Twenty-six years and more than 13 billion kilometers from Earth, the durable Voyager 1 spacecraft has journeyed a greater distance than any other human-made object, but scientists are not sure exactly what it has encountered at the far frontier of the solar system.
One team of scientists reported on Wednesday that radioed data show that the spacecraft apparently ventured across a turbulent boundary near the edge of the solar system, where supersonic "winds" of charged particles from the Sun collide with matter from interstellar space. No spacecraft has ever come close to the boundary, known as the termination shock.
At the same time, other scientists examining Voyager data argued that the boundary still lies ahead, though perhaps not too far.
The conflicting views were aired at a news conference of the US space agency NASA in Washington and are the subject of articles published yesterday in the journal Nature.
The team, led by Stamatios Krimigis of the Applied Physics Laboratory of Johns Hopkins University, described sharp changes in the velocity -- from supersonic to subsonic -- and in constituents of the so-called solar wind that Voyager 1 began flying through in August 2002. The spacecraft appeared to have entered the termination shock region and about six months later, it re-entered a region of more normal solar wind conditions. In that interval, the spacecraft traveled more than 643 million kilometers.
"This is our first direct look at the incredibly dynamic activity in the solar system's outer limits," Krimigis said.
The team analyzed Voyager data and found other arresting clues. Besides the braking solar wind, there was a hundred-fold increase in the number of charged particles detected over the six-month period and some of the observed particles were different from the usual solar-wind material, meaning that probably came from beyond the solar system.
"When we saw all that, we were pretty sure that we had reached the termination shock," said Louis Lanzerotti of Bell Labs, a research unit of Lucent Technologies.
A group headed by Frank McDonald of the University of Maryland concluded from the strength and behavior of high-energy particles and magnetic fields around Voyager 1 that the spacecraft may be in the neighborhood of the critical boundary, but has yet to cross over. In models of what the boundary should be like, the scientists said, the magnetic field should be more intense than Voyager has observed.
"It's just a matter of time," McDonald said, "but I think we will know when we get there."
The lack of consensus did not seem to upset other scientists. They suggested that some of the theoretical models of conditions toward the edge of the solar system, far from the nine planets, may be flawed or oversimplified.
"It means there's something new to learn in this final frontier of the solar system," Edward Stone, the chief Voyager scientist and a physicist at the California Institute of Technology, said at the news conference.
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