By Andrew Grant
Humankind has officially extended its reach to the space between the stars.
NASA’s Voyager 1 spacecraft exited the vast bubble of particles that encircles the sun and planets on August 25, 2012, mission scientists report September 12 in Science. At the time, Voyager was about 18.2 billion kilometers from the sun, or nearly 122 times as far from the sun as Earth.
“This is the beginning of a new era of exploration for us,” says Edward Stone of Caltech, who has headed the Voyager mission since 1972. “For the first time, we are exploring interstellar space.”
Confirmation of Voyager’s interstellar exploits came after determining that the probe is surrounded by a relatively dense fog of galactic particles rather than a thin mist of solar ones. It was a tricky measurement that required patience, clever detective work and a heavy dose of luck.
NASA launched Voyager 1 and 2 in 1977 to explore the outer planets, but from the beginning Stone’s team hoped the probes would survive long enough to investigate the region of space where our star’s dominance finally wanes. The sun unleashes a flood of hot, charged particles called plasma that jets out in all directions. The plasma forms a bubble called the heliosphere that is tens of billions of kilometers in diameter. Over the last decade, the solar plasma around Voyager 1 has thinned as the spacecraft hurtles toward the edge of the bubble at more than 60,000 kilometers per hour. Astronomers have been waiting for Voyager to cross this boundary — the heliopause, where solar particles give way to even speedier particles ejected by other stars — and enter interstellar space.