What Was the Arecibo Message, and What Did It Say?

Background

By 1974, the Arecibo Observatory in Puerto Rico had completed a major upgrade: a new, more powerful planetary radar transmitter and a resurfaced 305-metre dish, the largest single-aperture radio telescope in the world at the time. To mark the occasion and demonstrate the upgraded equipment, staff astronomer Frank Drake, fresh from a decade and a half of SETI work beginning with 1960's Project Ozma, designed a symbolic interstellar broadcast, with contributions from Carl Sagan and other Arecibo personnel. On 16 November 1974, the observatory transmitted the message for under three minutes toward the M13 globular cluster, a dense group of several hundred thousand stars roughly 25,000 light-years from Earth, chosen for being a large, relatively nearby target within Arecibo's declination range rather than for any specific reason to expect a listener there.

The message itself is compact: 1,679 binary digits. Drake chose that number deliberately, since 1,679 factors only as 23 times 73, both prime numbers, meaning any recipient attempting to decode it as a two-dimensional image has only one sensible way to arrange the bits: a grid 73 columns wide and 23 rows tall. Read that way, the digits resolve into a simple, deliberately unambiguous picture built from a handful of recognisable elements.

What the Message Contained

Arranged as intended, the message's grid depicts, from top to bottom: the numbers one through ten in binary; the atomic numbers of hydrogen, carbon, nitrogen, oxygen, and phosphorus, the elements that make up DNA; the chemical formulas of the sugars and bases in a DNA nucleotide; a graphic double-helix structure of DNA itself with the approximate number of nucleotides in the human genome; a schematic human figure with its average height indicated and the contemporary 1974 human population of Earth; a diagram of the solar system highlighting Earth's position; and a schematic of the Arecibo radio telescope itself, including its dish diameter, to give any recipient a physical sense of the instrument that sent the message.

The content was deliberately chosen to be decodable using only mathematics and physics, universal enough, its designers reasoned, that any technological civilisation capable of receiving a directed radio transmission would also be capable of recognising prime-number factoring, atomic numbers, and basic geometric representation, without requiring any shared language or cultural context.

The METI Debate

The Arecibo message is the most famous example of what researchers now call METI, Messaging Extraterrestrial Intelligence, or "active SETI": deliberately transmitting toward space rather than only listening, the approach that gives SETI its name generally. Proponents, including Drake and Sagan themselves, argued the practice is a natural extension of listening-based SETI and a meaningful cultural gesture regardless of whether it is ever received, and that the specific risk from any single transmission is negligible: Earth has been broadcasting radio and television signals outward unintentionally for over a century, so a single deliberate, low-power, briefly aimed transmission adds little to what any sufficiently advanced civilisation could already detect if it were listening.

A separate, more cautious position, associated most prominently with physicist Stephen Hawking and formalised in later scientific debate over subsequent, more powerful METI proposals, holds that deliberately announcing Earth's location and technological capability to unknown recipients carries a risk, however small or difficult to quantify, that listening alone does not: an advanced civilisation capable of receiving an interstellar signal might not be benevolent, and the decision to transmit affects the whole species, not just the scientists sending it, without any mechanism for broader public or political consent.

Neither position has produced conclusive evidence for the other, since both ultimately depend on unknown and likely unknowable facts about whether receiving civilisations exist at all, and if so, what their capabilities and intentions would be. The debate is better understood as a live disagreement over acceptable risk under deep uncertainty than as a dispute with an evidentiary resolution, and it has directly shaped later practice: some subsequent proposed METI transmissions have been paused or reconsidered following public objections raised on these grounds, while others, particularly brief, largely symbolic gestures like Arecibo's, have proceeded with comparatively little controversy given the target's extreme distance and correspondingly negligible practical risk.

Current Consensus

There is no scientific dispute about what the Arecibo message contained, who designed it, or why it was sent: the historical record, including Drake and Sagan's own 1975 published account, is complete and uncontested. The genuinely open question is the normative one debated under "The METI Debate" above, whether humanity should continue deliberately transmitting toward space at all, which remains an active, unresolved disagreement within the SETI and METI research community rather than a settled finding.

The Arecibo telescope itself, having sent this and decades of other SETI-related observations, suffered cable failures and structural collapse in December 2020 and was not rebuilt, closing the chapter of SETI history the 1974 message opened. The Arecibo message is part of this site's search for extraterrestrial life coverage, within the broader space mysteries cluster.