In 1932 Karl Jansky, an American researcher at Bell Labs, received radio waves apparently coming from empty interstellar space. Over the
next 15 years these radio waves remained a mystery, until more sensitive radio equipment, developed during World War II, was applied to the phenomenon. The first radio telescopes, designed to sort out and register such signals, were built in 1948, and researchers immediately found that the most powerful sources were two specific spots, in the constellations of Cygnus and Cassiopeia. Two “radio stars,” without visible light, were discovered.
Over the next five decades, several thousand more radio sources have been discovered. The nearest galaxy to ours, the Andromeda nebula, contains many suns that emit on the 1.80-meter radio wave frequency but not on the visible light frequencies. Other sources emit on wavelengths varying over a wide range of frequencies, from a few centimeters up to 20 meters or more. A source in Cygnus was identified as resulting from the collision of two nebulae about 200 million lightyears away. In 1951, radiation at a constant wavelength of 21 meters was discovered coming from clouds of hydrogen in interstellar space.
Bernard Lovell of Manchester University in Britain did some of the pioneering work in radio telescopy. After working in the 1930s on radar, in the postwar years he took over surplus equipment and set up
a team at Jodrell Bank, in Cheshire, where Manchester University already maintained a botany facility. Lovell’s original plan was to use radar beams to bounce off the Sun and planets. His first radio telescope, designed to bounce radar beams off the Sun and Venus, was built from 1947 to 1957. In 1957 it was used to track the first artificial satellite, Sputnik I, launched by the Soviet Union. The Jodrell Bank receiver was 80 feet in diameter, suspended from a circular track 350 feet in diameter that could be angled to point to various positions in the sky.
Soon, however, the Jodrell Bank receiver was used not to receive radar echoes but to assist in mapping the new radio wave–emitting stars, nebulae, and galaxies. Another British radio telescope receiver system, constructed at Cambridge University in Britain, was known as the Mullard Radio Astronomy Observatory. Rather than using a bowlshaped receiver, the Mullard system consisted of two cylindrical structures 2,400 feet apart. One was fixed to the ground, while the other moved along a 1,000-foot track.
The largest dish-type radio receiver or radio telescope in the world was completed in 1963 and operated by Cornell University at Arecibo in Puerto Rico. Funded by the National Science Foundation, Arecibo is built in a natural hollow on a mountain peak. One of the first discoveries at Arecibo was a corrected calculation of the rotation of the planet Mercury. In 1996 Arecibo received an upgrade at the cost of $25 million, increasing its sensitivity by a factor of 10.