Thirty-five years after leaving Earth, Voyager 1 is reaching for the stars.
Sooner or later, the workhorse spacecraft will bid adieu to the solar system — the first time a human-made object will have escaped to the other side.
When NASA's Voyager 1 and Voyager 2 first rocketed out of Earth's grip in 1977, no one knew how long they would last. Now, they are the longest-operating spacecraft in history and the most distant, at billions of miles from Earth.
Today marks the 35th anniversary of Voyager 1's launch to Jupiter and Saturn. It is now flitting around the fringes of the solar system, which is enveloped in a giant plasma bubble. This hot and turbulent area is created by a stream of charged particles from the sun.
Outside the bubble is a new frontier in the Milky Way — the space between stars. Once it plows through, scientists expect a calmer environment by comparison.
Voyager 1 is in uncharted celestial territory. The boundary that separates the solar system and interstellar space is near, but it could take days, months or years to cross it.
Voyager 1 is currently more than 11 billion miles from the sun. Twin Voyager 2, which celebrated its launch anniversary two weeks ago, trails behind at 9 billion miles from the sun.
The NASA Jet Propulsion Laboratory built the spacecraft. Their original goal was to tour Jupiter and Saturn, and they sent back postcards of Jupiter's big red spot and Saturn's glittery rings.
Voyager 2 then journeyed to Uranus and Neptune. It remains the only spacecraft to fly by these two outer planets. Voyager 1 used Saturn as a gravitational slingshot to catapult itself toward the edge of the solar system.
"Time after time, Voyager revealed unexpected — kind of counterintuitive — results, which means we have a lot to learn," said Ed Stone, 76, Voyager's chief scientist and a professor of physics at the California Institute of Technology.