The December 2020 collapse of the Arecibo telescope in Puerto Rico came at the end of a 39-month sequence that began with Hurricane Maria in 2017, according to a new report from the National Academies of Sciences, Engineering and medicine The committee behind the report found long-term zinc creep-induced failure in the telescope’s cable sockets as the main cause of the collapse. The mechanisms behind these failures, unprecedented in the more than a century that similar plugs have been used in other structures, can only be hypothesized, but the unique conditions of the radio telescope may have played a role.
Originally built between 1960 and 1963, the telescope featured a platform suspended nearly 500 feet above a 1,000-foot-diameter reflector or dish on three towers. Each tower was connected to the platform by four main cables 3 inches in diameter and 575 feet long. Five tin cables from each tower to a rear lock anchor offset the platform load on the towers. And in 1997, when additions to the platform increased its weight to 913 tons, the cable system was also updated with 12 auxiliary cables.
The committee identified Maria, which was a Category 4 storm at the time it hit the Arecibo telescope, as the start of the failure sequence, in part because inspections in 2003 and 2011 found no no measurable increase in zinc leading edge separation, while post-Maria inspections. in late 2018 and early 2019 they showed that cable slip had increased from 0.5 inches to over 1.5 inches at the auxiliary jacks at the ground end of a rear skid and 1.125 inches at the tower end of one of the main cables.
“The Arecibo telescope warned just after Maria that it was in structural difficulties due to increased pulling of the cable outlet,” the committee wrote in its report.
The first failure occurred on August 10, 2020, when the tower end of an auxiliary cable was pulled from its zinc-filled spelt socket despite being less than halfway down its design load and not even be the most loaded, according to the report. At the time, the cable had only been in service for 23 years, compared to about 57 years for the main cables.
Then one of the main cables from the same tower failed at the zinc-filled spelter dam on 6 November. Repairs were scheduled to begin a few days later, but a second cable was pulled, increasing the loads on the remaining three main cables. in the tower The National Science Foundation stated that safe repairs would not be possible and the telescope would have to be decommissioned.
Finally, on December 1, another of the three remaining cables failed, increasing the load on the last two above their rated resistance. The 913-tonne platform collapsed.
Unique place, unprecedented failure
Committee members note in the report that they could find no other recorded instance of a spelter dam failure. So what was different in this case?
Although the committee describes it as just a hypothesis, one theory points to the “plausible but unprovable response” that the telescope’s “uniquely powerful electromagnetic radiation environment”, in which the cables crossed the beam of a powerful transmitter of radio, could have contributed to the zinc socket creep.
Researchers have studied the electroplasticity of zinc and found that electric current flowing through zinc increases its creep rate. However, these laboratory experiments have only been done at much higher densities and for much shorter periods than the telescope socket zinc service. The committee found no data on the long-term electroplasticity of low-current zinc.
Therefore, the conditions of the telescope could explain why this failure occurred when no one like it had ever been seen, why the speed and pattern of wire pulling was not uniform, why a wire plug relatively young auxiliary failed first, as well as the general time of the cable breaks, according to the committee.
“The only hypothesis the committee could develop that provides a plausible but unprovable answer to all of these questions and the observed dam failure pattern is that zinc flow from the dam was unexpectedly accelerated in the radiation environment unique electromagnetic field of the Arecibo telescope,” the committee wrote.
