Three other instruments measured corresponding changes in cosmic rays, low-energy particles and magnetic field strength.
"Voyager has a very special place for us in our heliophysics fleet", said Nicola Fox, director of the Heliophysics Division at NASA Headquarters in the statement.
Voyager 2 will continue sending back scientific measurements until about 2025, before travelling onwards on a journey which will only stop if it smashes into something - or is captured by an alien intelligence. It is trailing Voyager 1, which reached interstellar space - the vast, mostly empty area between star systems - in 2012.
But reaching the interstellar medium is an impressive feat either way - it means both Voyagers would finally be flying through the void between the stars, where the Sun's solar winds are no longer able to reach them.
After comparing data from different instruments aboard the spacecraft, mission scientists determined the probe crossed the outer edge of the heliosphere on November 5, according to NASA.
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They might not call it just yet, as Voyager 1 entered the interstellar medium at around 121 AU (or 121 times the distance between the Sun and Earth), but Voyager 2 is now at only 119 AU.
Illustration shows the position of NASA's Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. NASA suspects it will take some 30,000 years for the Voyagers to travel beyond the Oort Cloud, and enter farther into uncharted territory. The greatest concerns about keeping the probes operating are power and thermal. "Those decisions will be made with getting the most science value back".
Suzanne Dodd, Voyager project manager at NASA's Jet Propulsion Laboratory in California, said: "We're looking forward to what we'll be able to learn from having both probes outside the heliopause".
"There is a lot of science data ahead, and we anticipate we can operate for five to six or nearly 10 more years, just not with all instruments on", Dodd said.
Each spacecraft carries a Golden Record of earth sounds, pictures and messages, which may one day be the only trace of human civilisation. During the 1970s and 1980s, Voyager 2 took on a "Grand Tour" with close flybys of Jupiter, Saturn, Uranus and Neptune, while Voyager 1 took a different course that featured a close-up of the Saturnian moon Titan.
Correction: A previous version of this story incorrectly described the freezing point of Hydrogen.