They wanted to show what humans are like and how life works on Earth. The message was sent to a star called Altair, which is very far away.
Now, 30 years later, scientists are hoping to hear a reply from aliens. They will use a big telescope in Japan to listen for an answer. But the chance of hearing anything is small. The star might not have any planets, and there's only an hour to hear the message.
The date to listen is August 22. This date is special in Japan because it's part of a star festival.
The scientist Shinya Narusawa thinks there might be aliens out there. He says that many planets have been found since the 1990s, and Altair might have a planet where life can live.
The original message was sent for fun along with a Japanese comic book. One of the astronomers even admitted that they had been drinking when they came up with the idea!
But even though it started as a joke, it has become something special. Many children have written to the scientists about the message, and that has made it all worth it.
Now everyone is waiting excitedly to see if there will be an answer from the stars. Will aliens reply? We'll have to wait and see!