Ever since it left Earth 41 years ago, the Voyager 2 spacecraft has sailed across the solar system on a stream of energetic particles from the sun.
But on Nov. 5, the whistling of the solar wind abruptly stopped. The current of charged particles dissipated. There was a period of turbulence, and then the probe found a strange new quality to the environment around it, the way the air smells of salt when you get close to the sea.
Voyager had crossed the heliopause, where the river of solar particles meets the vast ocean of interstellar space. It is now beyond the bubble of our sun's influence, NASA announced Monday.
For the second time, a human-made object has ventured into the void between the stars.
Its companion probe, Voyager 1, crossed that threshold in 2012. But Voyager 2 has a scientific leg up on its predecessor: It still possesses a working plasma instrument. This allows the spacecraft to sense a different kind of charged particle, called galactic cosmic rays.
After decades of looking at the galaxy "through the clouded lens of our heliosphere," NASA physicist Georgia de Nolfo said, "we're now able to take a step outside with Voyager and contemplate the vistas of our local galactic neighborhood."
The spacecraft is about 11 billion miles from Earth - so far it takes signals traveling at the speed of light 16.5 hours to reach mission control at the Jet Propulsion Laboratory in California. That's more than twice as distant as Pluto and 37 times farther than the journey from Earth to Mars.
Neither of the Voyager probes has technically left the solar system, said Ed Stone, who has been the project scientist for the mission since 1972. In about 300 years, they will reach the edge of the Oort cloud - a halo of icy bodies loosely bound by the sun's gravity that is thought to be the source of comets. It will take another 40,000 years for the spacecraft to exit that cloud and come under the influence of another star.
For all their long years of travel, both spacecraft are essentially still on the solar system's doormat.
But even from this vantage point, the probes are gaining new insights about how our corner of the cosmos works. Each left the heliosphere at a different location, and encountered slightly different conditions when they reached interstellar space. This suggests that there are complex interactions between the solar wind and interstellar space that affect the shape of the sun's bubble, said NASA heliophysicist Nicola Fox.
"It's a whole new view from the other side of that boundary," she said.
Out there, highly energetic particles blow through space's dark expanse like a strange cosmic breeze. These galactic cosmic rays have tremendous energy and travel close to the speed of light; if they ever reached astronauts or spacecraft, the consequences could be severe. Fortunately, most of these particles are deflected from our solar system by the sun's magnetic field.
But scientists have to get beyond the sun's protective bubble if they want to study these rays. So that's what Voyager did.
With its plasma instrument and other tools, Voyager 2 will use these cosmic rays as "galactic messengers," de Nolfo said, revealing clues about the stellar explosions that formed them.
But both spacecraft are living on borrowed time. "I like to say that they're healthy, if you consider them as senior citizens," said Suzanne Dodd, longtime project manager for the mission. Though the probes are functional, they are running out of the plutonium that powers them.
Voyager 2 is especially vulnerable, Dodd said, because it's so cold - a few degrees above the freezing point of the hydrazine that powers its thrusters. Soon, researchers must make trade-offs about how to use the reserves that remain: Will they keep the spacecraft warm or continue to do science?
"We have difficult decisions ahead," she said.
Scientists expect to receive their last signal from both Voyagers in the next five to 10 years. But their mission will not end then. Each spacecraft carries a copy of the Golden Record, a gold plated copper disk inscribed with sounds and images meant to communicate what life is like on Earth.
As they drift through space, to the next star and beyond, they will always bear this message - a postcard, to whomever finds them, from the world their creators call home.
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