NSF offers a closer look at how the Arecibo Observatory collapsed

Arecibo's Program Director called it an unexpected, "violent" event.

The scientific community is still reeling from the collapse of the world's second largest radio telescope in Arecibo, Puerto Rico two days ago, and the National Science Foundation is starting to share more about what happened that day. In a briefing with reporters this morning, Arecibo Program Director Ashley Zauderer and Ralph Gaume, director of NSF's Division of Astronomical Sciences, offered the most detailed account to date of the events leading up to — and including — the December 1st collapse.

The trouble at Arecibo began this August, when one of the auxiliary cables supporting the receiver platform slipped out of its socket atop Tower 4, one of the telescope's main support struts. Once free, the cable struck Arecibo's reflector dish, leaving behind a gash about 100 feet long. About three months later, one of that same tower's main support cables -- each comprised of about 170 district metal wires — also failed, placing substantial additional strain on the three remaining cables.

Engineers on-site began working to reduce the load placed on those cables, in part by trying to relax a series of "backstay" cables that helped the tower itself remain upright. According to Zauderer, other options involved using a crew tethered to a helicopter to remove weight from the support tower, but safety concerns eventually grounded that idea. Regardless, it soon became all too clear that the Arecibo complex was living on borrowed time.

"After the November 6th cable failure, the [remaining] cables could have failed at any time," said Gaume. "We were unable to predict when it would happen, [but] we knew it would happen." Gaume later noted that, no matter what the Arecibo ground crew tried, they "would have never been able to relieve enough load to get the cables back to the condition before November 6th."

The team's worst fears came to pass on December 1st, when — after monitoring individual wires snapping under the strain -- the second main cable stretching from Tower 4 to the receiver platform failed. The receiver's weight was then being borne at one end by just two cables, both of which failed fractions of a second after the first one did, sending the platform careening down into the massive reflector dish. Meanwhile, Tower 4 was suddenly free of weight to carry to support, but was still being pulled by seven backstay cables — that pressure sent the top 65 feet of the tower tumbling backwards. And across the dish, the same thing was happening to Tower 12; its top section was sent rolling down a hill near the Arecibo operations building. Fortunately, no one was injured.

Now, the only thing left to do is pick up the pieces — quite literally. Gaume said the NSF expects to have a “full assessment” of the damage caused by the collapse and its environmental impact by the end of this week. And while the NSF hasn’t ruled out the possibility of restoring or rebuilding the facility, it can’t happen without significant input from other parties.

“NSF has a very well-defined process for building major research equipment and facilities,” he said. “It involves Congress appropriating funding, along with assessment and input by the scientific community, including research and other stakeholders. That process would need to play out.”