For 57 years, the Arecibo Observatory, a radio/radar telescope facility located about 12 miles (19 kilometers) south of the city of Arecibo in Puerto Rico, was one of astronomy's great treasures.
Until recently, Arecibo had the biggest radio telescope in the world, and its ability to detect distant signals made it one of the world's most powerful tools for studying both planets and moons in our own solar system and mysterious objects in distant regions of the universe. Over the years, scientists used it to determine Mercury's rotation rate and map the surface of Venus, to discover the first binary pulsar and the first known exoplanet. Arecibo's researchers also made important findings about the properties and orbits of asteroids that are potentially hazardous to Earth.
Back in 1974, Arecibo was even used to broadcast a radio message toward the globular star cluster M13, 21,000 light years from Earth, filled with data that could have been deciphered by extraterrestrials to produce a simple illustration depicting a stick-figure human, our solar system, and DNA and some of the biochemicals of early life. (The message was designed by astronomer Frank Drake with the help of Carl Sagan and other scientists.)
The telescope's deterioration became evident back in August, when a support cable failed and slipped out of its dish, leaving a 100-foot (30-meter) gash in the dish, according to a National Science Foundation (NSF) media release. Engineers worked to try to figure out how to repair the damage and determine the integrity of the structure. But then, on Nov. 6, a main cable on the same tower broke as well — a hint that other cables might be weaker than initially believed.
At that point, an engineering evaluation determined that it would be too risky even to do more repair work on the telescope. On Nov. 19, NSF announced that the observatory's radio telescope would be decommissioned and dismantled.
But before that could happen, on Dec. 1, the instrument platform collapsed. The facility's 900-ton (816-metric ton) instrument platform, which was suspended by cables attached to three towers at a height of 450 feet (137 meters) above the telescope's massive 1,000-foot (350-meter) diameter radio telescope dish, suddenly broke away from its supports and fell, according to an account of the mishap from the NSF.
NSF has authorized repairs to Arecibo's LIDAR instrument and a smaller telescope used for atmospheric science. But rebuilding Arecibo's radio telescope would cost an estimated $350 million, the Associated Press reported. An NSF official indicated at a Dec. 3 press conference that it could take years for the federal government to make a decision about whether to do that, according to SpaceNews. The NSF said the observatory would not close completely — in addition to operating a 39-foot (12-meter) dish and the LIDAR instrument, the visitors center would remain open.
The sudden, shocking apparent end of Arecibo's radio telescope caused an outpouring of reminiscences on Twitter, with the hashtag #WhatAreciboMeansToMe, from both researchers who had used the telescope and ordinary people who had visited the observatory and been inspired by it. People have even chosen to get married there.
One of the commenters was astronomer Kevin Ortiz Ceballos, at the University of Puerto Rico. He fondly remembers visiting Arecibo as a young boy with his parents, who took him there to learn about the solar system. Years later, he was excited to get a chance to do science there.
"With Arecibo, I studied stars with potentially habitable planets, looking to observe flares on them and quantify their activity," Ortiz explains via email. "By better understanding the stars that host planets, we can characterize the environment that potentially habitable planets may be in. We used Arecibo's unique capabilities for very high resolution observations of flares to understand the magnetic fields and emissions of these stars.
"Last year, I observed the first interstellar comet, 2I/Borisov, with Arecibo. I looked for hydroxyl emission from the comet's tail, which is an indicator of the water-production rate of the comet — how dry or wet it may be. This measurement is important for understanding the full composition of the comet and the stellar system it may have originated from. Arecibo's huge collecting dish and specialized receivers can search for hydroxyl emission to an amazing precision, helping us constrain the comet's activity."
Arecibo's Stellar History
Arecibo was built back in 1963 at a cost of $9.3 million (close to $80 million in today's dollars), due in large part to the efforts of Cornell University physicist William E. Gordon, who was interested in studying the Earth's ionosphere. Gordon chose Puerto Rico for the site because the sun, moon and planets pass almost directly overhead. Plus, a natural sinkhole south of the city of Arecibo provided a cost-effective way to support his design of a spherical, bowl-shaped reflector planted in the ground, with a movable receiver hanging over it.
Pretty quickly, scientists realized the observatory also would be useful in the then-new fields of radio and radar astronomy. In 1965, one of the observatory's first great accomplishments was to discover that the true rotation rate of Mercury, the nearest planet to the sun, was just 59 days, not 88 as had been previously estimated, according to a list of accomplishments on the observatory website. In 1968, Arecibo scientists showed that sporadic radio pulses from the direction of the Crab Nebula supernova remnant came from a pulsar located at the center of the nebula.
Other important discoveries followed. In 1974, Arecibo was used to discover the first pulsar in a binary system, which provided important confirmation for Albert Einstein's theory of general relativity. Astronomers Russell Hulse and Joseph Taylor, Jr. were awarded the Nobel Prize in 1993 for that work.
In the 1980s and 1990s, scientists used Arecibo to make more discoveries about the solar system. They used the telescope's radar to produce the first-ever maps of the surface of the planet Venus, whose thick cloud layer had blocked optical telescopes' view. They also found that despite Mercury's high surface temperature, the planet still has ice in shadowed craters at its north and south poles.
In 2003, Arecibo provided evidence for the existence of hydrocarbon lakes on Titan, a moon of Saturn.
In recent years, Arecibo has continued to gather important information, including helping to calculate distances important for understanding the universe. It's also produced radar images of Mars that revealed lava flows and other geological features that hadn't been detected in visual images of the Red Planet.
Arecibo also played in important role in studying asteroids that could pose a danger to life on our planet.
"The Arecibo Observatory was the largest radar in the world, capable of measuring the size and position of any dangerous asteroids approaching Earth," Abel Mendez explains in an email. He's an astrobiologist and director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo.
"Most radio telescopes, like the biggest one in China, do not have the radar capability of Arecibo," says Mendez, who used the Arecibo telescope to observe the radio emissions of red dwarf stars, according to his university web page. "We now rely in less powerful radars around the world to study asteroids, which might reduce the warning period of any threat."