Humanity first left the solar system in 2012 when the Voyager 1 probe passed into interstellar space decades after leaving the planets behind. It's since spent almost four decades making its way out of the massive bubble called the heliosphere which is generated by the Sun and contains our entire Solar System.
The U.S. space agency previously announced that Voyager 2, the second human-made object ever to depart the solar system following its twin Voyager 1, had zipped into interstellar space on November 5, 2018 at a point more than 11 billion miles (17.7 billion km) from the sun. The two tests are physically indistinguishable, yet they took various ways through the close planetary system. They exploited the "Grand Tour", an arrangement of the planets that happens just once at regular intervals.
When Voyager 1 reached the edge of our solar system, known as the heliopause, it no longer had a functional plasma spectrometer. We didn't have much data about this region, obviously, and being able to study it at all is a breakthrough in itself. It accomplished that feat roughly a year ago, traveling into interstellar space and leaving our system for good.
Six years ago, when Voyager 1 crossed into the interstellar medium, it couldn't detect this cosmic ray boundary layer. This atmosphere protects the solar system from greater interstellar radiation (and who knows what else) but eventually it thins out. "We will see a transition from the magnetic field inside to a different magnetic field outside, and we continue to have surprises compared to what we had expected".
Voyager 2 has recently sent back information demonstrating that it has additionally crossed the heliopause, and it had a completely practical plasma spectrometer. "At least two points where the spacecraft Voyager crossed it", said bill Kurth of the University of Iowa. Now, the new measurements from Voyager 2 indicate that the boundaries between our solar system and interstellar space may not be as simple as once thought. However, their exit points were about 150 AU apart.
"There appear to be cosmic ray boundary layers on both sides of the heliopause, with the outer one only being evident at the position of Voyager 2", the researchers said, adding this external layer "was not evident at the place, time where Voyager 1 crossed".
Voyager 2 has provided new insights into the nature of the heliosphere's limits. Their original objective was to study Jupiter and Saturn specifically, sending back findings like active volcanos on Jupiter's moons and giving us our first detailed look at Saturn's rings. The only functional probe that has any hope of reaching the heliopause is New Horizons, which is now flying through the Kuiper Belt.