Courtesy of Cornell University

Late Prof. William Gordon (above) was a designer of the Arecibo Observatory.

February 9, 2021

The Arecibo Observatory: Astronomical Contributions and Legacy

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In her last days in office, Gov. Wanda Vázquez Garced signed an executive order committing Puerto Rico to reconstruct the Arecibo radio observatory — a telescope Cornell managed for 42 years. 

The order was passed less than a month after the collapse of the radio telescope in December 2020, when the cables supporting the 900-ton telescope snapped due to accumulated damages from Hurricane Maria in 2017 and multiple earthquakes in 2020. 

The collapse of the 1,000-foot-wide dish, which was the largest radio telescope on Earth until 2016, paused current research and sent shockwaves in the astronomy community.

It was unsafe to make any repairs on the damaged cables before the collapse, according to the National Science Foundation, which announced in November 2020 that the Arecibo Telescope would be decommissioned. 

The design of the Arecibo Observatory began in 1958, and was the brainchild of Prof. William Gordon, electrical engineering, who was known as the “father of the Arecibo Observatory.”

Gordon was interested in studying the ionosphere, a region of the atmosphere capable of reflecting radio waves. 

“Gordon’s concept of using a natural earth form to support a 1,000-foot-diameter reflector to focus radio waves was ingenious and challenging,” said former Cornell President Dale Corson when the telescope was being designed. “A spherical reflector does not focus the radio waves at a point but along a line, and he had to devise a way to collect those waves, which he and his staff did in brilliant fashion.”

The Advanced Research Projects Agency, under the Department of Defense, funded the construction. The agency was interested in monitoring the Earth ionosphere as a part of its Defender Program to develop ballistic missiles

Since its inception, the observatory has contributed to numerous scientific discoveries. In 1967, Arecibo discovered that the rotation rate of the planet Mercury was 59 days rather than 88 — marking a significant transition from geocentric to heliocentric theory in understanding the solar system.

And beyond heliocentric theory, the radio telescope was also a technological achievement signifying more accurate equipment for studying space.

“Arecibo was the perfect telescope to study pulsars after they were discovered,” said Prof. Donald Campbell, astronomy. 

Under subsequent directors in 1974, the observatory was used to discover binary pulsars — dense neutron stars that rotate rapidly and emit electromagnetic waves in pulses. In a binary pulsar, the two stars orbit around each other in an elliptical path. 

The telescope’s detection of the radio waves emitted by this binary pulsar helped to confirm Einstein’s predictions of general relativity, and garnered the 1993 Nobel Prize in physics.

Arecibo’s initial asteroid detection later led to one of the biggest programs at the observatory, Donald Campbell said. This program expanded to detect not only asteroid existence, but also size, shape and rotation. Campbell was also involved in mapping the surface of Venus using Arecibo’s radar data in the 1980s, becoming the first observatory to produce radar maps of the planet Venus. 

Arecibo has used radar data from pulsars to discover other phenomena in space, such as discovering exoplanets, planets that exist outside of the Solar System, from a pulsar they orbited.

Further, Arecibo has contributed to NANOGrav, a North American collaboration that aims to use pulsars to detect gravitational waves — which are disturbances in the curvature of the space time continuum, and are produced from the collision of black holes.

“Arecibo has been crucial to [NANOGrav],” said Prof. James Cordes, astronomy, one of the researchers involved with the project. “We’re right on the verge of detecting these gravitational waves.”

Arecibo had been supplying half of the data used by researchers at NANOGrav. However, this data supply will be discontinued, as the collapse put an immediate halt to any research fully dependent on Arecibo.

Cordes emphasized the importance of the observatory as not just a facility, but an overall environment. 

“It’s been a tremendous influence on me and not just as a physical place or a physical object,” Cordes said. “It was also a sanctuary to go to to be able to think. The radar was one of the world’s most powerful … so it will be hard to replicate.”