Launched in 1977, the Voyager 1 and Voyager 2 unmanned probes are now studying the edges of the heliosphere, the huge magnetic "bubble" around our solar system created by the solar wind as it runs up against the thin gas in interstellar space.

Voyager 2 in August 2007 crossed this boundary 7.8 billion miles from the sun.

Voyager 1 had crossed the boundary in December 2004 about 10 billion miles away from Voyager 1 and almost a billion miles farther from the sun.

Scientists think this indicates that the bubble carved into interstellar space by the heliosphere, which extends well past the distant orbit of Pluto, is not perfectly round, and the solar system is shaped a bit like an oblong.