Humanity first left the solar system in 2012 when the Voyager 1 probe passed into interstellar space decades after leaving the planets behind. Now, there’s a second spacecraft beyond the limits of our solar system: Voyager 2. Luckily, Voyager 2’s instruments are in somewhat better shape than Voyager 1’s, so scientists were able to observe the transition from the heliosphere, which is dominated by the sun, to the interstellar medium (ISM).

Both Voyager probes launched in 1977, with Voyager 2 heading into space a few weeks before Voyager 1. The two probes are physically identical, but they took different paths through the solar system. They took advantage of the “Grand Tour,” an alignment of the planets that occurs only once every 175 years. Voyager 1 visited and got gravity assists from Jupiter and Saturn before heading off toward the edge of the solar system. Voyager 2 swung past Jupter, Saturn, Neptune, and Uranus. It made its last planetary observation of Uranus in 1989, almost a decade after Voyager 1 had started its long march toward the edge of the solar system.

When Voyager 1 reached the edge of our solar system, known as the heliopause, it no longer had a functional plasma spectrometer. As a result, there was some debate about when, exactly, the probe left our solar system. So, we missed the expected transition from warm solar plasma to the denser cold plasma of the ISM. Eventually, measurements of local electrons and magnetic field shifts confirmed it was in interstellar space.

Voyager 2 has just sent back data proving that it has also crossed the heliopause, and it had a fully functional plasma spectrometer. The transition happened about a year ago in November 2018, and the changeover was roughly in-line with what scientists expected based on Voyager 1’s indirect readings. As Voyager 2 crossed from the heliosphere to the ISM, it detected a 20-fold increase in plasma density.

Voyager 1 and 2 crossed the heliopause at roughly the same distance from the sun, 121.6 AU and 119 AU, respectively. However, their exit points were about 150 AU apart. Scientists are studying the discrepancies in the data in hopes of gaining a better understanding of the boundary between our solar system and the wider galaxy. For example, Voyager 2 detected a continuous change in magnetic field directions as it crossed into the ISM, whereas Voyager 1 did not. Voyager 2 has also continued to see low-energy particles from the sun in the ISM, but Voyager 1 didn’t.

It will be some time before we have more data to study. The only functional probe that has any hope of reaching the heliopause is New Horizons, which is currently flying through the Kuiper Belt. It could leave the solar system around 2040, but we don’t know if it will maintain communication with Earth that long.

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