'We sent a spaceship to the stars!'

"We sent a spaceship to the stars!" 

Those were the words of famous astronomer, the late Carl Sagan, as the sound of the rocket carrying Voyager 2 faded into the Florida sky on Aug. 20, 1977. Today, the robotic probe is officially outside our solar system, embarking on the longest journey in human history.

The twin robotic probes Voyager 1 and 2 were sent out during a grand conjunction of the four giant planets — Jupiter, Saturn, Uranus and Neptune — which were all on the same side of the Sun. That was a matter of lucky timing, as it's a situation that happens only once every 176 years.

Using a technique called a gravity assist, the spacecraft used the powerful gravity of Jupiter, the largest planet in the solar system, to slingshot themselves outwards to Saturn. Then they repeated the manoeuvre, getting a second boost to send them out toward the outer solar system and deep space. Voyager 1 took a direct route out of the solar system, while Voyager 2 went on to fly past Uranus and Neptune.

The Voyagers are two of only five spacecraft to have enough speed leave our solar system entirely. Their predecessors, Pioneer 10 and 11 also flew past Jupiter (Pioneer 11 went to Saturn). The New Horizons probe that visited Pluto in 2015  is also outward-bound. All other artificial objects launched from Earth have remained somewhere within the gravitational field of the Sun. Only these five probes will go interstellar and they will wander among the stars of the Milky Way, potentially for billions of years.

Carl Sagan saw these probes as an opportunity to reach out to other civilizations that might exist in the galaxy. They might might spot these craft wandering through their star system in the distant future. The craft would be UFOs to them, and Sagan thought we might as well tell the aliens where the spacecraft came from and who built them. He convinced NASA to attach messages to the spacecraft that would act as cosmic greeting cards to the cosmos.

The Pioneers carried simple engraved plaques with an illustration of a man and a woman, a diagram of our solar system and a map of the Earth's position in the galaxy. For Voyager, the message was a more elaborate gold record (this was the '70s, so CDs and iPods had not been invented yet) containing an hour and a half of music, people saying hello in 50 different languages and 116 photos that had been digitally encoded. The records are enclosed in a gold cover with a galactic map and instructions on how to decode the pictures. We even gave them a stylus to play it. 

The small team that assembled the record, including Sagan, radio astronomer Frank Drake and space artist Jon Lomberg, gathered at the launch of Voyager 2 to see it off. I was fortunate to be among them.

Unmanned rockets take off much faster than those with people inside. As it clawed its way into the sky on a pillar of smoke with a thunderous roar that beat against your chest, we could see that Voyager was on its way in a big hurry. It was out of sight in a matter of minutes, and by six o'clock that evening, was already beyond the orbit of the moon. It took the Apollo astronauts three days to go that far.

As we gathered in the van to drive back to the hotel at Cocoa Beach, everyone was very quiet, thinking about how far that spacecraft was going, and how long it would last. After all, a billion years is hard to get your head around. No human artifact has ever lasted that long. The oldest Egyptian pyramids are only about 5000 years old and the earliest stone tools made by our ancestors go back about three million years. A billion years ago, complex life hadn't appeared on Earth yet.

A billion years from now, who knows what will happen? Wars will have been won and lost, civilizations could collapse, humans may well be extinct and an entirely new intelligent species could take over the planet. In that time, the continents will shift position so the Earth will look entirely different from today — and through it all, Voyager will still be out there. It's a sobering thought, and we were the last human eyes to ever see it. 

The chances of alien eyes, or whatever aliens use to see, spotting the Voyagers are infinitesimally small, and if it does happen, it won't be for a very, very long time. Our galaxy is so vast, it will take Voyager 1 about 40,000 years to get anywhere near another star. It'll pass within 1.6 light years of Gliese 445. And Voyager 2 will journey for more than 250,000 years before it approaches the star Sirius, the brightest star in our sky. 
 
Today, both Voyager probes are still operating, sending back information on the environment of interstellar space. But their nuclear powered generators are running down and the intrepid explorers will go silent within the next decade. At that point the scientific phase of the mission will end, but the longest human experiment ever conducted, on the longest journey humanity has ever undertaken, will have just begun.