Each character corresponds to a specific range of amplitudes. A space denotes amplitudes between 0 and 0.999, while the number 1 represents amplitudes between 1 and 1.999. Similarly, 2 represents the range of 2 to 2.999, and so on. For values of 10 and above, letters are used. For example, 10 to 10.999 is represented as A, 11 to 11.999 as B, and so forth. The character 'U' in the code represents an intensity value between 30.0 and 30.999, which was the highest recorded intensity. The intensity value reflects the strength of the signal compared to the general background noise.

The columns in the printout correspond to 10kHz-wide channels, starting with channel 1 on the left and channel 2 displaying the signal of interest. The general frequency being observed is around 1420.405MHz, commonly known as the hydrogen line.


I have plotted the values on the below graph to give a clearer view of the rise and fall of the signal.

WOW signal graph

Ohio State University's radio telescope, known as Big Ear, was a stationary antenna that relied on the Earth's rotation to scan the sky. Due to the beam width of the antenna and the Earth's rotation, the telescope could only observe a specific point in the sky for approximately 72 seconds. Consequently, an extraterrestrial signal would be expected to last exactly 72 seconds, peaking at 36 seconds as the signal passed over the antenna's window, and then gradually diminishing over the remaining 36 seconds.

While it is possible that the WOW signal could have been an Earth-based signal reflected off a piece of space debris, its duration, shape, and narrow band characteristics align with what would be anticipated from an extraterrestrial signal. The signal's occurrence within the protected frequency allocation for radio astronomy (1400-1427MHz) further supports the case for its authenticity.

However, there are factors that cast doubt on the WOW signal. If the signal had originated from a piece of space debris in a fairly high orbit, its duration would have been different, longer if the debris was tracking with the Earth's rotation or shorter if it was tracking in other directions relative to Earth's rotation. Unfortunately, there is a lack of information regarding checks for known space debris orbits in relation to the WOW signal. Considering that NORAD actively tracks tens of thousands of such objects, one would assume that this task could have been relatively straightforward.

The case for the WOW signal's credibility lies in its duration, implying a source beyond Earth, and its narrow bandwidth, suggesting artificial origin. However, there are two significant concerns. Firstly, despite its relatively strong signal, the WOW signal has never been detected again, which raises doubts. Secondly, the Ohio State radio telescope employs two side-by-side focal points. If the signal were of cosmic origin, it should have been detected by the second antenna approximately three minutes after the first detection, yet it was not. While it is conceivable that the signal ceased within the three-minute interval, such a scenario would be remarkably unlucky, albeit possible.

After nearly 50 years of searching, the WOW signal remains the most intriguing and possibly promising signal detected thus far.

Making ourselves known.

Every hour of every day for more than half a century, human civilization has been accidentally sending highly visible radio signals into space. From the viewpoint of a distant extraterrestrial observer, the rotating Earth radiates bright pulses of electromagnetic energy across the radio spectrum. The flashes of radio emissions are a result of the rising and setting of hundreds of powerful radio stations, television transmitters, microwave links, etc., peppered around the globe. Although these transmitters generally radiate parallel to the surface of the Earth, as these signals pass beyond the horizon, they ultimately radiate out into the cosmos. In fact, so much radiation is now leaking into space that the Earth is nearly as intense a radio source as the Sun.

Therefore, if anyone is listening from as far as 50 light-years away, they already know what we're up to. The big question is: will our new space friends interpret Big Brother as a sign of intelligent life?

A more conscious effort was made in the 1970s with the launch of four probes by the United States to survey the outer planets of the Solar System. These probes were given trajectories that would eventually take them out of the Solar System and into the depths of space for an indefinite period. This endeavour was more of an act of faith and symbolism. The spacecraft were equipped with plaques that, if found, would communicate information about us to the discoverer. However, at the time, there was some controversy surrounding the content of these plaques. The concern was that the depicted information, including our location, could potentially aid a hostile entity in locating us.

The Pioneer 10 and 11 probes, launched in 1972 and 1973 respectively, carried a plaque that depicted the location of the Earth and the solar system within the galaxy, as well as an image of the human form. On the other hand, the Voyager 1 and 2 probes, launched in 1977, carried two gold records. These records contained depictions of the human form, our solar system, and its location, along with recordings of pictures and sounds from Earth.

It is worth contemplating that long after the human race has ceased to exist and the Earth itself is devoid of life, these probes could serve as the longest-lasting remnants that we ever existed.

Voyager 1

As every amateur knows, if everyone is just listing potentially good DX contacts, they will go unrealized. Therefore, in 1974, the Arecibo radio telescope, one of the largest in the world, transmitted a series of messages in the form of digital images. The Arecibo message itself consisted of a 1679-pixel image with 73 rows and 23 columns. It included representations of the numbers one through ten, the atomic numbers of hydrogen, carbon, nitrogen, oxygen, and phosphorus, a depiction of a human being and its height, the population of Earth, our solar system, and an image of the Arecibo telescope along with its diameter.

What is happening now?

At present, the SETI Institute, a privately funded organization, has been collaborating with the University of California, Berkeley, to build the Allen Telescope Array, a SETI-dedicated array of telescopes that will have the equivalent capabilities of a 100-meter radio telescope. This array is a precursor to larger radio astronomy arrays planned for later in the decade. With advancements in telescope and SETI technology, it is possible that we may be able to detect signs of intelligence not through directed messages but through the unintentional "noise" we broadcast into the cosmos via radio, television, and radar signals.

Some argue that endeavours like SETI are a waste of money and effort, as our current technology is limited in range, requiring signals to be relatively close, perhaps within a 100-light-year radius. This covers only a small fraction, around 0.1%, of our Milky Way galaxy. Considering the Drake equation, which estimates the potential existence of two to ten extraterrestrial civilizations within our entire galaxy, the odds of success seem quite slim.

However, the implications of making contact with an extraterrestrial civilization would be profound for human culture and our perception of ourselves. Just imagine the possibilities if we were able to establish ongoing radio communication with a civilization located 20 light-years away. It would undoubtedly change everything we know.

For more information on the WOW Signal see: http://www.bigear.org/Wow30th/wow30th.htm

For more information on the SETI Institute see: http://www.seti.org/

For more information on SETI Australia see: http://seti.uws.edu.au/

For more information on Pioneer 10 & 11: http://nssdc.gsfc.nasa.gov/planetary/pioneer10-11.html

For more information on Voyager 1 & 2: http://nssdc.gsfc.nasa.gov/planetary/voyager.html


Page last revised 2 September 2008 


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