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In the 1970s, astronomers Carl Sagan and Frank Drake developed the first messages intentionally sent out of our solar system. But how do you describe yourself to beings who have no concept of life on Earth?
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Sources:
http://voyager.jpl.nasa.gov/spacecraft/sceneearth.html
http://www.bbc.com/future/story/20141112-will-et-understand-our-messages
http://web.archive.org/web/20080802005337/http://www.news.cornell.edu/releases/Nov99/Arecibo.message.ws.html
http://web.archive.org/web/20080802005337/http://www.news.cornell.edu/releases/Nov99/Arecibo.message.ws.html#footnote
http://voyager.jpl.nasa.gov/spacecraft/goldenrec.html
http://www.space.com/6370-attempts-contact-aliens-date-150-years.html
http://www.johnstonsarchive.net/astro/pulsarmap.html
http://voyager.jpl.nasa.gov/spacecraft/goldenrec1.html
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Hank: In the 1970's, if you wanted to describe life on Earth in a simple way, you probably knew who to ask: astronomers Frank Drake and Carl Sagan. Both were involved in astrophysics research and education, and Sagan had been consulted on NASA missions like the Venus probe Mariner two.

But when NASA was finalizing its plans for the Pioneer 10 and 11 missions, the first two spacecraft to pass through the asteroid belt, Sagan proposed a plan which had been suggested to him: Attaching a message to the probes explaining who we are and where we live in case they're ever found by another civilization. And NASA agreed.

Over the next few years, the two scientists put together three landmark representations of the human race, the first time that anyone had intentionally created messages to be sent outside the solar system to be read and interpreted by extraterrestrial life.

Two were physical messages, using NASA probes as couriers, the other one was more of a crafty mathematical code for aliens to crack, but all of them were attempts to describe ourselves to beings who would have no concept of Earth or humans, or even just simple units of measurement like  meters or seconds.

We know the odds of these messages being detected by another civilization are very small, they'd have to find a small probe flying through space or be listening to exactly the right frequency at exactly the right time, but the sent them anyway.

Pioneer 10 and Pioneer 11 launched in the early 1970's and each carry a copy of the same plaque meant to explain where they came from. For anything on the plaque to make sense, Drake and Sagan realized they needed to use a universal language, science. So, to describe length, the plaque uses units based on the energy change that occurs when an electron and a proton in a hydrogen atom switch the way they spin. Since it has a diagram of the transition, they hope that any space-faring civilization would understand what they were talking about.

To explain where the Earth is, they included a pulsar map. The map shows the Earth's location among 14 different pulsars, stars that regularly emit bursts of electromagnetic radiation almost like beacons. The plaque also includes other details, like pictures of humans standing in front of the probe. Which is kind of weird because now the whole universe knows what we look like with our clothes off.

But when it was time to launch the voyager probes a few years later, Sagan, Drake, and their team were a little more ambitious. Instead of just a plaque, the voyager's carry with them copies of the golden record - like the kind that plays music - forged from gold-plated copper instead of vinyl.

The record's cover contains basic information like the hydrogen and pulsar diagram's, plus some illustrations that explain how to play the message on the record. Once the alien recipients figured out how to build a record player, they'd be treated to sounds from all around Earth and greetings in 55 languages, from distinct Sumerian languages to modern Chinese dialects. That's followed by 90 minutes of different kinds of music throughout the world and throughout history and a host of natural sounds like birds chirping and whale songs, wind, and thunder.

Finally, 116 images are included on the disks, intended to explain nearly every conceivable aspect of life on Earth. Photos of plants and animals, different landscapes, illustrations of humans in various stages of life, diagrams of our sex organs, and demonstrations of how we drink water and chew food, and lick ice cream cones.

That was Sagan and Drake's attempt at creating a kind of encyclopedia of life on Earth, but they also included a third kind of message, and all it really said was "we are intelligent, and we're here." For this message, they didn't use a physical representation. Instead, they tried radio waves.

In 1974, Sagan and Drake put together a code to be transmitted by the Arecibo radio telescope in Puerto Rico, which was undergoing some upgrades. The data was sent in binary like in computers, but instead of being ones and zeroes, the information was encoded as two different radio frequencies.

It took 1,678 radio signals to transmit the message, which was a deliberate number, it's the product of 73 and 23, two prime numbers. And when you organize those signals into a 23 by 73 rectangle it looks like this (except that there weren't any actual colors in the actual message, they're just added to the picture for clarity)

Those white dots at the top represent the numbers one through 10, the purple dots are the atomic numbers of the most important elements for human life, hydrogen, nitrogen, carbon, oxygen, and phosphorous, the green blobs are the formulas for the building blocks of DNA and those blue strings are meant to be DNA's double helix. Some of the other information in the data are pictures of a human, our solar system, and the Arecibo telescope.

For three minutes, the telescope sent the message toward the M13 globular cluster, 25,000 light years away. They only picked that cluster on the night of the observatory's re-opening ceremony and that's basically where it happened to be pointed. By the time the message gets there, the cluster won't even be in that spot anymore because it'll have moved on in it's galactic orbit, so it's unlikely that we'll ever get a response, even if we wait 50 thousand years.

Since then we've broadcasted lots and lots of other messages at likely-looking star systems, though we haven't yet heard back, but I think at least we've learned a little bit more about ourselves in the process.

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