Details of what happened when Voyager 2 left the solar system and entered interstellar space have been revealed by scientists, with experts saying there is a "definitive boundary" between the two regions.
Voyager 2 was launched on August 20, 1977. In December last year, NASA announced the spacecraft had left the solar system, saying data returned suggested there had been a major change in the environment, with a marked increase in galactic cosmic rays and decrease in heliospheric particles from the Sun.
In a series of articles published in the journal Nature Astronomy, scientists have now shown how the spacecraft experienced a jump in plasma density in a similar fashion as Voyager 1 did when it entered the interstellar medium in 2012.
Our solar system is surrounded by a protective, wind sock-shaped bubble known as the heliosphere. Within this is the heliosheath, which is where the solar wind from the Sun starts slowing down and interacting with the interstellar medium. The edge of the heliosphere is known as the heliopause and beyond this is interstellar space. Here, the Sun no longer has any influence over the environment.
Voyager 1 entered this interstellar space in 2012, with instruments on board showing changes to the magnetic field and a jump in plasma density. Plasma is ionized gas. In our solar system, it comes from the Sun. In interstellar space, it comes from the material ejected by stars that exploded millions of years ago. However, Voyager 1's onboard plasma instrument was damaged, so the data returned was incomplete. This meant determining the point at which Voyager 1 left the solar system was more difficult.
With Voyager 2, the plasma instrument was undamaged. This meant scientists were able to show there was a significant change to the plasma surrounding it on November 5, 2018—suggesting this was the exact date Voyager 2 left the solar system. Voyager 1 left the solar system when it had traveled 122.6 AU—almost 11.5 billion miles. Voyager 2 left at a distance of 119.7 AU—just over 11 billion miles.
Because the spacecraft left the solar system at different parts of the heliosphere, this finding suggests the heliosheath is fairly symmetrical.
Researchers also say the corresponding changes to the density of plasma recorded in both Voyager 1 and 2 suggests there is a very clear edge to the solar system. "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," The University of Iowa's Don Gurnett, who is one of the study authors, said in a statement. "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."
Having another data set on what happens when an object exits the solar system will help scientists get a better understanding of the heliosphere and interstellar space.
"It's kind of like looking at an elephant with a microscope," Bill Kurth, also from the University of Iowa, said in the statement. "Two people go up to an elephant with a microscope, and they come up with two different measurements. You have no idea what's going on in between. What the models do is try to take information that we have from those two points and what we've learned through the flight and put together a global model of the heliosphere that matches those observations."
The Voyager spacecrafts will now continue along their trajectories. Voyager 1's next close encounter with another star will be in just over 40,000 years, when it will come within 1.6 light years of the star Gliese 445. Voyager 2 will pass red dwarf Ross 248 at a distance of 1.7 light years in around 40,000 years.