In December of 2020, the Arecibo telescope - one of the largest and most iconic astronomical instruments in the world - collapsed. This 57-year-old megastructure not only made many incredible scientific discoveries over its lifetime, it was also an emblem of humanity’s interest and curiosity about our place in the universe. Its loss was felt across the world. The National Science Foundation, who owns the observatory, recently released their report to congress on the cause of the failure and the events leading up to it. Why was this telescope so important, how did it work, and why did it fail? I’m Grady, and this is Practical Engineering. Today, we’re discussing the Arecibo telescope collapse.

The same way we observe visible light from celestial objects using our eyes and optical telescopes, we can also take advantage of the other parts of the electromagnetic spectrum in astronomy. Most of the gamma rays, x-rays, ultraviolet, and infrared portions of the spectrum are blocked out by the atmosphere. But long-wavelength radio waves are not. A radio telescope is basically an antenna that can tune in to some frequencies of electromagnetic radiation that emanate from celestial objects. These radio waves can be quite faint, complicating the task of separating them from the background noise. You essentially have two options to get high-quality radio astronomy data: more time or more space. The longer you focus on an object, the more resolution you get. But, there’s only so much time. To speed up observations, you can also gather radio waves from a larger area and focus them into a clearer signal. Arecibo took that strategy to the extreme with it’s 305-meter (or 1,000-foot) diameter dish - the largest in the world until China’s half-kilometer FAST scope took the title in 2016.

Located on the Caribbean island of Puerto Rico, the Arecibo Observatory was designed and constructed in the 1950s and 60s as a department of defense project to detect nuclear warheads in the upper atmosphere. The National Science Foundation took over the facility in 1969 to use it for more peaceful endeavors, with help from a few managing partners over the years. A big part of Arecibo’s mission is education and outreach programs to engage the public’s interest in astronomy and atmospheric sciences. If you grew up in Puerto Rico, you almost certainly visited this incredible facility on a field trip or two or three. The most iconic part of the observatory was the massive radio telescope. Not only could it receive the faintest of radio signals, it could transmit them as well, allowing Arecibo to work as a celestial radar. It could send out radio signals and measure the returning echoes from nearby objects in space, including planets and asteroids. Arecibo facilitated some of the most exciting astronomical discoveries of our age, including the Nobel-prize winning observation of binary pulsars providing the first evidence of gravitational waves.

The telescope’s dish was constructed inside an enormous circular sinkhole. Although it looks solid from a distance, the reflector was a series of aluminum panels carefully suspended on steel cables. Because the dish was fixed to the earth, it was constrained to point at whatever part of the sky happened to be overhead. Radio telescopes can be used during the day and night - so there’s more sky to look at over the course of a day or year - but a telescope that can’t steer is still pretty useless. The designers of Arecibo had a pretty clever solution to the problem. Rather than using a parabolic shape for the dish that would focus everything to a single point, they chose a spherical curve. Spherical reflectors don’t perfectly focus all the incoming rays. That might sound like a bad thing since you want to gather and focus as much signal as possible across the entire dish. The beauty of a spherical reflector is that, by changing the position at which you measure the reflected waves above the dish, you’re measuring those waves from different parts of the sky. You can essentially steer the telescope by choosing where to receive radio waves above the dish, allowing you to focus on various objects and track them as the earth rotates.

Focusing those waves to a narrow area above the dish doesn’t do much good unless you have a receiver up there to collect and measure them. The Arecibo telescope was designed with a triangular platform suspended by steel cables above the dish to support the various instruments used to gather radio signals. To keep the platform aloft, three reinforced concrete towers, named for their positions on a clock of 4, 8, and 12, supported each group of cables. There were originally 4 cables for each corner of the platform, 3 inches (or 8 cm) in diameter. Big cables. Additional cables, called backstays, were connected to anchorages behind each tower to balance the horizontal forces, similar to the way suspension bridges work with their towers and abutment anchorages.

Initially the telescope used line feeds, elongated receivers that could gather signals within the focal line of the spherical dish. But, they could only measure signals within a narrow bandwidth, so line feeds would have to be swapped to change the frequency of the telescope. Upgrades in 1997 included the addition of the Gregorian dome that uses two additional reflectors to focus radio waves. This dome allowed telescope operators to observe a much wider range of radio frequencies. But this Gregorian dome didn’t just add capabilities. It also added weight - lots of weight - about 50 percent of the original platform. All of this extra load required some more support. So, two auxiliary cables from the platform to each tower were added, plus more backstays to balance the load. In addition to that, the dome was far more sensitive to tiny movements. You can imagine the stiffness and rigidity of a gigantic wind-catching dome suspended in the air by narrow steel cables - not an ideal structural arrangement for a sensitive instrument. To compensate, three tie-down cables were added, one for each corner of the platform, increasing the forces even further. A laser ranging system could communicate with hydraulic jacks to carefully adjust the tension in these tie-downs and keep the platform perfectly stable within the precision of a millimeter.

