Voyager 2 Reaches Interstellar Space

Voyager 1 has a companion in the realm of the stars. (Image: NASA / JPL-Caltech)
Voyager 1 has a companion in the realm of the stars. (Image: NASA / JPL-Caltech)

Researchers at the University of Iowa report that the spacecraft Voyager 2 has entered the interstellar medium (ISM), the region of space outside the bubble-shaped boundary produced by wind streaming outward from the Sun. Voyager 2 thus becomes the second human-made object to journey out of our Sun’s influence, following Voyager 1’s solar exit in 2012.

In a new study, the researchers confirm Voyager 2’s passage on Nov. 5, 2018, into the ISM by noting a definitive jump in plasma density detected by an Iowa-led plasma wave instrument on the spacecraft. 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.

University of Iowa researchers report the spacecraft Voyager 2 has reached interstellar space, following Voyager 1’s historic passage six years ago. In the study, the researchers note a jump in plasma density detected by an Iowa-led plasma wave instrument on the spacecraft as evidence. (Image: NASA/JPL-Caltech)

University of Iowa researchers report the spacecraft Voyager 2 has reached interstellar space, following Voyager 1’s historic passage six years ago. In the study, the researchers note a jump in plasma density detected by an Iowa-led plasma wave instrument on the spacecraft as evidence. (Image: NASA/JPL-Caltech)

It’s also similar to the plasma density jump experienced by Voyager 1 when it crossed into interstellar space. Iowa’s Don Gurnett, corresponding author on the study, published in the journal Nature Astronomy, said:

“In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true.

“We show with Voyager 2 — and previously with Voyager 1 — that there’s a distinct boundary out there. It’s just astonishing how fluids, including plasmas, form boundaries.”

Gurnett, professor emeritus in the UI Department of Physics and Astronomy, 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 published in Science that confirmed Voyager 1 had entered the ISM.

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. The spacecraft were launched within weeks of each other in 1977, with different mission goals and trajectories through space. Yet they crossed into the ISM at basically the same distances from the Sun.

That gives valuable clues to the structure of the heliosphere — the bubble, shaped much like a windsock, created by the sun’s wind as it extends to the boundary of the solar system. Bill Kurth, University of Iowa research scientist and a co-author on the study, said:

“It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed.

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, which Gurnett likens to the effect of a snowplow on a city street.

The Iowa researchers say the heliosheath has a varied thickness, based on data showing Voyager 1 sailed 10 AU farther than its twin to reach the heliopause, 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. Kurth said:

The last measurement obtained 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 so far away that it takes more than 19 hours for information to travel from the spacecraft to Earth. Kurth said:

Gurnett added with a smile:

The Iowa study is one of five papers on Voyager 2 published in Nature Astronomy. These papers confirm the passage of Voyager 2 into interstellar space and provide details on the characteristics of the heliopause.

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