As the Sun plows through the local interstellar medium (ISM), the wind of supersonic charged particles continually emanating from its surface envelops the solar system in hot, low-density plasma. Called the heliosphere, this plasma bubble is presumed to have a distinct aspherical boundary with the ISM’s much cooler, higher-density plasma that comes within a few times 1010 kilometers of the Sun. (The orbital radius of Uranus is 5 × 109 km.) To considerable acclaim, NASA reported in September that its Voyager 1  spacecraft, launched toward the outer planets 35 years ago, crossed that boundary “on or about” 25 August 2012, at a distance of 1.8 × 1010 km from the Sun. The principal evidence for the spacecraft’s emergence into the ISM, the first by any human artifact, is the expected abrupt 40-fold increase in plasma density, measured by the rise of the plasma’s density-dependent oscillation frequency. Simultaneous...

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