On the interplanetary highway, there are no mile markers and no exit signs. Precious few clues indicate that you're nearing the edge of the solar system. Those clues, however, are revealing that the venerable Voyager 1 spacecraft, launched 26 years ago and now 90 times as far from the sun as Earth is, either has reached or will soon enter a turbulent region near the solar system's final frontier. There, the solar wind first slams into large numbers of atoms and molecules that have leaked into the solar system from interstellar space. The encounter puts the brakes on the solar wind, causing it to abruptly slow from supersonic speeds of 400 to 700 kilometers per second down to subsonic speeds of 100 km/sec, according to simulations.
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