The Voyager 1 space probe, launched in 1977, is now more than 8 billion miles from Earth, but whether it has actually left the outer limits of the solar system is still a matter of debate. In the Nov. 6 Nature, researchers led by Stamatios Krimigis of Johns Hopkins University report that the craft already has passed that historic milestone, entering the boundary marking the edge of the sun’s influence, known as the termination shock. The termination shock arises from supersonic particles streaming from the sun (the solar wind) violently intersecting with particles from deep space. Krimigis and his team offer a drop in solar wind speed, among other data, as proof that Voyager journeyed through that barrier.
In the same issue, however, researchers led by Frank McDonald of the University of Maryland disagree, saying that the probe is only approaching the barrier and the amount of data seen is only a prelude to the true termination shock that lies just ahead. An accompanying commentary acknowledged that “neither explanation is certain.” Wherever it actually is, Voyager is farther out than any object yet made by humans.