NASA announced that its Voyager 2 space probe could be close to interstellar space. Approximately 17.7 billion kilometers (almost 11 billion miles) from the Sun, it detected an increase in cosmic rays coming from outside the solar system, one of the criteria already used in the past to assess whether its twin Voyager 1 had reached interstellar space, an event confirmed in September 2013. The route of the two probes is different and is the reason why one of the two probes is farther than the other and the heliosphere doesn’t have a fixed size so NASA’s monitoring is continuing but there are no certainties yet.
The Voyager 2 spacecraft was launched on August 20, 1977, before its twin Voyager 1. After completing its mission to observe the solar system’s gaseous planets between 1979 and 1989, it kept on moving farther away from the Sun. The final mission for both Voyagers is to study the boundaries of the heliosphere, the region of space in which the density of the solar wind is higher than that of interstellar matter. Actually, the solar system is much larger than the heliosphere because the Oort Cloud is much farther away from the Sun, so according to definitions that cloud is part of the solar system but at the same time it’s in interstellar space.
Cosmic rays, energetic particles that move at very high speeds coming from outside the solar system, are partly blocked at the boundaries of the heliosphere, called the heliopause. As a result, when Voyager 1 detected a sudden increase in cosmic rays, at NASA they started assessing its position, especially since it was the first time that it was possible to obtain data on site from a spacecraft.
Now at NASA they’re seeing a similar phenomenon detected by Voyager 2 thanks to its Cosmic Ray Subsystem (CRS) instrument, one of the few still working. NASA scientists are carefully examining the data to see if it’s reaching interstellar space as well but they don’t expect a precise repetition of what happened to its twin spacecraft.
The heliosphere expands and contracts following the Sun’s 11-year cycle of activity in which its emissions vary in intensity with consequences that are felt even several billion kilometers away. In different areas the heliosphere’s boundaries could receive different amounts of cosmic rays so the detections by the two Voyager probes could be different. At NASA they accumulated experience following the Voyager 1 space probe moving into interstellar space but they are aware of the fact that there are still unknowns so they remain cautious.
Ed Stone, the Voyager project scientist who in 2013 announced the confirmation that Voyager 1 had entered interstellar space, stated that we’ll learn a lot in the coming months but we still don’t know when Voyager 2 will reach the heliopause. However, there’s no doubt that there’s a change in the environment around the probe.
All of this is creating excitement that for NASA scientists will only grow with the data that the Voyager 2 space probe will keep on sending. It’s difficult to say how long it will take to establish if it actually reached the heliopause so any change will be analyzed carefully. All data will increase our knowledge of the heliosphere’s outermost area and, when Voyager 2 enters interstellar space, of that frontier.