LOS ANGELES — NASA's long-running Voyager 1 spacecraft is barreling its way toward the edge of the solar system.
Since 2004, the unmanned probe has been exploring a region of space where solar wind — a stream of charged particles spewing from the sun at 1 million miles (1.6 million kilometers) per hour — slows abruptly and crashes into the thin gas between stars.
NASA said Monday that recent readings show the average outward speed of the solar wind has slowed to zero, meaning the spacecraft is nearing ever closer to the solar system's edge to a boundary known as the heliopause.
"It's telling us the heliopause is not too far ahead," said project scientist Edward Stone of the NASA Jet Propulsion Laboratory.
Scientists estimate it will take another four years before Voyager 1 completely exits the solar system and enters interstellar space.