In 1972 a young professor at the California Institute of Technology was asked to work part-time at NASA’s Jet Propulsion Laboratory as chief scientist for a new space mission, one that would probe the limits of the solar system and eventually enter interstellar space. Edward Stone accepted the assignment, and now, 33 years after the launch of the two Voyager spacecraft, he says the goal is in sight. He looks almost giddy as he talks about the implications of recent data received from Voyager 1. But first he must explain where the spacecraft is today.
"Voyager 1 is the most remote human-made object," Stone says. "It's now 115 astronomical units from Earth," that is, 115 times farther than Earth is from the sun, or "a bit more than 10 billion miles [16 billion kilometers]." Voyager 2 has traveled somewhat slower and in a different direction and is now around 14 billion kilometers from Earth.