Five articles published in the journal “Nature Astronomy” report various detections carried out by NASA’s Voyager 2 space probe in interstellar space. A number of teams of researchers examined the data collected by the five instruments still in use with each article dedicated to the data from a single instrument. Taken together, they help to understand interstellar space outside the heliosphere, the bubble in which the influence of the Sun is felt. One year after coming out of that bubble, Voyager 2 sent a lot of data on plasma and cosmic rays showing the differences compared to those within the heliosphere.
The Voyager 1 space probe had already entered interstellar space six years ago, having the Voyager 2 outside the heliosphere as well is allowing to collect data in a different region and compare it with its twin’s. For a few years, scientists hadn’t known whether what Voyager 1’s data indicated was normal or they were specific to the region in which it’s flying at that time. In December 2018, NASA announced that Voyager 2 had entered interstellar space by pinpointing the event on November 5, when its PLS (Plasma Science Experiment) instrument detected a sharp drop in the speed of solar wind particles and subsequently the flow of solar wind around Voyager 2 disappeared.
The detections carried out by the Voyager 2 space probe confirmed that the plasma in the area close to the heliopause, the heliosphere’s border area, is far denser and colder than that inside the heliosphere. Voyager 1 detected a plasma density slightly higher than expected just outside the heliosphere, indicating that the plasma is somehow compressed. Voyager 2 detected a similar phenomenon just before leaving the heliosphere, indicating a compression on the inner edge of the bubble. Scientists are still studying these phenomena to understand the mechanisms that determine the compression on the plasma on each of the two sides of the heliosphere boundary.
Among the problems there’s the size of the heliopause, where there’s the collision between the solar wind’s charged particles and those of interstellar winds. The Voyager 1 space probe passed through that border in a matter of hours while Voyager 2 took 80 days to do so. Another difference is that Voyager 1 detected interstellar cosmic rays that went in the opposite direction while Voyager 2 kept on detecting particles from the heliosphere, as if it were porous in that area.
Magnetic fields are another object of study by the two Voyager space probes. Voyager 1 had already detected a magnetic field in the region outside the heliosphere aligned with the one inside it, in the last year Voyager 2 confirmed that discovery.
The direct study of interstellar space is still in its beginning and is carried out thanks to two space probes designed for completely different purposes, which was the exploration of the outer solar system’s planets. This is extraordinary but it’s not the best situation due to the limitations of the still active instruments. There are projects for new space probes designed specifically to explore interstellar space but in the best-case scenario a launch could take place in the 2030s for a mission that would last several decades.