Trip to the tune of C as physics crosses a boundary Canadian pianist Glenn Gould: His performance of Bach's Prelude and Fugue in C Major from Book II of The Well-Tempered Clavier was included on the NASA Voyager Golden Record. Photo: Sony Interstellar space, scientists now know with certainty, is dense with particles, and the place is literally hissing. Or you could say it is whistling in the dark. It's almost a pure tone. Like middle C. But slightly varying, like your piano is not quite tuned right," said Donald Gurnett, a University of Iowa physicist who has been working on the Voyager mission most of his adult life. Dr Gurnett is the lead author of a paper published in the journal Science that provides what seems to be the final, incontrovertible evidence that NASA's Voyager 1 has crossed into a realm where no spacecraft has gone before. Scientists have long thought there would be a boundary out there, somewhere, where the million-kilometre-an-hour "solar wind" of particles would give way abruptly to cooler, denser interstellar space, permeated by charged particles from around the galaxy. That boundary, called the heliopause, turns out to be 18.2 billion kilometres from the sun, according to Voyager's instruments and Dr Gurnett's calculations. Beyond the boundary, space is - perhaps counter-intuitively - much denser with particles. There are 80,000 particles per cubic metre in the region where Voyager 1 is now, Dr Gurnett said. The sun's hot ejecta - a plasma of charged particles - forms a vast bubble, known as the heliosphere. In the outer regions of the heliosphere, the particles are relatively few and far between, with just 1000 particles per cubic metre in some regions, Dr Gurnett said. But the heliosphere has an edge. Voyager 1's epochal crossing of the boundary, into the cooler, denser plasma, took place on August 25 last year, the report says. This confirms earlier findings, published in three papers in Science in June, that on that date, Voyager 1 experienced a sudden drop in solar radiation and a spike in cosmic particles coming from all around the galaxy. Earlier Voyager data had been ambiguous. It continued to pick up magnetic signals that suggested it was still within the sun's magnetic field. Chief scientist for Voyager Ed Stone suggested Voyager 1 was flying through a transitional zone. Now scientists have a new set of measurements thanks in large part to a solar flare. On March 17 last year, the sun ejected a huge mass of particles, and when those solar particles arrived at Voyager more than a year later, on April 9, they triggered oscillations in the charged particles of matter - the plasma - surrounding the spacecraft. From the frequency of those oscillations - essentially the sound of space itself - the scientists could interpret the density of the plasma. That density, much higher than anything registered, offered compelling evidence Voyager 1 had, in fact, entered the interstellar zone. Although Voyager 1 is now in interstellar space, it hasn't technically left the solar system. That's because of the Oort Cloud - a region of comets in orbit around the sun.