More than 35 years after its launch and almost 33 years since it whizzed near Saturn, the Voyager 1 spacecraft may have officially left the solar system. On 25 August last year, when the craft was more than 18 billion kilometers from the sun, sensors noted a steep drop-off in the numbers of so-called anomalous cosmic rays, the charged particles that often become trapped in the magnetically turbulent region that separates the solar system from interstellar space. At the same time, the sensor detected a dramatic rise in the numbers of intense galactic cosmic rays that originate far beyond our solar system—the largest such jump since the craft was launched, the researchers report online and in a forthcoming issue of Geophysical Research Letters. Cosmic ray intensities had been fluctuating for several weeks prior to 25 August, a sign that the Voyager craft may have been moving through the turbulent boundary of the solar system—or that the boundary may have been shifting back and forth in space, sweeping across the craft as it did so, due to variations in solar activity. But in the past 6 months, cosmic ray levels have stabilized. Although scientists may debate whether Voyager 1 has truly left the solar system, it's clear that the craft has entered a region of space totally unlike that it has traversed since it left Earth.
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Update 21 March: Yesterday afternoon, NASA's Jet Propulsion Laboratory released a statement that read, in part, "It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space. ... A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed."