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The Arecibo Telescope: Puerto Rico’s Iconic Instrument (That Didn’t Survive 2020)

Shortly before 8 am on 1st December 2020 in the thick Puerto Rican forest 95km west of the capital San Juan, a booming crash broke the silence. This particular area of the world looks very different depending on where you are. Standing within the thick vegetation it could be easy to feel like you are miles, perhaps even hundreds of miles from civilization. However, from the air, you would see that the vast ocean of green below is broken by an odd sight. 

A giant telescope lies nestled in the undergrowth, its colossal dish hugging the floor while its receivers hang above pointing expectantly towards the stars. This is one of the largest single-aperture telescopes anywhere on the planet – or should I say, it was – because that booming crash in early December signalled the end of the Arecibo Telescope.  

Built in a natural sinkhole, the Arecibo Telescope was a breathtaking sight. The inverted spherical dome that lines the sinkhole floor measures a huge 305 metres (1,000 ft) in diameter – that’s almost three times the length of a Premier League football pitch. Suspended 150 metres (492 ft) above the dish, and held in place by three giant towers on the edge of the dome, was the receiver and radar transmitters. For 57 years this was one of the most important observatories on the planet, but time finally caught up with this ageing hulk. After being battered by Hurricane Maria in 2017 and suffering damage as a result of earthquakes in 2019 and 2020, the end appeared to be near as the final months of 2020 passed by. 

But near turned out to be much closer than anybody had anticipated. 

Early Days

While the Arecibo Telescope has been used for a wide variety of scientific functions over the years, when it was first envisioned in the 1950s, it wasn’t necessarily science and exploration that those involved had in mind. 

With the U.S and USSR beginning to both posture and with the nuclear arms racing hotting up, the threat of apocalyptic weapons raining down from the sky became a serious concern. This was still early days and the United States was almost a decade away from its North American Aerospace Defence Command (NORAD), which was founded in 1958 and would eventually provide one of the most complex and comprehensive early warning systems on earth. 

But in the early 1950s, those working on early warning systems focused on the unique physical signatures that a nuclear weapon would make if it was fired up into the atmosphere to then re-enter and attack a specific target. The focus was the ionization of the atmosphere which could be caused by hot, high-speed objects – specifically within the Ionosphere, the ionised portion of the upper atmosphere located at altitudes between 48 km (30 mi) to 965 km (600 mi). 

Unfortunately very little was known about the Ionosphere or indeed how these physical signatures might manifest themselves. One of the objectives of the planned Arecibo Telescope was to study just this. 

Two physicists working for Cornell University, William E. Gordon and George Peter were given the task of developing plans for the telescope and their interest was drawn to the karst region in western Puerto Rico. Karst areas typically come with plenty of caves and sinkholes which could provide the perfect location for a gigantic dome. One enormous cavity, located 18km south of the town of Arecibo, stood out.

However, original designs don’t quite fit with what appeared. Initially, the Arecibo Telescope would have come with a fixed parabolic reflector along with a 150 metre (492 ft) tower. But having a fixed reflector would have severely hampered scientific work as it wouldn’t have been able to move. While the tower located in the centre of the dome would have severely affected signals. 

Instead, they needed something that could move above the dome, without blocking too much of it. The answer came from George and Helias Doundoulakis, who devised a structure that called for a doughnut-shaped truss, which is a series of beams assembled to create a rigid structure, to hang above the dome, suspended by four towers located at the edge. This truss could be rotated 360 degrees and could be lowered in the event of a hurricane. 

Just a quick word on George Doundoulakis before we move on, because, well, he’s one of those little known, yet extraordinary people that we should probably know more about. His story begins in Detroit where he was born to a Greek family which moved back to the Crete when he was just four years old. During World War II he fought for the Greek resistance and was eventually recruited by Britain’s Special Operations Executive and played a leading role in the successful kidnapping of a Nazi Major-General, Heinrich Kreipe, before being forced to flee Crete. 

While undergoing training in Cairo he switched his services to the American Offices of Strategic Services (OSE) and was sent back to Crete where he eventually commanded a force of 7,000 guerilla fighters. Despite ferocious Nazi attempts to destroy the rebels, their sabotage of railway lines and maritime shipping proved to be hugely damaging for German supply lines. 

