Arecibo Observatory

A 305 metre dish in a sinkhole, and the day it fell

The Arecibo Telescope began with a happy mistake. Early in 1958 William E. Gordon conceived a radar to measure the density and temperature of the Earth's ionosphere up to a few thousand kilometres, and his calculations said the dish would need to be 1,000 feet across. His study group's design report that December described a limestone sinkhole in Puerto Rico that could hold it. Soon after, an experiment by Kenneth Bowles showed that a much smaller dish would have done the original job, but by then the team had a long list of other experiments only a giant could do, and the figure stuck. The new Advanced Research Projects Agency, interested because ballistic missiles travel through the ionosphere, paid for it. As a later history of the project put it, without the error such a large dish would probably never have been built.

Construction began in September 1960 in the karst hills of Barrio Esperanza, Arecibo, and the observatory opened on 1 November 1963. The dish, 305 metres wide, was made of 38,778 perforated aluminium panels, each about 3 by 7 feet, hung on a mesh of steel cables over the sinkhole, with shade-loving plants growing underneath; the receiver hung 150 metres above it from cables strung between three towers. For 53 years it was the largest single-aperture telescope in the world.

It earned the size. In April 1964 Gordon Pettengill's team used it to show that Mercury rotates in 59 days, not 88. In 1974 Hulse and Taylor found the first binary pulsar there, work that won them the Nobel Prize in Physics, and in 1990 Aleksander Wolszczan found the pulsar whose three planets became the first planets known beyond the solar system. Also in 1974 the telescope beamed the Arecibo message, a 1,679-bit picture of numbers, stick figures, chemical formulas and the telescope itself, toward the globular cluster Messier 13, 25,000 light-years away. Its looks put it in GoldenEye and Contact.

Funding thinned after 2006, and hurricanes, Maria above all in 2017, and earthquakes took their toll. An auxiliary cable broke in August 2020 and a main cable in November, and the NSF decided to demolish the telescope in a controlled way. It did not get the chance. Around 7:56 on the morning of 1 December 2020 the remaining cables from one tower failed, and the platform crashed through the dish, with no one hurt. In 2022 the NSF said it would not rebuild; the site now houses an education centre, Arecibo C3, beside a smaller radio telescope, a LIDAR facility and the visitor centre.