Well, maybe.
Today, not a paper but an engineering report. Eh ? This is obviously not my speciality at all, in any way shape or form. In fact reading this only revealed to me even further the tremendous depths of my own ignorance regarding materials science and engineering practises. The former is something I never cared for at undergraduate level and the latter is something about which I know literally nothing. Naturally, I wouldn't normally even glance at a report like this, except that it's about a topic that's personally important to me : why Arecibo collapsed.
There's an okay-but-short press release version here. It's interesting to see the extent of the deconstruction at the site, which was already well advanced in 2021; I couldn't find a more recent photo. Otherwise the Gizmodo version is the 30-second read and not much else. For this post I read most of the full 113 page report, which really is "jaw dropping", at least in parts, as Gizmodo described it. Unsurprisingly there are fairly hefty tracts where my eyes glazed over, but there's still plenty in here that's accessible and understandable to non-engineers like me.
In a nutshell, Arecibo collapsed due to a combination of factors, two of which are predictable enough but the third is something nobody expected. The first two are inadequate maintenance and the impact of hurricane Maria. But it's important not to oversimplify, as these are intimately bound with the third : the effects of the radar transmitter. This is not quite a case where one can simply say, "if they'd just done their jobs properly then it'd still be standing today", thought the report does contain some damming stuff.
Going through this linearly would end up being a shorter version of the report, which wouldn't really help anyone. If you want that level of detail you should go through it yourself; it's thorough to the point of going back to hand-written notes from the earliest days of the telescope. I have to say, though, that it's also highly repetitive in parts and in my view somewhat self-contradictory in places – but as it says, this is a preprint and still subject to editorial revision. Anyway, rather than doing a blow-by-blow breakdown, let me extract some broader lessons here.
Safety is not the same as redundancy
Probably the most general lesson is, I presume, obvious to anyone with an engineering background. But to an outsider me the distinction between safety and redundancy was interesting just because it makes a lot of intuitive sense but I'd never heard of it before. Safety, apparently, refers to the breaking point of any particular element. For example a cable with a safety factor of two could support twice as much as its current load before it would snap. Redundancy, on the other hand, is about how many elements could fail before the whole structure would come crashing down. Arecibo's three towers, they say, don't provide redundancy because a single failure would inevitably mean a total collapse (compare with the six of FAST).
Of course it's very unlikely that even a single tower would ever fail because their safety factor was massive, so redundancy there was unnecessary (at least regarding any failure of the concrete towers themselves). The same can't be said for the metal cables, where the safety factor generally seems to have been about a factor two or a bit less, in accordance with standard design practises – still plenty, but with a need for redundancy just in case. The report stops short of saying that there were any actual design flaws in the telescope, but does not that it obviously would have been better if there had been more towers. Safety factors, they say, were not the issue, although I think I detect some inconsistency here. Where they do issue an outright criticism, for example, is that while the original cable system had redundancy, this was no longer true with the 1997 upgrade that added the 900-tonne Gregorian dome and altered the cable system. Which is a little bit in contradiction to their claims that the telescope didn't suffer from design faults. It's a bit muddled.
Poor maintenance contributed to but did not cause the collapse
At least this is the overall gist I got. There's plenty of criticism levelled here, but it's hard to disentangle how serious the maintenance problems really were. As I read it, a more diligent maintenance program probably could have prevented the collapse, but this is partly with the benefit of hindsight – the failures which occurred were unprecedented (see below) but should have been spotted all the same. Of particular concern is that there wasn't enough knowledge transfer during the telescope's two changes of management (I'll speak from first-hand experience in declaring that management changes should be avoided for a host of other reasons; I went through one such change at Arecibo and God knows what the staff must have felt like when a second took place only a few years later). In addition, and probably worst of all, is that the post-Hurricane Maria repair efforts were both much too slow, taking months to even get started and would have lasted for years, and targeting a cable which never failed. Major repairs needed to happen far sooner but there was also a need to identify the failures more accurately.
