The Solar Wind And The Heliopause
Six billion kilometers from the Sun is the planet Pluto. At this distance, the Sun is only a brilliant point of light, and gives no warmth to heat the dead and icy surface of its most distant planet.
The solar wind still flows by, however. As it gets farther from the Sun, it becomes increasingly diffuse, until it finally merges with the interstellar medium, the gas between the stars that permeates the Galaxy. This is the heliopause, the distance at which the Sun's neighborhood formally ends. Scientists believe the heliopause lies between two and three times as far from the Sun as Pluto. Determining exact location is the final mission of the Pioneer and Voyager spacecraft, now out past Pluto, their flybys of the planets complete. Someday, perhaps in twenty years, perhaps not for fifty, they will reach the heliopause. They will fly right through it: there is no wall there, nothing to reveal the subtle end of the Sun's domain. And at that point, these little machines of man will have become machines of the stars.
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Jeffrey C. Hall