On November 16, 1974, a 305-meter dish set into a sinkhole in the karst hills fired a narrow beam of radio waves toward the constellation Hercules for two minutes and 49 seconds. The signal carried 1,679 bits of binary data aimed at a globular cluster called M13, roughly 25,000 light-years away in the constellation Hercules. The people in the control room understood the math. Even at light speed, the message would take 250 centuries to arrive, and any reply would take another 250 centuries to come back. No one in that room, no one’s grandchildren, no one’s distant descendants in any plausible sense would ever know if the message was received.
They sent it anyway.
What was actually in the broadcast
The signal was a string of 1’s and 0’s. Arrange those 1,679 bits into a grid of 23 columns by 73 rows, both prime numbers, and the binary resolves into a crude pictogram. Frank Drake, then director of the National Astronomy and Ionosphere Center, designed the message with input from Carl Sagan and a small group of colleagues at Cornell. The grid contained the numbers one through ten in binary, the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus, the chemical formulas for the nucleotides of DNA, a double helix, a stick figure of a human, the human population of Earth in 1974, a diagram of the solar system with Earth offset to mark our home, and a schematic of the Arecibo dish itself.
It was, in effect, a postcard. Name, address, species, biochemistry, return contact.
The transmission used the observatory’s newly installed radio transmitter. The SETI Institute still uses the Arecibo Message as a teaching tool, asking visitors to design their own grids of binary pixels and think about what a species would choose to say about itself in 1,679 bits.
Why M13, and why it matters less than it sounds
M13 was not chosen because anyone believed M13 was a good place to find life. It was chosen because it would be directly overhead at Arecibo at the moment of the dedication ceremony. The dish sat in a natural sinkhole and could not be steered. Whatever was at zenith when the transmitter fired was what got the postcard.
M13 is a globular cluster containing hundreds of thousands of stars packed into a sphere roughly 145 light-years across. It sits roughly 25,000 light-years from Earth. Light travelling at 300,000 kilometers per second takes 25,000 years to cross that distance. By the time the radio waves arrive in the year 26,974, the cluster will have shifted in its orbit around the galactic center, though Drake himself was emphatic that the proper motion of M13 would be only a small fraction of the beam’s width, and that the message would still arrive at the cluster.
The point was never to expect a reply. The point was that humans, in 1974, could now do this at all.
The math of a 50,000-year round trip
Consider the timescale. 25,000 years ago, the last glacial maximum was covering much of the northern hemisphere with ice sheets. Anatomically modern humans existed but had not yet domesticated a single plant. The cave paintings at Lascaux were still thousands of years in the future. Writing would not be invented for another 20,000 years.
That is the one-way travel time of the Arecibo Message.
A reply, assuming an alien civilization received the signal, decoded it, decided to answer, and possessed a transmitter pointed back at us, would arrive in the year 51,974. The civilization that sent the message will not be the civilization that receives the answer. There is no continuous institution on Earth that has lasted 50,000 years. The oldest continuously inhabited cities are thousands of years old. The oldest written records date back roughly 5,000 years. The Catholic Church, often cited as one of humanity’s most durable institutions, is roughly 2,000 years old.
Whoever opens the reply, if there ever is one, will be as distant from Frank Drake as Frank Drake was from the people who painted the bulls at Lascaux.
A gesture aimed at Earth as much as at the stars
The Arecibo Message belongs to a tradition of human gestures that look outward but speak inward. The recipients on Earth were the VIPs gathered at the observatory that day, the world press, and everyone who has since seen the grid reproduced in textbooks and on T-shirts. The message was, in part, a piece of performance: a demonstration that humanity had upgraded its hardware and could now reach across the galaxy if it chose to.
The transmitter on that day was, briefly, one of the most powerful directed signals humans had ever produced. For the duration of the broadcast, Earth on that frequency became enormously brighter than the Sun. Anyone in the beam’s path with a sufficiently sensitive radio receiver would have seen our star outshine every other point in their sky.
For 169 seconds, we were the brightest thing in that corner of the galaxy.
