On August 15, 1977, a radio telescope in Ohio detected a signal from space that was so strong and unusual, a scientist wrote “Wow!” next to it. This signal remains one of the biggest mysteries in the search for life beyond Earth. More than forty years later, scientists still do not know what caused it or where it came from.

The Big Ear radio telescope was built to listen for signals from space. It did not look for sounds like music or voices, but instead picked up radio waves, which are a type of energy that travels through space. Scientists believed that if intelligent life existed elsewhere, it might send out radio signals, just as humans do with TV and radio.

Every night, the Big Ear telescope scanned the sky and recorded the radio waves it found. A computer printed this data on long sheets of paper, using numbers and letters to show how strong each signal was. Most signals were weak and just natural noise from stars and space. But sometimes, something unusual appeared.

On that August night, the telescope was pointed at a group of stars near the constellation Sagittarius. At about 11:16 p.m., it picked up a signal unlike anything it had recorded before. The signal was extremely strong, about thirty times stronger than the usual background noise from space. It lasted exactly 72 seconds, which was the longest the telescope could track one spot in the sky as the Earth turned.

A few days later, a scientist named Jerry Ehman was reviewing the computer printouts. His job was to look through all the data for anything interesting. When he saw the numbers and letters for this signal, he was amazed. The signal was so strong and unusual that he circled it with a red pen and wrote “Wow!” in the margin.

What made this signal so special? First, it was very strong and came from only one direction in space, while normal space noise comes from everywhere. Second, it had a very narrow frequency, close to 1420 megahertz. This matters because it is the natural frequency of hydrogen, the most common element in the universe. Scientists have long thought that if aliens wanted to send a signal others would notice, they would use this frequency because any civilization searching for signals would probably be listening to it.

Third, the signal got stronger and then weaker in just the way scientists would expect if it came from a fixed point in space as the telescope moved past it. This pattern suggested it was coming from far outside our solar system.

Jerry Ehman and other scientists wanted to find the signal again right away. They pointed the telescope at the same area of space the next night, hoping to find it. But there was nothing. They kept searching over the next days, weeks, and months. Even after more than forty years, the signal has never returned.

This makes the mystery even greater. If the signal came from an alien civilization trying to communicate, why did they send it only once? If it was a natural event in space, what kind of event could create such a perfect signal and then never happen again?

Over the years, scientists have suggested many possible explanations. At first, some thought it might have been a signal from Earth that bounced off space junk and came back to the telescope. Others believe it could have been caused by a passing comet. When comets get close to the sun, they release clouds of hydrogen gas, which could create a reflected signal at that frequency.

However, other scientists disagree with these ideas. If it were a reflection from Earth, why did it have all the features of a deep-space signal? If it were a comet, why has no comet ever created a similar signal before or since? And since comets move, if a comet caused the signal, the telescope should have picked it up more than once as the comet moved through space. Some people believe the Wow! Signal was a message from an alien civilization. They think the aliens may have sent a brief, powerful signal and then stopped, waiting to see if anyone would answer. Since humans have never sent a reply to that area of space, perhaps the aliens think no one is listening.

The truth is, no one knows. The Wow! Signal is still unexplained. It is one of the strongest pieces of evidence that something unusual might be out there in space, but it is not enough to prove that aliens exist. Scientists continue to search for signals from space using more powerful telescopes and better technology. They hope that one day they will detect another signal like the Wow! Signal – or perhaps even better, a signal that repeats so they can study it and understand where it comes from.

Until then, the Wow! Signal from August 15, 1977, remains one of the greatest mysteries in the history of astronomy and the search for life beyond Earth.