Humanity’s second ambassador to interstellar space has reported back from the frontiers of the solar system — with the message that the border of the sun’s territory is a complex and ever-changing place.

Late last year, NASA’s Voyager 2 spacecraft broke through the heliopause, the boundary where the solar wind gives way to the plasma that permeates the galaxy (SN: 12/10/18). Six years earlier, its sister probe, Voyager 1, made its own heliopause crossing (SN: 9/12/13). Now, the combined results of these two journeys, published online November 4 in several papers in Nature Astronomy, offer the most detailed look yet at this largely unexplored region of space.

These two robotic explorers “are taking humankind to astonishing new places that 60 years ago we never imagined doing,” says Gary Zank, a space physicist at the University of Alabama in Huntsville who was not involved with this research.

The view outside Voyager 2’s window changed on November 5, 2018, when the craft was about 17.8 billion kilometers from the sun — 119 times farther away than Earth — and the density of the surrounding plasma jumped by about a factor of 20. The steady stream of low-energy atomic particles from the sun dropped away, replaced by a barrage of far more energetic particles known as cosmic rays. These changes told researchers that Voyager 2 had left the sun’s protective magnetic bubble, just over 41 years after beginning its sight-seeing expedition across the solar system.

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“It’s been a wonderful journey,” mission lead Edward Stone, a planetary scientist at Caltech, said at an Oct. 31 news conference on the findings.

Cosmic protons As Voyager 2 crossed the heliopause — the boundary where the solar wind yields to interstellar plasma — it measured many more high-energy protons striking it per second. These particles, known as cosmic rays, originate far out in the galaxy and are usually deflected by the sun’s magnetic field. Change in galactic proton rate as Voyager 2 crossed the heliopause S.M. Krimigis et al/Nature Astronomy 2019 S.M. Krimigis et al/Nature Astronomy 2019

Voyager 1 had gotten there first, but Voyager 2 had an advantage: a working sensor that could measure the speed, temperature and density of the surrounding plasma. “That makes a huge difference in providing us with a significantly greater level of understanding” of how the solar plasma intermingles with the interstellar medium, Zank says. Voyager 1’s sensor had shut down long before reaching the heliopause, so researchers had to infer many plasma properties from other measurements, which might not have been as accurate as a direct measurement.

Despite encountering the heliopause at different times and locations — the two spacecraft are farther from each other than each are from the sun — some things looked similar. The magnetic field looked pretty much the same on the inside and the outside of the boundary: Somehow, the sun’s magnetic field lines up nearly perfectly with the local galactic field, contrary to expectations. “We could dismiss that as coincidence in one case, but we can’t do that twice,” study coauthor Leonard Burlaga, of NASA’s Goddard Space Flight Center in Greenbelt, Md., said in the news conference.

There were notable differences in the two crossings as well. Voyager 1 sailed through a largely stagnant solar wind for two years before reaching the heliopause, whereas the stream of solar particles alongside Voyager 2 was quick and steady right up to the boundary. Voyager 1 encountered galactic material intruding into the solar bubble, while Voyager 2 instead witnessed solar particles leaking far out into interstellar space. “We’re seeing the same beast, but it’s behaviorally quite different,” Zank says.

Figuring out what many of these results mean will be challenging: The probes interrogated two boundary spots separated by nearly 24 billion kilometers. But this boundary is always changing. It breathes in and out in sync with the sun’s 11-year activity cycle, and eruptions on the sun’s surface make their way out to the heliopause and stir things up. “That complicates all these stories,” Stone said.

New data will have to wait. So far, five spacecraft have made it (or will make it) this far into space — but the Voyagers are the only ones sending back reports. Pioneer 10 and 11, launched in 1972 and 1973, stopped working years ago. New Horizons, which paparazzied Pluto in 2015 (SN: 12/15/15), recently detected a possible glow of hydrogen gas at the solar system’s edge (SN: 8/9/18). But that craft may run out of power before it reaches the heliopause. In the meantime, NASA is looking into launching a dedicated interstellar probe as early as the 2030s.

For now, the Voyagers are humanity’s eyes and ears in interstellar space, and the team estimates that both spacecraft have about five years of service left. Electrical power comes from heat generated by a nugget of plutonium, and as the probes cool, they lose the ability to keep their instruments running. When asked if he expected the Voyagers to last this long, Stone replied: “We’re certainly surprised.”