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 today, has ventured farther from Earth than any other spacecraft in history. Voyager 1 is now 18.2 billion kilometers from Earth—so distant that it takes radio commands traveling at the speed of light nearly 17 hours to reach the spacecraft. (The twin Voyager 2 probe launched first but took a more leisurely route through the solar system. It is now 14.8 billion kilometers from Earth.)

That’s quite an odometer reading, and all indications were that Voyager 1 was well on its way to an astonishing feat—exiting the solar system and venturing into interstellar space. But a new study suggests that the spacecraft is farther from taking that unprecedented step than had been assumed.

Nearly eight years ago, the spacecraft crossed into the heliosheath, the outer region of the solar system where the solar wind (plasma from the sun) begins to slow due to pushback 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 that been deflected by interstellar plasma flows and begun to turn northward (in relation to the sun’s axis of rotation).

But in the September 6 issue of Nature, Robert Decker of the Johns Hopkins University Applied Physics Laboratory and his colleagues report that Voyager 1 has measured the speed of poleward-flowing plasma at five intervals since March 2011. And the velocity of northbound plasma at the spacecraft’s location is basically zero, within the uncertainties of the measurements. (Scientific American is part of Nature Publishing Group.)

No one knows the thickness of the heliosheath, so no one knows how soon Voyager 1 might reach its outer edge. By one previous estimate, the heliopause could be just ahead of humankind’s most well-traveled emissary, or it could lie as many as four billion kilometers beyond—about seven years’ travel time. The new findings seem to favor the latter possibility. Decker and his colleagues conclude from the plasma measurements “that Voyager 1 is not at present close to the heliopause, at least in the form that it has been envisioned up to now.”

UPDATE (9/6/2012): In an email, Decker notes that the Nature paper was based on data collected before February 2012 and that newer observations further complicate the question of when Voyager 1 will leave the heliosphere and enter interstellar space. "I should point out that data received from Voyager 1 within the past two months show very interesting variations in the numbers of charged particles that enter our heliosphere from the interstellar medium and in the number that leave our heliosphere and enter the interstellar medium," Decker notes. "Whether these very new data are another feature of the broad transition region that Voyager 1 has been in for the past two years, or a new region or boundary of the heliosphere, remains to be seen."