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 Jupiter, 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.
The Voyager 2 mission has released its first scientific measurements of interstellar space, according to newly published research.