After a 35-year, 13-billion mile journey, NASA's Voyager 1 spacecraft has become the first human-made object to reach interstellar space, new evidence from a team of scientists shows.
"It's kind of like landing on the moon. It's a milestone in history. Like all science, it's exploration. It's new knowledge," long-time Voyager scientist Donald Gurnett, with the University of Iowa, told Discovery News.
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In the end, it was the sun itself that in essence "rang Voyager's bell," providing the definitive proof that had eluded scientists for the past year or so about whether or not the spacecraft had reached the space between the stars.
On Aug. 25, 2012, Voyager, which was launched in 1977 to study the outer planets, detected a sudden drop in the number of particles trapped in the bubble of space under the sun's influence, the so-called heliosphere, and a corresponding spike in the number of galactic cosmic rays from outside the solar system.
That evidence alone, however, was not enough to convince scientists Voyager had finally reached interstellar space. What they really wanted to know was how much plasma -- ionized molecules and atoms -- was around Voyager, but that measurement was not possible since the spacecraft's plasma detector stopped working more than 30 years ago.
Computer models had long predicted that within the heliosphere, which is filed with the sun's hot breath of solar wind, plasma density would be a small fraction of what exists in cold interstellar space.
But there was another way. Under very special circumstances, Voyager's two 10-meter (33-foot) antennas can detect vibrations in the plasma that scientists can then use to calculate density.
"If you displace the electrons from their normal position and release them, there is a restoring force that pulls them back and the electrons then oscillate. It's a very characteristic frequency -- like hitting a bell -- and if we can detect that we can compute the density," Gurnett said.
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"It's very straight-forward," he added.
But not very common. It happened nine years ago when Voyager 1 crossed a shockwave, a telltale sign that the solar wind was no longer moving at supersonic speeds.
Another hint of Voyager's whereabouts came in October and November 2012 when the spacecraft's antennas registered the effects of a solar flare. The bevy of particles emitted in the so-called coronal mass ejection traveled for about a year before reaching Voyager.
Conclusive proof came this spring when Voyager detected another solar outburst.
"We were able, for the first time, to measure the density of the plasma, the number of particles per cubic meter," Gurnett said. "As soon as we detected those oscillations, we knew that we were in the interstellar medium."
"The definition of the heliopause is based on the plasma density and they just couldn't measure that. And we, by some good fortune having to do with solar events, finally could do that," he said.
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Extrapolating back in time, scientists calculate that Voyager 1 likely crossed into interstellar space back in August 2012, the same time it measured changes in the prevalence of cosmic rays and solar particles.
While one step of Voyager's journey is over, a new expedition is beginning.
"We are now in interstellar space. This is a very exciting new phase of the mission," said lead scientist Edward Stone, with NASA's Jet Propulsion Laboratory in Pasadena, Calif.
"As usual, the most important thing we'll find is probably something that we didn't expect. That's what makes this mission so very special," he said.
The research appears in this week's Science.