NASA’s Voyager 2 spacecraft has crossed into interstellar space, agency officials announced in December.

The milestone makes Voyager 2 humanity’s second operating spacecraft in history to go interstellar after the Voyager 1 spacecraft did in August 2012. “One kind of feels like a lucky fluke,” says Justin Kasper, a scientist involved in the Voyager missions from the University of Michigan in Ann Arbor. “Two feels like we’re becoming a society that’s capable of exploring interstellar space.”

Voyager 2’s new interstellar status is based on data from its Plasma Science Experiment (PLS), which logged a decrease in particles around the spacecraft that had been ejected from our sun. The PLS measurements of this “solar wind” plummeted to zero on November 5—the official date of Voyager 2’s departure. Now the mission team is confident the spacecraft has joined its predecessor in passing beyond a key boundary called the heliopause and into interstellar space. “November 5 was the day that the galactic cosmic-ray intensity abruptly increased, and that same date was when the heliospheric particle intensity dropped significantly,” says Voyager project scientist Ed Stone. “That same day the magnetic field increased, and that’s also the point at which the plasma [instrument] quit measuring the solar wind. So that’s the correlation we were looking at [to confirm interstellar space].”

The heliopause is the region at which the solar wind’s outward expansion is countered by the influence of incoming interstellar particles. It is considered to be one of the limits of the sun’s influence on surrounding space—although the two spacecraft are still said to be inside the solar system, they are now in a region of space dominated more by the Milky Way Galaxy than our sun. Prior to this, Voyager 2 crossed a region known as the termination shock back in 2007, where the speed of the solar wind dropped dramatically as it began to encounter interstellar particles and radiation. The entire region of the sun’s bubble of influence in the galaxy is known as the heliosphere, whereas the region Voyager 2 has just traversed is called the heliosheath, which lies between the termination shock and the heliopause.

NASA launched the twin Voyager spacecraft in 1977 on a mission to explore the outer planets. After both studying Jupiter and Saturn—with Voyager 2 taking a detour to also visit Uranus and Neptune—the two probes continued their journeys toward the edge of the solar system. In August 2012 Voyager 1 became the first human-made vehicle in history to reach the heliopause and enter interstellar space. But both missions have raised questions about our solar system’s true boundary. Originally some scientists speculated our star’s winds would peter out in the vicinity of Mars, but the Voyager spacecraft have gradually pushed this boundary far beyond. The solar system’s actual limit remains contentious, however, with some researchers defining it not by solar winds but rather by the most distant objects thought to be held in thrall by our sun’s gravity—comets in the Oort Cloud up to about two light-years away. Regardless of where one believes the sun’s sphere of influence ends, both Voyagers are poised to greatly extend it—each has a famous Golden Record onboard in the event any other spacefaring species happens across them on their lonely, eons-long sojourns between the stars.

Voyager 2 has traveled about 120 astronomical units—one AU is the Earth–sun distance—which equates to just over 18 billion kilometers, a distance light itself takes more than 16 hours to traverse. Although far out, this is about one AU closer in than Voyager 1’s earlier exit from the solar system. The most obvious explanation for this disparity is that our solar system’s heliosphere is not perfectly spherical—instead it is oddly shaped and asymmetric, perhaps due to the influence of the Milky Way’s magnetic field. “You can think of the galactic magnetic field as an array of bungee cords,” says Eric Christian from NASA Goddard Space Flight Center, a scientist on the Voyager team. “The solar system is this soccer ball you’re pushing through these bungee cords,” flexing and distorting its shape as it moves.

Another possible explanation is the sun’s fluctuating activity, measured via outbursts such as solar flares and powerful explosions called coronal mass ejections. These events can affect the heliosphere much like gas pumped into a balloon, causing it to grow—conversely, when their numbers decrease the heliosphere can shrink, changing the location of the heliopause. It is possible that in coming years, as the sun reaches the peak of its roughly 11-year activity cycle, its outbursts could push the heliopause farther out again, perhaps even beyond Voyager 2. “There’s a chance [the probe could enter interstellar space twice],” Kasper says. “It’s all going to depend on how long the solar minimum lasts.” In fact, something like this happened when Voyager 2 crossed the termination shock in 2007. Fluctuating solar activity made the boundary oscillate, so the spacecraft ended up crossing the termination shock several times.

Voyager 2 is also entering interstellar space in a completely different region from its sister ship. Whereas the latter traveled out of the heliosphere’s northern hemisphere (the planet-filled ecliptic plane is the equator), Voyager 2 is heading out of the southern hemisphere. Here, the galactic magnetic field is thought to be weaker, which may also affect the shape of the heliosheath. Having outbound spacecraft from both heliospheric hemispheres opens up fascinating opportunities for science. Both spacecraft have a working magnetometer, to measure the local magnetic field, and two particle detectors—one for solar particles and another for incoming cosmic rays. Only Voyager 2, however, continues to have a functioning plasma instrument, which could tell us much more about this unexplored region, including the temperature, density and velocity of any electrically charged material flowing around the spacecraft.

The Voyager team will continue to take measurements as both spacecraft travel away from the sun into a region called the outer heliosheath, although neither probe has much time remaining to make observations. “We’re probably only going to have four to five years left of data,” Christian says. But they may later be joined in this region by other spacecraft in the coming years. Although NASA’s Pioneer 10 and 11 spacecraft launched before the Voyagers paved the way on similar journeys beyond the Asteroid Belt to the outer solar system, eventually leading into interstellar space, they are no longer communicating with Earth. But NASA’s New Horizons spacecraft, which flew by Pluto and is about to conduct the most distant-ever rendezvous in the solar system, could continue operating into interstellar space, giving us a third functioning interstellar probe in coming decades.