Scientific Instruments – With the death of Arecibo, an era of Radio Astronomy Science & Technology ends

November 25, 2020

ARECIBO OBSERVATORY was conceived in an era of space age monumentalism, an imposition of geometry into geology that is as impressive in its simplicity and size as the greatest brutalist architecture. When the James Bond franchise used the 306-meter bowl as its location in its Pomp, a showcase for iconic 1960s design, the only surprise was that it had lasted so long.

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The observatory wasn’t new to Spycraft. It was developed as a tool for studying the ionosphere, an electrically charged upper layer of the atmosphere, with radar. The US Department of Defense had an interest in such work, which could lead to new ways of characterizing incoming missiles or sniffing out enemy transmissions.

A freestanding dish big enough for the job would have been impractical. The designers therefore looked for a hole in the floor to reuse. They found it in northwest Puerto Rico, a sinkhole where the limestone landscape had collapsed just right. They built three towers on the edge of the sinkhole and lifted the electronic heart of the instrument – the bit that sends and receives radio waves – into the empty space between them. Signals going to or from this device bounce off eight acres of wire mesh that extends below.

As a radar, Arecibo used the world’s largest shell to study not only the ionosphere, but also the surfaces of nearby planets and passing asteroids. But as a radio telescope it really stood out, making some crucial discoveries in the 1960s and 1970s, the golden age of radio astronomy. The most famous was a pair of pulsars – spinning neutron stars – orbiting each other in a way that would prove Einstein’s general theory of relativity. Later data revealed planets around a different pulsar. This was the first definitive discovery of planets beyond the solar system.

Arecibo was also used for the wayward offshoot of radio astronomy, the search for extraterrestrial intelligence. Since 1960 radio astronomers have occasionally used their instruments to look for artificial signals from the stars. After an upgrade that replaced the original mesh with a shell made from 38,778 aluminum plates, Arecibo went a step further in 1974. It broadcast a 1,679-bit message to a star cluster 25,000 light-years away. This message had graphical representations of basic biochemistry and astronomy and the technology used to broadcast it encoded.

Over time, advances in technology have undermined the benefits of Arecibo’s size and funding has declined. The technique began to show its age. In August, one of the cables supporting the instrument platform broke and damaged the bowl. A second click in early November seemed to indicate an impending collapse. And so it should be closed.

But when the vegetation beneath the shell rises through its remains and the site falls into picturesque ruin, the sketch of its cross-section encoded in this message from the 1970s will continue on its way. It is already 46 light years from Earth. Its pixels now form the most distant monument to human achievement in the entire universe. And they always will.

This article appeared in the Science & Technology section of the print edition under the heading “Si monumentum requireis respicite”.

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