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.

In early November, Voyager 2 experienced the same transition, at about 16.5 light-hours from Earth (about 18 billion kilometers, or 11 billion miles). As the strength of the solar wind fluctuates, there's a chance that the boundary will catch up with it again before it enters interstellar space for an indefinite trip between the stars. But that will simply provide scientists with another opportunity to study the transition zone. And we may still have the magnetic field transition to look forward to, if the crafts' power holds out long enough.

The two are powered by the decay of a radioactive element and so see their power budget drop by four watts a year. A number of their instruments have been shut down to stay within that power envelope. Eventually, the power will drop low enough that the hardware will no longer be able to communicate with Earth.

While coming in second in the race to the edge of the Solar System may seem like a disappointment, Voyager 2 has a key advantage over its sibling. Back in 1980, Voyager 1's Plasma Science Experiment failed; Voyager 2's is still working. The charged particles of both the interstellar medium and the solar wind are plasmas, meaning this hardware can measure their properties, including their speed, density, temperature, and pressure.