In December 2004, the Voyager 1 spacecraft began to leave the Solar System, crossing the solar wind termination shock and entering the heliosheath, a region where the solar wind (a stream of ionized particles) slows down as it begins to interact with the interstellar medium. The Voyager instruments are still monitoring the bulk velocity of the heliosheath plasma, and the latest news is that plasma velocity has been decreasing almost linearly during the past three years from 70 kilometres per second to close to zero, where it has remained for the past 8 months. This means that Voyager 1 may be close to the heliopause, where what remains of the solar wind matches the pressure exerted by interstellar space. Theorists had predicted a sharp discontinuity at the heliopause, so the gradual nature of Voyager's Solar System exit comes as something of a surprise.
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