WASHINGTON — The National Science Foundation announced Nov. 19 the decision to perform a “controlled decommissioning” of the giant radio telescope at the Arecibo Observatory in Puerto Rico. Recent damage to the telescope’s support cables have made the dish, which takes advantage of a natural sinkhole to create the world’s largest parabolic antenna, unsafe to either operate or repair.

Ttwo giant cables used to support a 900-ton platform suspended over the telescope’s 305-meter main dish have broken, putting the entire structure at risk of collapse. One cable slipped out of its socket in August, falling to the dish below and damaging it, while the second broke Nov. 6

Both cables are attached to the same tower, one of three surrounding the main dish. “The engineers have advised us that the break of one more cable will result in an uncontrolled collapse of the structure,” said Ralph Gaume, director of the NSF’s Division of Astronomical Sciences, referring to cables attached to that same tower. That would result in the platform crashing down to the main dish and potentially toppling one or more of the towers.

Unfortunately, in order to repair the cables, workers would have to risk their lives by going out onto the already compromised platform to do the work, on a platform already at risk of imminent collapse. Fixing the broken cables isn’t possible.

“NSF has concluded that this recent damage to the 305-meter telescope cannot be addressed without risking the lives and safety of work crews and staff, and NSF has decided to begin the process of planning for a controlled decommissioning of the 305-meter telescope,” said Sean Jones, assistant director of the NSF’s Mathematical and Physical Sciences Directorate.

FILE – In this Tuesday, Aug. 11, 2020 file photo, provided by the Arecibo Observatory, shows the damage done by a broken cable that supported a metal platform, creating a 100-foot (30-meter) gash to the radio telescope’s reflector dish in Arecibo, Puerto Rico. Giant, aging cables that support the radio telescopes are slowly unraveling in this U.S. territory, threatening scientific projects that researchers say can’t be done elsewhere on the planet. (Arecibo Observatory via AP)

The use of explosives to bring down the structure is being contemplated. Though the platform would likely come down on its own eventually, engineers are working on a plan to decomission it in a controlled manner. NSF officials are working with the White House Office of Management and Budget, and Congress, to figure out how to pay for the decomission. Deconstruction costs are estimated in the $10.6 million to $18.7 range, depending on which structures are abandoned in place versus being completely removed. Those estimates, however, are four years old, and don’t reflect current proceral costs.

“While we are saddened by the loss of the facility, we commend [NSF Director] Dr. Sethuraman Panchanathan and his team for prioritizing the lives and safety of observatory staff and repair crews throughout this process,” Reps. Eddie Bernice Johnson (D-Texas) and Frank Lucas (R-Okla.), chair and ranking member, respectively, of the House Science Committee, said in a statement. The encouraged the NSF to continue to use the observatory for educational outreach and “to explore opportunities to use the site for exciting new science in the future.”

Engineers are still studying why the two cables broke. The first cable was one of set of auxiliary cables installed in the 1990s to reinforce the observing platform. The second was one of the main cables dating back to the observatory’s construction in the early 1960s. That main cable broke despite being subject to stresses of just 62% of its rated strength, and that fact led engineers to conclude that all the other cables are at similar risk of failure, despite having been maintained to specification.

After the initial auxiliary cable broke, the observatory had been working on a repair and stabilization plan that included installing a more modern monitoring system that could have caught issues with the main cables. “It is truly unfortunate that this main cable failed before we had a chance to get things stabilized,” she said.

Decommissioning of the telescope is not the same as completely closing the entire Arecibo Observatory, NSF officials said. A lidar there used for atmospheric studies will remain, as well as a visitors’ center. The decommissioning process will seek to preserve buildings used for observatory operations that are located at the base of one of the towers.

NASA said that it will rely on the Goldstone Observatory in California for future planetary radar observations. NASA recently brought that facility back to full operations after installing a new klystron for that radar.

Arecibo was the largest radio telescope in the world from the time it was built in the early 1960s until the completion of China’s Five Hundred Meter Aperture Spherical Telescope, or FAST, in 2016. It became part of popular culture through appearances in movies such as Contact and GoldenEye.

Update: The Center Platform Has Now Fallen

Deborah Martorell, a meteorologist in Puerto Rico, tweeted early Tuesday: “Friends, it is with deep regret to inform you that the Arecibo Observatory platform has just collapsed.”

The 900 ton central platform has now fallen an approximate 450 feet into the center of the thousand-foot dish, taking the tops of three of the remaining support towers with it.

The National Science Foundation said that an investigation into the platform’s fall was ongoing.

“Initial findings indicate that the top section of all three of the… telescope’s support towers broke off. As the 900-ton instrument platform fell, the telescope’s support cables also dropped,” it said in a statement.

“Preliminary assessments indicate the observatory’s learning centre sustained significant damage from falling cables,” it added.

Two cables had broken since August, damaging the structure and forcing officials to close the observatory.