In July 2015, the New Horizons probe became the first spacecraft to explore Pluto and its moons after a nearly decade-long journey from Earth. Three years later, New Horizons continues to uncover mysteries about the outer solar system, this time confirming observations of what appears to be a hydrogen megastructure first seen by the Voyager spacecraft almost 30 years ago.As detailed in a recent paper in Geophysical Research Letters, the New Horizons spacecraft observed ultraviolet light that many physicists think results from a hydrogen ‘wall’ at the edge of our solar system. In other words, if the solar system is an egg, New Horizons just got its first glimpse of what appears to be the shell.The sun is constantly producing ionized particles that are collectively known as the “solar wind,” which creates a bubble around the solar system that extends some 10 billion miles from the Sun. The leading theory about the source of ultraviolet light claims that when neutral interstellar hydrogen atoms encounter this bubble—called the heliosphere—they slow down and begin to build up at the threshold of the Sun’s influence. This wall of interstellar hydrogen particles should scatter ultraviolet light in a distinct way, which is what the Voyager spacecraft observed back in 1992.
New Horizons is the first spacecraft since Voyager 1 and 2 to observe this ultraviolet light, although its source is still far from certain. If it’s not indicative of a hydrogen wall at the boundary of the Sun’s influence and interstellar space, then astronomers must furnish an alternative explanation for why this ultraviolet light is observed so far from the Sun. Starting next year, the New Horizons spacecraft will begin looking for ultraviolet evidence of the hydrogen wall twice a year for the remainder of the mission—about another decade or two. This will help determine if the wall actually exists, or if the Voyager spacecraft and New Horizons have observed something even stranger deep in the galaxy.