The collapse of the Arecibo telescope in Puerto Rico in December 2020 came at the end of a 39-month streak 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 that long-term creep-induced failure of zinc in the telescope’s cable sockets was the root cause of the collapse. The mechanisms behind these failures, unprecedented in over a century of similar babbit sockets being used in other structures, can only be speculated upon, but the unique conditions within of the radio telescope may have played a role.

Originally built from 1960 to 1963, the telescope featured a platform suspended nearly 500 feet above a 1,000-foot-diameter reflector or dish from three towers. Each tower was connected to the platform by four main cables 3 inches in diameter and 575 feet long. Five backstay cables connecting each tower to a backstay anchor counterbalanced the platform load on the towers. And in 1997, when additions to the platform increased its weight to 913 tonnes, the cable system was also upgraded 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 increases measurable lead edge separation from the zinc, while post-Maria inspections in late 2018 and early 2019, cable slippage had increased from 0.5″ to over 1.5″ on the auxiliary jacks at the end to the ground of a backstay and 1.125 in. to the end of the tower of one of the main cables.

“The Arecibo Telescope gave a warning just after Maria that it was in structural difficulty due to the increasing removal of cable sockets,” the committee wrote in its report.

The first outage occurred on August 10, 2020, when the tower end of an auxiliary cable pulled out of its zinc-filled babbit socket while at less than half its rated load and that it was not even the most loaded, according to the report. The cable had only been in service for 23 years at the time, compared to around 57 years for the main cables.

Then, one of the main cables of the same tower failed at the zinc-filled babbit socket on November 6. Repairs were scheduled to begin a few days later, but a second cable pulled out, increasing loads on the remaining three main cables. on the tower. The National Science Foundation said 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 strength. The 913-tonne platform collapsed.

Single site, unprecedented failure

Committee members note in the report that they were unable to find another recorded example of a babbit socket failure. So what was different in this case?

While the committee describes it as a mere hypothesis, one theory points to the “plausible but unprovable answer” that the telescope’s “particularly strong electromagnetic radiation environment”, in which cables passed through the beam of a powerful transmitter radio, could have contributed to the appearance of the phenomenon. sleeve zinc creep.

Researchers studied the electroplasticity of zinc and found that electric current flowing through zinc increases its creep rate. However, these laboratory experiments were only carried out at much higher densities and for much shorter periods than the zinc service of the telescope mount. The committee found no data on the long-term, low-current electroplasticity of zinc.

Thus, telescope conditions could potentially explain why this failure occurred when a similar failure has never been observed before, why the rate and pattern of cable pullout was not uniform, why a plug relatively young auxiliary cable failed first, as well as the overall timing of cable tearing. the cable wire breaks, according to the committee.

“The only hypothesis that the committee was able to develop that provides a plausible but not demonstrable answer to all of these questions and the observed pattern of socket failure is that the creep of the zinc from the socket was unexpectedly accelerated in the environment particularly powerful electromagnetic radiation from the Arecibo telescope,” the committee wrote.