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.
The first evidence that Voyager had reached that boundary appeared on July 28, 2012, when the number of solar particles measured by Voyager plummeted. But the particle count rebounded a few days later. Three similar dips and recoveries occurred in the following weeks until August 25, when solar particles disappeared for good (SN Online: 6/27/13). The solar particle measurement, combined with a surge in higher-energy particles from other stars, suggested that Voyager had exited the heliosphere and reached the promised land. Several well-publicized studies made that claim.
Stone and his colleagues resisted that conclusion. They lacked evidence of what they thought would be the key signature of interstellar space: a shift in the direction of the magnetic field. Solar plasma produces a distinctive magnetic field because it all comes from the same source; scientists expected that the field would shift in interstellar space, where particles flit around in all directions. Despite the particle evidence that Voyager had departed the heliosphere, the magnetic field direction remained constant. “We felt we did not have the smoking gun to say that we had left the solar bubble,” Stone says.
What the Voyager team needed was another independent measurement to confirm the story implied by the particle data. One option was to prove that Voyager was surrounded by cold, dense plasma from interstellar space rather than hot, wispy plasma from the sun. Such a measurement would have been straightforward except that Voyager 1’s plasma instrument stopped working somewhere near Saturn 33 years ago.
Donald Gurnett, a Voyager scientist at the University of Iowa, found a way to get the measurement anyway. Poring over data from another instrument on the spacecraft, Gurnett discovered that in April 2013 a blast wave from the sun, the same kind that can cause solar storms on Earth, had reached Voyager’s neck of the woods and jostled electrons in the surrounding plasma. It was the first such energetic solar shock in nine years. “In that sense we were lucky,” Stone says.
Gurnett then used the frequency of the electron vibrations to calculate that plasma surrounding Voyager 1 was about 50 times as dense as scientists would expect inside the heliosphere, a sign that the spacecraft had entered interstellar space.
“The study very definitively shows that we’re in the interstellar medium,” says Gary Zank, a space physicist at the University of Alabama in Huntsville who was not involved in the research. “There’s no way of producing a density of that size within the heliosphere.”
Not everyone agrees, including a few holdouts on the Voyager team. George Gloeckler and Lennard Fisk, both from the University of Michigan in Ann Arbor, have written a paper demonstrating how plasma could become dense enough within the heliosphere to produce Gurnett’s measurement. “Gurnett definitely measured the density correctly,” Gloeckler says. “But I don’t believe you can say that what he measured is the interstellar plasma.”
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Barring a change in the magnetic field, Gloeckler believes the team should wait another two or three years for Voyager 2, which has a working instrument to measure the density and temperature of plasma, to reach a similar position in space. “Voyager 2 will experimentally answer this question,” he says. “Why rush to conclusions now?”
Zank and many other astrophysicists say the evidence is overwhelming that Voyager 1 has crossed the heliopause, but they acknowledge that they have to determine why the magnetic field direction didn’t shift. At the same time, scientists are combing through more than a year’s worth of data Voyager 1 has collected since entering interstellar space. NASA estimates that Voyager 1 has enough plutonium fuel to keep all its instruments powered for another seven years, giving the probe plenty of time to measure an environment littered with particles that originated in distant stars and violent supernovas. “All this will give us considerable insight into what’s happening in the far reaches of the galaxy,” Zank says.
For now, Stone and other scientists are excited about the robotic explorer’s accomplishment on August 25, 2012 — the same date, coincidentally, that the world lost its most famous human space explorer, Neil Armstrong.