Voyager 2 Just Became the Second Spacecraft to Ever Leave Our Solar System

After a 41-year journey, NASA’s Voyager 2 spacecraft is officially the second human-made object to leave our solar system. Researchers announced on Monday that on November 5, Voyager 2 broke through the heliosphere, the bubble of ionized particles that envelops the solar system. This spectacular outcome, the Voyager project scientists revealed at the fall meeting of the American Geophysical Union, was never guaranteed when the craft launched in 1977.

Before its twin Voyager 1 left the confines of our solar system in 2012, reaching its edges meant entering uncharted territory, both literally and figuratively. As of the time of this article’s publication, Voyager 2 was about 11,154,587,203 miles from Earth.

“When Voyager was launched, we didn’t know how large the bubble was, we didn’t know how long it would take to get there, and we didn’t know if the spacecraft could last long enough to get there,” Ed Stone, Ph.D., announced on Monday. Stone, a Caltech physicist, has served as a Voyager project scientist since 1972. With this latest development, his team showed the world a second set of vital data on what the boundary between the solar system and the rest of the universe is like.

Stone described the dynamic forces at the boundary of the heliosphere that Voyager 2 encountered as it exited the solar system and entered interstellar space.

“There are two winds pushing on each other: the solar wind from the inside pushing out, and the interstellar wind pushing back in, in balance,” he explained. As shown in the video above, starting at about 1:12, the bubble that Voyager 2 just left forms a boundary, against which interstellar winds from the Milky Way push. Using its onboard instruments, Voyager 2 gave scientists on Earth a clear readout of when it left the sun’s neighborhood and entered the outer reaches of interstellar space.

Voyager project scientists predicted that, as the spacecraft crossed the heliopause — the edge of the heliosphere — they’d see a rapid rise in interstellar particles and a corresponding drop in solar particles. And as Voyager 2’s instruments detected in the GIF above, that hypothesis was clearly visible. On November 5, after a gradual change in particle densities, the craft’s instruments detected an abrupt shift in both measurements, which led the researchers to conclude that this was the moment Voyager 2 left the solar system.

Stone noted that these measurements were a bit different than those taken by Voyager 1 as it left the heliosphere, but that this discrepancy was expected since the two probes exited at different points in the solar cycle and in different regions of the heliosphere.

“That’s what makes this interesting,” he said. “We still have things to explore in the nearby interstellar space as the two spacecraft continue their exploration.”

And while Voyager 2 sent back a slightly different set of radiation data from that sent back by Voyager 1, it also used its plasma science experiment (PLS) to send back a totally unique dataset. Voyager 1’s PLS was not working in 2012, making Voyager 2’s PLS readings an essential part of filling in that gap in our knowledge. That makes this dataset the first of its kind, helping NASA scientists map the flow of charged particles in the heliosphere.

As the graphs above show, the levels of plasma, charged particles emitted by the sun, dropped abruptly as Voyager 2 left the solar system. Taken with the rest of the data, these readings help scientists get a better idea of how plasma flows through the heliosphere.

Now that Voyager 2 has left the solar system, it will continue transmitting data back to Earth as long as it has the power to do so. At this point, though, everything it tells us is a gift, as scientists never fully knew whether they should expect it to reach where it is now. As long as Voyager 2 is still transmitting data, it will tell us more and more about the galactic cosmic rays that zip around our interstellar neighborhood, possibly paving the way for future crewed missions to other star systems — or at least helping us understand how quickly outer space will kill us.