The Sun constantly blows out a stream of charged particles, the solar wind, at hundreds of kilometres per second. It carries the Sun’s magnetic field with it and inflates a vast bubble in the thin gas between the stars. That bubble is the heliosphere, and its outer boundary, where the solar wind finally gives way to the interstellar medium, is the heliopause.
Three boundaries
Moving outward, the solar wind first slows abruptly at the termination shock, where it drops from supersonic to subsonic speed. Voyager 1 crossed it in December 2004, about 94 AU from the Sun; Voyager 2 crossed it in 2007, closer in, a sign that the bubble is not perfectly symmetrical. Beyond lies the heliosheath, a turbulent region of slowed, compressed solar wind. Then comes the heliopause itself, where the pressure of the solar wind and the interstellar gas balance.
Crossing over
On 25 August 2012 Voyager 1 crossed the heliopause 121 AU from the Sun, about 18 billion kilometres. The density of the surrounding plasma jumped, and particles from the Sun nearly vanished while cosmic rays from the galaxy increased. Voyager 2 followed on 5 November 2018, at 119 AU. They are the only spacecraft ever to reach interstellar space.
Leaving the heliosphere is not the same as leaving the Solar System. The Sun’s gravity still holds the Oort cloud, far beyond, and by NASA’s estimate Voyager 1 will not reach even its inner edge for about 300 years.
What shape is it?
The Sun moves through the interstellar gas, so the heliosphere was long pictured like a comet: blunt in front and trailing a long tail behind. Measurements from Cassini suggested instead that it might be closer to a bubble. A model by Merav Opher and colleagues suggests a shape like a deflated croissant, with two lobes of solar wind bent backwards. The question is still open.
In 2009 the IBEX spacecraft, mapping particles coming from the boundary, discovered a bright ribbon across the sky that nobody had predicted. It is shaped by the interstellar magnetic field draped over the heliosphere. A new NASA mission, IMAP, launched in 2025 to map the boundary in more detail.