Two spacecraft flying outside the solar system in interstellar space
Voyager 1 (top) and Voyager 2 (bottom) have now both exited the solar system and are flying through nearby interstellar space. Credit: NASA/JPL-Caltech

One year and one day ago, Voyager 2 left the solar system at a spot 119 Earth-Sun distances away, scientists confirmed today. Data from humanity’s second interstellar probe revealed that its exit spot from the solar bubble was unexpectedly similar to Voyager 1’s crossing point.

“Voyager 1 reached the edge of the bubble back in 2012, and we’ve been waiting since that time for Voyager 2 to catch up,” Edward Stone, a heliophysicist at the California Institute of Technology in Pasadena, told reporters at a press briefing.

“Both craft have now reached interstellar space,” he said. “There’s a lot to learn by comparing Voyager 1 and Voyager 2.”

Similarities Present a Puzzle

Voyager 1 and Voyager 2 were launched in 1970 and meandered through the solar system before making for interstellar space. The exact time that Voyager 1 crossed the heliopause, the boundary that separates the solar system from the rest of the galaxy, is still unknown. The craft’s plasma instrument, the one best suited to detect the transition away from solar material, was damaged in 1980. Confirmation of its exit came a few months after the fact.

Unlike its sibling, Voyager 2 has a working plasma instrument that recorded its outward journey. The spacecraft recorded changes in the magnetic environment, the flows of solar and galactic particles, and different plasma temperatures that confirmed its entrance into interstellar space. That transition from inside to outside the bubble took less than 1 day, the project scientists said, just like with Voyager 1.

Graphs showing changes in galactic and solar particles over time
As Voyager 2 approached the heliopause, it measured an increase in the flux of galactic cosmic rays (top line) and a simultaneous decrease in particles from the Sun (bottom line). Credit: NASA/JPL-Caltech/GSFC

The team was surprised at some of the similarities between the two points in the heliopause. “The most major [similarity] was that the distance at which Voyager 1 crossed, 121.6 astronomical units [AU], is very close to the distance at which Voyager 2 crossed, 119 AU,” according to Voyager scientist Stamatios Krimigis, a solar physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. An AU is the distance between Earth and the Sun.

The similarity in distance “is very strange in the sense that one occurred at the solar minimum when solar activity is the least and the other one occurred at solar maximum,” he said. “And if we take our models at face value, we expected that there would be a difference” because the bubble should expand or contract slightly from changes in solar wind pressure.

“The boundary layer we saw at Voyager 2 we couldn’t see at Voyager 1 because we didn’t have a working plasma instrument.”

Voyager 2 also detected a plasma transition and boundary layer just inside the heliopause. “The boundary layer we saw at Voyager 2 we couldn’t see at Voyager 1 because we didn’t have a working plasma instrument, so we couldn’t see the density go up and the temperature go up,” explained heliophysicist John Richardson of the Massachusetts Institute of Technology in Cambridge.

However, the width of the layer seen by Voyager 2 matches the width of a so-called stagnation region detected by Voyager 1. “The speed of the plasma was essentially zero,” Krimigis said. “And we also saw the boundary layer at Voyager 2 which was about the same width, but the speed of the plasma was not zero. And that remains a puzzle.”

Thinner, Smoother, Stronger, Leakier

The part of the heliopause where Voyager 2 passed was thinner and smoother than the spot where Voyager 1 left. And just beyond the border, Voyager 2 measured a stronger magnetic field than Voyager 1 did.

“Inside, the magnetic field comes from the Sun, carried out by the ionized solar wind, and outside the magnetic field is what is in the local region of the Milky Way galaxy,” Stone said. When Voyager 1 crossed, “we were surprised to find that the direction of the magnetic field was not what we had expected when we were outside, and with Voyager 2 we are finding a very similar result. So, it’s a puzzle.”

Material from the solar bubble was leaking outside upstream into the galaxy.

Voyager 2’s crossing point was also very “leaky,” Krimigis said. “In other words, material from the solar bubble was leaking outside upstream into the galaxy…and that was very different than what happened with Voyager 1 where hardly any material was leaking out” but some galactic material was leaking in, he explained. The scientists published these results in a set of Nature Astronomy papers on 4 November.

The differences in the two crossing points are “fascinating” and “will undoubtedly lead to deeper insight into the fundamental physics governing the Sun’s interstellar neighborhood,” said Jamie Rankin. Rankin is a space physicist at Princeton University in Princeton, N.J., who was not involved with the research.

Voyaging Onward

“Voyager 2’s interstellar arrival is a significant milestone because we are now able to look at our own star from outside in rather than inside out, and from not just one, but two perspectives,” Rankin said.

However, “this is the interstellar medium as it’s perturbed by the tsunamis that come from the Sun and propagate out into this region,” Stone said. The mission teams expect another 5 years or so of data from the two craft before they run out of power, which is not quite enough time to reach a part of space that is truly devoid of the Sun’s influence.

The team hopes that future missions to interstellar space will help clarify some of the lingering mysteries of the heliosphere, including its exact shape. “Depending on the instrumentation of a future interstellar probe,” Krimigis said, “we would have the possibility of taking images from outside the heliosphere” and seeing if it really is a sphere or more of a teardrop shape.

“Without the careful and wise planning by hundreds of visionaries along the way—for over 4 decades!—humankind would still be taking measurements from the confines of our own astrosphere,” Rankin said. “But now that both Voyagers are exploring interstellar space, we have made a small, but significant, step towards reaching the nearest stars.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer


Cartier, K. M. S. (2019), Voyager 2’s interstellar arrival was kind of familiar. That’s surprising., Eos, 100, Published on 04 November 2019.

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