(CNN) -- At the edge of the heliosphere, you wouldn't know by looking whether you left the cradle of humanity behind and floated out into interstellar space. You would just see unfathomably empty space, no matter which side of the invisible line you were on.
But scientists now have strong evidence that NASA's Voyager 1 probe has crossed this important border, making history as the first human-made object to leave the heliosphere, the magnetic boundary separating the solar system's sun, planets and solar wind from the rest of the galaxy.
"In leaving the heliosphere and setting sail on the cosmic seas between the stars, Voyager has joined other historic journeys of exploration: The first circumnavigation of the Earth, the first steps on the Moon," said Ed Stone, chief scientist on the Voyager mission. "That's the kind of event this is, as we leave behind our solar bubble."
A new study in the journal Science suggests that the probe entered the interstellar medium around August 25, 2012.
"It's still taking data and telling us what it's actually like out there," said Marc Swisdak, associate research scientist in the Institute for Research in Electronics and Applied Physics at the University of Maryland. Swisdak was not involved in the new study, but has worked with Voyager data.
The twin spacecraft Voyager 1 and 2 were launched in 1977, 16 days apart. As of Thursday, according to NASA's real-time odometer, Voyager 1 is 18.8 billion kilometers (11.7 billion miles) from Earth. Its sibling, Voyager 2, is 15.3 billion (9.5 billion) kilometers from our planet.
Technically, if you include the distant comets that orbit the sun, then Voyager 1 has not left "the solar system," Stone said. For that, we'll have to wait another 30,000 years.
How do we know?
Voyager never sent a postcard saying "Greetings from interstellar space!" So whether it has made the historic crossing or not is a matter of controversy.
"The spacecraft itself really doesn't know," Stone said. "It's only instruments that can tell us whether we're inside or outside."
Further complicating matters, the device aboard Voyager 1 that measures plasma -- a state of matter with charged particles -- broke in the 1980s, Swisdak said.
To get around that, scientists detected waves in the plasma around the spacecraft and used that information to calculate density. Vibrations in the plasma came from a large coronal mass ejection from the sun in 2012, resulting in what Stone called a "solar wind tsunami." These vibrations reached the area around Voyager this spring.
Measurements taken between April 9 and May 22 of this year show that Voyager 1 was, at that time, located in an area with an electron density of about 0.08 per cubic centimeter.
In the interstellar medium, the density of electrons is thought to be between 0.05 and 0.22 per cubic centimeter.
Last year, between October 23 and November 27, researchers calculate that Voyager 1 was in an area with an electron density of 0.06 per cubic centimeter. That's still within the interstellar space range, and it means that over time the spacecraft passed through plasma with increasing electron density.
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 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.