Goodbye, Arecibo

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UPDATE: On December 1, 2020, the Arecibo Observatory collapsed following its decommissioning by the National Science Foundation the previous month. No injuries were reported.

For a half-century, it was the largest radio telescope in the world. But the Arecibo antenna will soon be taken apart and trucked away.

One of small number of cables (each as thick as your arm) holding the receiver platform high above the dish failed last August. Despite this calamity, plans were quickly drawn up for repair. Then, on November 6, a sister cable gave way. Engineers realized that the mechanical health of the telescope was far poorer than they had believed. The National Science Foundation, which had long sponsored the instrument, declared it mortally wounded.

Arecibo was always the big guy on the team. Its 1,000-foot diameter main reflector, constructed of aluminum panels perforated like ceiling tile, is hunkered down in a natural sinkhole in the karst of northwestern Puerto Rico. This location was no accident. Since early plans for the instrument foresaw study of the solar system, a site close to the equator was desirable. The planets would then pass more or less overhead.

The most southern real estate in the U.S. is on Hawaii’s big island, but the Cornell team designing the instrument figured that Hawaii was too far from Ithaca, and opted instead for Puerto Rico. It’s said that the telescope’s principal architect, Cornell engineer William Gordon, chose its exact location by sliding a nickel around a topographic map of Puerto Rico until he found a natural depression in which the antenna would fit.

For a half-century, Arecibo’s size, and consequent sensitivity, were unrivaled. New Mexico’s Very Large Array has but one-fifth the collecting area, despite sporting 27 antennas. The VLA would get lost in the vast aluminum expanse of Arecibo’s main reflector.

The telescope has some limitations: It can only be aimed over a 40 degree range of angles at any given time. But the Earth’s rotation means that, over the course of a day, approximately one-third of the cosmos is accessible to Arecibo. For most observing programs, that’s plenty.

But while it can see much of the sky, Arecibo does so with a telephoto lens. Its typical field of view is ten times narrower than the full moon. Like other single-dish antennas, it doesn’t have the resolution – the ability to see small detail – that antenna arrays can deliver. Precise radio maps of the heavens are not its métier.

However, when your research is all about photons – about studying weak or distant emitters of radio waves – then Arecibo goes to the head of the class. It’s simply a bigger bucket for collecting radio waves coming to us from elsewhere in the universe. I spent a lot of time at Arecibo studying galaxies that were many millions of light-years away. The antenna is also frequently used for pulsar research and measurement of the emission from planets in our own solar system. Even sky-savvy folk seem to forget that it was Arecibo that Polish astronomer Alex Wolszczsan used to discover the first planet around another star in 1992.

There’s another thing that distinguishes Arecibo from its dishy peers: An ability to transmit. When it was first planned, Arecibo’s number one assignment was expected to be a routine aiming of its transmitter at the sky to measure the amount of energy reflected from the ionosphere. Better understanding of the idiosyncrasies of this charged atmospheric layer was important for long-distance radio communication, a matter of considerable interest to the military. This capability led to the moniker given the telescope by the Puerto Rican locals: El Radar.

While such studies have continued, Arecibo’s monster 2 megawatt transmitter has been used for other projects, including early mapping of the topography of cloud-covered Venus and more recent studies of asteroids. But perhaps its most celebrated transmitting experiment took place in 1974 when Frank Drake – the observatory director at the time – celebrated an upgrade to the antenna by transmitting a 3 minute-long, friendly message to the globular star cluster, M13. This was, and remains, the most powerful message ever sent from Earth. Given the cluster’s distance, you can expect a reply in about 44,000 years. Or not.

However, Arecibo’s efforts to locate alien societies have largely been its listening experiments: SETI. While there were some early SETI observations made on an ad hoc basis, in 1992 the NASA SETI Program began a study of 1,000 star systems at Arecibo. Some of these stars were several hundred light-years away, so Arecibo’s unrivaled sensitivity was a major selling point.

After the cancellation of the NASA program scarcely a year later, private- and university-funded projects kept Arecibo in the SETI game. The Berkeley SETI group ran a piggyback observing program that took advantage of a second receiver on the antenna to observe random parts of the sky while the main receiver was in use by other astronomers. The SETI Institute used Arecibo for three years beginning in 1998 as part of its Project Phoenix, a scrutiny of about 800 nearby star systems.

For those astronomers and SETI researchers who have spent time at the Puerto Rican installation, the loss of this telescope is akin to hearing that your high school has burned down. Observing at Arecibo was much like going to summer camp. You had one job: do the experiment, while enjoying the conviviality of the friendly local telescope operators and staff, as well as the kitchen’s first-rate chicken-and-rice dinners. For two weeks you were on-site while the rest of the world, with its incessant demands and petty annoyances, vanished in the haze of distance.

Losing Arecibo is like losing a big brother. While life will continue, something powerful and profoundly wonderful is gone.