Two studies published in the journal Nature this week say NASA's Voyager 1 spacecraft is starting to probe the outermost edge of the solar system. Its voyage of exploration started in 1977, principally to study Saturn. But now it's heading off into the unknown.
At 8.4 billion miles from the sun, Voyager is far beyond Pluto and plunging through cold, dark and nearly empty space. It's destination is the final frontier of the solar system, a region populated largely by particles streaming from the sun at amazing speeds. As NPR's Richard Harris reports, some scientists say Voyager has crossed over into the beginning of the end of the solar system; others say it still has some distance to go.