A One-Off Conjunction of Events Toppled the Arecibo Telescope

Not many major scientific installations claim a starring role in a major Hollywood movie—but the Arecibo Telescope in Puerto Rico holds the distinction of having been in multiple blockbusters, from “Contact” to “Goldeneye” and “Species,” among others, which gave it a little extra fame beyond the realm of science.

But its location 19 kilometers south of the city of Arecibo had purpose. As the only major National Science Foundation (NSF) installation on Puerto Rico, it’s viewed as something of an island treasure, explained Roger L. McCarthy, founder and owner of McCarthy Engineering and treasurer of the National Academy of Engineering (NAE), who specializes in the analysis of mechanical designs and incidents, failures, and accidents involving issues related to mechanical, thermal, machine, architectural, and control design. 

“Puerto Rico isn’t that big, but to the extent there is a middle of nowhere in Puerto Rico, Arecibo is in the middle of nowhere. There was a reason for that. The whole purpose of a radio telescope is to receive faint radio waves,” McCarthy said. “So, the last thing you want is everybody’s local walkie-talkies influencing the reception on the telescope.” 

But that remoteness would prove to be something of a hindrance following the telescope’s unexpected collapse in 2020. “Unfortunately, to repurpose it for something else other than something that doesn’t require nowhere is hard,” he observed. 
 

A storied history 

When first built in 1963, the Arecibo Observatory’s 1,000-foot (305-meter) fixed spherical radio telescope was the largest such device in the world. Work began on the site in 1960, with crews transforming what was a naturally formed sinkhole into this scientific marvel. 

“This telescope, when it was—incredibly—designed and built back in the 1960s, it was just a breakthrough in terms of human ability to explore the universe and what was amazing is the Defense Department first dreamed it up to detect missiles,” McCarthy said. “They came up with some much better technology, so they turned it over to the scientists. And the scientists started looking at the brilliance of the spherical design. What that gave it is multiple focal points, so you could move an emitter or a receiver around. It wasn’t just what the dish was pointed at—it was basically a wide-angle lens that was incredibly versatile.” 

At the base of the observatory sat the 1,000-foot fixed spherical reflector dish, its surface covered by tower-mounted aluminum panels. Suspended above the dish by steel cables was a steerable radio antenna that could be pointed in various directions.  

The goal of building the observatory was to make studying the highest portions of Earth’s atmosphere (the ionosphere) and beyond possible, according to Daniel Altschuler, who served as the observatory’s director for 12 years until 2003, in a tribute to the observatory as part of ASME’s Landmark Award Ceremony in November 2001. 

“The Arecibo Ionospheric Observatory (AIO) as it was then known, was inaugurated on the first of November of 1963,” Altschuler wrote. “On April 7, 1964, the first radar contact with Mercury was achieved. [...] The telescope has been used to study the properties of thousands of distant galaxies, helping to map the geography of our universe.”  

In 1971, the facility was renamed the Arecibo National Astronomy and Ionospheric Center. 

The telescope could detect frequencies from 50 megahertz to 11 gigahertz, allowing scientists to detect incredibly weak radio signals from stellar bodies such as pulsars and galaxies. But it wasn’t just a receiver. Arecibo was equipped with transmitters, including an S-band (2,380 megahertz) radar system for planetary studies and a 430-megahertz radar system for ionospheric research. Scientists famously used Arecibo’s transmitters to send a radio message with some basics about humanity and Earth to a globular star cluster 25,000 light years away, Messier 13, in 1974. 

The observatory was the site of many landmark discoveries, according to the NSF, including the first binary pulsar, discovered by Russell Hulse and James Taylor in 1974, for which they were awarded the Nobel Prize in 1993. It was at Arecibo that scientists learned that Mercury spins on its axis once every 59 days (not 88 days as previously thought) in 1965 and realized that the planet was host to polar ice in 1994. In 1981, researchers were able to generate the first radar maps of the surface of Venus and in 1992, the telescope revealed the first ever exoplanet, a pulsar called PSR 1257+12. 
 

