Ode to Arecibo

This month marks the 4^th anniversary of the epic, catastrophic failure of the Arecibo Radio Observatory. It was already marked for decommissioning before gravity decided to speed things up, but the collapse was awesome in the most literal sense of the word: you could do nothing but let your jaw drop when you saw the footage.

Up until 2016 Arecibo was the largest radio telescope in the world. Its dramatic setting, nestled into a sinkhole surrounded by green jungle in Puerto Rico, earned it screen time in several major Hollywood films, most famously “GoldenEye” and “Contact”. Its downfall was epic and represented a major loss the world’s radio astronomy community.

In honor of its memory and legacy, let’s dive into Arecibo’s history, its importance to astronomy, and the dramatic tale of its ending. 

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Astronomy as a Side Benefit

Despite its reputation as one of the world’s premier radio astronomy facilities, that’s not what Arecibo Observatory was originally designed for. The time was the late 1950s, when the Cold War was in full swing, and the Advanced Research Project Agency was hoping to figure out a way to detect incoming missiles in the ionosphere.

In order to do this, a better understanding of the ionosphere was required, and in 1959 Cornell University submitted plans for a massive ionosphere radar probe for just this purpose (the plans did note that this probe could double as a radio observatory for astronomy as a sort of bonus feature).

The project was approved, with Puerto Rico selected as the site both thanks to its low latitude (which provides access to both northern and southern skies) and the massive sinkholes that can be found in its northwestern quadrant which could provide a nest for the proposed giant dish. Construction stretched over three years, and the Arecibo Ionospheric Observatory (as it was then called) officially opened on November 1, 1963.

One Big Dish

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The finished product was quite the structure. The dish of the telescope was a full 1,000 ft (305 m) across. You might be wondering just how such a massive thing could be pointed. After all, radio telescopes (if you’ve never seen one) are usually steerable dishes designed to collect radio waves and focus them into a point just in front of the dish, where a receiver or secondary reflector is held in place over the dish itself by a tripod. The dish is then rotated to face whatever target the operator wants. 

That, of course, is not how Arecibo was designed. For one thing, the dish absolutely could not move—it was stuck in the ground. For another, it was determined that building a gigantic tripod over it to hold the receiver in place really was not going to work.

Instead, three huge towers were built at the dish’s edge. A triangular platform was suspended from these towers by huge bundles of cables (remember this point. It’s going to come back later). The actual receiver for the dish hung under this triangular platform on a track that allowed it to move around. This was how the telescope was “pointed”, not by moving the dish but by moving the receiver.

Listening to the Skies

Arecibo didn’t retain its military job for very long. By 1969 the Department of Defense had transferred ownership to the National Science Foundation, and it became a full-time science observatory, a status it held right up until the end. 

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As the world’s biggest radio eye (or, well, ear maybe is more accurate) on the sky for several decades, Arecibo was uniquely situated to provide unprecedented obasrvations of the cosmos. Within our own solar system it was Arecibo’s that first determined how quickly Mercury spins. The first look through the clouds of Venus came not from an orbiting spacecraft but from Arecibo. And many an asteroid has had its surface radar-mapped by the big dish in Puerto Rico.

Arecibo was not limited to the solar system, however. As a radio telescope it was very well suited for studying Fast Radio Bursts, a still mysterious type of event in which a huge amount of energy (think three days’ worth of energy from the Sun) bursts out in milliseconds. It was also very good at observing pulsars, the rapidly spinning remnants of massive stars. Nobody knew for sure if that’s what a pulsar was until Arecibo turned its ear on the one that sits at the heart of the Crab Nebula in 1968 and confirmed it.

It was actually Arecibo’s expertise at observing pulsars that led to one of the biggest discoveries in modern astronomy. In 1990, while being repaired, Arecibo was left unable to point. That was fine for doing pulsar surveys however, as the dish could just listen to the sky as it rotated overhead, without having to point. In doing so it found fluctuations in the spin time of the pulsar PSR B1257+12.

That’s not supposed to happen. In 1992 the source of the fluctuation was announced: no less than three planets were orbiting this pulsar, tugging on it slightly with their gravity and causing the blips in spin time. These were the first worlds ever found outside of our own solar system, the first proof we had that planets existed elsewhere. It was a Big Deal.

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Anybody Listening?

Being a gigantic radio dish, Arecibo was capable of transmitting as well as receiving. As a result it had a key role in the history of humanity’s search for life beyond our planet. In 1974 it sent a radio message out to the stars, the first such message ever sent specifically in case someone out there might be listening. 

Known as the Arecibo Message, the signal sent out included information about DNA, a very basic depiction of a human being, an extremely rudimentary diagram of the solar system, and a vague drawing of Arecibo itself. Other messages have been sent to the stars since then, but Arecibo’s was the first.

The Downfall

Despite its tremendous contributions to science, as time went on Arecibo began to fall into disrepair. The massive facility was aging and expensive to maintain, and in 2016 the National Science Foundation began considering options for decommissioning the telescope.

Then in 2017 Hurricane Maria hit Puerto Rico like a hammer. In the midst of the devastation that this storm rained down upon the island, the damage done to the telescope was overlooked, but it is suspected that this was when the cables connecting the suspended receiver platform to the three towers around the dish began to slip.

Certainly by 2019 images show that multiple cables had begun to slip out of the sockets holding them in place on the towers. In August 2020 an auxiliary cable escaped its socket completely and fell to the dish, breaking its surface. In November 2020, a major cable followed, increasing the strain on the remaining cables. The telescope was judged too unstable to allow humans on it anymore. In late November it was determined that a rapid decommissioning was needed to make sure the telescope got taken apart safely.

That never got the chance to happen. At around 8 am on December 1, 2020, one more cable slipped. This was the final straw. Cable after cable broke away from the affected support tower and the suspended platform began to fall. It took the tops of the two remaining towers with it as the entire structure fell onto the dish below. 

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After 57 years of service, with its dish shattered, its towers broken, and its receiver nothing more than a flattened pile of rubble, Arecibo Telescope came to an abrupt and violent end.

The Legacy

Of course, no telescope’s legacy ends with the loss of its physical structure, not so long as the data it collected exists. Discoveries are still being made today using archived Arecibo data. The small asteroid 4337 Arecibo, named in honor of the observatory’s many contributions to asteroid mapping, still orbits in the outer asteroid belt. And of course the Arecibo Message, humanity’s first deliberate communication to the stars, is still out there, spreading farther and farther into the cosmos with each passing day.

And you know what? As legacies go, that’s a pretty solid one to leave behind.