At the edge of the solar system, right before reaching interstellar space, robotic probes are still finding undiscovered mysteries. NASA’s New Horizons spacecraft, which is speeding on from its 2015 encounter with Pluto to study an object in the distant Kuiper Belt, appears to have detected what scientists are calling an interstellar “hydrogen wall.”
The space barrier was detected with New Horizons’ instrument Alice, a compact ultraviolet imaging telescope and spectrometer. Alice’s main goal on the New Horizons mission was to gather information on the atmospheric conditions of Pluto.
But scientists are now using Alice to study the edges of the solar system, and the instrument detected an ultraviolet light source in the distance. In a new paper published in Geophysical Research Letters, New Horizons scientists suggest that the light is scattered by a wall of hydrogen. It is “best explained if the observed ultraviolet light is not only a result of the scattering of sunlight by hydrogen atoms within the solar system but includes a substantial contribution from a distant source,” says the paper.
If the mysterious light does, in fact, come from a hydrogen wall, scientists believe that the wall could be formed by interstellar winds encountering our local solar wind. The New Horizons team plans to continue studying the distant region with Alice about twice a year.
“We assume there’s something extra out there, some extra source of brightness,” co-author Randy Gladstone of the Southwest Research Institute tells Gizmodo. “If we get a chance with New Horizons we can maybe image it.”
In 2013, the Voyager 1 spacecraft crossed through the barrier of the solar system after flying for almost 40 years, becoming the first human-made object to reach interstellar space. The Voyager data matches up with Alice’s, and as the years go on, Alice will get a closer and closer look at this barrier at the edge of the solar system, ultimately reaching interstellar space around 2040.
Before it leaves the solar system, though, New Horizons still has plenty of exploring to do. The spacecraft is going to fly near an object known as Ultima Thule—in the Kuiper Belt beyond Pluto—on New Year’s Day, 2019.