A few days after 15 August 1977, an astronomer named Jerry Ehman sat at his kitchen table in Ohio, working his way through a stack of computer printouts.

It was not glamorous work. The printouts came from Big Ear, a radio telescope run by Ohio State University, which was scanning the sky for signals from alien civilisations. Most of what it recorded was noise: columns of numbers and letters, mostly 1s, 2s and blanks.

Then Ehman found this:

6EQUJ5

He circled it in red pen. And next to it, in the margin, he wrote a single word.

Wow!

What the code means

Big Ear's printout used a simple shorthand for the strength of a signal compared to the background noise. A blank meant nothing. The numbers 1 to 9 meant progressively stronger signals. When the signal got stronger than 9, the computer switched to letters: A for 10, B for 11, and so on.

So "6EQUJ5" means: the signal was 6, then 14, then 26, then 30, then 19, then 5 times stronger than the background. A sharp rise and fall.

That shape is the remarkable part.

Big Ear didn't move. It relied on Earth's rotation to sweep its beam across the sky. Any fixed source in space would pass through the beam in about 72 seconds, getting stronger as it reached the centre and weaker as it left. That is exactly the shape of the Wow! signal. It behaved precisely like something that was out there, among the stars — not like interference from the ground.

Why the frequency mattered

The signal was also in a very special place on the radio dial: very close to 1420 megahertz.

That's the frequency at which hydrogen, the most common element in the universe, naturally emits radio waves. In 1959, physicists Giuseppe Cocconi and Philip Morrison suggested that any civilisation looking to be found might broadcast near this frequency, because any astronomer, anywhere, would know to listen there. It's also a frequency band that international agreements protect from transmissions on Earth.

So the Wow! signal was strong, narrow, came from the sky, and sat at the frequency SETI researchers had chosen as the most likely place for a message.

It came from the direction of the constellation Sagittarius.

Then it was gone

Here is the frustrating part. Big Ear had two detector "horns" that looked at slightly different patches of sky, a few minutes apart. The signal should have shown up in both. It appeared in only one.

And then nobody ever heard it again.

Ehman searched for it repeatedly. Others have pointed much more sensitive radio telescopes at the same region of the sky, more than once. Nothing.

So what was it?

Let's be careful, because this is exactly the kind of mystery that attracts confident claims. Here's what's on the table, honestly labelled.

The best mystery in radio astronomy

The Wow! signal has endured not because it proves anything, but because it's so perfectly almost. Almost the right shape. Almost the right frequency. Almost enough to make you believe.

Somewhere out in the direction of Sagittarius, something happened in 1977. Maybe it was a hydrogen cloud lighting up. Maybe it was something we haven't thought of yet.

We just know that for 72 seconds, it was there.