Before its collapse in 2020, this 305-meter dish sent humanity's first intentional message to aliens, mapped Venus through clouds, and discovered the first exoplanets. Orbiting a dead star.
Mariordo (Mario Roberto Durán Ortiz), CC BY-SA 4.0, via Wikimedia Commons
The Giant Ear in the Sinkhole
Located in a natural limestone sinkhole in Puerto Rico, the Arecibo Telescope was a large engineering project. Completed in 1963, its main feature was a 305-meter (1,000-foot) spherical reflector dish. The dish was not a single solid piece; it was made of 38,778 perforated aluminum panels, covering an area of about 18 acres. Above the collector, a 900-ton instrument platform was suspended 150 meters (492 feet) in the air by massive steel cables connected to three concrete towers. This design made Arecibo the world's largest single-aperture telescope for 53 years, until China's FAST telescope was completed in 2016.
Unlike many optical telescopes, Arecibo's main dish remained stationary. To observe different parts of the sky, astronomers moved the receivers on the suspended platform. The observatory was also equipped with powerful radar transmitters. This dual capability allowed it to passively listen to radio waves from deep space and to actively bounce signals off celestial bodies. In 1981, scientists used this radar to pierce the thick, opaque clouds of Venus and produce the first surface maps of the planet. The telescope also determined that Mercury's rotation period was 59 days, not 88 as previously thought.
A Message, Pulsars, and a Final Collapse
On November 16, 1974, to celebrate a major upgrade, Arecibo broadcast a pictorial message toward Messier 13, a globular star cluster 25,000 light-years away. The message, designed by a team including Frank Drake and Carl Sagan, was a 1,679-bit binary transmission that lasted less than three minutes. It depicted, in simple graphics, the numbers 1 to 10, the atomic numbers of key biological elements, the DNA double helix, a human stick figure, and a diagram of our solar system.
Arecibo's most unexpected discovery came in 1992, when astronomers Aleksander Wolszczan and Dale Frail confirmed the first planets ever found outside our solar system. These planets were not orbiting a sun-like star. Instead, they were found circling PSR B1257+12, a pulsar—the tiny, dense, rapidly spinning remnant of a massive star that has exploded. The discovery of planets that survived a supernova was an important astronomical finding. The observatory also was central to studying Fast Radio Bursts (FRBs), including identifying the first repeating burst, FRB 121102, in 2012.
After 57 years of operation, the observatory's life came to a sudden end. An auxiliary support cable failed in August 2020, followed by a main cable in November. On December 1, 2020, before a controlled demolition could be performed, the remaining cables failed and the massive instrument platform crashed into the dish below, destroying the telescope.
💡Fun Facts
The Arecibo Message will take approximately 25,000 years to reach its target, the star cluster M13.
Data from Arecibo was used by the SETI@home project, a volunteer computing initiative where people donated their home computers' processing power to search for signs of extraterrestrial intelligence.
The observatory was a filming location for the climax of the 1995 James Bond film *GoldenEye* and was featured in the 1997 movie *Contact*.
The discovery of the first binary pulsar at Arecibo in 1974 provided indirect evidence for gravitational waves and earned Russell Hulse and Joseph Taylor the 1993 Nobel Prize in Physics.
The site is currently closed to the public. A new science and education center, Arecibo C3, is anticipated to open in November 2024, but an exact date has not been set.
Admission
To be determined for the new center. The previous visitor center had an admission fee.
Accessibility
The observatory is located in a hilly, rural area with winding roads. Accessibility details for the new center have not been released.
