Arecibo Message: A 25,000-Year Beacon to the Cosmos

Broadcast in 1974, the Arecibo message was aimed at the M13 globular cluster about 25,000 light-years away and encoded 1679 bits in a 23-by-73 grid to reveal numbers, DNA components, a DNA helix, a human figure, a solar-system map with Earth highlighted, and the transmitter dish. Designed as a demonstration rather than a dialogue, its earliest possible reply would not reach Earth for roughly 25,000 years, with any response arriving no sooner than about 52,000 CE.