The Arecibo Broadcast was a message broadcast from the massive Arecibo Observatory in Puerto Rico, in 1974, to the M13 globular star cluster. It will reach that cluster in about 25,000 years, although, of course, it is not known whether anyone will be there to hear it. The Arecibo Broadcast is one of the best-known of a growing number of messages deliberately broadcast to the stars in hopes of making contact with an extraterrestrial intelligence.
Of course, no one today knows whether there is even a chance that the signal will eventually be intercepted by another intelligent species. It is assumed that any beings now using a communications system more advanced than radio would still be capable of listening to radio transmissions, but it is by no means certain they would have any interest in doing so. Moreover, some scientists, including Stephen Hawking, have warned that contacting alien civilizations would be a bad idea because they would probably be superior to us, and might exploit our planet - and us - in much the same way that the Spanish did to Latin America. Such a scenario is incredibly unlikely, of course - but then again, so is a successful contact with aliens in the first place. Still, Hawking's fears are probably exaggerated, since there is no reason to believe Earth has anything that would be of more interest to scavenging alien resource-seekers than any uninhabited sector, even our own Asteroid Belt.
The Arecibo Broadcast represents a more optimistic interpretation: that, if alien species exist, it is still worthwhile attempting to contact them. Instead, the chief problem considered by the participants in the project was a thought experiment: how to send a signal which will be understood on the other end. An alien civilization with a radio network would probably share some basic knowledge with us, like mathematics and an understanding of the basic structures in the universe, but probably there would be little or no means of comprehending one another's languages.
The Arecibo Broadcast's solution was to send a signal which, when written out in digital form, should contain at least some components which an advanced civilization would immediately recognize. It consisted of nearly 1700 digits, which when arranged in 23 columns of 23 digits each (prime numbers), counts out one to ten in binary, then lists the atomic numbers of several chemical elements necessary for life (including carbon, hydrogen, and oxygen). Next are a pair of much more complex but essentially similar diagrams identifying complex organic compounds and a DNA double helix. This is followed by a stick figure-like drawing of a human being.
Next are a series of astronomical references. The first is a diagram of the solar system, with rough sizes indicated (the Sun gets nine dots, the inner planets each get one, and Jupiter and Saturn get three each). Earth's digit is deliberately entered out of line with the others as a hint that this is the planet from which the signal originated. Finally, it closes with a diagram of a radio telescope roughly resembling Arecibo itself.
There is considerable skepticism about whether an alien civilization would be able to correctly interpret this as a binary message, and then - probably more or less by chance - "decrypt" the proper arrangement according to prime-number columns and rows. Nevertheless, the attempt to produce a meaningful message - one which was also repeated in devising the communications plates placed on the Voyager probes, for example - was an attempt to break down proofs of our own intelligence into a series of symbols simultaneously complex enough to demonstrate our intelligence, but simplistic enough (or at least universal enough) that they could be understood with a minimum of difficulty by an alien race.
Once it was devised, the Arecibo message was then broadcast to the M13 globular star cluster on November 16, 1974. M13, also known as the Hercules Cluster, was chosen because it was the closest large star cluster - many, many stars are far closer, but there was no reason to believe that any of them were candidates for life. The Hercules cluster is about 25,000 light-years away, meaning that the Arecibo Broadcast will reach its target in 25,000 years. By the time it does, however, we expect it to actually miss by a considerable margin (due to the cluster's movement in the meantime). Still, if someone in the vicinity of the cluster did hear the message, we may receive a response in as little as 50,000 years.
By then, even if humanity has survived, the Arecibo Observatory itself will of course be long obsolete. Indeed, its annual funding has already been under strain for several years, and will likely be eliminated entirely (so that the facility will be closed) within the next decade. Most of the calls to date have been from the Cosmic Call (nine) and Teen Age Message (six) projects. Although they were sent much after Arecibo, mostly within the last decade or so, they will actually reach their targets first, because they were sent to close by stars, like Cygnus (arriving 2069), Libra (arriving 2029), and Casseiopia (arriving 2036). A response would presumably take the same amount of time to return to Earth, although a species anything like us would take some time to consider an appropriate response, and in all likelihood might decide not to send one at all.