42 years of travel at 54,000 km/h, Voyager 2 enters interstellar space.
|First results of Voyager 2 probe from interstellar space|
Analysis of the data captured by the instruments of the ship sheds light on the mysterious bubble that surrounds the solar system
On November 5, 2018, it crossed the heliosphere, the protective bubble formed by the Sun's magnetic field and in which all the planets of the solar system are housed, as the scientists who studied the preliminary data from the five instruments aboard the spacecraft concluded a month later. And it began to explore an unknown territory, the kingdom of stars.
First results of Voyager 2Now, the in-depth analysis of all the measurements made by this probe - detailed in five studies published in Nature Astronomy - has allowed astronomers to confirm, first, that Voyager 2 has indeed crossed into interstellar space; it has thus become the second spacecraft to do so, after its twin, Voyager 1, succeeded in 2012.
And, secondly, it has shed light on the heliopause, the outermost structure of the heliosphere that acts as the boundary between the heliopause and interstellar space, and of which little was known so far.
"When we launched the Voyager mission with the two spacecrafts in 1977 we had no idea about the bubble that the Sun creates around itself with the supersonic solar wind, the heliosphere," explained Edward Stone, head scientist of the Voyager program since 1972.
"We didn't know how big it was, nor did we know if the spacecraft would survive after reaching the end of the heliosphere and leaving it to enter interstellar space," he added.
The Voyager mission
|Voyager 1 The Solar System 1977-2019|
Voyager 1 crossed the heliopause in August 2012, in the northern hemisphere, at 122.6 astronomical units (one astronomical unit equals the average distance between the Earth and the Sun). And Voyager 2 crossed the heliopause six years later at 119.7 astronomical units in the southern hemisphere, nearly 18 billion kilometers from the Sun.
The fact that the two Spaceships, although traveling on opposite trajectories, have crossed the heliopause at about the same distance from the Sun, "has to imply that the heliosphere is symmetrical, at least at the two points where the Voyager Spaceships crossed," said Bill Kurth, of the University of Iowa (USA), co-author of one of the studies.
Although Voyager 1 sent valuable data to Earth about that area of the end of the solar bubble, the heliopause, a failure of its plasma instrument in 1980, prevented it from taking complete data from that transition.
For that reason, scientists could not directly identify the transition from hot solar plasma to the coldest interstellar plasma. It took eight months for the Voyager 1 instruments to record electron plasma oscillations to confirm that the probe was entering interstellar space.
Voyager 2, on the other hand, has been able to record this step with direct and detailed measurements. Voyager 2's plasma wave instrument, which picked up a jump in plasma density: while inside the solar bubble the plasma is hot and less dense, in interstellar space it is colder and denser.
Voyager's observations around the heliopause show enormous magnetic fields, large enough to form a spherical bubble around the Sun. Also, unlike Voyager 1, the data from Voyager 2 suggest that the heliosphere boundary is thinner and smoother, with a stronger interstellar magnetic field on the other side.
Researchers believe that Voyager 2 crossed the heliopause in less than a day and consider that the interstellar medium closest to that boundary is warmer and more volatile than they expected.
The Voyager 2 data have thus shown similarities, but also some differences with regard to the crossing of the predecessor Voyager 1 and the researchers consider that it could be due to changes in the levels of solar activity and to the different trajectories of the spacecrafts.
It's amazing how fluids, including plasmas, can form boundaries," said Don Gurnett, a researcher at the University of Iowa, co-author of one of the papers and lead plasma instrument scientist for the two Voyager probes, referring to heliopause.
Now, "we are trying to understand the nature of that boundary between the two winds, where they collide and mix. We want to know how they mix and how much spillage there is from the inside out and the other way around. Also, we want to study cosmic rays, which are in interstellar space and only 30% reach Earth," he added.