The Sun in its rearview mirror, Voyager 2 is in interstellar space

Enlarge / The Plasma Science Experiment, and the change in the solar wind it detected as Voyager 2 reached interstellar space. (credit: NASA/JPL-Caltech/MIT)

On Monday, NASA announced that one of its longest-running experiments has started a new phase. Five years after Voyager 1 reached interstellar space, its sibling, Voyager 2, has joined it there. While the Oort Cloud of icy bodies extends well beyond the probes' current locations, they've gone past the point where the charged particles of the solar wind dominate space. Instead, their current environment is dominated by cosmic rays ejected by other stars.

Voyager 2 was actually the first of the two probes launched back in 1977, but its trajectory had it reaching its first destination, Jupiter, after Voyager 1. Its trailing position had a large impact on its future; after Voyager 1 obtained sufficient data at Saturn, Voyager 2 was switched from a route that optimized data gathering to one that sent it past the ringed planet on a trajectory that boosted it toward Uranus and Neptune. That difference meant that Voyager 1 reached interstellar space much earlier.

To a large extent, this made it much easier to figure out that Voyager 2 made the transition. Prior to Voyager 1's arrival, it was expected that two transitions would happen at roughly the same time: the Sun's magnetic field would fall off and the galactic magnetic field would dominate, and the particles of the solar wind would drop while cosmic rays would pick up. Voyager 1 experienced the latter transition but not the former. Put differently, there was a change in charged particles without an accompanying change in the magnetic field.

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