In front of the Sun, galactic debris builds up, including hydrogen.
Credit: Alder Planetarium/NASA
And now NASA researchers are quite certain that New Horizons (the probe that famously skimmed past Pluto in 2015) can see a ‘hydrogen wall’ that defines the boundary of our Heliosphere. That hydrogen wall is the outer boundary of our home system, the place where our Sun's bubble of solar wind ends and where a mass of interstellar matter too small to bust through that wind builds up, pressing inward. Our host star's powerful jets of matter and energy flow outward for a long stretch after leaving the Sun — far beyond the orbit of Pluto. But at a certain point, they peter out, and their ability to push back the bits of dust and other matter — the thin, mysterious stuff floating within our galaxy's walls — wanes. A visible boundary forms. On one side are the last vestiges of solar wind. And on the other side, in the direction of the Sun's movement through the galaxy, there's a build-up of interstellar matter, including hydrogen.
What New Horizons definitely sees, the researchers reported in a paper published 7th August 2018, in the journal Geophysical Research Letters, is some extra ultraviolet light — the kind the researchers would expect such a wall of galactic hydrogen to produce. That replicates an ultraviolet signal the two Voyager spacecraft — NASA's farthest-traveling probes, which launched in the late 1970s — spotted all the way back in 1992.
‘Alice,’ the instrument on board New Horizons responsible for this finding, is much more sensitive than anything the Voyagers had on board before beginning their own journey out of the solar system, the researchers wrote. And they said they expect Alice to function 15 to 20 more years. New Horizons will continue to scan the sky for ultraviolet light twice a year and report what it sees back to Earth.