40 Years Ago, Earth Beamed Its First Postcard to the Stars

Forty years ago, on Nov. 16, 1974, Earth beamed its first postcard to the stars.

The message left our home planet on a warm and sticky day in Puerto Rico and has been flying through the galaxy at the speed of light ever since. In about 25,000 years, it will collide with a cluster of more than 300,000 stars.

Unlike the radio signals that had been leaking from Earth since the late 1930s, this postcard was the first deliberate transmission to an alien civilization. Meant to be decipherable by extraterrestrial beings, the message contained some key information about the species that had sent it.

“It was a message that would actually inform anyone who did receive it that we existed, and tell them a little bit about what we were like,” says my dad Frank, who had the responsibility of constructing and sending what’s now known as the Arecibo Message. “And it was also a message to ourselves in that it showed what an intelligent civilization can do to contact other civilizations.”

Dad had been given just one month to write Earth’s first radio greeting to the stars.

It was 1974, and the Arecibo Observatory’s giant radio telescope had just gotten a major upgrade. Beamed into space by the Observatory’s powerful, one million-watt transmitter, the message would cap a ceremony marking the completion of the improvements (you can listen to it being sent, below). But it was a secret – only the ceremony’s organizers knew ahead of time what would happen, and they envisioned a transmission lasting about 3 minutes.

So, it needed to be relatively simple.

Texting Aliens

Fortunately, Dad had been thinking about how to write postcards to aliens for a while. Sending a message to other worlds, while simple in principle, becomes almost impossibly complex when it’s time to actually write the letter. Aside from the question of what to say, there’s the issue of how to say it.

For starters, not just any language will work – think about how difficult it is to understand the many languages spoken on Planet Earth. The chances of an alien civilization understanding, “Hi, we live on Earth and are friendly,” are vanishingly small, just like your chances of understanding “Vie minut johtajanne luo” are practically zero unless you speak Finnish. Numbers and equations, while more concrete, are still written using arbitrary symbols. And things like distance measurements are just as arbitrary as words.

So how does one compose a missive with the best chance of being universally understood?

“From day one, I’ve always thought pictures work,” Dad says. “Sending pictures is easy. We do that all the time, with great results.”

Black and White

Dad set up a grid and drew crude shapes and symbols by shading in squares. A 3-minute transmission at 10 bits of information per second meant he had at most 1,800 squares to play with.

Then, he drew a depiction of double helical DNA, wrapped around “3 billion” written in binary code. This is the approximate number of characters (“base-pairs”) in a human genetic sequence. Below that, he shaded a human into the message, with “14” on one side, and “4 billion” on the other. The first number represented the height of an average human, in units of 12.6 centimeters – the length of the radio waves used to send the message.

The second number? In the 1970s, that was the human population of Earth.

Next came a map of the solar system (which included Pluto, since it was still one of the nine classical planets), with the Earth offset from the others, toward the human, to signal that this was our home planet. And lastly, he sketched a representation of the Arecibo telescope itself, with its size noted in units of radio wavelengths, to indicate the level of technology used to post the letter.

Sending the message would mean translating the grid into binary code, where zeros and ones denoted which squares were shaded and which were open (black squares = 1, white = 0). To decode the message, recipients would need to figure out how to organize that string of numbers into a grid with the correct dimensions, recognize that the grid contained symbols, and then decipher the meaning of those symbols. The finished message had 1,679 bits; 1679 is the product of prime numbers 23 and 73, which offers a hint about how to lay out the grid.

Test Transmission

Dad tested his message by sending it to his friend and colleague Carl Sagan, who hadn’t been involved in writing it and had no knowledge of its content. He wanted to see if Carl could figure out what was in there.

“And more importantly,” Dad adds, “Make sure he would not find things in it that were equally plausible decryptions but were wrong, which would be misleading and give inaccurate depictions of our civilization.”

Perhaps not surprisingly, Carl got almost everything right, and quickly. But the chemistry eluded him, Dad recalls, adding that biochemists he sent it to later got that part almost immediately. “People recognize things that are in the area in which they have expertise,” he says.

To Hercules

And then it was time to choose a destination.

Sitting in a sinkhole, the Arecibo telescope is somewhat limited in its steering ability. So the postcard needed to be addressed to somewhere roughly over Arecibo at 1pm on November 16, 1974, the time of the ceremony.

Scouring the star charts, Dad identified a worthy target: M13, the Great Cluster in Hercules. With more than 300,000 stars – and probably at least that many planets! – M13 was perfect, and its heart would fit entirely in the telescope’s beam. An Arecibo-type receiver on any of those worlds should have no trouble detecting the message. (Some people, Dad notes, think the cluster will have moved in 20,000 years, and that the message will miss it. That’s not true, he says. “It will have moved a little bit, but only a fraction of the width of the beam of transmission,” he says.)


At 1pm, just as planned, the Arecibo transmitter came to life. For a few moments, a smooth tone filled the crags and valleys of the surrounding jungle terrain.

And then the transmitter began to sing a series of alternating tones. “It sounded like a bird warbling,” Dad recalls. Over nearly three minutes, the 1,679 bits of information hurtled into space, carrying this message from Earth – the first deliberate radio transmission from the humans on our little blue planet – to whatever beings might live in that great cluster in Hercules.

“When we sent the last character, and it stopped and we went back to that steady tone, everybody was crying,” Dad says. “We were hearing what it would be like to actually contact another world. That was what that sound was. It had the aura of human beings doing something marvelous that involved the whole cosmos.”

But there was another surprise to come. Unintentionally, Dad had coded another dimension into the message. As the transmitter began to send those alternating tones, one of the engineers in the control room recognized a bit of Morse code. Hiding in the first notes of the warble was the simplest greeting of all:


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