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Voyager 1 has officially left the solar system, having crossed the so-called termination shock in December last year. The craft crossed the boundary 94 astronomical units (the distance from the Earth to the sun is one AU) from the sun, according to NASA researchers based in Washington DC.

The key things to look for, scientists say, are an abrupt drop in the speed of the solar wind, along with a significant increase in the sun's magnetic field strength. The solar wind loses speed quickly at the termination shock, falling from at least 700,000 miles per hour down to 100,000mph.

Based on the strength of the solar wind, some researchers claimed Voyager passed this marker three years ago. But because there was no change in the magnetic field strength - caused by solar particles slowing down and crowding together at the termination shock - the claim was widely disputed.

On this occasion, Voyager 1's magnetometer has detected a two-and-a-half fold increase in the strength of the magnetic field, New Scientist reports. The researchers have ruled out the possibility that this is due to a solar flare's shock front. If a solar flare was behind the increase in the magnetic field strength, the craft would also have detected fewer cosmic rays, but instead the reverse is true.

Edward Stone, of Caltech told New Scientist: "This time the entire Voyager 1 team agrees we have crossed the termination shock."

However, there is still missing data. Theory predicts that Voyager should have seen an increase in so-called anomalous cosmic rays (ACR) when it left the solar system, but no such increase was detected.

ACRs are bits of neutral interstellar flotsam that are ionised when they enter the solar system. The Voyager 1 team proposes that ACRs might be made only in especially turbulent parts of the termination shock, as opposed to continuously along its edge. If this is the case, it is possible that Voyager just missed them. ®

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