Forty-three years after their launch, the venerable Voyager spacecraft continue to transmit data back to Earth, including their latest observation: The density of space is increasing as they move outward.
Both probes have departed the heliosphere, the vast magnetic bubble created by the sun and surrounding our solar system. They are now just outside the heliopause, the boundary marking the end of the sun’s influence and the beginning of the interstellar medium, sometimes called “empty space” but suffused with gas, dust, and cosmic rays.
The latest data comes courtesy of the Plasma Wave Science instrument aboard Voyager 2, now 11 billion miles away. The measurements illustrate that even “beyond the heliopause the sun can still modify the interstellar medium—it’s not a brick wall,” says William Kurth who, along with fellow University of Iowa scientist Donald Gurnett, recently published a paper on the probe’s findings.
These measurements indicate that the electron density of the interstellar medium is currently increasing as the spacecraft travels away from the sun.
There are two theories to explain this. One possibility is that incoming interstellar material is piling up in the region just outside the heliosphere. Another is that interstellar magnetic field lines grow stronger as they drape over the heliopause, triggering a process that depletes the plasma just inside the heliosphere.