For nearly six decades it has probed the heavens, watching for flashes of distant pulsars and the brief blasts of powerful fast radio bursts, tracked potentially hazardous asteroids, and listened for the faint signals of extraterrestrial civilizations. It even sent humanity’s first deliberate message for intelligent minds beyond Earth.

But to all great things, there must come an end. Arecibo has suffered catastrophic damage.

Following the failure in August and November 2020 of two of the cables holding the hanging instrument platform above the telescope’s bowl-shaped dish, the relative calm of the Puerto Rican jungle was shattered on the morning of December 1st, as the rest of the cables gave way and the entire platform fell crashing into the reflecting surface below.

Arecibo was ruined, and we haven’t just lost a great observatory, we’ve lost an iconic structure in the midst of mountains and jungle, a location from where we reached out to the Universe for the first time to say hello to whoever may be out there.

“There was nothing more inspiring than standing on the 900-ton instrument platform and surveying the massive dish below,” says Douglas Vakoch, who visited the telescope in the late 1990s as a SETI Institute scientist. “When that platform came crashing down, so did any hope of resurrecting the telescope.”

For many years Arecibo was unique, and until 2016 it was the largest single-dish telescope in the world. The first thing you would notice when driving to it are three towers, the tallest 111 meters high, rising out of the trees. The cables were slung between them, holding aloft the instrument platform over a huge bowl, 305 meters across and constructed from 38,000 aluminum panels, tucked deep in a natural depression in the mountains of Puerto Rico.

“It was a spectacularly visual thing, kind of magical, like seeing the Taj Mahal or something,” says Dan Werthimer, a radio astronomer from the University of California, Berkeley, when asked to try and sum up just what it was like to visit and experience such a remarkable observatory.

Werthimer focuses on building instrumentation, for both radio astronomy and its offshoot SETI, the search for extraterrestrial intelligence, and had visited Arecibo on many occasions.

“One of the wonderful things about Arecibo is that they encouraged experimentation,” he says. “Most observatories won’t let you touch anything, but at Arecibo, you could play around with their telescope, they let you tinker and play and occasionally break things."

One of those wild and crazy ideas came in 1977, from Berkeley scientists Stuart Bowyer, Mike Lampton, and Jack Welch. One of the perennial problems in SETI – well, in astronomy, really, given the shortage of telescopes – is getting time on telescopes to conduct observations. So they came up with the idea of ‘commensal SETI’, whereby a SETI instrument attached to a radio telescope would piggyback on regular astronomical observations. As radio astronomers looked at interesting objects and performed all-sky surveys, the SETI instrument would be there listening in the background for the chance to overhear an alien communication.

The first commensal SETI project, fittingly called SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations), was conducted at Hat Creek Observatory in California, but Werthimer, who was drawn to Berkeley for the chance to work in SETI, took SERENDIP to Arecibo in 1992. It has remained there ever since, continuing to collect data right up until that fateful August day when the first of the cables fell from the sky and gouged a hole in the dish, putting an end to any future observations. It was SERENDIP data that made up the bulk of observations analyzed by SETI@home.

The record-breaking volunteer computing project stopped sending new data to users in March 2020, but that was not the end of the project. The SETI@home team, including Werthimer, wanted to spend time instead of going through all the data accrued and analyzed by citizen scientists around the world, selecting the best candidates for follow-up observations.

“Not having Arecibo is actually going to be a problem because that is where we were going to check our best candidates,” says Werthimer.

Arecibo also came to SETI’s rescue in the 1990s when federal funding for the search was canceled. With private donations from benefactors such as Hewlett-Packard’s Barney Oliver, Project Phoenix arose from the ashes of NASA’s SETI program, and Arecibo was one of the observatories used to renew the search for ET.

“My first stay at Arecibo was as a SETI Institute scientist, taking part in Project Phoenix,” says Vakoch, who is now President of METI International, which is an organization devoted to beaming messages into space for alien life to detect. “For ten days twice a year, we had 24/7 use of the world’s largest radio telescope. It was breathtaking to imagine that each night could be the night we made first contact.”

