Professor Don Garnett of the University of Iowa, USA said, "This is a historic achievement".
At a distance of about 11 billion miles (18 billion kilometers) from Earth - well beyond the orbit of Pluto - Voyager 2 had entered interstellar space, or the region between stars. A period of low solar activity should have pulled the heliopause back a bit during Voyager 2's crossing a year ago.
Voyager 2's encounter with the boundary has always been anticipated by eager scientists keen to get their hands on data transmitted back by the probe, as although Voyager 1 did send some data back, somewhere en route in 1980 the probe's plasma instrument was damaged which meant that it couldn't gather complete data on this breakthrough moment. Its 22.4-watt transmitter has a power equivalent to a fridge light, which is more than a billion billion times dimmer by the time it reaches Earth and is picked up by Nasa's largest antenna, a 70-metre dish. As Voyager 2 crossed from the heliosphere to the ISM, it detected a 20-fold increase in plasma density.
One remaining mystery is the direction of the magnetic field on either side of the heliopause. "It's just astonishing how fluids, including plasmas, form boundaries".
We didn't actually know when Voyager 2 might follow suit - the heliosphere is a bit wobbly, and slightly changes shape all the time - but in October previous year, it started picking up an increase in cosmic radiation, similar to that experienced by Voyager 1 in 2012.
"It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed", says Bill Kurth, University of Iowa research scientist and a co-author on the study. "Once we left the heliosphere, we continued to see particles leaking from the inside out". Eventually, measurements of local electrons and magnetic field shifts confirmed it was in interstellar space.
Using data collected from the Low-Energy Charged Particle (LECP) instrument on Voyager 2, which is almost identical to that flown on Voyager 1, the team say that observations close to the heliopause show relatively large magnetic field magnitudes. When Voyager 1 crossed, "we were surprised to find that the direction of the magnetic field was not what we had expected when we were outside, and with Voyager 2 we are finding a very similar result". Again, all this data taken together raises more questions than it can answer. In a new study, scientists have confirmed that the spacecraft is now outside the bubble-shaped region created by the sun's wind, known as the heliosphere. Turns out the sun's impact goes beyond its own borders. Now they have a new plan that will keep Voyager 1 and 2 awake and operating so scientists may continue to receive vital data about a part of space we've never explored. The probe has been traveling through interstellar space - even riding the momentum of interstellar shock waves - for several years now. The one in our heliosphere is hot and scattered, and in interstellar space, it is colder and more condensed. The space between stars also contains cosmic rays, or particles accelerated by exploding stars.
The journey of the 42-year-old Voyager probe therefore provides rare new insights into the suburbs of our cosmic home.
The Voyager probes became world famous thanks to the gold plates they carry, full of music, sounds and images of the earth, meant as a kind of time capsule that tells the story of our world to any aliens they might encounter along the way. That material is undergoing natural decay. The astronomer believes that the probes give important clues about the details and structure of the heliosphere.
The missions also measured incoming cosmic rays that grew stronger as the probes approached heliopause, with direct implications for the health of manned space missions into deep space.
Both spacecraft run on nuclear power, and their generators are near the end of their lives.