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Voyager.

"As of Wednesday, 26 years after its launch, NASA's Voyager 1 was 8.4 billion miles from the sun. That's 90 times the distance separating the Earth from our star.

As the robotic spacecraft continues to push far beyond the reach of the nine planets, two teams of scientists disagree whether it passed into the uncharted region of space where the sun's sphere of influence begins to wane.

The sun sends out a stream of highly charged particles, called the solar wind, that carves out a vast bubble around the solar system.

Beyond the bubble's ever-shifting boundary, called the termination shock, lies a region where particles cast off by dying stars begin to hold sway. That region, called the heliopause, marks the beginning of interstellar space and the end of our solar system.

Whether Voyager 1 reached that first boundary or is still on approach remains unclear. On Wednesday, scientists provided evidence for both possibilities. Further details appear Thursday in the journal Nature.

"This is very exciting: Voyager is beginning to explore the final frontier of our solar system," said Edward Stone, the Voyager project scientist. "It's a totally new region we've never been in before."

Scientists have long theorized that a shock wave exists where the hot solar wind bumps up against the thin gas of the interstellar medium. A similar shock wave precedes aircraft flying faster than the speed of sound, causing a sonic boom.

In space, the violent encounter slows the solar wind from supersonic velocity to subsonic speed, and causes a pileup of particles.

As they accumulate, the particles increase in temperature. Also, as they skip back and forth across the shock boundary, they are accelerated and energized.

Scientists have pored over data from Voyager 1 for evidence of any of those activities, which would suggest the one-ton spacecraft has reached the termination shock. (The one instrument that could measure the solar wind velocity and give somewhat of a definitive answer ceased working years ago.)"

the rest is here.