4 minute read

In 1974, the Arecibo Observatory became capable of transmitting radio signals at a maximum effective power that was twenty times the combined power of all power plants on Earth at the time (\(2 \times 10^{13}\) W). A transmission at 1 Hz bandwidth or less would have been detectable by similar radio telescopes all over the Milky Way.

The first use of this transmitter (at 1700 GMT on November 16, 1974) was to send a brief message to the globular star cluster M13 (the Great Cluster in Hercules, NGC 6205), which lies 25,000 light years away and contains several hundred thousand stars. This was to demonstrate the advanced level of radio astronomy on Earth, rather than a realistic attempt to initiate interstellar communication with another civilisation. A paper published in 1975 by the staff of the Arecibo Observatory gives more details about the message [1].

Here are my notes from reading this paper. You can find my notes on other classic SETI papers in my roundup of the technosignatures literature.

Message transmission

The message was transmitted by the Arecibo telescope at a frequency of 2380 MHz (a higher frequency than the 1420 - 1662 MHz ‘water hole’, a quiet region of the spectrum recommended by the authors of the Cyclops report [2] as a convenient band for communication with extraterrestrial intelligences), a bandwidth of 10 Hz (which is broader than what others had proposed [3]) and at a rate of 10 characters per second. It took 169 seconds to send and 5 hours and 20 minutes to pass Pluto.

The effective power in the direction of transmission was \(3 \times 10^{12}\) W and by the time it reaches M13 (in about 25,000 years) it will make the Sun appear to be the brightest star in the Milky Way.

Message content

Constructed primarily by Frank Drake, Richard Isaacman, Linda May and James Walker, with suggestions for improvement by others that included Carl Sagan, the message described characteristics of life on Earth with 1679 characters in binary format (using two transmission frequencies that were continuously corrected for the Doppler effect of the orbital motion and rotation of Earth). The message was laid out in a grid, arranged from top right to bottom left in 73 rows of 23 characters per row (two prime numbers were used to facilitate decoding the message).

Binary number system

The first 4 rows give the numbers 1-10 in binary format. Each number is read from bottom to top, starting on the third row down (the 1s), followed by the second row (the 2s) then the top row (the 4s) (with the fourth row indicating a “number label”). A single column can represent numbers from 1-7 and an adjacent column to the left is used to continue the binary format (third row represents the 8s, second 16s and top 32s). A blank row was left between each number.

Terrestrial biochemistry

Using the binary number system described by the first four rows of the message, rows 6-10 (row 5 was blank) contains the numbers 1, 6, 7, 8 and 15, the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus. Twelve groups of five numbers then give the numbers of each of these elements in the building blocks that make up a DNA double helix: four units of deoxyribose, four phosphate groups and an adenine, thymine, guanine and cytosine, arranged in the same configuration that they occur in DNA.

The double helix conformation is illustrated in the next part of the message together with the number 4 billion (the approximate number of DNA base-pairs in the human genome) encoded in binary down the centre of the helix.

Human form

The DNA helix leads to the head in a picture of a human and to the left is the number 4 billion representing the size of the human population on Earth. To the right is a vertical line from the head to the feet of the human and a horizontal line, representing the number 14 when read right to left, which gives the number of units of length of a human (one unit being the wavelength of the transmission: 12.6 cm)

Planet Earth

Underneath the encoding of a human form is a diagram of the Solar System with the Sun to the right. Planet Earth is highlighted by displacing it upwards compared to the other eight planets (Pluto was still a planet back then).

Our technology

A telescope is depicted underneath the Solar System together with a number indicating its size (2430 units).


The Arecibo message was never supposed to be a serious attempt at communicating with extraterrestrial intelligence, and has a very small chance of being detected, but it demonstrated that humans had reached a level of technological advancement that made this a possibility. We are capable of sending information out into the galaxy for the benefit of other civilisations or to respond to, or initiate, interstellar communication.


  1. 1. The Staff at the National Astronomy: The Arecibo Message of November, 1974. Icarus 1975, 26:462–466.
  2. 2. Project Cyclops: A Design Study of a System for Detecting Extraterrestrial Intelligent Life. 1972.
  3. 3. Dixon RS: A Search Strategy for Finding Extraterrestrial Radio Beacons. Icarus 1973, 20:187–199.