For nearly six decades, Arecibo Observatory in Puerto Rico served as a scientific leader at the forefront of space and atmospheric sciences, radio astronomy, and planetary radar astronomy, as a marvel of science and engineering, as a source of inspiration, and as a cornerstone of the local community and the island itself.

In the late 1950s, the concept of Arecibo Observatory was the vision of William E. (Bill) Gordon of Cornell University for a large radar facility on the order of 305 meters (1000 feet) in diameter, necessary for detailed study of Earth’s ionosphere. The original design called for a stationary parabolic dish that only viewed the zenith point directly overhead; however, it was realized that a large spherical dish with a movable focusing platform suspended some 150 meters (500 feet) overhead would allow for tracking of astronomical objects. In a proposal written by Gordon in 1958, the main objective of the facility would be to study the electron density and temperature of the ionosphere, but it is noted that the same facility could perform radar observations of solar system objects and receive radio emissions from distant sources in outer space. In 1959, Cornell signed a contract to design and build the telescope with Air Force Cambridge Research Laboratories, which had been designated by the federal Advanced Research Projects Agency to manage the project. Many members of the neighboring communities around Arecibo helped construct the observatory, including author Rivera-Valentín’s grandfather. Finally, on November 1, 1963, Arecibo Observatory was inaugurated and took its place as a leader in multidisciplinary science.

Arecibo Observatory was built in northwest Puerto Rico at the southern edge of the municipality of Arecibo, nestled in a limestone sinkhole among the surrounding karst terrain abutting the Tanama River. The sinkhole provided a natural depression that could support the massive spherical-cap shape of the dish with minimal modification to the environment. The initial form of what became known as the William E. Gordon telescope (see Fig. 1) was made of a metal mesh, similar to chicken wire, allowing one to see straight through it into the sinkhole below. This is because the first observations with Arecibo were at sufficiently long wavelengths (low enough frequencies) that the mesh was opaque to radio waves. The triangular structure that held the line feeds, used to focus and receive the reflections from the spherical dish, was suspended by steel cables from three towers around the perimeter of the sinkhole. A circular track mounted below the triangle and an arc-shaped track below that gave the feeds the ability to move in azimuth and elevation to track specific regions of the atmosphere and astronomical objects in the sky.

As intended, early atmospheric radar observations at Arecibo studied the composition of the ionosphere and led to characterization of the electron temperature and electron density with time. However, with such a powerful facility, radar studies moved beyond the ionosphere, making several important observations in the nascent field of planetary radar astronomy. Radar observations of the planet Mercury showed that it rotates prograde (west to east) in 59 days rather than synchronously with its 88-day orbital period about the Sun, while radar observations of Venus confirmed it slowly rotates retrograde (in a reverse or backward direction) in about 245 days. The ability of radar to measure the distance to objects with extreme precision led to refinement of the astronomical unit, which sets the distance scale for the solar system, and measurement of the perihelion shift of Mercury in accordance with the theory of general relativity. When not actively transmitting radar signals, Arecibo was used for radio astronomy, studying the universe at frequencies of tens of megahertz (MHz) to 600 MHz (wavelengths of tens of meters to 0.5 meters). In 1968, Arecibo observations of the Crab Nebula determined the pulsar at its center (pulsars having been discovered only a year prior) rotated once every 33 milliseconds.

In the late 1960s, ownership of Arecibo Observatory was transferred to the National Science Foundation (NSF) and in the early 1970s, in partnership with NASA, the surface of the telescope was upgraded to 38,778 perforated aluminum panels. Shrinking the size of the holes in the dish allowed for higher-frequency (shorter-wavelength) observations including the 1.42-GHz (21-centimeter) line of hydrogen for radio astronomy and the addition of a powerful 2.38-GHz (12.6-centimeter) transmitter for planetary radar.

Over the next two decades, Arecibo continued to make breakthroughs in its main fields of study. The atmospheric radar was used to simultaneously heat and study the E and F regions of the ionosphere and detect an ionized layer of helium. For the first time, the radar was used to simultaneously measure two ion temperatures (O⁺ and H⁺) and three compositions (O⁺, H⁺, and He⁺), continuing the original objective of the facility to understand the abundance and temperature of species in the upper atmosphere.

Some of the seminal discoveries by Arecibo of this era for radio astronomy again related to pulsars. Arecibo was used to discover the first binary pulsar in 1974, which resulted in the 1993 Nobel Prize in physics awarded to Russell A. Hulse and Joseph H. Taylor. Periodic monitoring of this double-pulsar system provided indirect evidence for gravitational waves as predicted by the theory of general relativity. In 1982, the first millisecond pulsar was discovered at Arecibo, leading to the refined use of pulsars as extremely precise clocks in the search for direct evidence of gravitational waves. In 1992, precise timing of a pulsar led to the discovery of the first exoplanets outside our own solar system.

