The spacecraft has detected two changes in the surrounding expanse of empty space around it, exactly as scientist expected it to, before it enters interstellar space, TheRegister Web site reported Wednesday.
Voyager 1 has detected an increase in galactic cosmic rays since June 1, one of three expected events that will tell scientists the probe has flown beyond the heliosphere -- the outer shell of the sphere of charged particles that surround the sun.
It has also sensed a decrease in the amount of lower-energy particles streaming out of the solar system, with the two phenomena suggesting the probe is almost at the point of entering interstellar space.
Scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., are awaiting the third expected sign, a shift in the direction of magnetic field surrounding the probe, from east-west to north-south.
Voyager 1 was launched in September 1977, a month after its twin, Voyager 2. Voyager 1 has been a bit speedier in its trip to the edge of the solar system and will enter interstellar space first.
Their original 5-year mission was to study the planets Jupiter and Saturn, but they have kept sending data to Earth as they head into the darkness outside the solar system.
"Even 35 years on, our rugged Voyager spacecraft are poised to make new discoveries as we eagerly await the signs that we've entered interstellar space," Ed Stone, Voyager project scientist at the California Institute of Technology, said.