A 169-second broadcast
On November 16, 1974, a brief message left Earth, aimed at the stars. The broadcast originated from the Arecibo Observatory in Puerto Rico, following the facility's recent major upgrade. The message itself was a string of 1,679 binary digits—ones and zeros—transmitted over 169 seconds. This number, 1,679, was deliberately chosen because it is a semiprime, the product of two prime numbers: 23 and 73. The logic, devised by Cornell University astronomer Frank Drake and others including Carl Sagan, was that any intelligent recipient would recognize the mathematical hint and arrange the data into a 23x73 grid, revealing a pictogram.
The transmission was powerful. It was sent at a frequency of 2,380 MHz with an effective power of 3 terawatts, strong enough to be detected by a similarly sized antenna almost anywhere in the galaxy. The target was Messier 13 (M13), a dense globular cluster of about 300,000 stars located roughly 25,000 light-years away in the constellation of Hercules. The message was a demonstration of the upgraded telescope's capabilities. Given the 25,000-year travel time, any potential reply would take an additional 25,000 years to return. By then, the stars of M13 will have moved from the position the message was aimed at.
The pictograph from Earth
When the 1,679 bits are arranged correctly, they form a simple, blocky image that provides basic information about humanity. The pictogram is divided into seven sections. The first section depicts the numbers one through ten in binary format. Below this are the atomic numbers for the five elements considered fundamental to life on Earth: hydrogen, carbon, nitrogen, oxygen, and phosphorus.
The next part of the message shows the chemical formulas for the sugars and bases that make up the nucleotides of DNA. This is followed by a graphical representation of the DNA double helix itself, along with an estimate of the number of nucleotide base pairs in the human genome. Centered in the message is a simple stick-figure drawing of a human, flanked by a binary representation of average human height and the Earth's population in 1974—approximately 4.3 billion. The penultimate section contains a diagram of our solar system, with Earth displaced toward the human figure to indicate our origin. The final section is a schematic of the Arecibo radio telescope, accompanied by its diameter—305 meters.