The craft has reached a point 10.8 billion miles from the sun, where solar wind has shifted, and may be approaching interstellar space more rapidly than expected.
Scientists realized solar winds had slowed to zero in June when data from the craft's Low-Energy Charged Particle instrument showed that particles hitting the outward face of the craft matched its speed. This meant that there no longer was wind resistance on the craft, according to NASA.
They took four more monthly readings before reaching the conclusion that the solar wind had actually reached zero, the agency said.
Scientists presented the findings this week at the American Geophysical Union meeting in San Francisco.
Voyager 1 was launched on Sept. 5, 1977, and crossed what is called the termination shock into the heliosheath in December 2004.
The heliosheath is the turbulent shell that marks the furthest reach of the sun's sphere of influence. Winds from the sun travel at supersonic speed until passing into the area, where they begin heating up and slowing down.
Scientists suspect the zero reading means that pressure from the interstellar wind in the region between stars has turned the solar wind sideways, according to NASA.
They will know when Voyager 1 actually leaves the solar system and enters interstellar space when they measure a sudden drop in the density of hot particles and an increase in the density of cold particles, according to NASA.
Scientists currently are recalculating their models of the structure of the heliosheath to better determine when this event will occur. They had believed it would happen in about four years, but Voyager's current position could mean that estimate is incorrect.
Voyager 1 has a sister craft, Voyager 2, which is currently 8.8 billion miles from the sun. Voyager 2 was launched on Aug. 20, 1977, only about two weeks earlier, but is traveling at a slower speed, at 35,000 mph versus Voyager 1's 38,000 mph. Voyager 2 is following a different trajectory but should experience the same lack of solar wind as its sister craft in the next few years, according to NASA.
NASA's Jet Propulsion Laboratory in Pasadena, Calif., built both spacecraft and continues to operate them.