The study suggests that the plasma density is about 30 times higher in the interstellar medium than in the heliosphere, which is close to what scientists thought based on other kinds of measurements. The boundary is called the heliopause.
When did it happen?
Scientists have been using several kinds of measurements to figure out if and when Voyager 1 had reached the interstellar medium.
Evidence from particle data had already pointed toward the conclusion that the probe succeeded. In late July and early August of 2012, scientists saw dips in the concentration of particles made in the solar system, and peaks in particles made outside.
“If you just looked at that data, you’d think it’s pretty clear that we’ve actually crossed a boundary. We’re no longer in the place where the solar system particles are being made, and we’re actually out in the interstellar medium,” Swisdak said.
But magnetic field measurements suggested otherwise. Researchers had expected to see stark changes in magnetic field direction when the probe crossed out of the heliosphere, but that wasn’t supported by measurements from the probe.
Swisdak and colleagues published a modeling study suggesting that the particle data is more relevant, and that the magnetic field might not change as much as people thought. They proposed a crossing-over date of July 27 — about a month sooner than the new study.
The specific date will likely be debated for some time, Swisdak said. One possible explanation is that if the heliosphere is analogous to an air-conditioned room, Voyager stepped through the doorway into a hot room on July 27. For a month it was in a metaphorical room with a mixture of hot and cold air, and finally entered the truly hot part on August 25.
Puzzles still surround the magnetic field at the edge of the heliosphere, Stone said, and “We’re going to be prepared to have more surprises.”
What else is out there?
Voyager 1 now has a totally new mission, Stone said.
“We’re now on the first mission to explore interstellar space,” he said. “We will now look and learn in detail how the wind which is outside, that came from these other stars, is deflected around the heliosphere.”
Wind — made of particles — from these other stars has to go around the heliosphere the way a water in a stream flows around a rock, Stone said. Scientists are interested in learning more about the interaction between our solar wind and wind from other stars.
Natural radioactive decay provides heat that generates enough electricity to help Voyager 1 communicate with Earth. The first science instrument will be turned off in 2020, and the last one will be shut down in 2025, Stone said.
Voyager 2 will likely leave the solar system in about three to four years, Stone said.
Its plasma instrument is still working, Stone said, so scientists can directly measure the stellar wind’s density, speed and temperature. That also means that when it crosses out of the heliosphere, Voyager 2 will send a clearer signal.
At that time, it will join its twin in the vast nothingness between stars that used to be beyond our reach.
Elizabeth Landau | CNN