Fifty years ago, Earth’s first ever interstellar message was transmitted from the Arecibo radio telescope in Puerto Rico. Wrapped up in the message’s 1,679 binary digits are our hopes, dreams and fears about ourselves and our place in the cosmos, entwined with the aim of reaching out to the stars in search of someone to share the Universe with.

The Arecibo message “signaled the importance of complex and difficult discussions about human communication in the cosmos,” says Chelsea Haramia, Associate Professor of Philosophy at Spring Hill College in the US and Senior Research Fellow at the University of Bonn in Germany.

“It was important in making people think about aspects of communicating with extraterrestrial intelligence [ETI] and it was a demonstration that even a civilization like ours could really communicate across a large chunk of our galaxy – quite an achievement in some sense,” adds Michael Garrett, the Director of the Jodrell Bank Centre for Astrophysics.

The message was beamed towards the globular star cluster Messier 13 on 16 November 1974, as part of a lavish ceremony with 250 invited guests to commemorate the re-opening of the jungle observatory. The telescope had received a major upgrade that saw its wire mesh antenna replaced by a dish of 38,000 solid aluminum panels and the introduction of a transmitter. 

The Arecibo message wasn’t our first attempt to reach out to ETI, though. In 1962 Soviet astronomers sent a message in Morse code to Venus, spelling out the words ‘Mir’, ‘Lenin’ and ‘USSR’, though the effort was more of a propaganda stunt than a serious attempt at reaching out to other life. The transmission was designed to test the radar capabilities of the Yevpatoria planetary radar in Ukraine, and presumably nobody taught the Venusians Morse code beforehand.

Though I make light of it, the Morse message highlights some of the problems that Frank Drake – the father of the search for extraterrestrial intelligence and the main man behind the Arecibo message – had to tackle when he set about designing his signal to the stars. It’s not just a language barrier that is going to exist between humans and any possible ETI; the barriers could be social, cultural, biological, technological, even cognitive. Drake, with assistance from a few others including Carl Sagan, had to render his message as intuitively understandable to ETI as possible.

“As the first structured attempt to communicate with ETI, it marked a turning point, not just technologically but philosophically,” says Jonathan Jiang of NASA’s Jet Propulsion Laboratory at Caltech. “The simplicity and elegance of its binary format, combined with fundamental scientific information, provided a blueprint that inspired subsequent projects.”

Those projects included Jiang’s own ‘Beacon to the Galaxy’, which aimed towards building a message that could be understood by ETI. It’s inspired by the Arecibo message in many ways, including its use of binary code, which Jiang argues would be a universal concept.

Originally suggested by Drake’s administrative assistant, Jane Allen, as a way to top off the re-opening ceremony, the Arecibo message is written in binary code incorporating 1,679 bits. What’s special about this number is that it can only be divided by 1, by itself, and by two other numbers: 23 and 73. Both these 23 and 73 are prime numbers, and Drake figured they would be the clue as to the dimensions of the pictogram that the message described. It also meant that it was a short message – writing in his 1991 book Is Anyone Out There? co-written with journalist Dava Sobel, Drake describes how, as the director of the National Astronomy and Ionosphere Center (NAIC) that oversaw the Arecibo radio telescope, he wanted to keep the message short enough to transmit within three minutes so that the invited guests didn’t grow bored. It also was not repeated – if potential recipients are not listening at the exact time the radio waves oscillate past them, they’ll never know the message was sent in the first place. Yet if they are looking, they’ll notice that at a frequency of 2,380MHz, the signal will shine brighter than the Sun for about three minutes.

On the first line of the pictogram are the numbers 1 to 10 in binary code. These are important, because they help unlock the rest of the message. 

Next are the numbers 1, 6, 7, 8 and 15, representing the atomic numbers of the five elements essential to the chemistry of all life on Earth: hydrogen, carbon, nitrogen, oxygen and phosphorous. These are the five elements that build the nucleotides that assemble into DNA molecules, the double helix of which is also represented in the message as a crude drawing.

“Drake’s approach was groundbreaking, and the decision to include basic mathematical and chemical principles remains fundamental,” says Jiang.

However, perhaps the message is a little bit scant on details. If ETI’s biochemistry is exactly like ours then perhaps they’ll recognize the atomic numbers, but the nature of these numbers is not immediately obvious.

