Nearly two years ago one of the most iconic telescopes in the world came crashing down on itself. The Arecibo telescope—located in Arecibo, Puerto Rico—was the world’s largest single-dish telescope for most of its more than half-century of existence. A series of cable failures caused the telescope’s 817-metric-ton receiver platform to collapse onto the dish below on December 1, 2020, putting the telescope out of commission.

Last week, the National Science Foundation (NSF)—which owns and finances the Arecibo Observatory—announced that it would not be funding the telescope’s reconstruction. Likewise, the NSF’s funding plan will not provide “operational support for current scientific infrastructure” at the Arecibo Observatory, such as its 12-meter radio telescope or its Lidar facility. Instead the NSF is soliciting proposals from universities or other groups that could establish a new center for STEM (science, technology, engineering and mathematics) education and outreach at Arecibo with an annual budget of $1 million per year for five years.

Many astronomers were disappointed—but not surprised—by the announcement. “We were aware that the NSF was going to make a tough decision..., but we were expecting a little bit better than that,” says Abel Méndez, a planetary astrobiologist at the University of Puerto Rico at Arecibo. Méndez didn’t expect that the Arecibo telescope would be rebuilt but says he had hoped that the observatory would continue to receive funding for its other on-site instruments.

Still, there’s no doubt that the Arecibo telescope was the flagship instrument of the Arecibo Observatory and what allowed the site to stand out on the global stage. The telescope was built into a natural sinkhole in the northwest of Puerto Rico, which offered the perfect geography to accommodate its curved, 305-meter-diameter reflector dish. The Arecibo telescope was completed in 1963, and for decades, it was the most sensitive radio telescope in the world.

Thanks to its ability to detect faint radio signals, the Arecibo telescope was able to advance many areas of astronomy. One such area was the search for extraterrestrial intelligence, or SETI.

“I think it’s probably fair to say that, historically speaking, the Arecibo telescope has been the most important radio telescope for SETI, period,” says Andrew Siemion, director of the Berkeley SETI Research Center. Siemion explains that much of the Arecibo telescope’s early research centered on studying Earth’s atmosphere and ionosphere, but late astronomer Frank Drake pushed for the expansion of its radio capabilities—in part to scan the skies for potential alien signals. Besides trying to detect messages, the Arecibo telescope famously sent one of its own in 1974. That “Arecibo message,” designed by Drake, could be decoded into a simple pixel-art image providing basic information about life on Earth.

In addition to its contributions to SETI, the Arecibo telescope was used to identify the first confirmed exoplanets. Observing the millisecond radio pulsar PSR B1257+12, the giant dish revealed slight deviations in the rapidly-rotating star’s radio pulses, betraying the presence of at least two planets, both just a few times larger than Earth. These planets were announced by astronomers Aleksander Wolszczan and Dale Frail in 1992.

The Arecibo telescope’s radio observations continued until its collapse in 2020. “It is extraordinary—maybe unprecedented—for a radio telescope to stay as productive as it has for so long,” says Frail, who works at the National Radio Astronomy Observatory.

Yet in its later years, the Arecibo telescope began to fall out of favor among many radio astronomers. “There’s been a move towards multielement telescopes—constructing large telescopes with lots of small pieces,” Siemion explains. Rather than go to the trouble of building radio telescopes with single, massive dishes, it has become easier and more effective to set up arrays of smaller telescopes working in conjunction, he says.

That’s not to say that huge single-dish radio telescopes have been abandoned altogether. In 2016 China unveiled its Five-Hundred-Meter Aperture Spherical Telescope (FAST), which dethroned Arecibo as the world’s largest single-dish telescope. Although multidish arrays are more complicated in some respects, they are also correspondingly more functionally flexible and have thus become the preference for most modern radio astronomers. Today such arrays can be found around the world.

“I am sorry to see the Arecibo telescope go the way of so many other radio telescopes in the past,” Frail says, “as they all have had to make way for newer, more powerful telescopes.”