The telescope's last few years were pretty rough. The 2017 Atlantic hurricane season sent two massive storms - Irma and Maria - across Puerto Rico. Maria was one of the strongest storms ever to hit the island and caused nearly 3,000 fatalities and close to 100 billion dollars in damage. Arecibo wasn’t spared from that devastation. It suffered a broken line feed that fell from the instrument platform and crashed through the dish, among other damage. More consequential than hurricanes, though, Arecibo was slowly losing its funding. The National Science Foundation had been trying for years to divert Arecibo funds to newer projects. In 2018, the University of Central Florida stepped up to take over the management and funding of the observatory, not knowing what was soon to come.

Only a few years later, in August 2020, one of the newer auxiliary cables on Tower 4 broke free from its socket unexpectedly in the middle of the night. As the cable failed, it crashed through the reflector dish, tearing a gash through the aluminum panels. These sockets, called spelter sockets, used to attach the cables to the tower are a common way to terminate wire ropes and cables, but they have to be installed correctly. You have to broom the end of the cable, making sure that every strand is separated from the others, clean them meticulously, then carefully pour molten zinc into the socket to create a permanent wedge that only gets tighter with more tension. If done properly, the termination should be stronger than the cable itself. In other words, there’s no good reason a cable should ever pull out of a spelter socket. And yet, this was not the first incident of cables slipping in their sockets at Arecibo. Maintenance staff at the observatory had been concerned about the problem for years. This failure was the beginning of the end of the telescope, though we didn’t know it yet.

The damage from the failed cable was significant, but the engineers brought on to assess the structure believed it could be repaired. The suspended platform was designed with some redundancy, so losing a single cable wasn’t necessarily catastrophic. Managers put a temporary stop to the science at the facility while a remediation project could be installed. But first, it had to be designed. As a first step, engineers developed a structural computer model of the platform and towers to evaluate options for repair.

One nice aspect of cable-supported structures is that you can estimate the tension in each one just by looking at it. All cables sag under their own weight, following a curve called a catenary. The more tension in the cable, the tauter it becomes, and so the less it sags. If you know the weight of the cable, you can use the catenary equation with the measured sag distance to estimate cable force fairly accurately. A sag survey was conducted at Arecibo using lasers, and this is how the structural model was calibrated. To make sure the model could predict how changes in forces would affect the structure, the engineers performed some pretty clever validation as well. Since they had measurements of the instrument platform before and after the first cable failed, they could remove that cable in the model and compare the predicted behavior of the platform to what actually happened. When the first cable failed, that corner of the platform dropped by two-and-a-half feet or about a meter, and the model was able to predict this within a couple of inches.

While all this design was taking place, further trouble was just around the corner. In November, only 3 months after the first cable broke at Tower 4, a second one failed. This time it was one of the original cables installed in the 1960s. It didn’t pull from its socket but simply broke. And it broke at a force well below what it should have been able to handle (about 62% of its rated strength to be precise). The falling cable again damaged parts of the telescope, and again, the platform remained standing. However, optimism about the structure was quickly declining. The question went from, “how do we fix the telescope?” to “can we fix the telescope?” And there were differing opinions.

The engineers used the structural model to evaluate options that could relieve the tension in the remaining cables and reduce the possibility of a complete failure. They could cut the broken cables since they’re pretty heavy and not doing anything useful anymore. They could move the Gregorian dome so that the other towers were holding more of its weight. They could loosen the backstays, causing the towers to lean inward by 18 inches or half-a-meter. And of course, they can add some temporary cables at Tower 4 to take up some of the weight. All of these options showed reductions in the forces carried by the remaining cables, but the problem was figuring out how to do work safely. At this point, they had two failures, both well below the specified breaking strength of the cables. Not only had the telescope lost its structural redundancy, but the engineers also didn’t trust the strength of the remaining cables, and for good reason. Crews couldn’t access the site due to the risk of another cable failing, but nearly all the options to relieve the load on the cables would require having personnel on the platform and towers.

