The transmission lasted 169 seconds. It consisted of 1,679 binary digits, a number chosen because 1,679 is the product of two prime numbers, 73 and 23. The hope was that an intelligent recipient would arrange the digits into a 73-row by 23-column grid. The message, designed by Frank Drake with help from Carl Sagan and others, was beamed from the 1,000-foot dish of the Arecibo Observatory in Puerto Rico. It was aimed at the globular star cluster M13 in the constellation Hercules, approximately 25,000 light-years from Earth. The ceremony marked the rededication of the observatory after a major upgrade.
The content was a basic pictorial primer. It depicted the numbers one through ten in binary, the atomic numbers for hydrogen, carbon, nitrogen, oxygen, and phosphorus (the components of DNA), the formulas for the sugars and bases in DNA nucleotides, a double helix representing DNA, a stick figure of a human, the human population of Earth, our solar system with Earth offset, and a schematic of the Arecibo telescope itself. The broadcast frequency was 2,380 MHz with a power of 450 kilowatts, making it the most powerful human-made signal ever sent into space at the time.
The event is often misunderstood as a serious attempt at contact. Its architects knew the futility. By the time the message reaches M13, the cluster will have moved. The ceremony was primarily a demonstration of human technological achievement, a symbolic gesture for a terrestrial audience. It was a postcard sent to a neighborhood we would have left long before it arrived. The real legacy is its cultural weight. The Arecibo Message codified a specific vision of humanity—as carbon-based, DNA-driven, and technologically adept—that we chose to project into the cosmos. It remains a benchmark for all subsequent discussions about interstellar communication, less for its content than for the act itself.