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After many claims and statements over the past few years that Voyager 1, our most distant operating spacecraft, has 'left the solar system' (it hasn't, as I explain here), it does now seem that as of August 2012 this extraordinary vehicle has entered the interstellar medium. This is the tenuous atmosphere that permeates the space between the stars in our galaxy, and is distinctly different from the tenuous atmosphere that comes from, and envelopes, the Sun.
Writing in a paper in Science, Gurnett et al. describe how, in late 2012, the plasma wave instrument on Voyager began receiving data consistent with a very low density of surrounding electrons, about 1 for every 13 cubic centimeters of space - a telltale signature of the medium between the stars because as tenuous as it is, this is still significantly more dense than the so-called heliosheath that Voyager had been traversing.
Rather ironically this measurement could only be made with confidence because a massive coronal ejection event from the Sun had streamed out to Voyager's location on April 9th 2013 and, in passing, set the electrically charged particles (the plasma) surrounding it in motion. This ethereal pulse caused oscillations in the plasma, and the frequency of these waves revealed the plasma's true density.
By looking at earlier data the scientists realized that such events had been seen before, with similar oscillations seen in both November and October of last year. Extrapolating backwards reveals the shift in environment occurred on about August 25th 2012.
These are tricky measurements to make, but it does seem that our species has finally dipped its toes into the great ocean of interstellar matter that pervades our galaxy.
