On the island of Puerto Rico, three mountains not far from the city of Arecibo, there is a gigantic antenna 305 meters wide. It was built in the early 1960s, exploiting a large hollow, and for almost sixty years it was the main instrument of the Arecibo Observatory, thanks to which we discovered many things about the Universe and we have been listening for decades, trying to capture any alien communications. Two serious accidents this year, however, made the large antenna useless, to the point of making it necessary to demolish it for safety reasons.
From alien messages to neutron stars
The Arecibo radio telescope became quite famous in the late 1990s thanks to the film Contact by Robert Zemeckis with Jodie Foster. It was inspired by the novel of the same title published in 1985 by Carl Sagan, the great American popularizer, and told of the first hypothetical contact between humans and an alien species, addressing above all the ethical and religious implications of this discovery.
For his book, Sagan had taken inspiration from several initiatives undertaken to pick up alien messages and communicate with them, including the most famous of all: SETI (Search for Extra-Terrestrial Intelligence), the program for the search for intelligent extraterrestrial life. In its years of existence, the Arecibo radio telescope would have been part of several experiments related to this project.
In 1964, less than a year after its inauguration, the Arecibo radio telescope was central to recalculating the rotation period of the planet Mercury, allowing to discover that it lasts just 59 days and not 88 as previously estimated. A few years later, the observatory made it possible to obtain the first concrete evidence on the existence of neutron stars, the last stage of life of stars with very large mass (about ten times that of the Sun), which occur when the nuclear fusion reactions inside them end due to the end of the light elements that feed them, in practice their fuel.
Radar image of asteroid 4769 Castalia, created thanks to the instruments of the Arecibo observatory
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In the following decades, thanks to the Arecibo observatory and its instruments it was possible to obtain the first radar image of an asteroid, to guess the existence of the first exoplanets (planets outside our solar system) and to identify SHGb02 + 14a, a radio source that according to some it could be among the best candidates as a message of an alien nature. It was picked up for the first time in March 2003, and announced by the SETI @ home project (project for the analysis of radio signals by exploiting the computing capacity made available by those who have their computers connected to the Internet) with great caution and invitation not to reach hasty conclusions, as it could be a trivial reception error linked to interference, background noise or technological problems.
Structure
In its nearly 60 years of existence, the Arecibo radio telescope has withstood the violent storms and hurricanes that sometimes affect Puerto Rico, and several earthquakes. The large antenna is made up of nearly 40,000 aluminum panels, mounted on a network of steel cables. The disk is topped at a height of 150 meters by a triangular platform suspended by 18 cables anchored to three high concrete towers along the circumference of the antenna. Building the whole complex was not easy, nor was it maintenance, and now due to two accidents it seems to have become even more difficult, if not impossible.
Arecibo radio telescope antenna still whole, photographed in June 2019 (Wikimedia)
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Settlements
Last August one of the support cables slipped out of its anchor and fell on the antenna, destroying dozens of panels. The cable had been installed in the 1990s and was therefore relatively recent compared to the age of the radio telescope. Ashely Zauderer, responsible for activities at Arecibo on behalf of the US government agency National Science Foundation (NSF) which owns the observatory he said in a press conference that that cable “shouldn’t have broken the way it broke.”
The Arecibo radio telescope in a satellite image taken on November 17, 2020, with evident damage to the antenna caused by the failure of the cables (EPA / MAXAR via ANSA)
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The damage was not indifferent, but researchers and engineers had immediately set to work to study a repair plan that would allow to save the radio telescope. Work was supposed to have started a few weeks ago, but before they started another cable installed in the 1960s had failed, causing further damage to the antenna. It had been analyzed recently: it showed the signs of aging, but according to the technicians it was still able to withstand the stresses it was subjected to. It was scheduled to be replaced by the end of the year, but it caved before any action could be taken.
Other NSF managers they explained that the Arecibo radio telescope risks “catastrophically collapsing” following the failure of the two cables, and the damage they have caused to the rest of the structure. The level of risk is such that even inspections of the valley by technicians are not possible. The antenna is now unsafe and the installation of new cables would lead to new dangerous stresses, with unpredictable results.
Damage caused by the collapse of one of the cables on the radio telescope structure (© El Nuevo Dia de Puerto Rico via ZUMA Press / ANSA)
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Having found it impossible to fix the antenna, the engineers and observatory managers are now working on a plan to demolish it in a controlled manner, preventing it from collapsing on itself. The end of Arecibo does not mean the end of research with large radio telescopes. For a few years in China FAST (Five hundred meter Aperture Spherical Telescope) went into operation, a radio telescope with a diameter of 500 meters and up to three times more sensitive than Arecibo, which will remain in the memories of researchers for a long time and not only.
In 1974, thanks to its antenna it was possible to send a message, which later became known as “Arecibo message”, Towards the globular cluster of Hercules (M13) 25 thousand light years away from us. The radio message – which contained schematic representations of the solar system, the DNA molecule, a human being and the radio telescope itself – is still on its way and will continue to propagate for many millennia. In the event that it was actually picked up by someone able to answer us, we could receive a reply message no earlier than 50 thousand years. In a way, the Arecibo radio telescope’s journey isn’t over either.
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