The arguments that have come after
The 1974 broadcast was sent in what was, by the standards of the field, an innocent era. The opposition to interstellar messaging that exists today, the worry that announcing ourselves to the cosmos might be a poor idea given what we don’t know about who might be listening, was largely absent. Drake’s team did not hold a public consultation. They did not consult the United Nations. They wrote a message, aimed the dish, and pressed the button.
The debate began almost immediately. Martin Ryle, a prominent British astronomer, objected to the transmission, arguing that broadcasting Earth’s location was reckless. That objection has only grown louder. Space Daily has covered the ongoing controversy over deliberately broadcasting Earth’s coordinates, a practice now known as METI, or Messaging to Extraterrestrial Intelligence.
The counter-argument is that Earth has been leaking radio signals for over a century. Any civilization within roughly 100 light-years that possesses an Arecibo-class receiver has already heard us. The Arecibo Message was a focused beam, but the background hum of human broadcasting, television, military radar, has been propagating outward since the early 20th century. By the time the Arecibo Message reaches M13, the sphere of Earth’s accidental transmissions will be 50,000 light-years across.
The telescope that sent it no longer exists
The dish that broadcast the message is gone. In 2020, after cable failures, the instrument platform suspended above the Arecibo reflector collapsed, crashing through the aluminum dish below. A team led by Jonathan Jiang of NASA’s Jet Propulsion Laboratory has since proposed an updated successor message, the Beacon in the Galaxy, designed for eventual transmission from FAST in China or the SETI Institute’s Allen Telescope Array. Space Daily has written about the proposed Beacon in the Galaxy message and the questions of authorship and consent it raises.
Arecibo itself contributed to astronomy in ways that had nothing to do with the 1974 message. The observatory was central to planetary radar, mapping the surface of Venus through its clouds, refining the orbits of near-Earth asteroids, and conducting work that contributed to the discovery of the first binary pulsar. Space Daily has covered some of the late Arecibo-era work, including the observatory’s discoveries about asteroid rotation.
The dish is rubble now. The message is still travelling.
Where the signal actually is, right now
As of May 2026, the leading edge of the Arecibo Message is roughly 51 light-years from Earth, a thin fraction of one percent of the way to its target. The beam is narrow and directional, pointed where M13 was when the transmitter fired in 1974. There are stars closer to Earth than 51 light-years, but the Cornell team behind the broadcast has been clear that none of them lie in the path of the message. The first star projected to fall inside the beam is hundreds of years of travel away.
The signal will continue to travel at the speed of light, expanding very slightly as it goes due to the angular width of the original beam, for the next 24,950 years before any part of it reaches M13. The radio waves themselves do not decay. They simply thin out, spreading the same fixed energy across an ever-larger spherical shell, until the signal becomes indistinguishable from cosmic background noise to any receiver not specifically designed and pointed to catch it.
Whether anything ever catches it is a question with an answer none of us will live to learn.
What the gesture actually was
Frank Drake died in 2022. Carl Sagan died in 1996. Most of the engineers in the Arecibo control room that November day are gone. The Soviet Union, which had sent the first deliberate radio transmission in 1962 (the Morse message bounced off Venus, carrying the words “mir,” “Lenin” and “USSR”), no longer exists. New Scientist has catalogued the handful of intentional interstellar transmissions humans have sent since, from the Cosmic Call broadcasts of 1999 and 2003 to the Across the Universe transmission of a Beatles song toward Polaris in 2008.
None of them will be answered by anyone now alive.
The Arecibo Message was a peculiar kind of act: a communication whose audience could never respond to its author. It treated the human species, rather than any individual or generation, as the sender. It assumed that what humanity is in 1974 is continuous enough with what humanity will be in 26,974 that the gesture remains coherent. That is a strong assumption. It is also, in a sense, the only assumption available to anyone who wants to speak across geological time.
Radio waves don’t slow down. The signal from that November afternoon in Puerto Rico is still out there, expanding through the disk of the Milky Way, carrying a stick figure and a double helix and a count of how many of us there were when the button got pressed. It will arrive at its target long after every person who watched it leave is gone, and long after the institutions that built the dish, the country that funded the observatory, and quite possibly the language the message was meant to teach, have all become history or stopped existing.
It is, in that sense, one of the longest letters ever written.
