Message in a wave: The Arecibo Observatory in Puerto Rico is the world's largest single-dish radio telescope and most powerful radar. It was used by Frank Drake in 1974 to send the first radio message intentionally aimed at extraterrestrials.

Credit: Courtesy of the NAIC - Arecibo Observatory, a facility of the NSF.

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Furthermore, several 19th century scientists contemplated how we might communicate with possible Martians and Venusians.

These early proposals - which predate by 150 years the first intentional message aimed at extraterrestrials, beamed out from Earth in 1974 - were based on signals in visual light, as the invention of radio was still decades away.

In fact, as history shows, ideas for interplanetary communication have largely been driven by whatever the contemporary technology allowed - be it lamps, radios or lasers. "You go with what you know," says Steven Dick, NASA's Chief Historian.

Over two thousand years ago, the ancient Greeks argued over the existence of life on other planets, but the idea really took off after the Copernican revolution, says Dick. "Once it was realised that all the planets go around the Sun, it was not hard to imagine that the other planets could be like Earth."

Galileo, Kepler and others considered the inhabitability of the planets, while being careful not to upset Church authority. Later "the idea blossomed in the 17th century into the 'plurality of worlds' debate, but it remained controversial," adds Dick, who has written several books on the topic.

One of the most influential proponents for extraterrestrial life was Bernard le Bovier de Fontenelle, who wrote a book called Conversations on the Plurality of Worlds in 1686. Despite the interest, there was no recorded discussion of how we might locate or contact these potential aliens until more than a century later.

Florence Raulin-Cerceau of the Alexandre Koyré Centre in Paris, France, has documented the early attempts at communication with extraterrestrial intelligence (CETI), or what is now often called active SETI (search for extraterrestrial intelligence).

"As early as the 19th century, inventors imagined "sky telegraph" equipment to communicate with the supposed inhabitants of the Solar System's planets," Raulin-Cerceau recently wrote in the French magazine Pour la Science.

The first of these inventors was Carl Friedrich Gauss, a German mathematician. In the 1820s, he spoke of reflecting sunlight towards the planets with his land surveying invention, the heliotrope. He is also credited with the idea of cutting a giant triangle in the Siberian forest and planting wheat inside.

"The size and colour contrast should have made the object visible from the Moon or Mars, and the geometric figure could only be interpreted as an intentional construction," says Raulin-Cerceau.

Twenty years later, the astronomer Joseph von Littrow came up with a similar idea to pour kerosene into a 30-kilometre-wide circular canal that would be lit at night to signal our presence. The second half of the 19th century saw more realistic proposals develop, however.

In a more modern fashion, French inventor and poet Charles Cros was one of the first to propose sending coded messages to other planets by flashing beams of light. In 1869, he imagined using a parabolic mirror to focus the light from electric lamps towards Mars or Venus. He figured the light could be flashed on and off to encode a message.

"Cros granted that the planets could be inhabited by beings not able to respond, but he was still persuaded that 'the eternal isolation of the spheres [will be] vanquished,'" wrote Raulin-Cerceau.

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