Sunday marks the 40th anniversary of the launch of Voyager 2 which, at the time, confused the heck out of the press and public because it actually launched before Voyager 1. Why did they launch the second probe first? Because Voyager 2 was going to follow a longer trajectory to reach the Jupiter system, allowing it to fly by Saturn, Uranus, and Neptune. Voyager 1 launched 16 days later on a faster track optimized to fly by Jupiter, Saturn, and make a relatively close pass of Saturn's intriguing moon Titan.

The missions, of course, were smashing successes. Voyager 1 reached Jupiter on March 5, 1979, about four months before its twin. Scientists weren't sure what they would find out there. Pioneer 10 and 11 had given them some insights about Jupiter and Saturn as gas giants, but little information was known about the many moons of these worlds. Most scientists thought they would probably be a lot like the cold, dark, and lifeless moons of Earth and Mars.

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They were anything but. When you talk to the scientists involved with the Voyagers and ask when they knew their missions would reveal something entirely different and truly otherworldly, they point toward the discovery of volcanic activity on Jupiter's moon Io. Until then, the only known active volcanoes in the Solar System were on Earth. Here was a small moon with 10 times the volcanic activity.

"That was really the wake-up call that we were in for a journey that was even much more spectacular in terms of what we were going to discover than we could imagine," said Ed Stone, who has been Voyager's chief scientist since the program started in 1972. "There were many. That's really the wonderful thing about Voyager. But if I had to pick one that was symbolic, Io was it."

Io really was the beginning of a special journey. Thereafter, the Voyager spacecraft found a deep water ocean on Europa, methane seas on Titan, and likely geysers on Triton. Later, Voyager 1 took an iconic photo of Earth among the other planets in the Solar System. It later left the system entirely into the void of interstellar space.

"Time after time, we discovered things that we hadn't even imagined," Stone told Ars. Indeed. We still marvel today.


Listing image by NASA