The National Science Foundation (NSF) will decommission Arecibo Observatory's massive radio dish after damage has made the facility too dangerous to repair, the agency announced on November 19, 2020.

The announcement came as scientists awaited a verdict about the fate of the iconic observatory after damage to the complex cabling supporting a 900-ton science platform suspended over the dish. In August, a cable slipped out of its socket, but engineers evaluating the situation deemed it stable; earlier this month, a second cable unexpectedly snapped, leaving Arecibo's fate much more perilous. After considering three separate engineering reports, the NSF, which owns the property, has decided the facility is unstable enough that there is no way to repair the damage that does not put personnel at undue risk.

"Our goal has been to find a way to preserve the telescope without placing anyone's safety at risk," Sean Jones, assistant director for the Mathematical and Physical Sciences Directorate at the NSF, said in a news conference today. "However, after receiving and reviewing the engineering assessments, we have found no path forward that would allow us to do so safely. And we know that a delay in decision making leaves the entire facility at risk of an uncontrolled collapse, unnecessarily jeopardizing people and also the additional facilities."

"The telescope is currently at serious risk of an unexpected, uncontrolled collapse," Ralph Gaume, director of NSF's Division of Astronomical Sciences, said. "According to engineering assessments, even attempts of stabilization or testing the cables could result in accelerating the catastrophic failure. Engineers cannot tell us the safety margin of the structure, but they have advised NSF that the structure will collapse in the near future on its own."

Safety concerns loom

During the news conference, officials emphasized that the decision was based on prioritizing safety, not a reflection of the scientific work that Arecibo has done over the past decades or could continue to do in the future, and that some hoped-for science will be lost with the facility.

"This decision has nothing to do with the scientific merits of Arecibo Observatory," Gaume said. "That is not a consideration. It's all about safety."

Guame added that the agency will work with scientists who had been planning to use the Arecibo telescope and its other facilities in order to relocate those planned research projects wherever possible. However, the facility was unique, particularly in its radar capability, which was heavily used to study near-Earth asteroids and other solar system objects.

"Some of the Arecibo science will transfer; some of it will not," Gaume said.

The officials also emphasized that if they can manage to decommission the telescope in a controlled manner, the other assets at Arecibo Observatory — primarily, the visitor center, an atmospheric science instrument on site and a second atmospheric tool on the neighboring island of Culebra — should survive.

Collapse threat a challenge for decommissioning

Both of the failed cables on Arecibo were connected to the same support tower. An engineering analysis completed after the second cable failed in November found that if one more cable on that tower, dubbed Tower 4, failed, the platform would collapse into the dish and likely cause the towers to topple. And because the cable support system is already so fragile, the engineers didn't see any way to safely evaluate the situation in more detail, much less stabilize it.

"What we're challenged with is a structure for which we don't understand the safety margins, the engineering approaches to better understand the safety margins involve considerable risk, and the engineering approach to repair the structure appears to be highly unsafe."

That means the situation is volatile enough that the NSF can't guarantee the telescope will be decommissioned in a controlled manner. Gaume and Ashley Zauderer, the program director for Arecibo Observatory at the NSF, said that the agency has hired engineers to develop a plan for a controlled decommissioning. Creating that plan and gathering the necessary approvals for it will take multiple weeks, they added.

The NSF officials said that right now, their priority is focused on safely decommissioning the radio dish and protecting as much of the other facilities on-site as possible. But they did emphasize that they are not decommissioning Arecibo Observatory as a whole.

"We're discussing the decommissioning of a structure made of steel and cables, but it truly is the people on that have the ideas," Zauderer said. "It's the idea of discovery that led to the construction to start with, it's the passion of the people that work at the observatory … to continue to explore, to learn, that is the true heart and soul of Arecibo."

"It's not the telescope that's the heart and soul, it's the people."

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