The one deliberate high-power interstellar message humanity has transmitted: 1,679 bits broadcast from the Arecibo Observatory in Puerto Rico on 16 November 1974, at 2380 megahertz, during the ceremony marking the resurfacing of the dish. It was written by Frank Drake with Carl Sagan and others, ran for under three minutes, and was aimed at the globular cluster M13, roughly 25,000 light-years away.
What is in it
The bit count is the product of two primes, 23 and 73, so the only sensible way to lay it out is a grid of those dimensions, and the picture that results carries the numbers one to ten, the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus, the formulas for the sugars and bases of DNA, the number of nucleotides in the human genome and the shape of the double helix, a human figure with its height and the population of Earth, the solar system with the third planet displaced toward the figure, and the telescope itself with its dish diameter. The effective radiated power in the beam was of the order of twenty terawatts, which for those three minutes made the transmitter brighter than the sun at that frequency.
What it was for
It was a demonstration of the refurbished instrument rather than an attempt at contact, and its authors said as much at the time. M13 will not be where it was aimed when the signal arrives in about 25,000 years, and no reply is expected or possible inside fifty millennia. Its importance is that it happened at all: it is the reference point for every later argument about METI, and Martin Ryle, then Astronomer Royal, objected in print that announcing the planet’s position to an unknown listener was not a decision for one observatory to take.
Connections
SETI · METI · Drake Equation · Wow! Signal · Dark Forest Hypothesis · Kardashev Scale · The Three-Body Problem