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Voyager 1 hears plasma ‘hum’

The distant space probe has found a signal from the interstellar gas.

Circle with solar system in it. Two dots are ouside of the circle. The dots and connect ted to circle with arrows pointing outward. The dots say "vayager 1" for the top dot. Bottom dot says"voyager 2".
Voyager space craft journeys out of the heliosphere. Credit: NASA / Wikimedia Commons.