A bubble in the cosmos
Our solar system moves through the Milky Way inside a protective bubble called the heliosphere. This vast structure is inflated by the solar wind, a constant stream of charged particles flowing from the Sun at several hundred kilometers per second. The heliosphere extends far beyond the orbit of Neptune, pushing back against the gas and dust of the interstellar medium. The edge of this bubble, where the solar wind's outward pressure balances the inward pressure of interstellar space, is known as the heliopause. It marks the boundary between our solar system's environment and the galaxy beyond.
NASA's Voyager 1 and Voyager 2 spacecraft, launched in 1977, have been traveling toward this frontier for decades. Voyager 1 crossed the heliopause into interstellar space in August 2012, with Voyager 2 following in November 2018. As they approached this boundary, they made a peculiar discovery. Their ultraviolet spectrometers detected a glow that suggested a buildup of hydrogen, a feature scientists have named the Interstellar Hydrogen Wall.
An unexpected barrier
The hydrogen wall is not a solid surface; it is a region where neutral hydrogen atoms from the interstellar medium slow down and accumulate as they encounter the heliosphere. This pile-up creates a zone of denser-than-average hydrogen. The first hints of this structure came in 1992, when the Voyagers detected an excess of ultraviolet light known as Lyman-alpha emission. This type of light is produced when solar photons scatter off hydrogen atoms. An increase in this scattered light pointed to a wall-like concentration of hydrogen located roughly 100 astronomical units (AU) from the Sun.
For years, the leading explanation was that the heliosphere creates a bow shock as it moves through the interstellar medium, like the wave a boat makes in water. This shock would heat and compress the hydrogen ahead of it. Data from other missions, including NASA's New Horizons spacecraft, has complicated this picture. Observations by New Horizons, which confirmed the ultraviolet glow in 2018, suggest the feature is likely not a bow shock. The debate continues, with alternative models proposing that the local interstellar magnetic field drapes around the heliosphere, shaping the hydrogen into a different kind of structure. More recent data from New Horizons has also suggested that the wall may not be a significant source of Lyman-alpha emissions at all, with the glow possibly coming from a more distant galactic source.