NASA has confirmed that its Voyager 2 space probe has entered interstellar space. The instruments still operating confirmed that it passed through the outer boundary of the heliosphere, called heliopause, where the influence of solar wind ends meeting interstellar particles. These results were presented by members of the Voyager mission team today at the meeting of the American Geophysical Union (AGU) being held in Washington these days.

At the beginning of October 2018 NASA announced that the Voyager 2 space probe’s Cosmic Ray Subsystem (CRS) instrument had started detecting a sudden increase in cosmic rays, one of the clues that it was reaching the heliopause. The only real experience is the one accumulated in the last years when the Voyager 1 reached interstellar space, an event confirmed in 2013, but the route of the two probes is different, the moment in the solar cycle is different and also for this reason the heliosphere doesn’t have a fixed size, therefore every situation is different and therefore must be assessed carefully.

Now the Voyager 2 space probe is about 18 billion kilometers (11 billion miles) from Earth and, like its twin, still works in part thanks to a radioisotope thermal generator (RTG). Compared to its twin, it has the advantage that its Plasma Science Experiment (PLS) instrument still works and this was of considerable help to the mission’s scientists to have other detections of the space in which the Voyager 2 is traveling. Until recently, that space contained above all plasma coming from the Sun, the solar wind that creates the heliosphere. On November 5, PLS detected a sharp decline in the solar wind particles’ speed and subsequently the solar wind flow around Voyager 2 disappeared.

The measurements of the PLS instrument are the first of that type obtained at such a high distance from the Earth and are the strongest evidence that the Voyager 2 space probe has entered interstellar space but there are also confirmations from the other working instruments. The Cosmic Ray Subsystem (CRS), the Low Energy Charged Particle (LECP) and the Triaxial Fluxgate Magnetometer (MAG) are providing detections consistent with that conclusion.

Nicola Fox, director of the Heliophysics Division at NASA, stated that the heliophysics studies carried out thanks to the various space probes and satellite start from the Sun and extend outwards. The consequence is that having the Voyagers sending information on the borders of solar influence provides an unprecedented view of an uncharted territory that can help to gain a better knowledge of the heliosphere.

The Oort cloud is much farther from the Sun than the two Voyager probes and is considered part of the solar system. This means that the Voyagers are in interstellar space while still being in the solar system. It’s a matter of definitions that can lead to a bit of confusion. For scientists it’s even more an opportunity to study even better everything that’s outside the influence of the Sun thanks to these probes that will keep on sending information for years.