div data-type=”body-text”>On November 5, 2018, NASA's Voyager 2 probe crossed the border of the solar system and entered interstellar space, as its twin Voyager 1 had done about six years earlier. Unlike its predecessor, however, Voyager 2 found something new and very strange.
Throughout this last year, the scientists examined the data collected in the moments before and immediately following the passage of the probe from the area of influence of the Sun to the void between the stars, called interstellar medium.
The border between these two realms, known as heliopause, is a quite dynamic environment. Here, the galactic cosmic rays — which are high-energy particles from alien stars and distant galaxies — collide with the magnetic shield created as a bubble from the Sun, which extends throughout the solar system.
The Voyager probes are the only objects made by man to have ever crossed this tumultuous border between the Sun and other stars. And although the two sisters sent back much information and similar observations back to Earth, Voyager 2 also came across entirely new phenomena, according to five new studies published this week on Nature Astronomy.
Figure illustrating the heliopause. Image: NASA / JPL-Caltech
Among other things, Voyager 2 discovered a previously unknown border just outside the heliopause, reports a team led by Edward Stone, a professor of physics at the Caltech Institute who has also been part of the Voyager program since its conception, in the 1970s.
Researchers define this threshold as a "boundary layer of cosmic rays," because here the probe recorded a change in the gradient of cosmic rays from deep space and low-energy particles typical of our system.
There is evidence that Voyager 1 also encountered one of these border layers, but, bizarre to say, in that case it was located inside to the heliopause.
"There would appear to be boundary layers of cosmic rays on both sides of the heliopause, with the outer one evident only from where Voyager 2 was located," Stone's team said in the study. "This layer outside the heliopause was not perceptible at the point and time when Voyager 1 exceeded the threshold."
It is not clear why the probes crossed these layers on opposite sides of the heliopause. It could be due to the contrasting trajectories they have, so Voyager 1 would have come out of the heliopause passing through a hemisphere of the heliosphere, while Voyager 2 left it passing from the opposite one.
The boundary of interstellar space is also an environment that changes rapidly and we know that the activity of the Sun has decreased in the six-year interval that separated the two transits, which certainly influenced the conditions.
Regardless, Voyager 1 recorded a flow of high-energy cosmic rays before passing through the heliopause. "There were two episodes (for Voyager 1) that she was connected to," Stone said on a conference call last Thursday. "In that case, however, we saw the dispersion from outside to inside."
Voyager 2 recorded exactly the opposite phenomenon: an increase in low-energy particles from the heliosphere after crossing the boundary line with interstellar space.
"We can look at the data again to understand what is the process by which the particles that are inside start to overflow outwards," Stone explained in the teleconference. "There seems to be a region just outside the heliopause where there is still a connection to the interior."
The fact that there is a contamination of particles on both sides of the heliopause only underlines how strange the environment that is beyond the area of magnetic influence of the Sun. But, despite its apparent permeability, most of the cosmic rays never exceed the heliopause to enter the solar system.
This obviously played a lot in favor of life on Earth, because cosmic galactic radiation would be very harmful to biological beings. In other words, a better understanding of the heliopause would have implications in the search for potentially hospitable solar systems and exoplanets beyond our system.
The Voyager probes are the first to send us direct observations of this important border layer, and also the first that will spend eternity there. We hope they are not the last.
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https://www.vice.com/it/article/d3a987/nasa-voyager2-confine-spazio-interstellare
