1. The Dish Was First Meant to Study the Air High Above Earth
The idea came from William Gordon, an engineering professor at Cornell University in New York State. His research interest was the ionosphere, a very thin layer of the upper atmosphere where sunlight and other energy have knocked electrons loose from atoms [2]. Gordon proposed using radar: send a strong radio beam upward, then listen for the faint echo scattered back by the electrons. From that echo, scientists could work out how dense and how hot the layer was [1][2].
There was a catch. The echo is extremely weak, so the dish has to be enormous to catch enough of it. The design the planners settled on was an antenna about 305 meters (1,000 feet) across [1]. That is roughly the length of three American football fields laid end to end. The big question became how to build and aim something that large without spending a fortune.
2. A Sinkhole and a Hanging Receiver Made the Giant Dish Affordable
The solution was to keep the dish fixed in the ground. The engineers used a spherical, bowl-shaped reflector fixed in the ground, with a movable receiver hanging above it [1]. Building a dish that big on a frame that could tilt would have been far too expensive with the designs available then [1]. Instead, the team used a natural sinkhole in the limestone hills of northern Puerto Rico, which gave an economical way to support the bowl. Construction began in 1960, when crews moved into the sinkhole [1][2].
A fixed bowl can still be aimed, thanks to its shape. A spherical reflector focuses incoming radio waves along a line rather than at a single point, so moving the receiver to a new spot on that line points the telescope at a different part of the sky [2]. The price of this trick was a narrow view: the beam could be pointed only within about 20 degrees of straight overhead [1][2]. So the same choice that made the telescope affordable and huge also limited how much sky it could see. The dish's surface covers about 18 acres [1].
That was the answer to this article's question in a nutshell: a very large collecting area, plus a strong transmitter, turns out to be useful for far more than the ionosphere. The next sections show how.
3. Bouncing Radio Waves Off Mercury and Asteroids
The same radar that probed the ionosphere could be pointed at planets. Mercury was an early target. For a long time, astronomers believed Mercury spun once every 88 days, the same time it takes to go around the Sun. Radar measurements showed that its rotation takes only about 59 days instead [2].
Later, the radar was used on small bodies near Earth. By the early 2010s, the Arecibo radar had been used to study many near-Earth asteroids [2]. Sending a signal and reading the echo is one way to use the dish. The other is to simply listen, which is where pulsars come in.
4. A Pulsar With a Partner Became a Test of Einstein's Gravity
A pulsar is a rapidly spinning, very dense star whose radio "beacon" sweeps past Earth like a lighthouse beam [3]. In 1974, Russell Hulse, a graduate student, and his professor Joseph Taylor were searching for pulsars with the Arecibo dish. They found one whose beat did not stay steady. From the way the signal changed, they could tell that the pulsar was circling a companion of roughly the same mass, at a distance of only a few times the distance from Earth to the Moon [3]. It was the first pulsar found to be part of a binary system [2].
That pair behaved in a way that Newton's theory of gravity could not fully explain, so it became a natural laboratory for testing Einstein's general theory of relativity. According to the Nobel committee, Einstein's theory passed the tests with flying colors [3]. Hulse and Taylor shared the 1993 Nobel Prize in Physics for this discovery [3].
5. Tiny Timing Errors in a Pulsar Revealed the First Known Planets Beyond Our Solar System
A pulsar's beat is so regular that it works like a clock. In 1992, Aleksander Wolszczan and Dale Frail used the Arecibo telescope to time the pulses of a fast-spinning pulsar called PSR 1257+12. Small, repeating shifts in the arrival times told them that the pulsar was being orbited by at least two bodies about the size of planets [4]. Their minimum masses were about 2.8 and 3.4 times the mass of Earth, and they circled the pulsar in roughly 67 and 98 days [4].
These planets orbit a pulsar, not a star like our Sun. Even so, Physics Today describes the find as the first known extrasolar planetary system [2]. The planets were found not by taking a picture, but by studying tiny changes in the timing of a signal.
6. The 1974 Arecibo Message Was a Demonstration, Not a Phone Call
The dish could transmit as well as receive. In 1974, during ceremonies marking a major upgrade, it sent the most powerful broadcast ever deliberately beamed into space up to that time. It was aimed at M13, a globular star cluster that holds about a third of a million stars and lies roughly 21,000 light-years away [5].
The message was 1,679 bits, sent as two alternating tones at about 10 bits per second, and the whole broadcast took less than three minutes [5]. The number 1,679 equals 23 × 73, and both 23 and 73 are prime numbers, so apart from a single row there is no other rectangle besides 23 by 73 (or 73 by 23). Arranged in 73 lines of 23 characters, the bits form a picture that includes the telescope, our solar system, DNA, and a stick figure of a human [5]. The SETI Institute notes that the prime numbers may help any alien receiver decode it [5].
Nobody expected a quick answer. The signal needs about 21,000 years to reach M13, and a reply would need as long to come back. The SETI Institute says a reply is unlikely, and describes the value of the experiment as getting people to think about the difficulties of communicating across space, time, and a presumably wide culture gap [5]. Its effective power was equivalent to a 20-trillion-watt broadcast sent equally in all directions, because the dish squeezed the energy into a very small patch of sky [5].
7. What Happened When the Telescope Collapsed in 2020
In August 2020, one of the cables holding the receiver platform unexpectedly came loose. On November 6, a second cable broke [6]. On November 19, the National Science Foundation (NSF), the United States agency that funded the observatory, announced a plan to take the telescope apart for safety reasons [6]. Before that could happen, the 900-ton instrument platform fell onto the dish at about 7:55 a.m. local time on December 1. No injuries were reported [6].
The site did not close entirely. The University of Central Florida, which helped run the observatory, reported that NSF would keep paying the staff, repair a smaller 12-meter telescope and a laser-based atmosphere-sensing facility, and bring other scientific and educational facilities back online [6]. Years earlier, Physics Today had noted that the visitor center was the only science museum serving Puerto Rico's population [2].
8. Check the Numbers Yourself, Then Read the Originals
You can test some of the numbers in this article with a pencil. The dish is 305 meters across, so its radius is about 152.5 meters. Multiply 152.5 × 152.5 to get about 23,256, then multiply by 3.14. You should get roughly 73,000 square meters, which matches the 18 acres reported (18 acres is about 72,800 square meters).
For the message, work out 23 × 73 on paper: 23 × 70 = 1,610 and 23 × 3 = 69, which add up to 1,679. Then try a number such as 15. How many different rectangles can you make from 15 dots? Compare that with 1,679: apart from a single row, its only rectangle is 23 by 73 (or 73 by 23). If you want to go deeper, the Physics Today article in the sources is a good place to start, and it is written for a general audience.