Until I collapse: Arecibo’s more than 50 years of discoveries

This post was written by ScIU guest writer Laura Congreve Hunter, a graduate student in IU’s Astronomy Program.

Famous for its appearances in movies and television including GoldenEye (1995), Contact (1997) and an episode of The X-Files (“Little Green Men”), the Arecibo telescope is a massive 305m (1,000 ft) radio telescope.  Built into a natural karst sinkhole on the island of Puerto Rico in 1963, it was in fact the largest single dish telescope in the world until China completed its Five-Hundred-Meter Aperture Spherical Telescope (FAST) in 2016.  Unfortunately for the astronomical community, Arecibo suffered severe damages this year that led to its eventual collapse.

While Arecibo has previously endured rough treatment at the hands of Hurricane Maria in 2017 and an earthquake in January of 2020, its most significant damages occurredthis past August when the cables holding up the 900-ton instrument array started to fail.  First an auxiliary cable snapped and left a hundred foot gash in the telescope, and then in November one of the main cables broke, leading to an inspection of the other main support cables.  The inspection revealed further damage to other support cables and led the National Science Foundation (NSF) to begin decommissioning the telescope.  But before the NSF could begin the process of safely dismantling the telescope, the final cables snapped, dropping the instrument array 500 feet onto the telescope’s main dish.  Video footage of the collapse can be seen here (Credit: Courtesy of the Arecibo Observatory, a U.S. National Science Foundation facility).

Due to Arecibo’s massive size (covering nearly 20 acres) and instrumentation, it has been at the forefront of radio astronomy and planetary science.  Among Arecibo’s unique capabilities were its planetary radar system which bounced S-band (12.6 cm) radio waves off nearby bodies such as planets and asteroids.  The returning radar signal could then be observed by Arecibo or another radio telescope.  This radar played an essential role in identifying and tracking near-Earth asteroids and is not easily replaced by other radio telescopes.  Other telescopes either lack the power of Arecibo, have a smaller field of view, or have other obligations that take priority.

In the 50-plus years of operation, Arecibo’s radar capabilities were well-used to advance our understanding of the solar system.  One of Arecibo’s first major accomplishments was determining that Mercury’s rotation period was 59 days, not 88 days as previous observations found.  Continuing through the solar system, Arecibo used its powerful radar to create maps of Mars in 1975 (supporting NASA’s Viking mission), and the first radar maps of Venus’ surface in 1981.  Arecibo’s success, however, was not limited to our solar system; the telescope also has the honor of discovering the first extrasolar planet or exoplanet in 1992.  

This first exoplanet was found during Arecibo’s work monitoring and studying pulsars.  Pulsars are neutron stars, which are the incredibly dense remnants of massive stars.  They rotate extremely quickly and emit radio jets from their poles. These features make pulsars extremely precise clocks. Astronomers can discover companions to pulsars by timing their repeating radio signal.  If the time between signals is a little off, this indicates the pulsar has an unseen companion.  This is how Aleksander Wolszczan and Dale Frail discovered the first exoplanet, which orbits the star PSR B1257+12. This discovery launched the search for exoplanets and since 1992, 4,367 total exoplanets have been discovered including 2 additional planets around  PSR B1257+12.

Arecibo’s work studying pulsars resulted in another equally impressive achievement: the 1993 Nobel Prize in Physics.  This prize was awarded to Russell A. Hulse and Joseph H. Taylor, Jr. for observations they made with Arecibo in 1974.  What the two astronomers discovered was a binary pulsar, or a pair of neutron stars orbiting each other at a distance a few times larger than the separation of the Earth and the Moon. By tracking the timing of the radio pulses from the system, they found that it deviated strongly from what is predicted by Newton’s theory of gravity.  The binary instead followed predictions made by Einstein’s Theory of General Relativity and was decaying by radiating gravitational waves.

With all that Arecibo  helped accomplish, it is clear that it will be dearly missed by the astronomical community.  Discoveries such as exoplanets and binary pulsars are just a small sampling of Arecibo’s most noteworthy accomplishments.  With well over 50 years worth of discoveries in fields including planetary science, stellar and galactic evolution, and astrophysics, Arecibo had a long and productive lifetime as one of the largest telescopes ever built.  

Edited by Lana Ruck and Jennifer Sieben