After a 33-year odyssey, NASA's Voyager 1 has reached a zone where there is no outward-blowing solar wind.
NASA's Voyager 1 spacecraft has reached the outside edge of the solar system.
Scientists think it will enter interstellar space in about four years.
The spacecraft launched in 1977 and has taken photographs of Earth and other planets.
A NASA space probe dispatched 33 years ago for the first close-up studies of Jupiter and Saturn has entered the tail of the solar system, a place where the constant stream of charged particles flowing from the sun ebbs.
This final phase of solar system exploration should last another four years, computer models show, though scientists overseeing the two Voyager spacecraft really don't know what to expect.
Voyager 1 is now about 10.8 billion miles from the sun, traveling in a region of space known as the heliosheath, a turbulent area between the sphere of space influenced by the sun and magnetic forces from interstellar space that lies beyond.
In June, Voyager 1 relayed data that showed it was no longer traveling amongst the outward flow of solar wind particles. Instead, the solar stream has turned a corner, literally, as it sweeps, like an exhaust, into the tail of the heliosheath.
Flying along at about 38,000 mph, Voyager 1 is continuing its beeline toward interstellar space. It is so far from Earth that radio signals, traveling at the speed of light, take 16 hours to reach the spacecraft.
Its twin, Voyager 2, is traveling at a more leisurely 35,000 mph and will leave the solar system in a southerly direction a few years after Voyager 1, traveling in a northerly path, becomes the first human-made object in interstellar space.
Scientists hope to have five years for studies beyond the solar system before Voyager 1's plutonium power source is depleted.
"Every time we go somewhere new we find something totally unexpected," Voyager scientist Rob Decker, with Johns Hopkins University Applied Physics Laboratory in Maryland, told Discovery News.
"We never expected to make it this far," he said.
"When we launched, the space age was only 20 years old, so we had no evidence that things last that long," added California Institute of Technology's Ed Stone, Voyager project scientist. "It's still going strong."
The Voyager team presented their findings at the American Geophysical Union conference in San Francisco this week.