“Would an extraterrestrial receiving a list of the numbers 1, 6, 7, 8 and 15 really know we’re talking about the chemical elements central to life on Earth?” asks Doug Vakoch, who is President of the organization METI International, METI standing for ‘messaging extraterrestrial intelligence’. “Probably not. Today, when designing an interstellar message to convey core principles of chemistry, we’d embed those numbers in the Periodic Table to give aliens more context.”

Below the representation of the DNA double helix is a stick figure of a human being. Whereas the naked humans on the Pioneer plaques stirred up controversy among prudes and people criticizing the appearance of the woman acting subservient to the man, surely a simple stick figure couldn’t be criticized beyond its simplicity?

Drake had hoped that the figure would be suitable androgynous, but it has been pointed out that the stick figure appear suspiciously male. Drake explained this away as him having difficulty drawing a stick figure that would appear female. He also included the average height of a human being as 1.76 meters/5 feet and 9 inches (in units of the transmission frequency – aliens won’t know anything about meters or feet). This also happened to be be Drake’s height, and he wondered, in hindsight, whether he had subconsciously inserted himself into his message.

“Placing the human stick figure toward the visual center of the message gives Earthlings something we can readily recognize, even if this might be the most opaque part of the message to any otherworldly recipients,” says Vakoch.

This perhaps tells us who the real beneficiaries of the Arecibo message will be. The transmission was directed at the globular cluster Messier 13, a great ball of hundreds of thousands of stars some 22,500 light years away from us in the constellation of Hercules. The chance of anyone detecting the message is slim, and even if somebody does, it would take 45,000 light years before we receive a reply. However, we don’t have to wait that long to gain meaning from the sending of the message, says Shin-ya Narasuwa, an astronomer at the Nishi-Harima Astronomical Observatory at the University of Hyogo in Japan.

“If the residents of M13 send a reply, it will arrive in 45,000 years’ time,” he says. “Will Earthlings still be around to receive it? Frank Drake and Carl Sagan hoped Earthlings would survive to that time, so the Arecibo message is also a message to the Earth, I think.”

Indeed, inherent within the Arecibo message project is the underlying hope that humanity will still be around in 45,000 years time to receive a reply. It’s a positively optimistic message to ourselves and a rare step into long-term thinking, essential if we are to cultivate a better world and a more positive future. 

The message ends with the number 4.3 billion representing the population of Earth in 1974, a very simple schematic of the Sun and nine planets (including Pluto) with Earth highlighted, and another crude drawing, this time of the Arecibo dish.

The population of the Earth has in that time doubled, while the number of planets in the Solar System is today a more mutable concept – Pluto has been reclassified, another possible Planet Nine exists, and the distinction between planets, dwarf planets, giant planets and brown dwarfs is becoming increasingly blurred. Even the destination – and there’s been debate as to whether it will miss M13 given the cluster’s proper motion through the sky, though Drake was adamant that the transmission was on target – is no longer as promising as had been thought. Though globular clusters pack a huge amount of stars into a relatively small volume, in theory increasing the chances of somebody detecting the signal, we now know that globular clusters are not the ideal places to find habitable planets. Their ancient age – most formed around the same time as the galaxies themselves, 13 billion years ago – means a lack of the heavy elements required to build planets or enable the complex chemistry of life. Those five elements necessary for life that are described in the Arecibo message are in short supply in globular clusters.

How successful we deem the Arecibo message to have been depends on  how we view it. Was it as inclusive and detailed as it could have been? Possibly not. “I don’t think they thought about everything that deserved consideration, but early drafts rarely achieve that,” says Haramia.

Was it a great prototype for future messages to base themselves on? You bet. “I believe it established an excellent foundation for future METI attempts,” says Jiang. “It demonstrated how we might use shared principles of science and mathematics as a basis for universal communication.”

Is it a message that is likely to find a recipient and prompt a response? Probably not, and on that basis one might question its merits, but “Given the improbability of its success at establishing communications, viewing the Arecibo message instead as a work of art allows us to appreciate its symbolic and expressive qualities,” says Rebecca Charbonneau, who is a historian of science at the American Institute of Physics and the National Radio Astronomy Observatory. “By framing it as art, we can explore the message’s deeper implications about the nature of communication, human identity, and the landscape of science.”

Charbonneau draws us back to the stick figure of a human in the message. If it was indeed a form of self-portrait by Drake, whether subconscious or not, then it is revealing of the person and the culture who sent it, shaped by the gender biases, values and identities of the time, says Charbonneau. The message has, if not greater meaning, then certainly a different meaning to ourselves than it might have to aliens. 

