A brief broadcast to the galaxy
On November 16, 1974, a group of scientists including Frank Drake and Carl Sagan gathered at the Arecibo Observatory in Puerto Rico to celebrate a major upgrade to its radio telescope. To demonstrate the new power of the instrument, they beamed a three-minute message towards Messier 13 (M13), a dense globular cluster of about 300,000 stars on the edge of the Milky Way galaxy. The broadcast was a technical demonstration than a serious attempt to contact aliens; M13 is roughly 25,000 light-years away, so a round-trip communication would take 50,000 years.
The message itself was a simple infographic, composed of 1,679 binary digits. This number was chosen because it is the product of two prime numbers, 23 and 73, suggesting the data could be arranged in a rectangular grid of 73 rows by 23 columns. The transmission, sent at a frequency of 2,380 MHz, had an effective power of about 20 gigawatts, making it detectable by a similarly sized telescope almost anywhere in the galaxy. The content, designed by Drake and his colleagues, included depictions of the numbers one to ten, the atomic numbers of important biological elements, the formulas for the components of DNA, a graphic of the double helix, a stick figure of a human, the 1974 Earth population (4.3 billion), a map of our solar system, and an image of the Arecibo telescope itself.