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. Fortunately, Voyager 2’s instruments are in considerably better form than Voyager 1’s, so scientists have been capable of observing the transition from the heliosphere, which is managed by the sun, to the interstellar medium (ISM).
Both Voyager probes were launched in 1977, with Voyager 2 heading into the area a couple of weeks earlier than Voyager 1. The two probes are physically identical. However, they took different paths via the solar system. They took benefit of the “Grand Tour,” an alignment of the planets that happens only once every 175 years. Voyager 1 visited and received gravity assists from Jupiter and Saturn earlier than heading off towards the edge of the solar system. Voyager 2 swung past Jupiter, Saturn, Neptune, and Uranus. It made its last planetary remark of Uranus in 1989, nearly a decade after Voyager 1 had begun its long march towards the edge of the solar system.
When Voyager 1 arrived at the edge of our solar system, referred to as the heliopause, it no longer had a fully functional plasma spectrometer. Because of this, there was some debate about when, precisely, the probe left our solar system. So, we missed the anticipated transition from warm solar plasma to the denser cold plasma of the ISM. Finally, measurements of local electrons and magnetic field shifts confirmed it was in interstellar space.
Voyager 2 has just despatched back information proving that it has also crossed the heliopause, and it had a completely functional plasma spectrometer. The transition occurred about a year ago in November 2018, and the changeover was roughly in-line with what scientists anticipated based on the indirect reading showed by Voyager 1’s. As Voyager 2 crossed the heliosphere to the ISM, it detected a 20-fold increase in plasma density.