Blaming the intakes: a forensic analysis of the Arecibo collapse

After nearly three years of the hasty, unplanned dismantling of the Arecibo radio telescope, We finally have the reason behind the crash: bad plugs.

In case you somehow missed it, in 2020 we started receiving ominous reports that the cables supporting the 900-ton instrument platform atop the 300-meter primary reflector of what was at the time the world’s largest radio telescope were slowly unraveling. From the first sign of trouble in August, when a broken cable bore a hole in the reflector, to the second cable failure in November, it seemed sure that Arecibo’s days were numbered, and that it would fall victim to all the others. Bad luck seemed to build up quickly in that fateful year. The inevitable finally happened on December 1st, when the overstressed cables on the fourth support tower finally collapsed, sending the platform into a graceful swing alongside the natural depression embracing the reflector, damaging the telescope beyond all hope of repair.

The long period leading up to the telescope’s final work had an upside in that it gave engineers and scientists an opportunity to carefully monitor failures in real time. So, there was no real mystery as to what happened, at least from a big picture perspective. But one always wants to know the nitty-gritty of such failures, a task that falls to the criminal investigation firm Thornton Tomasetti. They enlisted the help of Columbia University’s Strength of Materials Laboratory, which sent pieces of the faulty cable to Oak Ridge National Laboratory’s high-flux isotope reactor for surgery. Neutron imagingWhich is similar to the X-ray study but uses streams of neutrons that interact with matter’s nuclei rather than its electrons.

Full report (PDF) It reveals five direct causes of the collapse, the most important of which are “[T]“There was an inconsistent manual scattering of the wires while connecting the cables,” which we take to mean that the individual strands of the cables were not properly spread out before the molten zinc splitter socket formed around them. The resulting shear stress caused the zinc to slowly flow around the cable strands, allowing them to slip out of the surrounding steel socket – well, you can see the rest below for yourself.

As is often the case with such failures, there are multiple causes, all of which are covered in the 300-plus page report. But being able to blame the bulk of failure on a single, easily understood—and easily remedied—flaw is comforting in a way. Perhaps this is a cold consolation for the astronomers and Arecibo employees, but at least it is a lesson that might prevent cable-supported structures from failing in the future.