After the war, he was awarded the American Legion of Merit as well as the British King’s Medal for his services during the conflict. He went on to become a brilliant physicist responsible for no fewer than twenty-six U.S Patents in the fields of radar, electronics, and narrowband television. But he is best remembered for his design of the Arecibo Telescope. A truly remarkable human being. 


Construction of the vast telescope began in the mid-1960s. The dome itself was initially built with a half-inch galvanized wire mesh but this was changed in 1972 when the mesh was replaced with 38,778 perforated aluminium panels, each measuring 1 by 2 metres (3 by 7 ft). This boosted the maximum operating frequency from 500 MHZ to 5000 MHZ. 

The receiver, weighing a hefty 820 tons, was suspended above the dome by 18 cables attached from the three towers. The initial design had called for four towers but three were eventually deemed adequate. These were reinforced concrete towers, one measuring 111 metres (365 ft) in height, while the other two stood at 81 metres (265 ft) high – this was because of the natural height difference in the area, but meant that all three towers had the same overall height.  

The platform in the centre came with a 93 metre (305 ft) long bow-shaped track, known as the Azimuth arm and it was here that the antennas and secondary reflectors were located. 

In 1997, a Gregorian Reflector System was installed at the Arecibo Telescope. This style of telescope was first developed way back in the 17th Century and includes two concave mirrors positioned opposite each other. The primary (and larger) mirror is responsible for collecting the light and bringing it into focus before reflecting it back across to the secondary mirror where it is again bounced back to the primary and passes through a small hole and on to the eyepiece located behind. 

On the Arecibo, these mirrors were located in the receiver hanging above the dish, with one mirror measuring 21 metres (72 ft) in diameter and the other 7.9 metres (26 ft). The entire structure is housed in a 90-ton enclosure which is the equivalent to six stories in height. As well as the new reflector system, the update added more advanced receivers now capable of covering 1–10 GHz which greatly improved the telescope’s accuracy. 

When we’re talking about Hertz in relation to astronomical instruments it’s perhaps easier to think in terms of distances. The receivers are attempting to detect radio-frequency radiation between wavelengths coming from outer space. These lengths typically range between 10 metres (30 MHz) and 1 mm (300 GHz). So the higher the GHz range, the more accurate the telescope can be. 

The Arecibo Telescope could operate between 50 MHz and 10,000 MHz (10 GHz), meaning the wavelengths were between 6 meters and 3 centimetres. It could pick up signals as close as 6 km (4 miles) above the Earth, and as far several billion light-years away. And it was extraordinarily sensitive, said to be able to pick up mobile phone conversations as far away as Venus. Not that there are any mobile phones on Venus, but if there were the Arecibo could have eavesdropped. 

Lastly, this upgrade saw the power of the Arecibo’s transceivers significantly boosted, going from 420,000 watts to 1 million watts. Among other things, this resulted in a much better resolution and could study Venus with a resolution of 1km (0.5 miles) while asteroids and comets were even better at 15 metres (50 ft). Amazingly, that’s good enough to detect a steel golf ball on the surface of the moon.  


Throughout its 57-year life span, work at the Arecibo Telescope led to countless discoveries. The most significant early finding came just a year after it opened with the discovery that Mercury’s rotation period was not 88 days as had long been thought, but rather 59 days. 

In 1968, a team at the telescope discovered the Crab Pulsar – a young neutron star 7,175 light-years from Earth. Today we still only know of around 2,000 of these pulsar stars which have an optical pulsar rotating around them. The term often used is a lighthouse star because of its rotational pattern. The Arecibo was able to accurately measure the size, 20 kilometres (12 miles) in diameter and rotational speed, 33 milliseconds – meaning that its beams of light circle 30 times every second. 

Another pulsar was discovered in 1974, this time the first known binary pulsar star (a binary system involves two stars orbiting a common centre of mass) which was named PSR B1913+16 – a find that eventually led to Russell Hulse and Joseph Taylor winning the Nobel prize for physics. Two further pulsars were discovered which pushed the boundary of our understanding even further. The first-millisecond pulsar, PSR B1937+21, was discovered in 1982, spinning 642 times per second, which was a record until 2005 when PSR J1748-2446ad appeared, tearing around at 716 times per second. 