An engineering marvel 

In addition to being named an ASME Mechanical Engineering Landmark, Arecibo’s radiotelescope was also dubbed an IEEE Milestone in Electrical Engineering and Computing. On the electrical engineering side, the telescope’s achievements led to new developments in antenna design, signal processing, and electronic instrumentation. And for mechanical engineering, it set the stage for advances in antenna suspensions and drive systems. 

Arecibo’s drive system positions all active parts of the antenna with millimeter precision, with zero impacts due to temperature change (such as thermal expansion). This gave the telescope its ability to maintain focus. The spherical antenna sat suspended 150 meters above the fixed reflector dish and thanks to the drive system, the focusing device could be steered anywhere over of the dish. As a result, the observatory could receive signals from a wider portion of the sky—day or night, since of course, it captured radio waves, not light waves. 

“It’s an incredible story of success begets success and as people used this more, they thought of more and better ways to use it,” McCarthy added. “Scientists started adding more and more power and they could see more and more things and it became more and more useful. Eventually it ended up being the most powerful radio transmitter in the world.” 

During its nearly 60 years of service, the Arecibo telescope saw two major upgrades. The first in the 1970s to resurface the dish reflector and add the S-ban radar transmitter. The second occurred in the 1990s, when Cornell University’s National Astronomy and Ionosphere Centre (NAIC) added a Gregorian reflector system (suspended 450 feet above the reflector dish inside a 90-ton dome) for focusing radio signals, a more powerful 1-million-watt radar transmitter, and a 50-foot-high steel mesh ground screen to reduce ground interference. 

“This was such a powerful tool that astronomers were sitting around thinking, ‘Well, if the other guys had an Arecibo, how far away could it be and we’d still figure out they were looking at us with an Arecibo?’ The answer, by the way, is 791 light years,” McCarthy said. “So, if aliens had an Arecibo and they were pumping out radio waves with as much power as Arecibo pumped out, we would detect them 791 light years away, which is an incredible distance when you think about it.” 

Arecibo’s record as the world’s largest radio telescope lasted until 2016, with the completion of the Five-hundred-meter Aperture Spherical radio Telescope (FAST)—or 1,640 feet—in Guizhou Province, China. But the FAST telescope may soon be dethroned as well. In 2022, construction began after 30 years of planning on the Square Kilometre Array (SKA), which is set to be the world’s largest radio-astronomy observatory and will span sites across Australia and Africa. These two giant telescopes, or interferometers, will bring together hundreds of antennas to detect the radio signals between 50 megahertz and 15 gigahertz when completed in 2029. 
 

Uncovering the unexpected 

Given the observatory’s status as a local landmark, Puerto Rican caretakers took great pride in caring for the facility. 

“The staff of local people were terribly dedicated to the mission of that telescope,” McCarthy recalled. “I started engineering school in the 1960s, and when I walked into this control room, it was like deja vu—here was all this stuff from when I was a freshman. It’s remarkable how well they had maintained it. They worked so hard to keep it pristine.” 

But despite that care and diligence, the telescope collapsed on Dec. 1, 2020, and it would take four years of analysis to uncover the reasons why. 

During the telescope’s second upgrade, crews doubled the weight of the central receiver, so it was nearly 1,000 tons by the time of the collapse. But they also added more cables to help accommodate the extra weight. 

“These cables were three inches in diameter. The wire strands in these cables are quarter-inch steel rods,” McCarthy said. “They added six more of these because they wanted to add the Gregorian Dome to the center so they could collect a wider range of wavelengths, a much wider frequency response.” 

But those cables, which stretched anywhere from 500 to 700 feet, ran right through the telescope’s powerful radar beam. 

“In all of recorded history, no one gave any thought to stretching a cable across the radar. Never caused any problems. But these cables were socketed in spelter sockets, which are really very clever, invented 120 years ago. The trick is you want to grab a cable in a way that introduces no stress concentration,” explained McCarthy, who chaired the NAE committee on the “Analysis of Causes of Failure and Collapse of the 305-Meter Telescope at the Arecibo Observatory,” which published its findings in 2024, four years after the collapse. 

