NASA announced Monday that one of its longest-running experiments has entered a new phase. Five years after Voyager 1 reached interstellar space, its brother, Voyager 2, has joined it. As the Oort cloud of icy bodies extends far beyond the probes’ current locations, they have moved past the point where the solar wind’s charged particles dominate space. Instead, their current environment is dominated by cosmic rays emitted by other stars.
Voyager 2 was actually the first of the two probes launched in 1977, but its trajectory made it to its first destination, Jupiter, after Voyager 1. Its lagging position had a major impact on its future; after Voyager 1 obtained sufficient data from Saturn, Voyager 2 switched from a route that optimized data collection to one that sent it past the ringed planet on a trajectory that propelled it toward Uranus and Neptune. That difference meant Voyager 1 reached interstellar space much earlier.
In large part, this made it much easier to figure out that Voyager 2 made the switch. Prior to Voyager 1’s arrival, two transitions were expected to occur at about the same time: the Sun’s magnetic field would drop and the galactic magnetic field would dominate, and the solar wind particles would fall while picking up cosmic rays. Voyager 1 has experienced the last transition, but not the first. In other words, there was a change in charged particles without an accompanying change in the magnetic field.
In early November, Voyager 2 made the same transition, about 16.5 light hours from Earth (about 18 billion kilometers or 11 billion miles). As the solar wind’s strength fluctuates, there’s a chance the frontier could catch up with it again before entering interstellar space for an indefinite journey between the stars. But that just gives scientists another chance to study the transition zone. And maybe we’ll still have the magnetic field transition to look forward to, if the craft’s power lasts long enough.
The two are powered by the decay of a radioactive element and see their energy budget drop by four watts per year. Some of their instruments have been shut down to stay within that wealth envelope. Eventually, the current will become so low that the hardware can no longer communicate with the earth.
While finishing second in the race to the edge of the solar system may seem like a disappointment, Voyager 2 has a significant advantage over its sibling. In 1980 Voyager 1’s Plasma Science Experiment failed; The Voyager 2 still works. 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.
List image by NASA