In 1980, the Arecibo successfully detected Comet Encken, which was a first by radar observation. In 1989, the observatory recorded an image of an asteroid for the first time in history. Just in case you are interested, 4769 Castalia is a peanut-shaped asteroid approximately 1.4 kilometres (0.87 miles) in diameter and falls under the category of ‘Potentially Hazardous Asteroid’ (PHO) – but don’t worry, though it did pass within 4,029,840 km; 2,504,020 mi) of the earth – that’s eleven distances from us to the moon if those figures don’t mean a whole lot – we have a pretty good idea of its orbit for the next couple of hundred years, so we should be safe. From that particular PHO at least!

In 1990, the Arecibo caught a glimpse of the first known extrasolar planet (planets outside of our solar system), while in 1994 it was used to map the distribution of ice in the polar regions of Mercury. In 2010 and 2011, the observatory detected strong bursts of radio emission from the T6.5 brown dwarf 2MASS J10475385+2124234 – you’ve got to love these names right? Before you get too excited, this simply means that the star has an incredibly strong magnetic field and a magnetic activity similar to our own sun. 

The Arecibo message

From substantiated science to a real hit and hope – a Hail Mary if you will. Since the 1970s data from the Arecibo telescope has been used in the ongoing Search for Extraterrestrial Intelligence (SETI). Much of this has been done through passive scanning of the skies, but on 16th November 1974, things were not quite as passive. 

It has come to be known as the Arecibo message. An interstellar radio message that was beamed out from the Arecibo telescope in the direction of the globular star cluster M13 – around 25,000 light-years from Earth. This wasn’t so much a message but rather a small compilation of information designed to showcase humanity set out in different colours. It included,   

  • The numbers 1 to 10. 
  • The atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus, which make up deoxyribonucleic acid (DNA) 
  • The formulas for the chemical compounds that make up the nucleotides of DNA
  • The estimated number of DNA nucleotides in the human genome, along with an image of the double helix structure of DNA 
  • The physical height of an average man, a figure of a human, and the human population of Earth 
  • A graphic of the Solar System, which showed roughly where the signal came from
  • An image of the Arecibo radio telescope

The message included 1,679 binary digits, approximately 210 bytes, and was transmitted at a frequency of 2,380 MHz. Needless to say – well, as far as I know, anyway – no response has been received. 


The Arecibo’s downfall began long before its demise came in December 2020. Financial cuts began eating into it and its budget of $10.5 million in 2007 fell to just $4.0 million in 2011. Funding was cobbled together from a variety of sources including the Puerto Rican government, NASA, private funding and even assistance from the American Recovery and Reinvestment Act of 2009 – but the writing was on the wall. 

In 2017, Hurricane Maria tore off parts of the receiver which damaged the dish below. While the scale of the damage was small, it proved to be another nail in the storied telescope’s coffin. In August 2020, a support cable snapped leaving a 30 metres (100ft) gash in the dish and in November, one of the support towers collapsed inflicting even more damage to the Arecibo. An announcement was made that there was not a great deal that could be done except ensuring the safety of those on-site. Plans for the safe decommissioning of the observatory were still being developed when on 1st December 2020, a second tower collapsed, this time bringing down the massive receiver in the centre, causing a huge amount of damage. 


It’s difficult not to feel like this is a rather sad ending for such a wondrous device that has taught us and showed us so much. Yes there are certainly other telescopes around the world that have caught up, and indeed now work better than the Arecibo, but its contribution to our knowledge of space has been enormous. From the groundbreaking exploration of the Pulsar Stars to the wonderfully ambitious Arecibo message, this gigantic observatory located in the Puerto Rican forest has certainly played its part in space exploration.   

It’s not immediately clear what happens now to the Arecibo Observatory, but it’s days of searching the universe are presumably over. But I do like to think that one-day extraterrestrial life might reply to the Arecibo message or even visit the spot that it was sent from. And maybe they will stare down at the vast forest below and wonder – where the hell did that message come from?

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