Messages to outer-space have become somewhat contentious lately. Are we seeking technologically superior, benevolent extra-terrestrial species or setting ourselves up as victims to more scientifically and militarily capable predators?
While the Arecibo broadcast was the first electromagnetic METI (message to extraterrestrial intelligences), only preceded by the gold plaques bolted to the Pioneer 10 and 11 space probes launched in 1972 and 1973 respectively, it is not the only radio transmission sent into outer-space. Three transmissions have subsequently been transmitted from Eupatoria or Yevpatoria, a Ukrainian Black Sea port city where major facilities of the National Space Agency of Ukraine are located. These are known as Cosmic Call 1999, Teen Age Message 2001 and Cosmic Call 2003.
In addition, there is the "Invitation to ET" website on the Internet, established by Dr. Allen Tough of the University of Toronto, inviting contact from any extraterrestrial civilization sufficiently advanced to be monitoring our world wide web communications. Although the later cannot really be considered a "message to the stars."
Nevertheless, although the Arecibo broadcast was the first rather than the only, we should not expect a quick reply to it! It was aimed at a globular star cluster designated M13, which is 21 thousand light years distant. If someone, on a planet circling a star that faces us on the outskirts of that star cluster, responded straight away, we might get a reply in 43974 AD. Our message was transmitted in 1974 AD, it will take 21,000 years to reach the outer "surface" of M13 and we would expect a radio transmission response to take 21,000 years to come back.
It was broadcast on November 16, 1974 as part of the ceremonies commemorating the upgrade of the Arecibo radio telescope in Puerto Rico. The message consisted of 1679 bits of data, each bit representing either a zero or a one, what is referred to as binary code. The transmission represents a two dimensional picture; to be deciphered properly the 1679 bits need to be arranged in a plane, a two dimensional rectangle, that is 23 bits wide and 73 bits long. These numbers were chosen because they are prime numbers, a mathematical concept that we believe should be universal and therefore is hoped to be decipherable by an alien intelligence. The bits were transmitted at what we would consider a very slow speed in today's world, just 10 bits per second using frequency shifting.
The images selected for inclusion in the message, graphically displayed, were those of the Arecibo Dish that sent the transmission, a binary number sequence, a human figure with associated height scale, hydrocarbon chemistry, the structure of deoxy-ribonucleic acid (DNA) and that of the solar system. The sending of such information out into the unknown raised a lot of concern and paranoia, which is probably why so few such public messages have been sent since. Determining whether secret, private or government endeavors have successfully transmitted messages since is impossible, although as we have steadily increased the numbers of communications satellites in our skies since Arecibo's broadcast, doing so without someone knowing becomes more difficult.
We also need to recognize that we could receive an earlier response. The M13 globular star cluster is basically a small galaxy neighboring ours. Our Milky Way galaxy has between 80 and 400 billion stars, generally estimated at 250 billion stars. M13 has between 300 to 400 million stars in a spherical shape rather than the spiral shape of the Milky Way. Both are groups of stars with relatively empty space around them, so in all fairness a star cluster is just another name for a small galaxy. We do tend to be somewhat size orientated at times. Nevertheless, M13 is separated from the Milky Way by a relatively empty region of space, but our message will pass through some of the Milky Way before reaching that empty space.
If the message is received by an extraterrestrial civilization able to detect it and decipher it within the Milky Way, circling perhaps a relatively close star, we might receive an answer within a thousand years. Hoping for a response within a human generation is probably less likely than having a win at the lottery.
Despite these realities, the Arecibo message is assigned an 8 on the San Marino scale. The San Marino scale is a ten point scale that assesses the risk of alien detection and interpretation of our communications based on two factors: the intensity of the signal and the information it carries. The Arecibo broadcast rates a maximum 5 for intensity and a 3 for information load. The fact that the San Marino scale fails to include any determination of the likelihood of detection based on such factors as the duration of the signal, demonstrates its ineffectiveness and the paranoia of security agencies.