Sir Martin Ryle, the Astronomer Royal at the time and Director of the Mullard Radio Astronomy Observatory in Cambridge, took his own meaning from the message: that in secret (nobody was told about the transmission until the day of the ceremony) Drake had embarked on some unauthorized diplomacy, finding the arrogance within himself (in Ryle’s eyes) to speak for everyone on Earth, and perhaps place us in danger by revealing our existence to potentially hostile alien life. Within days of the transmission, Drake received an angry letter from Ryle, who was never shy of wearing his heart on his sleeve and who had also complained to the International Astronomical Union demanding that they condemn Drakes’ actions in sending the message. They did not, and Drake calmly replied to try and assure Ryle that the great distance between us and any inhabited exoplanets would surely be too far for an invasion fleet to reach us.

However, Ryle’s concerns have proven long-lasting, with each new METI transmission met with a chorus of disapproval from many in and associated with the SETI community. The whole topic of whether we should send signals into space that could reveal our existence to the Universe at large is divisive, spurring arguments at conferences and in email exchanges. It’s not so much the worry of invasion, which is often mocked by pro-METI activists but is actually a straw-man argument, that is the real concern, but rather “The damage that METI can cause socially on Earth is another topic,” says Charbonneau.

“I have mixed feelings about the Arecibo signal,” adds Michael Garrett. “It’s clearly an important event and the birth of METI.”

Although Garrett feels the risk is very low, especially given that we are frequently loudly broadcasting into space, with various military and interplanetary radar, “I don’t think that the people that practice METI understand the risks properly,” he says. “I also personally don’t feel that any individual or group or institute has the right or is entitled to speak for Earth.”

Doug Vakoch naturally has an opposing view, given his role at METI International who are dedicated to trying to call out to the stars. He describes the Arecibo message as validation for both SETI and METI.

“If even humans can send such messages, aliens might be sending something similar, so we need to be listening,” he says. “[And] if we’re expecting extraterrestrials to continuously send intentional radio signals our way, it’s only fair that we should continue transmitting messages as well.”

The debate will only grow more intense the more groups such as METI International look to transmit to the stars. After the Arecibo message, there was no deliberate attempt to transmit a message for another 25 years, when in 1999 the Russian radio astronomer Alexander Zaitsev beamed into space the first of his four interstellar messages, fittingly from Yevpatoria, the site from which the Morse message was beamed towards Venus. There have been other, sporadic efforts, including METI International’s 2017 transmission to Luyten’s Star, which is a red dwarf 12 light years away. Conscious that the Arecibo message was specifically designed as a visual message, Vakoch and his team designed the 2017 signal to be interpretable by life-forms that do not have eyes. And even if they do have eyes, pictures might not be as easy to interpret as we had thought.

“Drake assumed that pictures would be self-evidently interpretable,” says Vakoch. And indeed, being a picture has helped the Arecibo message reach a kind of cultural critical mass here on Earth. “But we now know that our interpretation of pictures relies on cultural assumptions that extraterrestrials might never imagine.”

Jonathan Jiang agrees that messaging needs to be a little more sophisticated than just sending a postcard. “Modern messages may place greater emphasis on visual and contextual clarity to account for potential differences in perception and cognition among extraterrestrial recipients,” he says. “For instance, with Beacon to the Galaxy, we aimed to incorporate binary-coded visual representations and structured timestamps to improve the accessibility of our intended message. Today, we can leverage a more sophisticated understanding of data encoding and transmission, enhancing both message content and the probability of meaningful interpretation.”

This is just one example of how one of the Arecibo message’s lasting legacies is to help us develop the sophistication of our interstellar message design. Meanwhile Chelsea Haramia, who is particularly interested in the ethics of interstellar messaging and post-detection consequences, sees growing sophistication across all areas of the topic.

“The precedent that the Arecibo message set has been helpful in setting the stage for important debates about messaging,” she says. “The signal was brief, but the effects of its transmission here on Earth are ongoing.”

It announced ourselves onto the interstellar scene, led to us considering our longevity and ability to survive into deep time to be here to receive any reply, and caused us to question our own biases and influences that can find a voice in the messages that we construct to represent ourselves with. And the Arecibo message is the lynchpin that is central to our discussions about how and even if we should attempt to communicate to the stars.

“The Arecibo message set a precedent that messaging should be done under careful consideration,” says Haramia. “Now that we’re gaining an ever-greater understanding of many of the overlooked considerations surrounding messaging, I hope that we take seriously the need for caution, discussion and expertise that such cosmically representative activities require.”