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 has only 68 KB of memory on board -- far less than a smartphone, said Suzanne Dodd, Voyager project manager. Scientists communicate with the spacecraft every day.
"It's the little spacecraft that could," she said in a NASA press conference.
The probe 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.
Both Voyager probes carry time capsules known as "the golden record," a 12-inch, gold-plated copper disc with images and sounds so that extraterrestrials could learn about us. Let's hope they can build appropriate record players.
Voyager 2 will likely leave the heliosphere 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.
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