Arecibo Observatory

Arecibo Observatory

The largest semi-steerable radio-radar observatory on the planet Earth is the Arecibo facility, which Cornell University operates for the Us National Science Foundation. In the remote hinterland of the island of Puerto-Rico, it is 305 meters (a thousand feet) across, its reflecting surface a section of a sphere laid down in a pre-existing bowl-shaped valley. It receives radio waves from the depths of space, focusing them onto the feed arm antenna high above the dish, which is in turn electronically connected to the control room, where the signal is analyzed.

Alternatively, when the telescope is used as a radar transmitter, the feed arm can broadcast a signal into the dish, which reflects it into space. The Arecibo Observatory has been used both to search for intelligent signals from civilizations in space and, just once, to broadcast a message – to M13, a distant globular cluster of stars, so that our technical capability to engage in both sides of an interstellar dialogue would be clear, at least to us.

Advanced civilizations may have graduated far beyond radio for the own communication. Image : Superexteraterrestrial Sorceress on Rive Shore – Fantasy Art © Megan Jorgensen

In a period of a few weeks, the Arecibo Observatory could transmit to a comparable observatory on a planet of a nearby star all of the Encyclopaedia Britannica. Radio waves travel at the speed of light, 10,000 times faster than a message attached to our fastest interstellar spaceship. Radio telescopes generate, in narrow frequency rages, signals so intense they can be detected over immense interstellar distances. The Arecibo Observatory could communicate with an identical radio telescope on a planet 15,000 light-years away, halfway to the center of the Milky Way Galaxy, if we knew precisely where to point it.

And radio astronomy is a natural technology. Virtually any planetary atmosphere, no matter what its composition, should be partially transparent to radio waves.

Radio messages are not much absorbed or scattered by the gas between the stars, just as a San Francisco radio station can be heard easily in Los Angeles even when smog there has reduced the visibility at optical wavelengths to a few kilometres. There are many natural cosmic radio sources having nothing to do with intelligent life – pulsars and quasars, the radiation belts of planets and the outer atmospheres of stars; from almost any planet there are bright radio sources to discover early in the local development of radio astronomy. Moreover, radio represents a large fraction of the electromagnetic spectrum. Any technology able to detect radiation of any wavelength would fairly soon stumble on the radio part of the spectrum.