NASA’s Voyager 2 spacecraft beamed back unprecedented data from interstellar space. Scientists detect a jump in plasma density.

NASA’s Voyager 2 probe reached interstellar space, nearly a year ago, becoming the second spacecraft to ever enter that region. Now, one year later, scientists have published the first results from the data Voyager 2 gathered as it passed out of the bubble of gases expanding from the sun and into the wild of the Milky Way.

Its sister spacecraft, Voyager 1, reached the edge of the solar system in 2012. But its broken plasma instrument made it hard for scientists to gather crucial data about the transition from our solar system into interstellar space.

Now, data from Voyager 2 show a 20-fold increase in the density of the plasma. It was an Iowa-led plasma wave instrument on the spacecraft that made the detection. The density is similar to the plasma densities inferred by Voyager 1’s scientists, with small discrepancies likely due to their differences in location.

The transition from our solar system to interstellar space, the researchers say, may take less than a day to cross. The researchers published data from Voyager 2’s crossing November 4 in a series of five papers in Nature Astronomy.

“In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true,” says Iowa’s Don Gurnett, corresponding author on the study, published in the journal Nature Astronomy. “We show with Voyager 2—and previously with Voyager 1—that there’s a distinct boundary out there. It’s just astonishing how fluids, including plasmas, form boundaries.”

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The heliopause is the theoretical boundary at which the sun’s solar wind meets interstellar winds. These winds were caused by supernovae that have exploded millions of years ago.

Voyager 2’s entry into the interstellar medium occurred at 119.7 astronomical units, more than 11 billion miles from the sun. Voyager 1 passed into that region at 122.6 AU.

The Voyagers were launched within weeks of each other in 1977 with different trajectories through space. Yet, for both spacecraft, the plasma densities changed at similar distances from the sun. This indicates that the heliopause doesn’t change much between these two radically different parts of the sky.

“It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed,” says Bill Kurth, University of Iowa research scientist and a co-author of the study. “That says that these two points on the surface are almost at the same distance.”

“There’s almost a spherical front to this,” adds Gurnett. “It’s like a blunt bullet.”

Unlike Voyager 1, the Voyager 2 spacecraft detected a continuous change to the direction of the magnetic fields as it crossed the heliopause. While both missions saw sudden increases in the number of high-energy cosmic rays, Voyager 2 continued to see lower-energy particles from the Sun.

We have learned that every star has its own boundary between its sphere of influence and the local interstellar medium. The latest data is crucial for astronomers’ understanding of space more generally.

The researchers are now working on ways that will help them better understand the region of space outside the bubble-shaped boundary where solar particles still leak out of the heliosphere.

Voyager 1 and 2 have roughly five years before they’ll lose the use of their scientific instruments, said Ed Stone of the California Institute of Technology during the press conference. “When the two voyagers were launched, the space age was only 20 years old, Stone said. “So it was hard to know at that time that anything could last over 40 years.”

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