Working more tirelessly than the Energizer Bunny, Voyager I has, after a 36-year journey, become the first man-made object to enter interstellar space. In doing so, scientists now have proof that Interstellar space is dense with particles and could be said to be whistling in the dark.
“It’s almost a pure tone. Like middle C. But slightly varying, like your piano is not quite tuned right,” said Donald Gurnett, a University of Iowa physicist who has been working on the Voyager mission most of his adult life. Gurnett is the lead author of a paper published Thursday in the journal Science that provides what seems to be the final, incontrovertible evidence that NASA’s Voyager I has crossed into a realm where no spacecraft has gone before.
Scientists have long believed there was a boundary out there somewhere where the million-mile-per-hour “solar wind” of particles would give way abruptly to cooler, denser interstellar space. That boundary, called the heliopause, turns out to be 11.3 billion miles from the sun.
Beyond that boundary, space is much denser with particles. The sun’s hot ejecta forms a vast bubble, known as the heliosphere. In the outer regions of the heliosphere, the particles are relatively few and far between, with just 1,000 particles per square meter in some regions, Gurnett said. But the heliosphere has an edge. Voyager I’s crossing of the boundary, into the cooler, denser plasma, took place on Aug. 25, 2012.
On that date Voyager I experienced a sudden drop in solar radiation and a spike in cosmic particles coming from all around the galaxy. However, the data from the spacecraft was somewhat ambiguous, seeming to give evidence Voyager I was still within the sun’s magnetic field.
Scientists, however, have new data due to a solar flare which took place on March 17, 2012. The sun ejected a huge mass of particles during that flare. When those solar particles arrived at Voyager more than a year later, on April 9, 2013, they triggered oscillations in the charged particles of matter — the plasma — surrounding the spacecraft.
From the sound of space itself, scientists could interpret the density of the plasma. That density, much higher than anything ever registered in the outer solar system, offered compelling evidence that Voyager I had entered the interstellar zone.
“For the first time we’ve actually measured the density of the plasma,” said Ed Stone, the chief scientist for Voyager. He said he’s convinced by the new data that his spacecraft has fully penetrated interstellar space.
“It’s great. This is exploration. This is wonderful,” said Stone, who has overseen the Voyager project since the early 1970s.
Although Voyager I is now in interstellar space, technically it has not left the solar system. That’s because of the Oort cloud — a region of comets in orbit around the sun.
“We’ll get to the inner edge of the Oort cloud in about 300 years,” Stone said. “Of course the spacecraft will not still be transmitting then.”
Stone said the spacecraft will pass through the far side of the Oort cloud in about 30,000 years.