Sad thing to watch :(

https://www.space.com/arecibo-observatory-collapse-drone-video-nsf-evaluation

It took 17 seconds for Arecibo Observatory's massive radio telescope to crumble. It will take much longer for the dust to settle.

The iconic structure in Puerto Rico collapsed on Dec. 1 after cable failures in August and November made the telescope too delicate to safely repair. The U.S. National Science Foundation (NSF), which owns the site, knew that the structure could fall any time and was evaluating how to go about decommissioning the telescope. Now, the agency has shifted to evaluating what to do with its wreckage.

"We're in the assessment phase," Ralph Gaume, director of NSF's Division of Astronomical Sciences, said during a news conference held today (Dec. 3).

Related: Losing Arecibo Observatory creates a hole that can't be filled

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Arecibo Observatory to be decommissioned

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He said that the University of Central Florida, which operates the site for the NSF, has hired a clean-up contractor who arrived at the telescope yesterday. "They're beginning to make preliminary plans moving forward to provide us with a plan for what the cleanup looks like," Gaume said. "It's too early to say exactly what that cleanup looks like."

What's certain is that the collapse itself was brutal. During the news conference, NSF personnel shared two videos taken during the telescope's collapse: one from the observatory's control room just beyond the massive dish and one from a drone hovering near the tower that had lost two cables over the past four months.

Arecibo's troubles began in August, when a supporting cable connecting the 900-ton science platform to Tower 4 slipped out of its socket. An initial analysis suggested that, with a few repairs, the structure would be fine.

But then in November, just as staff were preparing for those repairs to begin, a second cable snapped. This time it was one of the primary cables and also connected to Tower 4, leaving the platform without one-third of its support at that corner.

Engineers swarmed in to evaluate the structure, using drones to remain at a safe distance from the unstable telescope. But they couldn't figure out a way to safely figure out just how stable the structure was, much less to safely repair it. The second cable had failed under much less stress than should have been required to snap it, so engineers lost their confidence in all the other cables as well.

Related: The Arecibo Observatory radio telescope in Puerto Rico has collapsed

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Arecibo Radio Telescope explained by NASA

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It's too early to say exactly what that cleanup looks like.

"We knew at that point in time that it was just a matter of time," John Abruzzo, the lead engineer for the Arecibo damage evaluation process and a managing principal at the firm Thornton Tomasetti, said during the news conference. "It was a very dangerous and precarious situation because it could have gone really at any time."

So on Nov. 19, the NSF announced it would begin the process of decommissioning the telescope — but that response, too, was complicated by the precarious state of the platform. Engineers needed a few weeks to develop a plan to safely demolish the structure.

They didn't have that kind of time, it turns out: The telescope collapsed less than two weeks later.

The newly released drone footage opens on already-snapped cables, then shows the failure of additional cables: first slow, then faster, with paint chips flying as individual wires within the thick bundles give way. Once all the cables at that tower snap, the triangular metal platform lumbers down to hit the opposite side of the dish, pulling off the tips of the other two supporting towers as it goes.

During the news conference, officials emphasized their commitment to the telescope throughout the difficult autumn, the telescope's dire precarity after the second cable failure, and their gratitude that no one was injured during the collapse.

The agency had authorized spending on evaluating and stabilizing the structure soon after the August failure on the condition that human safety remained top priority at the site. Preparations for repairs to the first cable failure were underway when the second cable snapped, and staff were exploring potential stabilizing approaches when the telescope collapsed.

Related: Arecibo isn't the first radio telescope to unexpectedly fail. Here's what we can learn from Green Bank's collapse.

A still image from video provided by the NSF shows the Arecibo radio telescope's massive platform falling to the ground. (Image credit: Arecibo Observatory, a U.S. National Science Foundation facility)

"It was a dangerous situation: After the Nov. 6 cable failure, those cables could have failed at any time — you were unable to predict when it would happen." Abruzzo said. "It was risky to try to do what we were going to do, and frankly, the probability of success was really not that high. It was just basically a last-ditch effort if you could to try and preserve it a little bit longer to allow us to do more work."

Agency officials emphasized that safety remains the top priority in dealing with the aftermath of the collapse.

"Our focus [going] forward has to remain safety," Gaume said. "We need a full accounting of how stable the site is, in particular those three towers and the remaining structure, along with putting together a plan to safely remove the wreckage." Gaume did not provide a timeline for that work.

Some limited science will resume at the observatory soon, NSF officials said. The agency has authorized repairs to the facility's LIDAR instrument and a smaller telescope used for atmospheric science, Gaume said, using money allocated to the facility after Hurricane Maria damaged it in 2017.

But whether the massive radio telescope will be replaced remains an open question.

"With regards to replacement, NSF has a very well defined process for funding and constructing large-scale infrastructure, including telescopes," Gaume said. "It's a multi-year process that involves congressional appropriations and the assessment and needs of the scientific community. So it's very early for us to comment on the replacement."

Email Meghan Bartels at mbartels@space.com or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.