1963

Arecibo Radio Telescope

Going back to the time of the first astronomical telescopes used by Galileo and others in the early seventeenth century, astronomers have known that the way to increase a telescope’s sensitivity and resolution is to simply make it larger. But physical limitations on the strengths of materials and the ability to grind and polish large glass lenses or silvered mirrors make it technically impossible to build useful single-lens telescopes larger than about 3 feet (1 meter) in diameter, or single-mirror telescopes larger than about 16 feet (5 meters) in diameter.

In radio astronomy, however, “mirrors” that reflect or transmit radio waves can be made out of metal—just as antennas are. Thus they can be made much larger. A group of radio astronomers and atmospheric scientists at Cornell University in the late 1950s and early 1960s realized that it would be possible to build a giant stationary radio telescope out of wire mesh inside natural, bowl-shaped depressions. A suitably “radio quiet” location was identified in the mountains near the town of Arecibo on the island of Puerto Rico, also conveniently located near the equator, which enables it to have a view of most of the sky. With the help of the Advanced Research Projects Agency, the US military’s parallel agency to NASA, construction of the enormous semispherical dish and support structures for steerable radio transmitters and receivers hanging above the dish began in 1960. The Arecibo radio telescope became operational in the fall of 1963 and remains the largest single-dish telescope in the world.

Many important astronomical discoveries have been made at Arecibo. The rotation rate of Mercury and the topography on the surface of cloud-covered Venus were both discovered using radar observations early in the telescope’s operation. One of the first known pulsars—rapidly spinning neutron stars—was discovered by Arecibo astronomers in the heart of the Crab Nebula, and the first millisecond pulsar (rotating around 500–1,000 times per second) was discovered at Arecibo as well. Arecibo astronomers bounced the first radar waves off near-earth asteroids to determine their size and shape, and the telescope is still the premier observatory on Earth for determining the orbits of potentially hazardous asteroids that could represent an impact threat to our planet.

The semispherical radio dish of the Arecibo Observatory, 1,000 feet (305 meters) in diameter, nestled into the mountains of northwestern Puerto Rico. The platform hanging above the dish is a 900-ton structure that contains radio and radar transmitters, receivers, and support structures.