Arecibo’s association with SETI hasn’t just been in the act of listening. In 1974 the father of modern SETI, Frank Drake, used the observatory’s transmitter to blast a radio signal towards the globular star cluster M13, in the constellation of Hercules. This famous message was both a publicity stunt and a proof-of-concept — the transmission was sent as part of the re-opening ceremony for the telescope after undergoing upgrades, but it also provided an opportunity to think about how to construct a message that could be received and understood by extraterrestrial life.

“That one message set the course of my life’s work,” says Vakoch. “This three-minute missive to the stars provides a snapshot of who we are, written in the language of math and science. The Arecibo message is a bold statement that we humans are moving out of our technological adolescence as a species, and we’re ready to contribute something of value to other worlds.”

The message was in the form of a pictogram, starting with the numbers 1 to 10 in binary code, and then using those numbers as the key to the rest of the message, which includes the binary numbers of the elements most important to life (hydrogen, carbon, oxygen, nitrogen, and phosphorus), depictions of organic molecules that build DNA and RNA, and a drawing of DNA’s double helix, before showing a stick figure of a human, the world’s population at the time (4.3 billion), the location of Earth and a crude representation of Arecibo itself.

Whether the strange message could actually be deciphered by extraterrestrials when it reaches its destination in about 22,000 years' time is immaterial. What it proved is that Arecibo — and humankind — had the ability to reach out to others in the Universe. And whether we agree that sending our own messages is safe and wise, or not, the loss of Arecibo means this ability is wrenched from our hands.

Vakoch agrees. “The collapse of the Arecibo telescope is a huge loss for METI,” he says. “I had hoped that one day we could use the Arecibo radar to send out more messages to alien worlds. Earth has lost its most powerful beacon to tell other civilizations out there that we want to start a cosmic conversation.”

Thanks to the Arecibo message, and the work of projects SERENDIP and Phoenix, Arecibo had become synonymous with SETI in the public’s consciousness. In some ways, this is doing Arecibo a disservice — while Arecibo played a large role in SETI’s history, SETI did not play a large part in Arecibo’s work, which was mostly spent conducting more mainstream astronomy and interplanetary radar observations.

The collapse is a hindrance, certainly, but SETI is in a much better position than it would have been six or seven years ago. In that time Breakthrough Listen has come onto the scene, spending $100 million over an initial ten-year period and making radio observations with the telescopes at Green Bank in West Virginia and Parkes in New South Wales, Australia. There’s also the Chinese Five-hundred meter Aperture Spherical Telescope (FAST), which usurped Arecibo’s record as the largest single-dish telescope in 2016, and which will also be conducting SETI experiments.

Things aren’t as simple as switching from Arecibo to FAST, however. There was an ‘open skies’ policy on Arecibo, meaning astronomers from any country in the world could submit a proposal to use it for observation. With FAST, the open skies time is greatly reduced in comparison, meaning that it will be difficult for astronomers outside of China to get time on the telescope.

Werthimer’s group at Berkeley has the advantage of a decades-long link up with Chinese astronomers, which has already gained them some exclusive time on FAST for SETI experiments, but the telescope is going to be so over-subscribed that the opportunities might not always be there in the future. Furthermore, Arecibo was unique in how it welcomed experimentation with instrumentation, paving the way for SERENDIP, whereas the scientists running FAST, as at most big observatories, are more circumspect about that sort of thing.

Perhaps the biggest damage caused by the loss of Arecibo is to how the public understands the search for ET. Given how strongly connected Arecibo is to SETI in people’s minds, the news of Arecibo’s destruction has led many to believe that it is also the end for SETI. Researchers must raise awareness of the fact that SETI is far from dead, and that other telescopes, including the largest in the world, still do SETI.

Although Arecibo has gone, its accomplishments and legacy will remain, and the search for alien life will continue.