The first data from Voyager 2 as it crossed into interstellar space are reported in several papers published in Nature Astronomy this week. These papers confirm that the passage of Voyager 2 to interstellar space occurred on 5 November 2018, at a distance of 119 times the Earth - Sun distance. The papers also provide details on the characteristics of the heliopause: the outermost solar structure and boundary between the heliosphere and interstellar space.
Voyager 2 is only the second spacecraft to travel beyond the confines of the heliosphere. The craft was launched slightly ahead of its twin Voyager 1 in 1977 and has been travelling through space for the past 42 years. Voyager 1 made the crossing into interstellar space in 2012, reporting valuable data about the heliopause region. However, damage to its plasma instrument meant that complete data on the transition could not be gathered.
Initial measurements taken from Voyager 2 as it crossed into interstellar space, detailed in five research papers, highlight similarities and differences to the Voyager 1 transition. Unlike Voyager 1, readings from Voyager 2’s instruments suggest a thinner and smoother heliosphere boundary with a stronger interstellar magnetic field beyond. In one paper, John Richardson and colleagues suggest that the transition through the heliopause occurred in less than one day and that the interstellar medium closest to the boundary is hotter and more variable than expected. In another study, Edward Stone and co-authors report observations indicating a layer between the heliopause and interstellar space where solar and interstellar winds interact. Such a layer was not observed by Voyager 1.
Together the results suggest that some of the differences between the Voyager 1 and Voyager 2 transitions may be because of changes in the Sun’s level of activity; others may be related to the spacecrafts’ differing trajectories.
There are still unanswered questions about the properties of interstellar space, and the structures of the unexplored regions further from the Sun are subject to debate. “This debate can only be settled by more direct observations from the Voyager spacecraft as they move further into interstellar space, exploring new regions and sending back new and unique data,” writes R. Du Toit Strauss in an accompanying News & Views article.