Launched on September 5, 1977, Voyager 1 has logged about ten-point-eight-billion miles.
Since the solar system is a rather big place, NASA says it may take another four years before finally crossing into interstellar space.
Still, Space.com cites scientists as saying the spacecraft has entered a region of space in the outer solar system where the speed of solar wind, charged particles from the sun, is effectively zero.
The announcement was made Monday at the fall 2010 meeting of the American Geophysical Union in San Francisco.
Voyager 1 is currently in a region known as the termination shock.
Its next stop will be a region called the heliosheath, which forms an outer shell of the sun's cosmic reach, which scientists refer to as its "sphere of influence." Voyager will have to cross a cosmic border called the heliopause to officially be in interstellar space.
The spacecraft is traveling at a rate of about 38-thousand miles an hour.