The way to avoid stress concentration is by making the end connection a loop with a socket, he explained. That socket is conical, so when the cable is fed through into the cone, the wires are spread out in a process called brooming, then molten zinc is poured over top to coat every wire, much like solder. 

“So, every wire gets coated on every surface, and then when you pull on the cable, the conical socket squeezes the zinc and it gets a grip on the cable—but there’s no stress concentration,” McCarthy said. “The end connection develops 100 percent of the strength of the parent cable because it doesn’t see a stress concentration. It’s very difficult for an end connection to develop 100 percent of the strength of the member, but a spelter socket does it brilliantly.” 

That was the reason spelter sockets were chosen for this design, as they worked so well. By the time the second upgrade took place, the existing cables had been in place for around 30 years with zero problems. When the super radar was added in the 1990s, slightly different sockets were purchased for the upgraded cables, but after the new additions, Arecibo had a total of 78 zinc-filled spelter sockets. 



“On the original sockets, if current was generated in the cable, it actually had several paths to ground, so not all the current went through the zinc in the original cable sockets,” McCarthy explained. “But in the auxiliary, the new ones added in the 1990s, all the current in the cable had to go through zinc to get to ground.” 

That proved to be the telescope’s undoing. After 23 years of having currents travel through the newer cables, the zinc began to creep. One of the newest sockets pulled out—even though it was only at 44 percent load, McCarthy noted. 

“No one ever thought about the electromagnetic radiation problem. Maybe a bad socket, so they were going to just replace the cable and move on. It was only when a second cable pulled out that they said, ‘Houston, we've got a problem.’ Then of course, when the third cable pulled out, it collapsed,” he said. “It had redundancy, but every time a cable pulls out, all the remaining cables get more load. They were all weakened by the electromagnetic current, but the newest sockets were most weakened because all of the current through the new socket design had to go through zinc so that zinc crept faster. And that’s what led to the collapse.” 

Remarkably, the Arecibo collapse marked the first ever recorded spelter socket failure in history, McCarthy pointed out. 

This answer was a long time coming. At first, the NSF hired NASA to figure out why the telescope fell, then Thornton Tomasetti, then Wiss, Janney, Elstner, and when these teams were unable to find the answer, finally the NSF went to the NAE.

“We sat there and looked at it for two years saying, why did this thing come down? One of our committee members suggested early on the issue of electromagnetic radiation, but we all looked at him like are you crazy? Electromagnetic radiation does not bring down civil structures,” McCarthy recalled. “It took us two years to finally look at the pattern of the electrical grounds of the cable, and when you start looking at the electrical grounding of the cables, it explains everything: explains the patterns of the pullouts, explains the failure, everything.” 
 

Leaving a legacy

In October 2022, the NSF issued a solicitation for a new multidisciplinary, world-class educational center at the Arecibo Observatory to serve as a hub for STEM education and outreach, dubbed the NSF Arecibo Center for Culturally Relevant and Inclusive Science Education, Computational Skills, and Community Engagement (NSF Arecibo C3). The center opened to coincide with NSF’s 75th anniversary in 2025. 

But the effort did not include funding to rebuild the 305-meter telescope or operational support for current scientific infrastructure, such as the 12-meter radio telescope or Lidar facility. Today, the site is still host to the damaged dish, but it’s unlikely that it will ever be rebuilt. In fact, the NSF had already started phasing out the Arecibo’s use in 2011 because of its outdated technology. 

“It was a remarkable tool and ultimately, its versatility led to its downfall because of course, no one had ever thought about radio waves taking down a civil structure before. In fact, to my knowledge, this is the only structure that’s ever failed from electromagnetic radiation,” McCarthy said. “When we finally figured out what really happened, we all sat there wondering what revision needs to be made. But as long as you don’t stretch cables across the most powerful radio transmitter in the world, you’ve got no problem. It was just an incredible one-off conjunction of events and it took us four years to figure it out.” 

Louise Poirier is senior editor.