One of the engineering firms working on the problem suggested some last ditch efforts to save the structure. After that, they could perform proof tests remotely using the tie-down jacks to check if the remaining cables had at least 10% extra strength. If the engineers could gain some confidence in the strength of the remaining cables, crews might be able to enter the site and implement further measures to save the structure. However, no one could get comfortable with the risks of the helicopter work or proof testing the already distressed cables.

It is a tough thing to say, when such an important and iconic structure is still standing, that there’s no path forward to repair. This quote from the engineer tells the whole story. “It is unlikely any of these methods will yield sufficient reductions without placing crews in jeopardy...Although it saddens us to make this recommendation, we believe the structure should be demolished in a controlled way as soon as pragmatically possible.” They wouldn’t get the chance, though.

On the morning of December 1st, a third cable at Tower 4 broke, starting a chain of events that would quickly collapse the structure. Amazingly, one of the observatory staff was flying a drone at the top of the tower when it happened, capturing incredible footage of the event. It happens almost instantly. Two of the main cables are already clearly in distress when the video starts. All the chipped paint is from individual strands of the cable failing. Observatory staff could hear these breaks and knew what was likely imminent, which is why the drone was up there in the first place. The third cable snaps, and the remaining cables, forced to bear the additional load, quickly follow. The drone turns around to reveal the platform crashing into the side of the dish.

The observatory also had a Gopro set up in the control room that captured the failure. You can see the cables let go from Tower 4, the platform swinging downward, the support cables crashing through the suspended catwalk, and the top section of Tower 4 breaking off from the unbalanced force of the backstays. All three towers suffered failures, major portions of the dish were destroyed, and the platform and instruments it supported were a complete loss. Several buildings, including the visitor center, were damaged by falling debris. Thankfully, even though there were people on site during the collapse, the engineers had established safe zones away from the structure, and no one was injured.

Several forensic investigations are still underway to examine the causes of the failed cables. Those results could be years away, so we don’t yet know for certain why the first two cables gave way when they should have had more than enough strength to carry the load. Engineers involved during the event suggested the first cable to fail likely was not fabricated correctly. Whether the spelter sockets were installed in the field or a shop, there are a lot of details required to do it properly, and it’s certainly possible that something was missed. And once those sockets are installed, they are difficult to inspect. As for the second cable, the engineers suggested a likely failure mode to be corrosion of the steel. The cables were painted regularly and reportedly had a dehumidification system that could blow dry air between the strands (although those systems usually require an airtight sleeve around each cable). Even so, Arecibo sat nearly 60 years only a short distance from the northern coast of Puerto Rico, and exposure to that salty sea air could have accelerated the demise of the main cables.

Another element worthy of scrutiny is the factor of safety used in the original design. This is the quotient of a structure's demands and its strength. The whole point of a factor of safety is to accommodate uncertainty. We predict the demands on a structure. We compare them to the strength. We recognize that there might be extra forces or less strength than expected for a variety of reasons outside our control, so we give our structures some margin. The Arecibo suspended platform cables were designed to have a factor of safety of two, meaning they were twice as strong as the expected static loading from the platform.

That might seem like a lot, but consider that elevator cables use a factor of safety of 11, and many bridges use safety factors above 3. In aerospace engineering, where weight is critical, they do tons of modeling and testing to build enough confidence in designs to get their factors of safety down to around 1.5. Arecibo was a unique facility, unlike any other structure in the world. It didn’t go through a rigorous structural testing program. And, it was designed before computer modeling could be used to accurately characterize all the static and dynamic forces it could experience. I think it’s worth considering whether the structure should have been designed with a little extra margin, especially considering the possible circumstances in which that margin would be required. A cable failure is a violent event. All the load the failed cable was carrying doesn’t just redistribute itself to the other cables evenly and gently. The dynamic loads that occur as the structure shakes and vibrates can be significantly higher than the static loads. It wouldn’t be surprising at all to find out that the peak stress in the remaining cables on Tower 4 actually did exceed their rated strength when that first and second cable broke, even if only for an instant.

Despite the collapse, Arecibo Observatory is not closed and science continues at the other facilities on site. As of this writing, crews are still working to clean up the debris from the collapse, and the National Science Foundation is holding workshops to discuss the future of the site. I hope that eventually they can replace the telescope with an instrument as futuristic and forward-looking as the Arecibo telescope was when first conceived. It was an ambitious and inspiring structure, and we sure will miss it. Thank you, and let me know what you think!