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.