The Arecibo Observatory collapsed this week. What now?

2020-12-04 14:00:00

Arecibo’s disk suffered damage in November, before the suspended receiver collapsed completely in December.

Arecibo’s disk suffered damage in November, before the suspended receiver collapsed completely in December. (University of Central Florida/)

On December 1, a crucial cable snapped at the iconic Arecibo Observatory in Puerto Rico. After 57 years catching radio waves from around the cosmos, the 900-ton receiver fell through the air to the ground below, slashing through thousands of the aluminum panels that made up the 300-meter-wide dish.

While the observatory’s collapse shocked and horrified the astronomical community, it did not come as a surprise. The reality of the instrument’s decay had become fully apparent in November, after the National Science Foundation reviewed two recent cable breaks and decided to decommission the world-class radio telescope. But for a field that thinks in decades, a month passes in a flash, and stunned researchers are only just beginning to contemplate their uncertain futures.

Redundancy is a luxury that astronomers don’t have. To maximize the value of sparse funding for expensive projects, planners never build the same instrument twice. As a result, even as the community welcomes a new cutting-edge facility in China and looks forward to the next generation of radio telescopes, Arecibo—like all observatories—filled an essential niche. While many Arecibo projects may theoretically be able to relocate (although they may have trouble in practice), others have been brought to a halt. The loss of Arecibo’s unique broadcast capabilities and frequency range—not to mention its social role as a hub of scientific activity—will hamstring radio astronomy for years to come.

“There’s a lot of projects that right now cannot be done as well at any telescope in the world,” says Maura McLaughlin, an astronomer at West Virginia University.

Arecibo was more than an exquisitely sensitive ear tuned to pick up faint radio waves from deep space. It also had a booming radio voice, unmatched by any other facility on the planet, one that researchers used in 1974 to broadcast a radio message to any inhabitants of the bundle of stars known as M13. The message described our solar system, human anatomy, and the design of the Arecibo dish itself.

Since then, the facility has communed mainly with objects a bit closer to home: asteroids. When broad surveys at other telescopes find new space rocks, NASA has used Arecibo’s radar abilities to figure out how dangerous the objects really are. The transmitter would beam out radio waves at an asteroid and, based on how they were reflected back, researchers could determine the rock’s size, shape, and path.

One of Arecibo’s final observations, at the end of July, was to take a closer look at one of the more threatening asteroids discovered this year. Asteroid 2020 NK1 is a house-sized object that initially had an estimated 1 in 70,000 chance of crashing into Earth toward the end of the century. But with Arecibo’s radar readings, NASA was able to conclude that the asteroid will never get closer than 2.5 million miles from the planet.

The only other radio dish that can effectively bounce radio waves off of asteroids, according to Anne Virkki, the principal investigator of Arecibo’s planetary radar program, is the Goldstone Deep Space Communications Complex in California. As a station in NASA’s Deep Space Network, however, the agency keeps Goldstone rather busy communicating with its fleet of robotic missions scattered throughout the solar system. Arecibo studied roughly 100 asteroids annually, but Goldstone will be able to handle just half as many objects, Virkki estimates. The Green Bank telescope in West Virginia is planning to add radar to its dish, but the beam will be weaker and narrower than Arecibo’s. For the foreseeable future, Earth will be flying blind.

“The whole planetary radar program was run at the [Arecibo] Observatory,” Virkki says. “That will be heavily affected.”

Many Arecibo projects don’t need radar and could conceivably move to other radio telescopes. But directors don’t exactly let their multi-million-dollar facilities sit idle, so there’s little slack in the system to take in the now homeless Arecibo studies.

McLaughlin participates in the NANOGrav collaboration, which teeters on the brink of detecting the first gravitational waves emanating from collisions between the supermassive black holes that lie at the hearts of most galaxies. Subtle irregularities among the beats of millisecond pulsars—spinning neutron stars that flash a lighthouse-like signal toward Earth hundreds of times each second—might reveal these ripples in spacetime. NANOGrav researchers have spent more than a decade patiently watching 80 pulsars, 40 from Arecibo and 40 from Green Bank. They recently saw hints of a discovery in the first dozen years of data and hope for a conclusive result soon when they process the entire 16-year run, which will become part of Arecibo’s scientific legacy.

But to achieve their ultimate goal of pinpointing specific clashes between titan black holes, the collaboration needs more time. Arecibo devoted 800 hours a year to the project, and the group simply can’t afford to buy that level of observation on the Green Bank telescope (and if they did, it would come at the price of other research). The astronomers currently hope to glean additional information about the now unmonitored 40 pulsars from datasets collected by international partners, but their expectations for the program’s long potential have collapsed along with Arecibo’s receiver.

“That’s where we’re really going to suffer from the loss of Arecibo, McLaughlin says. “We’ll really start seeing the hit in three, four, five years from now.”

Nevertheless, the astronomical community is resilient. Arecibo’s absence will soon become another feature of the radio astronomy landscape for the NSF and other organizations to consider when deciding which new projects to fund, and for researchers to weigh when they apply for observing time.

And that landscape continues to evolve. Researchers mourn the loss of Arecibo while welcoming the arrival of China’s Five-hundred-meter Aperture Spherical Radio Telescope (FAST), the world’s biggest single-dish radio telescope, which started accepting proposals from international astronomers last year after more than a decade of construction and testing. With more than twice the area of Arecibo, the facility will have sharper reception for some celestial targets, although in its current configuration it can’t pick up as many frequencies.

Radio astronomers also hope for progress on the Square Kilometer Array, an international project aiming to amass a square kilometer of data-collecting area by joining thousands of smaller dishes in the deserts of South Africa and Australia. The observatory would take images of the radio universe fifty times crisper than any other telescope. After thirty years of planning, construction could begin next year and the facility could be operational by the end of the decade, although whether the member countries can raise the billion dollars needed to get started remains uncertain.

Researchers in the US have their sights set on a more modest project, the Deep Synoptic Array – 2000. Spearheaded by the California Institute of Technology, the DSA-2000 would boast 2,000 dishes each roughly 5 meters across. The array would come relatively cheap, at an estimated $100 million dollars. The project is in the conceptual design stage, but with proper funding could begin surveying the sky around the middle of the decade. The combined collecting area would be much larger than what’s possible with a single dish, and the distributed array would be less vulnerable to catastrophic failures.

In the meantime, however, astronomers emphasize that the end of the large Arecibo dish must not mean the end of the Arecibo scientific community. After decades of operations, McLaughlin says, the area has become home to extremely talented engineers with valuable expertise in cryogenics, receivers, and other machinery necessary to keep a giant telescope running. If funding to rebuild Arecibo’s dish in full cannot be found, both she and Virkki hope that the site can keep participating in research in other ways. Astronomers could use the infrastructure to hold conferences, for instance, or they could erect a small array of radio dishes that could at least partially fill the hole left by the fallen giant.

“In time, that void will begin to be replaced,” McLaughlin says. “I do hope that Arecibo will continue to contribute.”

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