In the abyssal depths of interstellar space, a NASA probe launched from the U.S. in 1977 is still passively exploring the galaxy, returning all data to its makers, here on Earth. Called Voyager 1, the craft is far beyond the heliopause — an invisible "shield" that marks our solar system's border with the rest of the galaxy. Out there, in the interstellar medium, Voyager 1's instruments have detected the haunting hum of interstellar gas — plasma waves moving through a region of space far beyond the human imagination, according to a new study published in the journal Voyager 1 can help scientists study the composition of interstellar space