The Furthest Manmade Thing

The furthest manmade thing, the Voyager 1 spacecraft, has recently detected a change in its local environment in the form of a greatly enhanced density of energetic particles. Two papers published last week in Nature give different interpretations of the change. Krimigis et al. believe that Voyager has finally begun to encounter (at a distance of 85 astronomical units or 85 times the Earth-Sun distance) our solar system's "termination shock," the region of space where the outward going river of solar particles flags from supersonic to subsonic speeds in its confrontation with the interstellar medium. One would expect the shock front to be a good accelerator of particles, and the observed upswing in fast particles is suggestive. McDonald et al., however, argue that Voyager has not yet reached the termination shock, citing the relatively unimpressive presence of one species of energetic particles, namely so called anomalous cosmic rays. (Nature 6 November 2003.) Voyager 1 and its twin, Voyager 2 (some 20 AU behind in the effort to leave the solar system) were launched way back in 1977.