Voyager 2 finally enters interstellar space on its way to the stars; probe becomes second human-made object to make solar exit

An instrument on the spacecraft detected plasma density jump, confirming that it has entered the realm of the stars, say scientists


                            Voyager 2 finally enters interstellar space on its way to the stars; probe becomes second human-made object to make solar exit

NASA's Voyager 2 spacecraft has crossed into interstellar space, in effect leaving the solar system and joining its twin — Voyager 1 — both of which are now sailing beyond the reach of the sun's influence.

This makes it the second time in history when a human-made object has reached the space between the stars, following Voyager 1's solar exit in 2012.

The researchers from the University of Iowa confirm Voyager 2's passage on November 5, 2018, into the interstellar medium by noting a definitive jump in plasma density detected by an Iowa-led plasma wave instrument on the spacecraft. 

According to the team, data from Voyager 2 has helped further characterize the structure of the heliosphere — the wind sock-shaped region created by the sun's wind as it extends to the boundary of the solar system.

"Voyager 2 has entered the interstellar medium, the region of space outside the bubble-shaped boundary produced by wind streaming outward from the sun. The marked increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space. It's also similar to the plasma density jump experienced by Voyager 1 when it crossed into interstellar space," say the researchers in their findings published in Nature Astronomy.

Source: NASA

The first confirmation that Voyager 2 has entered interstellar space came in December 2018 when a NASA release stated that "Voyager 2 probe now has exited the heliosphere — the protective bubble of particles and magnetic fields created by the sun."

The Iowa study — funded by NASA through a contract with the Jet Propulsion Laboratory — is one of five papers on Voyager 2 published in Nature Astronomy. These papers confirm the passage of Voyager 2 to interstellar space and provide details on the characteristics of the heliopause. 

"The two Voyagers will outlast Earth. They are in their own orbits around the galaxy for five billion years or longer. And the probability of them running into anything is almost zero," says Bill Kurth, University of Iowa research scientist and a co-author of the study. 

Voyager 2's entry into the ISM occurred at 119.7 astronomical units (AU), or more than 11 billion miles from the sun. Voyager 1 passed into the ISM at 122.6 AU.

According to the researchers, the two Voyagers will outlast Earth. They are in their own orbits around the galaxy for five billion years or longer. (NASA JPL)

The spacecraft was launched within weeks of each other in 1977, with different mission goals and trajectories through space. Yet, says the team, they crossed into the interstellar medium at basically the same distances from the sun.

The scientists say that this gives valuable clues to the structure of the heliosphere. "It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed. That says that these two points on the surface are almost at the same distance," says Kurth.

According to Don Gurnett, professor emeritus in Iowa's Department of Physics and Astronomy, "There's almost a spherical front to this. It's like a blunt bullet." 

Gurnett is the principal investigator on the plasma wave instrument aboard Voyager 2. He is also the principal investigator on the plasma wave instrument aboard Voyager 1 and authored the 2013 study that confirmed Voyager 1 had entered the interstellar space.

The researchers say that data from the Iowa instrument on Voyager 2 also gives additional clues to the thickness of the heliosheath, the outer region of the heliosphere, and the point where the solar wind piles up against the approaching wind in interstellar space.

The heliosheath has a varied thickness, says the team, based on data showing Voyager 1 sailed 10 AU farther than its twin to reach the heliopause. "It is a boundary where the solar wind and the interstellar wind are in balance and considered the crossing point to interstellar space. Some had thought Voyager 2 would make that crossing first, based on models of the heliosphere," says the team.

The last measurement that scientists got from Voyager 1 was when the spacecraft was at 146 AU or more than 13.5 billion miles from the sun. The plasma wave instrument is recording that the plasma density is rising, in data feeds from a spacecraft, now far away. According to researchers, it takes over 19 hours for the information to travel from the spacecraft to Earth.

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