For planetary radar, increased sensitivity allowed for detection and characterization of near-Earth asteroids, main-belt asteroids, and the nuclei and comae of comets. Meanwhile, radar observations of the terrestrial planets continued with improved resolution, producing geologic maps of the Moon and the surface of Venus as well as finding evidence for ice at the poles of Mercury. This was a bountiful time for science at Arecibo and it continued to improve with the Gregorian upgrade in the mid-1990s, which, for planetary radar specifically, increased sensitivity by a factor of 15.

The Gregorian upgrade gave the William E. Gordon telescope the form that most are familiar with (see Fig. 2). The golf-ball-like dome was the equivalent of a four-story building suspended above the dish and housed the planetary radar, a suite of receivers up to 10 GHz (3 centimeters), and the secondary and tertiary reflectors used to remove the spherical aberration caused by the spherical primary dish. While a line feed remained in use for the atmospheric radar, the optics of the Gregorian dome allowed for wider-bandwidth receivers and increased sensitivity at the expense of adding weight to the suspended structure, totaling some 900 tons.

In addition to improvements in sensitivity, space and atmospheric sciences benefited from the long baseline of observations over time with Arecibo. Observations of neutral winds in the thermosphere show strong variations with time and season based on data collected over three decades. A similarly lengthy baseline found that the momentum balance between ionospheric O⁺ and oxygen in the thermosphere varies with local time, season, and solar cycle, whereas trends with solar cycle were only detectable on the decadal timescales provided by the long-term operation of Arecibo. With the upgraded facility, radio astrometry milestones with Arecibo included using a multi-beam receiver to efficiently map hydrogen in the galactic and extragalactic neighborhood; detecting organic molecules, the building blocks of amino acids, in other galaxies; discovering the first repeating fast radio burst (extragalactic millisecond pulses); and timing a pulsar in a triple-star system, providing the most stringent test yet to the strong equivalence principle of the theory of general relativity, among many other accomplishments.

Planetary radar efforts, especially the study of small bodies, accelerated after the Gregorian upgrade with over 850 detections of near-Earth asteroids (see Fig. 3) and several historic results, including the first direct evidence for binary near-Earth asteroids (asteroids with their own moon); the first direct evidence for triple near-Earth asteroids (asteroids with two moons); evidence for non-gravitational accelerations affecting the orbits and spin states of asteroids; detection of asteroids as small as 2 meters (described at the time as detecting President Obama at five times farther away than the Moon); and extensive spacecraft mission support, providing precise astrometry to aid in navigation as well as physical characterization and shape modeling of small bodies eventually visited by NASA spacecraft. Radar observations of the Moon produced maps of the lunar nearside with resolution (~100 meters) surpassed only by the most recent generation of spacecraft. While spacecraft imaging can provide finer-resolution images, radar provides the added dimension of penetration into the subsurface, revealing geologic structure below the dusty regolith unseen by optical cameras. Radar of the terrestrial planets was pushed to kilometer-scale resolution to understand the distribution of ice at the poles of Mercury, search for active volcanism on Venus, and characterize spacecraft landing sites on Mars (see Fig. 4).

Fittingly, one of the final acts of Arecibo planetary radar was the removal of potentially hazardous asteroid 2020 NK1 from the list of possible future Earth impactors. As a newly discovered asteroid in the summer of 2020, the exact orbit of 2020 NK1 was uncertain and impacting trajectories were possible later this century. Arecibo radar observations on July 31, 2020, provided precise measurements of the asteroid’s velocity and distance from Earth that greatly reduced its orbital uncertainties and allowed accurate prediction of its trajectory more than 400 years into the future, ruling out an Earth impact. Just 10 days later, observatory operations were shut down. The final observations completed were searching for new pulsars, timing known pulsars to detect gravitational waves, monitoring repeating fast radio bursts, and mapping extragalactic hydrogen, all appropriate representatives of the science of Arecibo Observatory.

On August 10, an “auxiliary” cable that supported the suspended platform slipped free from its socket atop one of the three towers and slashed through the dish below. The auxiliary cables were added between the towers and platform in the 1990s to help support the weight of the Gregorian dome. The cause of the failure remains under investigation. Observations were immediately halted and engineers began studying the stability and safety of the structure and how to proceed with repairs. On November 6, in the midst of the study to stabilize the structure, one of the main cables from the telescope’s original construction gave way, snapping at the top of the same tower as the auxiliary cable. After this failure, on November 19, NSF announced the decommissioning of the observatory in an abundance of caution given that another cable failure would likely result in the collapse of the structure. In the early morning of December 1, that break happened and the platform fell. The rapid change in tension ripped the tops off of all three towers. The falling cables gouged the dish. The 900-ton platform crushed everything below it as it plummeted like a pendulum into the side of the sinkhole. [Note:  Following the writing of this article, NASA released the findings of an investigation into the failure of the auxiliary cable: https://ntrs.nasa.gov/citations/20210017934.]

