More than 11.6 billion miles away from Earth, NASA's Voyager 1 has finally reached interstellar space, scientists confirmed Thursday.
"It's clearly a major milestone in the history of exploration — there's no doubt about that," Edward Stone, Voyager 1 project scientist, told Mashable Tuesday. "This is sort of like the first circumnavigation of the globe or the first footprints on the moon."
Stone and his team at NASA's Jet Propulsion Laboratory launched Voyager 1 into space 36 years ago at a time when the space agency relied on paper, chalkboards and raw computational genius to build spacecrafts. There was so much room for error — and yet Voyager 1 is arguably one of NASA's most successful missions and most certainly the biggest bang for its buck.
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As it whizzed past Jupiter and Saturn on its journey through space, Voyager 1 showed us the diversity of the planets and their numerous moons. For Stone, the most memorable moment was when Voyager 1 revealed eight active volcanoes on Jupiter's moon lo. Up until that point, we had thought Earth was the only home to volcanoes.
"That just set the tone for the rest of the mission," Stone said. "Time after time, we were surprised to find things that we hadn't even suspected that we were going to see. And the result of all that was we realized how diverse the bodies are in our solar system — they are familiar in form but diverse in detail."
Stone went on to become director of JPL and eventually retired in 2001, but he never left the Voyager mission. While most of the original team is now remote, a few members still work in an office at JPL's headquarters in Pasadena, Calif., where the control room downloads information sent from Voyager 1 (which takes 17 hours to travel back to Earth from the spacecraft's current location).
Last August, the spacecraft detected a sharp change in cosmic ray data, suggesting it had reached the very edge of our solar system. In December, NASA confirmed Voyager 1 had entered a new region of the solar system that was the "final area the spacecraft has to cross before reaching interstellar space."
If this story sounds familiar, it's because many news outlets jumped the gun earlier this year when a report — based on that cosmic ray data — claimed Voyager 1 had exited the heliosphere. However, for Stone, the cosmic ray flip was just one piece of the puzzle — an unsteady piece that could change at any moment. Despite popular opinion, it wasn't enough to confirm the spacecraft had indeed reached interstellar space. So, Stone waited patiently for another signal he expected — one that would show a significant change the magnetic field.
That signal never materialized. Instead, Stone and the Voyager team got lucky in April when an unexpected gift arrived. A giant coronal mass ejection, also known as a solar flare, had erupted from the sun in March 2012. It was so massive that it reached the Curiosity rover while it was en route to Mars.
When that same flare hit Voyager 1 nearly 13 months later, it "jiggled" the plasma around the spacecraft, generating oscillations that were picked up by the spacecraft’s plasma wave instrument. The frequency of the waves proved it was now traveling into an area with high-density plasma that continued to steadily thicken — exactly what they expected to find in interstellar space. (You can actually hear the waves in the recording here — the higher the density, the higher the pitch.)
Voyager project scientist Ed Stone and editor Christopher Harris review a video marking the milestones and achievements of the Voyager spacecraft. Image: Veronica McGregor, NASA
It's important to note that while Voyager 1 is in a new region of space, it has not left our solar system. Our solar system is defined as everything that is affected by the sun's gravitational force. That includes the Oort cloud, a spherical cloud that outlines the boundary of our solar system. It will take Voyager 1 another 300 years to reach the beginning of the Oort cloud and another 30,000 years or so to exit. It will be 40,000 years before Voyager 1 reaches the nearest star, Gliese 445, and adopts its gravitational pull.
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However, Voyager 1's battery power will run out between 2020 and 2025, so the future human race won't be able to track it. (Though the spacecraft does hold a recording of our "greetings from Earth" to anyone who could potentially find it down the road.) That still gives JPL scientists a few good years to crank through Voyager 1's new data from an unexplored region of space.
As for Stone, 77, this isn't a time to sit back and bid farewell to Voyager 1. In fact, he may once again become a familiar face around the JPL campus.
"It's not the end," he said. "It's actually the beginning of a whole new journey.

Images: NASA