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Artist's concept of NASA's Voyager 1 spacecraft entering interstellar space

Data collected by the Voyager 1 spacecraft, which launched in 1977, has helped scientists to calculate the density of the interstellar plasma. Credit: NASA/JPL-Caltech

Astronomy and astrophysics

Voyager 1 captures faint ripples in the stuff between the stars

The first spacecraft to visit interstellar space has now become the first to make continuous measurements of waves in that remote realm.

The Voyager 1 spacecraft has detected persistent ripples in the interstellar plasma, through which it has been travelling since it left the Solar System in 2012. By measuring these waves, astrophysicists have made the first continuous measurements of the density of the interstellar plasma, the rarefied medium between the stars.

Launched by NASA 44 years ago, Voyager 1 became the first human-made object to enter interstellar space — the region between star systems — and is still motoring along. Stella Ocker at Cornell University in Ithaca, New York, and her collaborators detected the interstellar-plasma waves by examining regular variations that Voyager recorded in the electric field it encounters as it flies away from the Solar System.

The waves consist of displacements between the plasma’s two components: positively charged ions and negatively charged electrons. Despite this displacement, the plasma tends to stay put and the waves go in no particular direction — like the stationary ripples in a lake on a windy day, Ocker says.

In the past, astrophysicists had made similar measurements of interstellar-plasma waves that had been triggered by solar events, but this is the first time that they have measured plasma density continuously.

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