It's been a long, strange trip out of the solar system for NASA's Voyager 1 spacecraft, and it may be a bit longer still.

Voyager 1, which launched 35 years ago, has ventured farther from Earth than any other spacecraft. The probe is now 18.2 billion kilometers from the sun—more than three times the average distance of Pluto. Voyager 1 is well on its way to an astonishing feat—escaping the sun's jurisdiction and venturing into interstellar space. Yet a new study suggests that the craft is further from taking that unprecedented step than had been assumed.

Nearly eight years ago the probe crossed into the heliosheath, where the solar wind (plasma from the sun) begins to slow because of push back from interstellar plasma. And in 2010 the velocity of the solar wind at Voyager 1's back unexpectedly dropped all the way to zero. Researchers expected that as Voyager 1 drew near to the boundary between the heliosheath and interstellar space, known as the heliopause, it would encounter solar plasma deflected sideways by interstellar plasma flows.

Yet in the September 6 issue of Nature, Robert B. Decker of the Johns Hopkins University Applied Physics Laboratory and his colleagues reported that no deflection is taking place. The new study raises two possibilities: either Voyager 1 has not yet approached the heliopause, or else plasma moves in unexpected ways there.

By one previous estimate, the heliopause could be just ahead of humankind's most well-traveled emissary, or it could lie as many as seven years' travel time ahead. The new findings favor the latter possibility. Decker, however, has newer data that further complicate predictions. In recent months Voyager 1 has registered a mixing of local and interstellar particles that could mark Voyager's arrival at another unexpected boundary—or a new domain of space.