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Chapter 5: Sociology

Did We Transmit on the Wrong Frequency?

Dear Dr. SETI:
My question concerns why the "Arecibo Message" was transmitted at 2380 MHz. Everything I've read indicates that the "water hole" for listening to potential messages is at 1420 MHz. If we decide messages would be at 1420, why would be purposefully transmit at 2380? Was there a specific reason why this frequency was selected?

David Ritchey, James S. McDonnell Planetarium, Saint Louis

The Doctor Responds:
David, the short answer to your question is: that's the frequency on which the existing radar transmitter at Arecibo operated.

A longer answer would involve considering the true purpose of the Arecibo transmission. Let's put it in historical perspective. Think back to November 1974. The Apollo program had ended two years prior. US military involvement in VietNam had just wound down a year ago, and the fall of Saigon was five months ahead. The OPEC oil embargo had ended eight months earlier, but the US economy was still reeling from soaring energy costs. Against this backdrop, the Arecibo Observatory had just undergone a massive renovation, its half-inch galvanized steel mesh replaced with a precision surface to allow higher frequency operation at greater efficiency, at a not insignificant cost. The funding agencies (National Science Foundation and Associated Universities) wanted to garner some favorable publicity to encourage continued public support, so they staged a dedication ceremony. Central to that ceremony was a symbolic transmission to the stars, using the existing 2380 MHz high power radar transmitter actually designed not for interstellar communications at all, but rather for surface studies of the Moon and Venus. The transmission's target was the cluster of stars that just happened to be overhead at the time of the ceremony. Those stars being some 25,000 light years away, there was really no expectation that anyone at the receiving end would actually receive or decode the message (in fact, there was no good reason to believe anybody was home at the target stars at all). This was a publicity stunt, pure and simple.

So, if the Arecibo Message was transmitted on the wrong frequency, toward the wrong target too far away, exactly who was its intended audience? I believe the transmission was a powerful message to humans, about our place in the cosmos. And it's become a landmark in human history, widely portrayed in popular culture and much discussed by all, despite its limited scientific value. So, to that extent, the Arecibo Message transmission has been received, and was a resounding success.

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