Yet the Arecibo telescope wasn’t just a radio telescope. It was also a radar telescope, capable of transmitting powerful beams out to objects in the solar system. Once reflected back to the dish, those beams allowed researchers to precisely measure the sizes, spins, surfaces and several other properties of the objects. Such radar probes have proved especially vital for studying potentially Earth-threatening asteroids.

Radar data from the Arecibo telescope “has been used to help us understand the range of physical properties that near-Earth asteroids could have and the range of situations we might need to be prepared for,” says Andy Rivkin, a planetary astronomer at Johns Hopkins University. Rivkin notes that the Arecibo telescope helped scientists plan out NASA’s Double Asteroid Redirection Test (DART) mission—the agency’s attempt to knock an asteroid off its course, which Rivkin helped lead. Prior to the mission’s launch, the Arecibo telescope collected precise radar data on the shape and size of the asteroid Didymos and on the size and orbit of its moonlet Dimorphos, Rivkin says, which helped NASA to successfully impact Dimorphos on September 26.

“Arecibo was absolutely still producing important data for asteroid science at the time of its collapse, and there is no replacement for its specific capabilities for high-resolution radar studies,” Rivkin says.

After the Arecibo telescope’s collapse, the NSF was faced with a few options: rebuild the telescope as it was, rebuild it in some upgraded fashion or don’t rebuild it at all. The first two options had projected price tags of several hundred million dollars. (It cost $30 million to $50 million just to clean up the collapse, according to an NSF estimate.) Preferring to spend its budget on other facilities many astronomers deemed more desirable and viable, the NSF opted not to rebuild the telescope.

Much of the radio astronomy capabilities “that were lost as a result of the [telescope’s] collapse can be recovered through additional investments in existing facilities and through international partnerships,” wrote a spokesperson for the NSF in an e-mail to Scientific American. As for radar, “NSF engaged NASA and other federal agency partners to explore next-generation ground-based radar needs.”

Regarding proposals for a new center for STEM education and outreach at Arecibo that the agency will field, “NSF recognizes the scientific, educational, historic, cultural and economic significance of the Arecibo Observatory site to Puerto Rico and the global scientific community and is focusing on leveraging the incredible STEM educational potential of the facility,” the NSF spokesperson wrote. It is not yet clear how this educational center would incorporate the observatory’s existing visitor center, which already runs educational outreach programs.

Méndez is glad that the NSF has decided to invest in local educational outreach, but he emphasizes that having the capacity for cutting-edge astronomy research on the island is likely even more critical for creating more Puerto Rican scientists. When it was still operational, visits to the Arecibo telescope inspired many local students to pursue a career in STEM—and astronomy specifically—Méndez says. When Méndez himself was younger, he already knew he wanted to study astronomy, but visiting the Arecibo telescope “gave [him] confidence,” he says. “Having this huge, important astronomical place nearby made me feel included” in the astronomical community, he explains.

Méndez hopes that the winner of the NSF’s $5-million grant to establish an education center will independently raise additional funds to support some of the site’s still-operational research instruments, including its 12-meter radio telescope, its solar radio spectrometer and its Lidar facility, which uses lasers to probe the upper atmosphere.

For now, though, it’s unclear whether the Arecibo Observatory will continue to function as a research site in any capacity. The observatory currently employs more than a dozen on-site scientists, who were unavailable to comment on the NSF’s announcement. It also employs engineers, who keep the existing instruments operational, and dozens of other staff members, from groundskeepers to building maintenance workers. “Most of those people will be out of jobs,” Méndez says.

“NSF is grateful to current and past employees of the Arecibo Observatory, who have been integral to the outstanding work at the site,” the NSF spokesperson wrote. The agency is extending the observatory’s current funding award by an extra six months to September 2023 “to provide additional time to support the transition to the proposed new Center.”

For now, the exact future of the Arecibo Observatory remains uncertain. But barring a massive fundraising campaign or a coordinated push to Congress, Méndez says, it’s unlikely that anything like the Arecibo telescope will ever be rebuilt in Puerto Rico.

“Arecibo occupies a very special place in the hearts of a lot of astronomers,” Siemion says. “Although maybe it was an expected day, it nevertheless is a sad day. It’s definitely the end of an era.”