In the months since, crews have worked to prevent environmental damage from potentially harmful materials such as oil and lead that fell to the ground inside the sinkhole, to salvage pieces of historical and sentimental interest, and to remove the fallen structure. While the iconic image of the platform suspended above the dish is gone, a significant fraction of the dish itself remains. The dipole antennae for the ionospheric heating facility that peeked through the bottom of the dish were relatively undamaged as well as the instruments not located on the platform, e.g., the 12-meter radio telescope and other atmospheric monitoring equipment on neighboring hills. While this is a tremendous loss of an incredibly sensitive instrument for space and atmospheric sciences, radio astronomy, and planetary radar astronomy, the site infrastructure and much of the primary surface remains, giving hope for possible reopening for science and the public in the future.

Of course, there is no way to do justice in this article to the generational scientific advancements made over six decades at Arecibo Observatory. And the legacy of Arecibo goes beyond pure science; it extends to the people and students inspired by the observatory. Arecibo Observatory has hosted a research experience for undergraduates program since 1972, including this summer, serving hundreds of students plus many more funded through other means. These programs have been especially helpful in serving underrepresented groups in science, technology, engineering, and mathematics (STEM). Additionally, hundreds of undergraduates, graduate students, postdocs, faculty, and staff at other institutions have visited Arecibo over the years for observer training and performing their own observations. Many of these students have gone on to complete undergraduate and graduate degrees in STEM fields, including writing nearly 400 Ph.D. theses involving research done at Arecibo.

Over the last several years, Arecibo extended its student development to hosting a pre-college research and supplemental education program for high school students from across Puerto Rico called the Arecibo Observatory Space Academy (AOSA). As part of the program, students experienced the full breadth of the research cycle by proposing a work plan, conducting their research, and presenting their results at an onsite symposium. AOSA served more than 300 students, and of those that are of college age, more than 90% have pursued degrees in STEM fields. Former students have developed space exploration clubs at their universities and won the NASA Revolutionary Aerospace Systems Concepts Academic Linkage mission architecture design competition three years in a row.

The awe-inspiring structure of the William E. Gordon telescope has motivated so many to seek careers in science and engineering. Part of this process is facilitated by the Ángel Ramos Science and Visitor Center, which includes exhibit space and an auditorium overlooking the telescope. The visitor center brings in up to 120,000 visitors per year; some are tourists, but many are Puerto Rican school children on field trips to see the observatory, often called El Radar, that has become part of their community identity. Arecibo Observatory is more than a landmark for Puerto Ricans; it is a source of pride and inspiration and an invaluable connection to science at the national and international level. In the wake of the tragic events of last December, many Boricuas shared on social media the momentous impact Arecibo had on them by using the hashtag #WhatAreciboMeansToMe. The loss of Arecibo is truly, deeply felt by the community in Puerto Rico and its diaspora.

The collapse of the William E. Gordon telescope, at the forefront of space and atmospheric sciences, radio astronomy, and planetary radar astronomy for six decades, is a tremendous loss to the science, engineering, and local communities. For all three fields, the loss of Arecibo is immense in terms of sensitivity, flexibility, and redundancy. For instance, without Arecibo, planetary radar relies solely upon the Goldstone Solar System Radar (GSSR) in California, whose primary objective is spacecraft communication and is less sensitive than Arecibo by a factor of 15 or so. While tremendously capable in its own right, the GSSR cannot replace the workhorse nature of Arecibo for planetary radar in tracking near-Earth asteroids, characterizing main-belt asteroids, studying the polar deposits of Mercury, or mapping the surface of Venus. These losses will act as the motivation to develop a new telescope at Arecibo Observatory. The observatory’s science and technical staff has already begun looking into designs for a next-generation Arecibo telescope; the government of Puerto Rico has pledged several million dollars to pursue design studies and NSF hosted a workshop to discuss pathways forward for the Arecibo Observatory site. Whatever the next steps for a next-generation telescope at Arecibo Observatory, whether a single dish, an array of smaller dishes, or something completely different, all concepts will require extensive design, evaluation, community input, and, most of all, funding.

In the wake of Hurricane Maria’s devastation of Puerto Rico in 2017, a common social media hashtag, #PRSeLevanta, emerged, saying that Puerto Rico will recover, and it remains apt here. The island of Puerto Rico and its people are resilient and Arecibo Observatory has countless supporters among those who have used it and been inspired by it. Many of us watched a huge part of our professional careers come crashing down that fateful day last December and a community suffered the loss of its beloved, inspirational icon. There may be some solace in knowing that the Arecibo legacy of science, engineering, and education cannot be taken away by this unfortunate event. The legacy of Arecibo will live on for decades into the future, just as Arecibo served for decades prior, in whatever form the future of the observatory takes.

For more information, visit the official website of Arecibo Observatory at https://www.naic.edu. Updates from the National Science Foundation on the status of Arecibo Observatory can be found at https://www.nsf.gov/news/special_reports/arecibo/index.jsp.

About the Authors

Patrick A. Taylor spent nine years as a staff member at Arecibo Observatory, including as group lead for planetary radar, before joining the Lunar and Planetary Institute. Edgard G. Rivera-Valentín is the first Arecibo-born staff scientist to work at Arecibo Observatory and spent four years there working with planetary radar, education, and public engagement before joining the Lunar and Planetary Institute.