Arecibo Observatory, Puerto Rico – The World’s Largest Radio Telescope For Over 50 Years

The Arecibo Observatory was home to the largest single-aperture telescope from the date of its completion in 1963 until July 2016, when the Chinese Aperture Spherical Telescope was built. Unlike most traditional telescopes, which use optical light, the Arecibo Observatory uses radio waves to understand the nature of our universe. It is used in three major areas of research: radio astronomy, atmospheric science, and radar astronomy. The radio telescope works 24 hours per day and can function in all kinds of weather. It has also been featured in popular films and TV series such as the X-Files, Contact and Golden Eye.

The construction of the Arecibo Observatory began in 1960 and it was inaugurated in 1963. The telescope’s designer was engineer William E. Gordon, and its original purpose was to study the properties of the ionosphere as part of a project for the Department of Defense. In the late 1960s, the National Science Foundation assumed the facility’s operation, naming it the National Astronomy and Ionosphere Centre (NAIC). It is the only place in the world doing research in radio astronomy, planetary radar, and space and atmospheric sciences.

The radio telescope was built in Arecibo, Puerto Rico for three main reasons. Being part of a US government project, it was required to be located within the United States territory. In addition, in order to study near Earth orbit bodies, a place near the equator was required. Puerto Rico is located 18.5ºN, which made it ideal for this purpose. Another factor that contributed to the decision was the discovery of a large sinkhole big enough to fit the huge reflector (which considerably reduced the cost of construction) in the Esperanza area, in Arecibo.

The huge reflector is approximately 305 metres (1,000 feet) in diameter and made up of 38,778 perforated aluminium panels measuring around 2 by 1 metres (6 by 3 feet) each. Each panel is individually adjusted to produce a spherical curvature with a 2mm RMS precision. The total weight is 272,155 kilograms (300 tons). A special shoe is required to walk on the reflector, which protects the panel by distributing the person’s weight.

A total of 39 cables are required to support the 907,185 kilogram (1,000 ton) platform that hangs above the reflector. The combined cable length is 6.7 kilometres (4 miles) and the cables strung between the towers and the platform each weigh around 9,072 kilograms (10 tons).

In 1997 the Gregorian Dome was installed on the Arecibo Telescope’s suspended azimuth arm. Since then, the new radar system has been used for a wide range of Solar System studies. Radar images reveal a wealth of information about the shapes, surface properties, and rotation of solid bodies in the Solar System.

For some experiments the radar technique is used to produce radio waves, and then reflect them off celestial objects; this is known as the ‘active mode’. For others, radio waves that have been emitted by celestial objects are captured; this is known as the ‘passive mode’.

Radar reflecting off planets and their satellites reveal their nature and properties, and allows the production of detailed images of their shallow surfaces. Radar also helps to protect the Earth by monitoring the orbits of asteroids, precisely measuring the object’s distance from Earth and its velocity, which helps to predict the object’s future position.

The Arecibo 430 MHz radar scatters radio waves off our planet’s ionosphere, revealing details about its physical properties such as composition and temperature. The ionosphere is a region of Earth’s atmosphere that lies between approximately 75 and 1,000 kilometres (46 and 621 miles) above the Earth’s surface. The number of electronically charges particles (ions and electrons) in the ionosphere is large enough to affect the propagation of radio waves.

In an attempt to enhance the capabilities of the radio telescope, a high frequency facility with six transmitters was built. This facility was completed in 2015 and aims to widen the range of the ionospheric experiments done at the Arecibo Observatory. Transmitters provide energy to feed two triangular antenna arrays, located at the centre of the main reflector. Each antenna is equipped with three cross-dipoles and operates in 5MHz or 8MHz. The new equipment allows studies of the plasma behaviour in the ionosphere, which provides a better understanding of the upper atmosphere.

LIDAR (Light Detection and Ranging) is used to study the middle and upper atmosphere. This is done by detecting atoms from sodium and other metal vapours, which are the results of meteors. Using this technique it is possible to study the dynamics and climate of the upper neutral atmosphere at the edge of space, between 80 and 100 kilometres (50 and 60 miles).

At the observatory, scientists discover pulsars by finding the electromagnetic pulses they emit. Pulsars are rapidly spinning neutron stars that can generate beamed radio emission that may regularly flash over the Earth like the beam from a celestial lighthouse. They are extremely dense objects composed almost entirely of neutrons and having diameters of only 20 kilometres (12 miles) or less, despite possessing masses greater than that of the Sun. Such as star transported to Earth would weigh as much as a mountain. A neutron star is formed  when the core of an exploding star (a supernova) collapses inwards resulting in an enormous level of compression.

Radio and television broadcasts, traveling at the speed of light, radiate from Earth every day. Arecibo broadcasted deliberate messages into space in 1974 and 2009 and astronomers at the observatory search for accidental or deliberate transmissions from other worlds. So far the Search for Extra-Terrestrial Intelligence (SETI) has been unsuccessful.

Many important scientific discoveries have been made at Arecibo. In 1964 Gordon Pettengill’s team discovered that the rotation period of Mercury is not 88 days, as formerly thought, but only 59 days.

In 1974, Joseph Taylor and his student Russell Hulse used the Arecibo Observatory data to measure the changing orbits of a paid of neutron stars. As these stars orbit one another, theory said they would give off gravitational waves and the energy carried away by these waves would cause the stars’ orbits to shrink. Taylor and Hulse’s measurements magnificently confirmed this idea and won them the Nobel Prize for Physics in 1993.

Among other accomplishments of the Arecibo Observatory are:

  • Direct imaging of an asteroid for the first time in history.
  • Discovery of water ice deposits at Mercury’s poles.
  • Tracking of near-Earth asteroids to monitor impact risks.
  • Mapping the cloud-covered surface of Venus.
  • Radar imaging of the rings of Saturn, revealing new details of the ring structure.
  • First detection of methane lakes on Titan, a moon of Saturn.
  • First detection of an asteroid with a moon.

Since celebrating its 50th anniversary in 2013, the future of the Arecibo Observatory is now uncertain. A harsh funding climate may result in the closing of the observatory and the dismantling of the telescope. Despite its iconic status, the increase of traditional telescopes across the United States, which are more frequently used and cheaper to run, has contributed to the concern for the future of the observatory. However, losing Arecibo would mean losing the ability to precisely monitor a large quantity of pulsars that are currently being detected by the observatory for projects that require extremely sensitive telescopes. Puerto Ricans hope that NASA or another institution will put up the funds to allow Arecibo to continue its work and former observatory director, Robert Kerr, has begun the process for listing it as a national historic site in an attempt to protect the observatory.

The visitor centre is open on Monday to Sunday from 9:00 to 16:00 (except holidays). Entrance for adults costs 12$ and for children and seniors the cost is 8$.

N.B. Opening hours and costs correct as per the official website at time of posting.


  • Information signs at Arecibo Observatory

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Arecibo Observatory, Puerto Rico – The World’s Largest Radio Telescope For Over 50 Years

by Uncover Travel time to read: 5 min
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