More than a quarter century after its launch, the Voyager 1 space probe--now some eight billion miles from the earth--has reached the solar system¿s final frontier, the unexplored territory bordering the interstellar medium. In fact, researchers writing today in the journal Nature, claim that the craft has encountered the so-called termination shock marking the edge of the sun¿s sphere of influence. Their assertion has not gone unchallenged, however. Another team writing in the same issue contends that Voyager 1 has not yet crossed that turbulent zone, but is on the approach.

The termination shock is predicted to arise from the violent intersection of supersonic particles streaming from the sun (the solar wind) and subsonic interstellar particles. Scientists haven't been certain exactly where this crossroads lies--estimates range from a few astronomical units (AU) to more than 100 AU (an astronomical unit is the distance between the earth and the sun). The results of the first study suggest that Voyager 1 hit that region at about 85 AU. Analyzing data from the craft¿s low energy charged particle detector, Stamatios Krimigis of Johns Hopkins University and his colleagues found three indicators that Voyager 1 journeyed through the termination shock starting in August 2002¿namely, signs of decreased solar wind speed, increased density of ionized particles, and the presence of ions from the interstellar medium. Some 200 days later, they say, the probe appears to have re-entered the supersonic solar wind, presumably because the shock moved outward. "Voyager 1 is giving us our first taste of interstellar space," Krimigis comments. "This is our first direct look at the incredibly dynamic activity in the solar system¿s outer limits."

The second team, led by Frank B. McDonald of the University of Maryland, also observed a surge in the intensity of ions and electrons at 85 AU. But their analyses of cosmic rays suggest that this increase is only a prelude to the termination shock, which they argue lies farther out.

"Neither explanation is certain, and we must hope that Voyager 1, on its march ever outwards, will encounter this interesting region again soon," writes Len A. Fisk of the University of Michigan in an accompanying commentary. "Once the termination shock has definitely been passed, the adventure enters a new phase. The region of subsonic solar wind beyond the termination shock will be fascinating, characterized by turbulence and particle acceleration, and by many unusual plasma phenomena." --Kate Wong