(Intro)

Deep in an archive in Columbus, Ohio, there’s a slip of paper with a bunch of random-looking letters and numbers printed on it. A few of the characters are circled, and one word is handwritten in red ink: "WOW". Those circled characters are the signs of an unusually strong signal from outer space detected in 1977, but we still don't know exactly what it was, and we never found it again.

There's a theme that tends to come up a lot when you're talking about space: With so many planets and moons scattered throughout this universe of ours, where are all the aliens? So some astronomers spend their time looking for extraterrestrial life by broadcasting carefully crafted messages to space. But what if there's intelligent life out there that's doing the same thing - sending messages to us - or even just broadcasting signals that we could pick up from here? That's why other astronomers and some entire organizations spend their time looking for alien signals using radio telescopes.

Lots of things in space naturally emit radio waves, and radio telescopes are designed to capture, focus, and read that energy. By analyzing the data collected by these telescopes, researchers can learn new things about the universe like where there might be undiscovered galaxies. But organizations like SETI, or Search for Extraterrestrial Intelligence, also like to monitor the data. They're looking for something out of the ordinary like patterns or especially strong signals that might mean they were sent out purposely or otherwise by an advanced civilization on another world.

These days we can comb through the data automatically using computers, but back in the 1970s it was done by hand which is where a volunteer named Jerry Ehman comes in. A computer hooked up to the telescope would print out the data it collected as characters arranged in a grid, and on August 15, 1977 Ehman was looking through some of these printouts when he saw a sequence of characters that meant the telescope had detected a signal 30 times the strength of the usual background radio waves. A signal that went on for 72 seconds.

38 years later we still don't really know what it was. Once Ehman found and accidentally named the "WOW" Signal, scientists wanted to see if they could find it again. They were able to narrow down the possible sources to somewhere in the constellation Sagittarius, possibly near the M55 Star Cluster, but never detected it again.

So where did the signal really come from? Scientists aren't sure. For one thing it could have just been a computer glitch, but it probably wasn't because the signal wasn't a sudden constant spike in the data that you'd expect to see if it was just a glitch. Instead, it gradually rose to a peak and then fell again as the telescope passed a certain area of the sky.

It also could have been caused by something natural, like a pulsar - a rotating star that sends out beams of radio waves in pulses - but then astronomers would expect to see the change through a lot of different radio frequencies. Instead, the "WOW" Signal was only detected on one channel, and a special one: 1420 megahertz, the same band that's naturally emitted by hydrogen.

If you want to send a message that says, "Hey, there's intelligent life over here", picking the same frequency emitted by the simplest element in the universe is a pretty good way to do it. In fact, it's such a strong possibility that extraterrestrial life would use that band, that using the frequency on Earth is illegal under international law. Nobody wants to get all excited about a possible message from aliens only to find out that it was a local radio station broadcasting the nightly news.

In 2012 on the 35th anniversary of the detection of the "WOW" Signal the National Geographic Channel decided, as part of a promotion for an upcoming TV series, to use the Arecibo Observatory in Puerto Rico to send what they called a reply to the "WOW" Signal. They compiled all sorts of messages, like Tweets from the public, and videos from people like Stephen Colbert and Leila Lopes who was Miss Universe at the time.

Then they sent them to the places the WOW Signal might have come from; but space is big - even traveling at the speed of light the message still wouldn't have gotten there yet. Proxima Centauri, the closest star to our solar system, is about 4 light-years away. The M55 Cluster, on the other hand, is more like 17,000 light-years away. Even if there is someone listening, and they decide to reply, it'll be a future generation of scientists that get to hear the reviews of Stephen Colbert's sense of humor. And until then, or until we detect more clues, the WOW Signal will remain a mystery.

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