168 Seconds of Emission, 25,000 Light-Years of Travel: The 1974 Arecibo Message Aimed at the Star Cluster Overhead at 1 PM

We often imagine that sending a signal to the stars is about aiming at a fixed target, as one would mail a letter to a known address. That is not the case. In 1974, the Arecibo Observatory aimed at the M13 globular cluster, knowing that it would have moved long before the message’s arrival, planned for roughly 25,000 years later.

A Telegram to the Stars: Why Arecibo Chose M13

On November 16, 1974, the Arecibo Observatory in Puerto Rico hosted a ceremony to celebrate the upgrade of its immense 305-meter dish. To mark the occasion, the scientific team decided to broadcast a radio message into space. What the guests heard that day was not spectacular: a sequence of two tones lasting nearly three minutes, precisely 168 seconds.

Behind this apparent simplicity lies a carefully crafted message, designed by a team led by Frank Drake, with the participation of Carl Sagan. The target was not chosen at random. The Arecibo dish, being fixed, can only point at things passing overhead. That day, at 1:00 p.m., the M13 globular cluster occupied that position in the sky, providing an ideal window of opportunity.

M13 offered another major advantage, so to speak: it is a particularly vast cluster, housing hundreds of thousands of stars. Statistically, the chances of finding a planetary system there were higher, at least on paper. The transmission occurred at a frequency of 2,380 megahertz, with an effective bandwidth of barely 10 hertz, and an emission power of 450 kilowatts.

When expressed as isotropic radiation, this power equated to 20 trillion watts. In other words, any antenna comparable to that of Arecibo, placed anywhere in the Galaxy, could have detected this signal. A seemingly modest technical gesture, yet capable of reaching extraordinarily far.

1,679 Bits to Summarize Humanity: What the Message Decodes

The message fits into 1,679 bits, or about 210 bytes, a size tiny compared with the files we exchange today in a single second. This figure is not arbitrary: 1,679 is the product of two prime numbers, 73 and 23. The idea is clever: an extraterrestrial intelligence capable of decoding a radio signal would logically understand that these bits must be arranged into a grid of 73 rows by 23 columns, the only rectangular arrangement possible with that total.

Once assembled in this form, the grid reveals an image. It contains the numbers from 1 to 10 written in binary, the chemical formula of DNA, a stylized human silhouette, a diagram of the solar system indicating Earth, and a simplified drawing of the Arecibo telescope itself. A cosmic business card, in short, summarizing a few essential references about our species and our position in space.

The bits were transmitted by frequency offset, at a rate of 10 bits per second, which explains the duration of the broadcast. If Frank Drake had enlisted graduate students from Cornell to help compose this message, one of them, Richard Isaacman, later claimed that 90% of the final content came from Drake alone, then 44 years old. Carl Sagan, for his part, helped popularize the approach with the general public.

A Moving Target: The Paradox of a Message with No Fixed Recipient

Here is the paradox at the heart of this experiment. M13 lies about 25,000 light-years from Earth, which means the message, traveling at the speed of light, would take roughly 25,000 years to traverse that distance. Estimates place its arrival in the region of the cluster around the year 25974. In other words, no one at Arecibo ever expected to receive a reply during their lifetime.

The problem is that star clusters are not stationary. M13 has proper motion, though small, but real. By the time the signal reaches the targeted region, the cluster will have shifted slightly from its current trajectory. Calculations show, however, that this movement remains modest enough for the message still to reach near the center of the cluster, not missing it entirely.

The observatory from which this signal originated is no longer in existence. The 305-meter radio telescope collapsed in 2020, and the American National Science Foundation announced in October 2022 that it would not be rebuilt. The site ceased to be an active observatory after six decades of service, before four institutions took over its management to turn it into an educational center. The message, meanwhile, continues its journey, indifferent to the fate of the antenna that broadcasted it.

This idea of sending a signal into the unknown is by no means an isolated episode. On May 31, 2025, the European Space Agency broadcast Johann Strauss II’s The Blue Danube from a large antenna in Spain toward the Voyager 1 probe to celebrate the agency’s 50th anniversary. The signal was expected to overtake the probe in about 23 hours, a reminder that the urge to speak to the stars, or at least to the emptiness that separates us from them, remains very much alive.

The Arecibo message was never intended to yield a quick reply, nor even any reply within our lifetimes on Earth. It was a gesture as much symbolic as technical, a way to test our ability to condense the essence of who we are into a few thousand bits. Whether M13 still exists, whether humanity remains here to receive any possible echo, no one can guarantee. But the signal itself continues its journey, quiet and patient, toward a rendezvous set in a future we will never witness.

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