The journey of NASA's dauntless Voyager 2 spacecraft through our solar system's farthest reaches has given scientists new insight into a poorly understood distant frontier: the unexpectedly distinct boundary marking where the sun's energetic influence ends and interstellar space begins.
The U.S. space agency previously announced that Voyager 2, the second human-made object ever to depart the solar system following its twin Voyager 1, had zipped into interstellar space on Nov. 5, 2018, at a point more than 11 billion miles (17.7 billion km) from the sun. Several research papers published Monday provided scientific details of that crossing.
Both Voyager 1 and Voyager 2 were launched in 1977, designed for five-year missions. Voyager 1 left the solar system at a different location in 2012. Both are now traversing the Milky Way galaxy's interstellar medium, a chillier region filling the vast expanses between the galaxy's stars and planetary systems.
The solar wind — the unending flow of charged particles emanating from the outer atmosphere of the sun — creates an immense protective bubble called the heliosphere that envelopes the solar system. The boundary of the solar system — the place where the solar wind ends and interstellar space begins — is called the heliopause.
Voyager 2's scientific instruments detected abrupt differences in plasma density and magnetic particles upon crossing the heliopause, the researchers said. The researchers said the heliopause appeared to be much thinner than expected.
Plasma — the fourth state of matter after solids, liquids and gases — exists in the solar system as a soup of the charged particles beaming continuously outward from the sun and clashing with interstellar plasma that darts inward from other cosmic events like stellar explosions.
"This is a very exciting time for us," California Institute of Technology physicist Edward Stone, project manager of the Voyager program, told reporters. "We will see a transition from the magnetic field inside to a different magnetic field outside, and we continue to have surprises compared to what we had expected."
The electromagnetic junction just outside the heliosphere was thought to be a deeper transitional place of intermingling cosmic weather, but Voyager 2's plasma wave instrument — built by University of Iowa researchers — detected sharp jumps in plasma density, much like two different fluids coming into contact with one another.
"Think of a cold front that forms when a very cold air mass comes down to the U.S. from Canada," said Don Gurnett, professor of physics at the University of Iowa. "Here we find a very hot plasma mass coming outward from the sun that encounters the cold plasma in the interstellar medium. It does not surprise me that a sharp boundary forms."
Scientists are still trying to understand the nature of interstellar space wind and how much of it can seep through the heliopause to reach planets in our solar system.
"We also have galactic cosmic rays, which are out in the interstellar space trying to flow in," Stone said, referring to the fast-moving, high-energy atomic particles whizzing around the universe. "And some of them, only about 30 percent of what's outside, can actually reach Earth."
Voyager 2 entered the interstellar medium far beyond the orbit of Pluto at a spot about 120 times further from the sun than Earth's orbit.
The research was published in the journal Nature Astronomy.