Doug Bonderud

Jan 27th 2021

Ready, SETI, Go! Analyzing the Arecibo Message


Is anyone out there? It’s a question that’s captured human interest since we first took to the stars — are we alone in the universe, or are there other intelligent lifeforms gazing up at the sky and wondering the same thing?

Instead of waiting around for aliens to get in touch, stellar scientists decided to make the first move with the Arecibo Message — a brief, focused effort at interstellar communication aimed at the M13 globular star cluster just over 20,000 light-years from Earth. While we haven’t heard back yet, the message remains a pivotal moment in the search for extraterrestrial intelligence (SETI) and has helped to inform the future of human space exploration.

The Universal Translator

On November 16th, 1974, Puerto Rico’s Arecibo Observatory turned its transmitter to the sky and beamed the most powerful and deliberate interstellar communication ever created into space. It took less than three minutes to complete the broadcast, which contained 1679 bits arranged into 73 lines with 23 characters per line. The 305-meter antenna produced transmission power equal to a 20 trillion watt multidirectional broadcast, but instead of sending information randomly out into the cold darkness of our universe, the team picked the globular star cluster M13 some 20,000 light-years away.

Of course, it’s not enough to just send a message; the Arecibo team also needed to craft a communication that civilizations with comparable technological capabilities of 1970s-era Earth could not only receive but understand. They settled on a pictorial postal package that contained crude images of the Arecibo telescope itself, our solar system, a stick-figure human and the familiar DNA helix. In addition, the total number of message bits, lines and bits per line were all purposely made prime numbers to help reduce the risk that receiving researchers would dismiss the signal as mere cosmic noise.

Express Delivery

With the capacity to send such a powerful message, why did scientists pick M13, also known as the Great Cluster in the constellation Hercules, instead of a system closer to home?

Both physical constraints and communicative potential played a role; not only would the constellation be located directly above the observatory at the time of the broadcast, but the cluster also contained at least 300,000 stars and potentially as many exoplanets, making it an ideal target.

As Nadia Drake — daughter of Frank Drake, who composed the Arecibo message — notes in a National Geographic article, the “heart” of M13 would also fit perfectly within the narrow beam produced by the transmitter, and even after the 20,000 year travel time, the cluster will remain firmly within the scope of the message’s delivery radius.

Cutting Through the Noise

While there’s some concern that potential extraterrestrial recipients might discover our message, track it back to Earth and arrive with less-than-noble intentions, the bigger worry is that we won’t hear anything at all — that we’re actually alone in the universe or that our message is simply missing the mark.

Of course, there was never a realistic chance that the Arecibo Message was going to return a reply in our immediate future. Even if aliens at the other end of the line detected the transmission, decoded the details and had a way to circumvent light speed to send a reply, it would still take at least 20,000 for this digital delivery to happen. Even worse? As the New York Times notes, more than 250 aluminum panels were damaged when a support cable fell and tore a 100-foot long gash in the Arecibo antenna dish. While there are plans to repair the dish, both total costs and timelines are uncertain.

According to Smithsonian Magazine, there might be a better option: building a SETI observatory on the moon. Not only would this take the burden off Arecibo as the sender and receiver of extra-solar contacts, it could also solve the bigger problem of radio noise. And, as the Smithsonian notes, not only does our atmosphere block a significant amount of the radio spectrum (great for life on Earth but not optimal for interstellar communication), but the rapid uptake of radio-based devices also makes an already loud receiving room that much louder. What happens if aliens intercept our message in-transit or discover one of the Voyager probes, then send a reply or reach out on their own — and we miss the message?

The moon offers a calm and quiet alternative. As astronomer Phillipe Zarka notes, “The far side of the moon during the lunar night is the most radio-quiet place in our local universe,” making it the ideal location for an orbiting SETI station and, eventually, a ground-based lunar observatory capable of listening intently for any universal replies.

Can You Hear Us Now?

The Arecibo Message represents humanity’s first foray into deliberate delivery of interstellar communication. While it’s unlikely that we’ll hear back anytime soon, the medium — not the message — is what really matters when it comes to making first contact. From increasing investment in ground-based telescopes to the potential for lunar observatories, the more planetary packages we dispatch, the better our chances of finding new interstellar neighbors.