More than 35 years after it first launched, NASA's Voyager 1 spacecraft is about to cross the boundary from our solar system into interstellar space – maybe.
The spacecraft, first launched in 1977, is now more than 11 billion miles, or 124 astronomical units, from the sun, according to data published in the journal Science this week. But it could still take several months, or even years, before the spacecraft leaves the region around the sun known as the heliosphere, which extends at least 8 billion miles past all the planets in our solar system, scientists said.
The specific region Voyager is currently sailing through has been dubbed the "magnetic highway," according to National Geographic. The magazine says it's located within the outermost part of the heliosphere's border, known as the heliosheath, an area in which there appears to be a connection between the two magnetic lines, and where the flow of charged particles moves in and out of the solar system.
"If you looked at the cosmic ray and energetic particle data in isolation, you might think Voyager had reached interstellar space, but the team feels Voyager 1 has not yet gotten there because we are still within the domain of the sun's magnetic field," said Ed Stone, Voyager project scientist at the California Institute of Technology, in a statement.
Scientists expected that the spacecraft would detect three signs as it passed through the edge of the solar system. They have already seen two of the signs: charged particles disappearing as they leave the solar magnetic field, and cosmic rays from far outside appearing as they zoom in.
Scientists observed low energy cosmic rays, which are typically swept out by the wind from the sun and can't get in, Stone told the journal Science.
But scientists are still waiting to see the third sign they predicted: an abrupt change in the direction of the magnetic field that would indicate the spacecraft has left the heliosphere.
"The edge may be somewhat turbulent. We just don't know," Stone told BBC News. "This is exploration after all, and we will find out how Nature makes this interface. But it will be moving because the Sun does 'breathe' in and out."
Stone said in an interview with Science, however, that the magnetic field did experience a sudden surge in intensity at the same time particles disappeared.
"It's just as though the solar field had become compressed but we had not yet crossed into the interstellar region where the interstellar field would be in a different direction," he said in the interview.
The increased intensity happened in the span of about 24 hours, according to a NASA statement. But the magnetic field changed direction by no more than 2 degrees, and scientists say they will continue to observe the area.
"A day made such a difference in this region with the magnetic field suddenly doubling and becoming extraordinarily smooth," said NASA scientist Leonard Burlaga in the statement. "But since there was no significant change in the magnetic field direction, we're still observing the field lines originating at the sun."