Arecibo Message (1974): Humanity's Radio Signal to the Stars

The Arecibo message was a brief radio signal beamed into space from the Arecibo Observatory in Puerto Rico on 16 November 1974. Designed mainly by the astronomer Frank Drake with help from Carl Sagan and colleagues, its 1,679 binary digits encoded a simple picture of humanity and its home planet. It was aimed at the star cluster M13, about 25,000 light-years away, and remains one of the best-known attempts to communicate with extraterrestrial intelligence.

Background

The Arecibo radio telescope, completed in 1963, was for more than half a century the largest single-dish radio telescope in the world. Its fixed reflector, about 305 metres (1,000 feet) across, filled a natural sinkhole in the karst hills of northern Puerto Rico. Above it hung a platform carrying receivers and transmitters, so the telescope could be used both to listen to cosmic radio sources and to send powerful radar pulses towards planets and asteroids.

In the early 1970s the dish was resurfaced with precisely fitted aluminium panels and given a new high-power radar transmitter. The observatory, run by Cornell University for the US National Science Foundation as the National Astronomy and Ionosphere Center, planned a ceremony to mark the upgrade. Frank Drake, director of the centre, proposed that the highlight should be a message transmitted to the stars.

Drake was already a pioneer of the search for extraterrestrial intelligence (SETI). In 1960 he had run Project Ozma, the first modern radio search for alien signals, and in 1961 he devised the Drake equation, a framework for estimating how many communicating civilisations might exist in our galaxy.

Designing the message

Drake and his colleagues faced a puzzle: how to write to a reader who shares no language with us. Their answer was to rely on mathematics and physics, which any technological civilisation would presumably understand. The message was written in binary, as a string of 1,679 zeros and ones.

The number 1,679 was chosen because it is the product of just two prime numbers, 23 and 73. A recipient who noticed this could lay the bits out in a grid. Arranged as 73 rows of 23 columns, the ones and zeros form a crude picture, read from top to bottom.

Carl Sagan, then a Cornell astronomer, and other staff members helped to refine the design. The message was deliberately compact so that it could be sent during a short ceremony.

What the message says

The picture is divided into sections, each building on the one before:

• Numbers: the numbers one to ten written in binary, teaching the reader the counting system used in the rest of the message.
• Chemistry of life: the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus, the elements that make up DNA.
• DNA: the formulas of the sugars and bases in the building blocks of DNA, an estimate of the number of base pairs in the human genome, and a sketch of the double helix.
• Humans: a stick figure of a person, with the average height of a man and the approximate human population of Earth in 1974, about 4 billion.
• The Solar System: the Sun and nine planets as then counted, with Earth displaced towards the human figure to show where the senders live.
• The telescope: an outline of the Arecibo dish and its diameter.

Heights and sizes were expressed in units of the transmission’s own wavelength, about 12.6 centimetres, so that the recipient would have a ruler built into the signal.

The transmission

The message was sent on 16 November 1974 during the rededication ceremony for the upgraded telescope. The telescope’s transmitter broadcast at a frequency of about 2,380 megahertz, switching between two slightly different frequencies to represent the zeros and ones. Because the dish concentrated the beam so tightly, the effective power in the direction of the target was enormous, many trillions of watts.

The whole message took less than three minutes to send, at a rate of 10 bits per second. Its target was the globular cluster Messier 13 (M13) in the constellation Hercules, a ball of several hundred thousand stars. It was chosen chiefly because it would be overhead at the time of the ceremony and offered many stars in a single beam.

Reception and criticism

Drake and his colleagues always presented the transmission as symbolic. Its real purpose was to demonstrate what the upgraded telescope could do and to stimulate thinking about interstellar communication. At the speed of light the signal needs around 22,000–25,000 years to reach M13, and any reply would take just as long to return.

Some scientists objected in principle. The British radio astronomer Martin Ryle, then Astronomer Royal, protested that announcing Earth’s presence to unknown civilisations could be dangerous and should not be done without international agreement. The question of whether humanity should actively transmit, sometimes called METI (messaging extraterrestrial intelligence), is still debated.

Myths and facts

Myth: The Arecibo message was a serious attempt to start a conversation with aliens. Fact: Its designers described it as a demonstration. They knew a reply could not come for tens of thousands of years.

Myth: Aliens have replied to the message. Fact: In 2001 a crop formation appeared near the Chilbolton radio telescope in Hampshire, England, resembling a modified version of the Arecibo picture. It was a hoax, and no genuine reply has ever been detected.

Timeline

• 1960, April: Frank Drake carries out Project Ozma, the first modern radio search for alien signals.
• 1963, November: The Arecibo radio telescope is inaugurated in Puerto Rico.
• 1974, 16 November: The Arecibo message is transmitted towards M13 during the rededication ceremony.
• 1975: The observatory staff publish a description of the message in the journal Icarus.
• 1977, August–September: NASA launches the two Voyager probes carrying the Golden Record, another message for possible alien finders.
• 2020, 1 December: The Arecibo telescope’s instrument platform collapses into the dish, ending its working life.

Legacy

The Arecibo message became an icon of the space age. Its pixelated design has appeared on posters, album covers and textbooks, and it inspired later projects, including the records carried by the Voyager probes. Like the earlier work of large radio dishes such as Jodrell Bank, it showed how radio astronomy had turned the search for life beyond Earth into a scientific project.

The telescope itself was damaged by cable failures in 2020 and collapsed on 1 December that year. Its destruction gave fresh attention to the message, which is still travelling towards Hercules.

Why it matters

The Arecibo message was humanity’s first deliberate, powerful radio greeting to the stars. It distilled human knowledge, from counting to the structure of DNA, into 1,679 bits, and it continues to raise the question of whether we should announce ourselves to the universe.