Today, the National Science Foundation released video taken at the moment the Arecibo Radio Observatory’s cables failed, allowing its massive instrument platform to crash into the dish below. In describing the videos, the NSF also talked a bit about the monitoring program that had put the cameras in place, ideas it had been pursuing for stabilizing the structure pre-collapse, and prospects for building something new at the site.

A quick recap of the collapse: the Arecibo dish was designed to reflect incoming radio radiation to collectors that hung from a massive, 900-ton instrument package that was suspended above it. The suspension system was supported by three reinforced concrete towers that held cables that were anchored farther from the dish, looped over the towers, and then continued on to the platform itself. Failure of these cables eventually led to the platform dropping into the dish below it.

Let’s go to the video

The video of that collapse comes from a monitoring system put in place in the wake of the cable failures. Due to the danger of further cable breaks, the NSF had instituted no-go zones around each of the three towers that supported the cables. With no personnel allowed to get close enough to inspect the cables, the staff started monitoring them using daily drone flights, one of which was in progress during the collapse. In addition, a video camera was installed on top of the visitor’s center, which had a clear view of the instrument platform and one of the support towers.

As you can see from the video, the drone was examining the area where the cables looped over the support towers. Specifically, it was examining the tower that had supported the one main cable that had failed earlier—note that one of the gaps that the cables pass through is unoccupied. While it was filming, individual wires in the cable started snapping, and the cable failed completely shortly afterward. The remaining connection visible there, which was connected to the scientific instrumentation, survived a bit longer before the plunging platform pulled it apart.

In the second segment of video, the view from the visitor’s center shows how the failure of the cables at that tower affected the rest of the system. With one of the three support anchors gone, the instrument platform dropped toward the dish in between the remaining two. This created off-axis forces that caused the tops of those towers to be wrenched off the rest, resulting in about 60 feet of reinforced concrete plunging to the ground below. At the same time, backstay cables that ran from the tower to the ground came loose and swung around wildly.

Despite all this destruction, the NSF’s decision to keep the areas around the towers clear of personnel ensured that nobody was injured. And the visitor’s center, which is near one of the towers, managed to escape without significant damage.

Before and after

Ashley Zauderer, who was NSF’s program manager for Arecibo, described some of the ideas the NSF had considered once the cables started breaking. These were mostly focused on releasing some of the strain on the remaining supports. Ideas that were initially analyzed included easing tension off the backstay cables, which would allow the platform to droop toward the dish a bit, reducing the forces on the cables. Another idea she mentioned was to take some of the hardware off the platform, reducing its weight. But this would require a helicopter and placing personnel on the platform, which was considered a high-risk activity.

While these plans were being evaluated, the telescope’s operators started monitoring the cables using a drone in order to avoid putting humans at risk. Over the past weekend, Zauderer said, the drone footage had revealed several individual wires in the cables snapping, and she implied that this meant that everyone knew a collapse was inevitable.

Even before that, however, there wasn’t a lot of optimism about the ideas Zauderer mentioned. John Abruzzo, an engineer at the firm hired to evaluate options for Arecibo, said, “the probability of success was not that high.” In almost every case where operations and repairs of this sort had been attempted, Abruzzo said, the structure at risk was in relatively good condition, something that was not true at Arecibo.

What are the future prospects for the site? Ralph Gaume, director of NSF’s Division of Astronomical Sciences, said that this isn’t an easy question to answer. While funds had been allocated to stabilize and potentially repair the observatory—replacement cables had been ordered when the first one snapped—the NSF can’t simply reallocate the money to anything new without congressional approval. And those funds are well short of what’s needed to build anything new at the site, which means Congress would also have to get involved in determining what’s possible.

And nobody on the press call was interested in speculating about how interested Congress might be in acting.

https://arstechnica.com/?p=1727382