The failures were of the cable sockets rather than the cables themselves. In these "spelter sockets", there's normally some degree of cable pullout after construction is complete and the structure assumes its full load : these sockets are widely used so this is known to be absolutely normal and no cause for concern. But the report is somewhat ambiguous as to whether the extra pullout which happened could have been noticed. Sometimes it sounds quite damning in describing the extra movement as "clear" but elsewhere describes it as "not accessible by visual inspection". The amount of movement we're talking about, until the point of the collapse itself, was small, of the order 1cm or so. It certainly isn't something you'd spot from a casual glance, but you could measure it by hand easily enough with a ruler. Not noticing this, if I understand things correctly, meant that the cables were estimated to still have their original high safety factors whereas in fact they were much lower. They say this "should have raised the highest alarm level, requiring urgent action". Perhaps most damming, they also say that it is "highly unlikely" that this excessive pullout went unnoticed. They also note that there was a lack of good documentation of maintenance records and procedures.
The contribution of the recent hurricanes, especially Maria, was extremely significant in precipitating the collapse. In fact, "absent Maria, the Committee believes the telescope would still be standing today". Pullout from the sockets shortly after installation is entirely normal as the structure takes the weight, but after that, any further movement isn't normal at all. This did in fact happen, and should have been spotted – but even this, as we'll see, apparently wasn't enough to bring down the telescope by itself.
One final point is that Arecibo wasn't well liked by backend management. I often had the impression of a behind-the-scenes mood of delenda est Arecibo – or at the very least, that that was what some staff members sincerely believed was happening even if it wasn't true. The report notes that a 2006 NSF report recommended closing Arecibo by 2011 if other funding sources couldn't be found, which I found truly bizarre. This was less than ten years after a major upgrade and exactly at the point the biggest surveys were just beginning. As to why anyone would think that closing it at that particular moment was a good idea, I'm truly at a loss. Nothing about it, even with some familiarity of the large-than-life politics behind the place, has ever made a lick of sense to me.
This is not, I hasten to add, any suggestion of deliberately shoddy maintenance; inasmuch as that was inadequate, there is no need to attribute that to anything besides an incompetently low budget. One strikingly simple recommendation in the report is that funding sources for site operations (e.g. science and development) and maintenance be entirely separate, so there is no chance of any conflict of interest or competition for resources which are essential to both.
The failure was unprecedented
The final and most interesting point of the report, the big headline message, is that Arecibo may have failed because of its radar transmitter. The report is emphatic, and repeats almost ad nauseum, that the kind of socket failures seen here have never before occurred in a century of operations of identical sockets used in bridges and other structures around the world. The damage from the hurricanes was significant, but not enough by itself to explain the failure. There is a crucial missing factor here.
The explanation suggested in the report is electroplasticity. In laboratory conditions, material creep (stretching) can be induced by electrical currents, apparently directly because of the energy released by the flow of electrons. As they note, in the lab this has been found under much higher currents operating for much shorter times, but could presumably work at lower currents if sustained for much longer periods. If correct, this would be Arecibo's final first, another effect of its unique nature. Such currents, they hypothesise, would have been induced by the powerful 1MW radar transmitter used for zapping asteroids and other Solar System objects. This would explain why the cables failed while still having apparently high safety factors, and possibly account for why the failures occurred in some of the youngest cables with no evidence of manufacturing defects (and weren't even the ones with the highest load). It would also, of course, explain why no such other socket failures have ever been seen. Hardly anything else has this combination of radar transmitter and spelter sockets, let alone in tropical conditions in an earthquake zone.
The report goes quite deep into the technical details of electroplasticity. Interestingly, it notes that even less powerful sources can induce currents in human skin that can be directly sensed a few hundred feet from the transmitter. The problem is that understanding the effects of these currently requires highly detailed simulations accounting for the complicated structure of Arecibo's cables, the exact path the current would follow, and using data on low, long-term currents that at present doesn't exist. The most obvious deficiency seems to me that they don't estimate just how long the radar was ever transmitting for. Sure, it was up there for decades, but it wasn't used routinely : regularly, to be sure, but not daily. This is something where a crude estimate should be relatively easy by searching the observing records; even the schedule of what was planned (which didn't always match what was actually done, usually because the err, radar broke) would give a rough indication.
