NASA's Voyager 2 probe exited our solar system nearly a year ago, becoming the second spacecraft to ever enter interstellar space.
It followed six years behind its sister spacecraft, Voyager 1, which reached the limits of the solar system in 2012. But a plasma-measuring instrument on Voyager 1 had been damaged, so that probe could not gather crucial data about the transition from our solar system into interstellar space.
Voyager 2, which left the solar system with its instruments intact, completed the set of data. Scientists shared their findings for the first time on Monday, via five papers published in the journal Nature Astronomy.
The analyses indicate that there are mysterious extra layers between our solar system's bubble and interstellar space. Voyager 2 detected solar winds â flows of charged gas particles that come from the sun â leaking from the solar system. Just beyond the solar system's edge, these solar winds interact with interstellar winds: gas, dust, and charged particles flowing through space from supernova explosions millions of years ago.
"Material from the solar bubble was leaking outside, upstream into the galaxy at distances up to a billion miles," Tom Krimigis, a physicist who authored one of the papers, said in a call with reporters.
The new boundary layers suggest there are stages in the transition from our solar bubble to the space beyond that scientists did not previously understand.
The place where solar and interstellar winds interact
An image of Uranus taken by Voyager 2 on January 14, 1986, from a distance of approximately 7.8 million miles. NASA/JPL On November 5, 2018, Voyager 2 left what's known as the "heliosphere," a giant bubble of charged particles flowing out from the sun that sheathes our solar system. In doing so, the probe crossed a boundary area called the "heliopause." In that area, the edge of our solar system's bubble, solar winds meet a flow of interstellar wind and fold back on themselves.
It took both spacecraft less than a day to travel through the entire heliopause. The twin probes are now speeding through a region known as the "bow shock," where the plasma of interstellar space flows around the heliosphere, much like water flowing around the bow of a moving ship.
This illustration shows the position of NASA's Voyager 1 and Voyager 2 probes outside the heliosphere, a protective bubble created by the sun. NASA/JPL-Caltech Both Voyager probes measured changes in the intensity of cosmic rays as they crossed the heliopause, along with the transition between magnetic fields inside and outside the bubble.
But because so much of the transition from our solar system to the space beyond is marked by changes in plasma (a hot ionized gas that's the most abundant state of matter in the universe), Voyager 1's damaged instrument had difficulty measuring it.
Now the new measurements from Voyager 2 indicate that the boundaries between our solar system and interstellar space may not be as simple as scientists once thought.
The data indicates that there's a previously unknown boundary layer just beyond the heliopause. In that area, solar winds leak into space and interact with interstellar winds. The intensity of cosmic rays there was just 90% of their intensity farther out.
"There appears to be a region just outside the heliopause where we're still connected â there's still some connection back to the inside," Edward Stone, a physicist who has worked on the Voyager missions since 1972, said in the call.
An illustration of a Voyager probe leaving the solar system. NASA/ESA/G. Bacon (STScI) Other results from the new analyses also show a complicated the relationship between interstellar space and our solar system at its edges.
The scientists found that beyond the mysterious, newly identified layer, there's another, much thicker boundary layer where interstellar plasma flows over the heliopause. There, the density of the plasma jumps up by a factor of 20 or more for a region spanning billions of miles. This suggests that something is compressing the plasma outside the heliosphere, but scientists don't know what.
"That currently represents a puzzle," Don Gurnett, an astrophysicist who authored one of the five papers, said in the call.
What's more, the new results also showed that compared with Voyager 1, Voyager 2 experienced a much smoother transition from the heliopause to a strong new magnetic field beyond the solar system.Â
"That remains a puzzle," Krimigis said.
The scientists hope to continue studying these boundaries over the next five years before the Voyager probes run out of fuel.
"The heliopause is an obstacle to the interstellar flow," Stone added. "We want to understand that complex interaction on the largest scale as we can."
5 more years of Voyager data
The Voyager 2 spacecraft launches from NASA's Kennedy Space Center on August 20, 1977. NASA/JPL NASA launched the Voyager probes in 1977. Voyager 2 launched two weeks ahead of Voyager 1 on a special course to explore Uranus and Neptune. It is still the only spacecraft to have visited those planets.
The detour meant that Voyager 2 reached interstellar space six years after Voyager 1. It is now NASA's longest-running mission.
"When the two Voyagers were launched, the Space Age was only 20 years old, so it was hard to know at that time that anything could last over 40 years," Krimigis said.
Now, he said, scientists expect to get about five more years of data from the probes as they press on into interstellar space. The team hopes the Voyagers will reach the distant point where space is undisturbed by the heliosphere before they run out of fuel.
After the spacecraft die, they'll continue drifting through space. In case aliens ever find them, each Voyager probe contains a golden record encoded with sounds, images, and other information about life on Earth.
In the future, the researchers want to send more probes in different directions toward the edges of our solar system to study these boundary layers in more detail.
"We absolutely need more data. Here's an entire bubble, and we only crossed at two points," Krimigis said. "Two examples are not enough."
