42 years on, Voyager 2 charts interstellar space

Pablo Tucker
November 5, 2019

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. The spacecraft is so far away that it now takes 19 hours for data to be transmitted to Earth. This is based on data showing that Voyager 1 had to travel further than its twin to reach the heliopause, where the solar wind and the interstellar wind are in balance.

Each paper has been dedicated to one of the five operating science instruments onboard Voyager 2. These papers confirm the passage of Voyager 2 to interstellar space and provide details on the characteristics of the heliopause.

When the probe made its historical crossing, researchers were only able to confirm this fact eight months later, through electron plasma oscillations from which an interstellar plasma density could be inferred.

This plasma density jump was also experienced by Voyager 1 when it entered into the interstellar space.

But Voyager 2 exited the solar system with all its instruments intact and had a complete set of data.

When Voyager 2 exited the heliosphere past year, scientists announced that its two energetic particle detectors noticed dramatic changes: The rate of heliospheric particles detected by the instruments plummeted, while the rate of cosmic rays (which typically have higher energies than the heliospheric particles) increased dramatically and remained high. This was when scientists knew it had entered a different region in space.

Before 2012 when Voyager 1 reached the edge of the heliosphere, researchers were not sure how far the boundary was from the Sun. Thankfully, Voyager 2's instruments worked well enough to measure the particles and magnetic fields present in this distant region. The boundary of the solar system - the place where the solar wind ends and interstellar space begins - is called the heliopause. Yet they crossed into the ISM at basically the same distances from the Sun.

We didn't actually know when Voyager 2 might follow suit - the heliosphere is a bit wobbly, and slightly changes shape all the time - but in October a year ago, it started picking up an increase in cosmic radiation, similar to that experienced by Voyager 1 in 2012. But scientists don't yet fully understand what is causing the compression on either side. In case aliens ever find them, each Voyager probe contains a golden record encoded with sounds, images, and other information about life on Earth. Voyager 1 hit the boundary at about 122 astronomical units from the Sun (an AU is the average distance from the Earth to the Sun); Voyager 2 hit it at 119 AU. These influences are limited by the influence of our galaxy, which has its own magnetic field and an interstellar medium full of its own charged particles. The density is similar to the plasma densities inferred by Voyager 1's scientists, with small discrepancies likely due to their differences in location. Voyager 2 will pass red dwarf Ross 248 at a distance of 1.7 light years in around 40,000 years.

Now Voyager 2 has sent back the most detailed look yet at the edge of our solar system - despite Nasa scientists having no idea at the outset that it would survive to see this landmark.

During that transition, Voyager 1 saw a gradual enhance of substantial-electrical power cosmic rays, but these had been punctuated by two spikes wherever higher-energy cosmic rays instantly rose.

Voyager 2 was launched on August 20, 1977.

"There appears to be a region just outside the heliopause where we're still connected - 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.

When Voyager 1 crossed the heliopause in 2012 and measured magnetic fields inside and outside the boundary, there was no significant change in the direction of the magnetic fields.

Voyager 2 also made some observations that don't square up with a sharp boundary--at least not what we'd expect.

"That says that these two points on the surface are nearly at the same distance", Kurth said. Yet when Voyager 2 crossed this thin surface, "there was essentially no change" in the direction of the field--something Voyager 1 observed as well, says Leonard Burlaga from NASA Goddard Space Flight Center and lead author for this paper.

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