In 1963, the Arecibo Observatory became operational on the island of Puerto Rico. Measuring 305 meters (~1000 ft) in diameter, Arecibo’s spherical reflector dish was the largest radio telescope in the world at the time – a record it maintained until 2016 with the construction of the Five-hundred-meter Aperture Spherical Telescope (FAST) in China. In December 2020, Arecibo’s reflector dish collapsed after some of its support cables snapped, leading the National Science Foundation (NSF) to decommission the Observatory.
Shortly thereafter, the NSF and the University of Central Florida launched investigations to determine what caused the collapse. After nearly four years, the Committee on Analysis of Causes of Failure and Collapse of the 305-Meter Telescope at the Arecibo Observatory released an official report that details their findings. According to the report, the collapse was due to weakened infrastructure caused by long-term zinc creep-induced failure in the telescope’s cable sockets and previous damage caused by Hurricane Maria.
The massive dish was originally called the Arecibo Ionospheric Observatory and was intended for ionospheric research in addition to radio astronomy. The former task was part of the Advance Research Projects Agency’s (ARPA) Defender Program, which aimed to develop ballistic missile defenses. By 1967, the NSF took over the administration of Arecibo, henceforth making it a civilian facility dedicated to astronomy research. By 1971, NASA signed a memo of understanding to share the costs of maintaining and upgrading the facility.
During its many years of service, the Arecibo Observatory accomplished some amazing feats. This included the first-ever discovery of a binary pulsar in 1974, which led to the discovery team (Russell A. Hulse and Joseph H. Taylor) being awarded the Nobel Prize in physics in 1993. In 1985, the observatory discovered the binary asteroid 4337 Arecibo in the outer regions of the Main Asteroid Belt. In 1992, Arecibo discovered the first exoplanets, two rocky bodies roughly four times the mass of Earth around the pulsar PSR 1257+12. This was followed by the discovery of the first repeating Fast Radio Burst (FRB) in 2016.
The observatory was also responsible for sending the famous Arecibo Message, the most powerful broadcast ever beamed into space and humanity’s first true attempt at Messaging Extraterrestrial Intelligence (METI). The pictorial message, which was crafted by a group of Cornell University and Arecibo scientists, which included Frank Drake (creator of the Drake equation), famed science communicator and author Carl Sagan, Richard Isaacman, Linda May, and James C.G. Walker, was aimed at the globular star cluster M13.
According to the Committee report, the structural failure began in 2017 when Hurricane Maria hit the Observatory on September 20th, 2017:
“Maria subjected the Arecibo Telescope to winds of between 105 and 118 mph, with the source of this uncertainty in wind speed discussed below... Based on a review of available records, the winds of Hurricane Maria subjected the Arecibo Telescope’s cables to the highest structural stress they had ever endured since it opened in 1963.”
However, inspections conducted after the hurricane concluded that “no significant damage had jeopardized the Arecibo Telescope’s structural integrity.” Repairs were nonetheless ordered, but the report identified several issues that caused these repairs to be delayed for years. Even so, the investigation indicated that due to the misdirection of repairs “toward components and replacement of a main cable that ultimately never failed,” these would not have prevented the Observatory’s collapse regardless.
Moreover, in August and November of 2020, an auxiliary and main cable suffered a structural failure, which led to the NSF announcing that they would decommission the telescope through a controlled demolition to avoid a catastrophic collapse. They also stated that the other facilities in the observatory would remain operational, such as the Ángel Ramos Foundation Visitor Center. Before that could occur, however, more support cables buckled on December 1st, 2020, causing the instrument platform to collapse into the dish.
This collapse also removed the tops of the support towers and partially damaged some of the Observatory’s other buildings. Mercifully, no one was hurt. According to the report, the Arecibo Telescope’s cable spelter sockets had degraded considerably, as indicated by the previous cable failures. They also explain that the collapse was triggered by “hidden outer wire failures,” which had already fractured due to shear stress from zinc creep (aka. zinc decay) in the telescope’s cable spelter sockets.
This issue was not identified in the post-Mariah inspection, leading to a lack of consideration of degradation mechanisms and an overestimation of the potential strength of the other cables. According to NSF statements issued in October 2022 and September 2023, the observatory will be remade into an education center known as Arecibo C3, focused on Ciencia (Science), Computación (Computing), and fostering Comunidad (Community). So, while the observatory’s long history of radio astronomy may have ended, it will carry on as a STEM research center, and its legacy will endure.
Further Reading: National Academies Press, Gizmodo