Researchers at the University of Iowa report that the spacecraftVoyager 2 has entered the interstellar medium (ISM), the region of space outside the bubble-shaped boundary produced by wind streaming outward from the sun.
Each paper has been dedicated to one of the five operating science instruments onboard Voyager 2.
Altogether, the instruments assist scientists in painting a picture of what lies beyond our Sun's heliosphere: interstellar space.
Artist impression of the solar system and interstellar space.
Nobody knew how long it would take for the probes to reach interstellar space. The space between stars also contains cosmic rays, or particles accelerated by exploding stars. Each paper details the findings from one of Voyager 2's five instruments: a magnetic field sensor, two instruments to detect energetic particles in different energy ranges and two instruments for studying plasma.
Though NASA continues to monitor, communicate with, and collect data from both Voyager probes, converting this data into useful scientific insights is largely the responsibility of scientists based at different institutions throughout the US.
While scientists were aware of the inner layer, the presence of the outer layer became evident only after Voyager 2 crossed into interstellar space. "What the models do is try to take information that we have from those two points and what we've learned through the flight and put together a global model of the heliosphere that matches those observations". The edge of the heliosphere is called the heliopause.
It crossed the outer edge of the Sun's protective bubble, known as the heliopause, on November 5, 2018.
The second set of measurements, by Voyager 2, give new insights into the nature of the heliosphere's limits because on Voyager 1 a crucial instrument created to directly measure the properties of plasma had broken in 1980.
"The Voyager probes are showing us how our Sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy".
"We show with Voyager 2 - and previously with Voyager 1 - that there's a distinct boundary out there".
But Bill Kurth, of the University of Iowa, said: "They will outlast Earth". Voyager 2 traveled from the hot, lower-density plasma of the heliosphere to the cold, higher-density plasma of interstellar space.
It has taken Voyager 2 more than four decades to reach it.
Voyager 1 is still up and about, and its instruments are recording that the plasma density is rising. But scientists don't yet fully understand what is causing the compression on either side.
When Voyager 2 crossed the boundary, it observed simultaneous increase in the intensity of cosmic rays, particles moving thought the space at almost the speed of light. Voyager-1 entered ISM at a distance of 122.6 AU. Voyager 2, on the other hand, is located closer to the flank, and this region appears to be more porous than the region where Voyager 1 is located.
Thanks to Voyager 2's magnetic field instrument, findings from Voyager 1 have been confirmed. Another surprising revelation was the magnetic field in the region just beyond the heliopause is parallel to the magnetic field inside the heliosphere.
Unlike Voyager 1 however, Voyager 2 did not come across this phenomena, reports Edward Stone at the California Institute of Technology, Pasadena and co-authors.
It is unclear when or if a follow-up mission will again reach the distances the Voyager craft have achieved, but plans for an interstellar probe are being looked at by the COSPAR (Committee on Space Research) Panel on Interstellar Research. Voyager 2 swung past Jupter, Saturn, Neptune, and Uranus.
The team hopes that future missions to interstellar space will help clarify some of the lingering mysteries of the heliosphere, including its exact shape.
The two probes' heliopause crossings occurred at similar distances from the Sun: For Voyager 1, it was at 121.6 astronomical units, and for Voyager 2, it was 119 astronomical units (one AU equals the average distance from Earth to the Sun).
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