The main platform of the Arecibo Observatory crashed into the dish below on December 1, 2020. Credit: Michelle Negron, National Science Foundation
The Arecibo Observatory in Puerto Rico was brought down by the combination of a hurricane, an equipment failure never before seen in the annals of engineering, and an “alarming” lack of concern from engineers and inspectors when wires had been seen years earlier crucial support devices slip out of their sockets. the main platform of the famous radio telescope collapsed in 2020, says a report published on October 25 by the National Academies of Sciences, Engineering, and Medicine (NASEM).
The publication is the most comprehensive analysis yet of the legendary observatory’s downfall, which occurred over several months in late 2020, when several cables holding the telescope’s main platform above its 305-meter (1,000-foot) wide dish failed.
The loss of Arecibo was a devastating blow to astronomers, especially those in Puerto Rico. But it was also, from an engineering perspective, “one of the most staggering failures of the modern era,” the report said. And while damage from winds from Hurricane Maria in 2017 triggered the sequence of events that led to the collapse, the outlets may also have been weakened by current induced by Arecibo’s powerful radio transmitter, the report suggests.
The report also notes that cables that ultimately failed were seen slipping out of sockets in 2018 and 2019, but contracted consultants did not deem them a threat. The slips were not even taken into account in post-Maria assessments of the structure’s structural integrity.
An anomaly like the one seen at the Arecibo holds “should raise all kinds of alarms,” says committee member Habib Tabatabai, a structural cable expert at the University of Wisconsin-Milwaukee. Tabatabai tells Astronomy: “I’ll give this analogy: A patient goes to a doctor and his eyes are protruding 1 inch. The doctor looks at it and he’s never seen anything like it before, and he looks up his medical books and there’s no mention of this in their books. So this anomaly doesn’t have to mean it’s okay.”
Missed signals
The first cable failure that led to the collapse occurred on August 10, 2020. This cable was an auxiliary cable that supported the telescope platform, and at the time observatory officials were optimistic that it could be repaired. However, this was followed by a main cable failure on November 6, 2020. This led the telescope’s owner, the National Science Foundation (NSF), to conclude that the telescope could no longer be safely repaired and had to be dismantled. But before work could begin, another auxiliary cable failed, causing the 900-ton receiving platform to immediately crash into the dish below on December 1, 2020.
The cables were held in place by spelter sockets, a common type of cable socket that can be welded by filling it with molten zinc and allowing it to harden. But according to the report, these outlets began to fail after Category 4 Hurricane Maria hit the observatory in 2017 with winds of up to 120 mph. “Gradually the zinc lost its grip on the cables and allowed many of them to fall off,” the report states.
“In Maria’s absence, the committee believes the telescope would still be standing today, but may have ultimately collapsed if its unique, accelerated zinc creep had not been addressed before it was dismantled,” the report states.
Warning signs were also missing, the report said. In 2019, Arecibo staff recorded that key cables had begun to slip out of sockets by up to 1.5 inches. But neither the staff nor the contracted structural engineers seemed to recognize the threat this posed to the entire structure.
The slip “should have raised the alarm level higher, requiring urgent action,” the report said. “The lack of documented concern from contract engineers regarding the insignificance of cable pulls or safety factors between Hurricane Maria in 2017 and the failure is alarming.”
If cable slip had been identified as a problem well before the first cable broke, it would have been safe to access the tops of the towers to install devices to relieve the load on the outlets, Tabatabai says.
Poorly directed repairs
NSF reported to the NASEM committee that plug slip was never brought to the agency’s attention as a problem for consideration for further inspection, analysis, or repair.
According to the report, Ray Lugo, who directed the Florida Space Institute at the University of Central Florida in Orlando – the operator of the observatory at the time of the collapse – told the committee: “I used wire ropes and cable lifting devices with notches, and in my experience as an engineer, I’ve never seen a cable, a receptacle cable, with that amount of pull-through.” But, he added, “I was told that it had been reviewed by an outside engineer and that there were no real concerns about the cable failure.”
The report does not identify the outside engineer.
In the aftermath of Hurricane Maria, planned repairs to the telescope ran into bureaucratic delays and funding was delayed. But because inspectors had failed to identify cable slip as the most pressing problem, “none of the proposed repairs would have saved the Arecibo Telescope from collapse,” the report concludes. Instead, repairs focused on one of the telescope’s remaining main cables, which ultimately never failed.
The engineering firm WSP was to supervise the repairs and perform the structural analysis. According to the NASEM report, NSF representatives told the committee in 2022 that “NSF was informed that WSP structural engineers visited the site in February 2020 to work on spliced main cable replacement and performed inspection of the towers , cables and primary platform structural elements while there, but no additional damage was detected during those inspections.
NSF also said it had no evidence that the WSP looked specifically at socket extractions.
The report states that the committee was “not afforded the opportunity to question the WSP”. Tabatabai says the committee was unable to coordinate a discussion because the WSP was no longer under contract with the Arecibo Observatory.
At the time of publication, WSP did not respond to a request for comment.
Unprecedented behavior
The collapse is the first documented case of zinc creep resulting in cables detaching from such outlets in more than a century of use, the report said. The committee says it doesn’t know and can’t prove what caused the extraordinary creep rate of Arecibo’s zinc. But the committee ruled out some obvious candidates, such as steel fatigue or corrosion caused by the humid climate and salty sea air, finding no evidence that they contributed to the collapse. Preventative measures taken by Arecibo to maintain intakes have proven effective in this respect, the report said.
The only plausible hypothesis the committee could think of is that electromagnetic radiation from Arecibo’s radio transmitter – for decades the most powerful on Earth – may have accelerated the failure. When subjected to an electric current, metals and ceramics can deform, a phenomenon called electroplasticity. This is used in some manufacturing processes where high levels of current are applied to the zinc, says Tabatabai. In the case of Arecibo, induced current levels at outlets would have been several orders of magnitude lower. But over decades of use, the zinc securing the cables may have warped, allowing the cables to slip.
In other words, every radio signal Arecibo has ever sent – every radar beam bounced off Mercury, Venus, or near-Earth asteroids, and even the famous interstellar radio message sent to the globular cluster M13 in an attempt to communicate with intelligent alien life. – may have cumulatively contributed to the demise of the telescope.
Tabatabai says the unique nature of the collapse means it’s important for engineers to learn from the collapse. Tabatabai directs the Structural Engineering Laboratory at UW-Milwaukee and plans to launch small-scale experiments to verify the process. If experiments could demonstrate that low current levels can accelerate the flow of zinc, it would not only improve understanding of the material’s limitations, but “open up opportunities to exploit it for potentially beneficial uses as well.”
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