Following the footsteps of its little sister Voyager 1, when for the first time a human-made device reached interstellar space, Voyager 2 has entered the interstellar medium, leaving the solar system’s frontier behind and no longer officially under is influence.
A historic milestone which will make this the second human-made object to do so.
The news has been confirmed by researchers at the University of Iowa and has been published in two articles in the journal Nature Astronomy.
The Voyager 2 probe reached the limit of the solar system on November 5, 2018, one year ago. Now, the probe has crossed this limit and entered the interstellar medium (ISM), a region beyond the limit of the bubble produced by the solar wind. This region could be defined as the space that separates the stars from each other.
How do we know where the probe is? The solar wind is characterised by a warmer, less dense plasma compared to the plasma in the interstellar space, a cooler more dense plasma.
Scientists have found that Voyager 2 has crossed this border by detecting the increased density of the plasma it has experienced along its journey. It did this through an instrument that measures plasma waves. This density jump was similar to that experienced by Voyager 1 years ago.
How did this income come about?
Voyager 2's entry into the interstellar environment occurred at 119.7 astronomical units (AU), that is, almost 18 billion miles from the Sun. Voyager 1 entered the ISM at 122.6 AU.
Both spacecrafts were launched a few weeks apart in 1977, with different mission targets and trajectories through space. However, they crossed the heliopause (the magnetic border that separates the Sun, the planets of the solar system and the solar wind from the rest of the galaxy) at about the same distance from the Sun.
Beyond the Voyager 2 journey milestone, researchers have been able to find out more details about the composition of our own solar system at its outer limits.
The heliosphere looks symmetrical
The contributions of Voyager 2's journey give valuable clues to understanding the structure of the heliosphere, the bubble, with a shape very much like a sleeve of wind, created by the solar wind as it extends to the limit of the solar system.
"We can deduce that the heliosphere is symmetrical, at least at the two points where the Voyager spacecraft crossed," says Bill Kurth, a research scientist at the University of Iowa and co-author of the study.
"This indicates that these two points on the surface are at about the same distance.
Fluids form limits
"The old idea that the solar wind will gradually diminish as you move into interstellar space simply isn't true," said Don Gurnett of Iowa, corresponding author of the study, published in the journal Nature Astronomy.
"We showed with Voyager 2, and earlier with Voyager 1, that there is a different limit. It's amazing how fluids, including plasma, form boundaries.”
Gurnett himself was also the author of the 2013 study published in Science that confirmed that Voyager 1 had entered interstellar space.
The data from the Iowa instrument on Voyager 2 also provides information about the thickness of the outer region of the heliosphere (what we might call its 'evolution') and the point where the solar wind accumulates against the approaching wind in interstellar space, which Gurnett compares to the effect of a snow plough on a city street.
Iowa researchers also say the heliosphere envelope is of varying thickness, the data show, since Voyager 1 sailed 10 AU farther than its twin to reach heliopause.
Interestingly, some thought Voyager 2 would make that crossing first, based on previously devised models of the heliosphere.
Where is Voyager 1 now?
The last measurement obtained from Voyager 1 recorded that the spacecraft was 146 AU, or more than 13.5 billion miles from the Sun.
The plasma wave instrument is recording that the density of the plasma is increasing (so we know it is moving away).
The two Voyagers will last longer than Earth. Right now they are in their own orbits around the galaxy for five billion years or more.
Although Voyager 2 is equipped with a gold disc with information about the human being, greetings in all languages, a map of coordinates to find the Earth... Researchers say (sadly) that the probability of encountering something is almost zero.
"They may look a little worn out by then," adds Gurnett (with a smile, as described in the press release)