The vacuum between the stars is not empty. The interstellar medium consists of dust and gas, which in turn can be in atomic, molecular and ionized form. Its density varies widely. Interestingly, it is greatest in cool, dense regions where matter is mainly in molecular form and one could count up to 1 million molecules per cubic centimeter. In hot, diffuse regions, on the other hand, matter is mainly ionized and one finds only a single ion per 10,000 cubic centimeters. Compared to, say, the capabilities of a vacuum chamber constructed by humans, with still ten billion particles per cubic centimeter, that’s pretty darn thin. But space is also pretty darn big, so despite its low density, the interstellar medium contributes about five percent of the total mass.

That five percent, of course, is of great interest to researchers. That’s why they’re pleased that Voyager 1 and 2 are now the first probes to penetrate the interstellar medium. Voyager 1 has now delivered interesting results, as a paper in Nature Astronomy shows. The probe has succeeded in recording the constant noise of the interstellar medium. Gas and dust that it contains are excited over and over again, sending signals in different ranges that Voyager 1 was able to pick up. By examining data sent back by the probe from more than 14 billion miles away, astronomer Stella Koch Ocker was able to detect the emission. “It’s very faint and monotonic because it’s in a narrow frequency bandwidth,” Ocker says. “So we’re detecting the faint, persistent hum of interstellar gas.”

This work allows scientists to understand how the interstellar medium interacts with the solar wind, Ocker says, and how the protective bubble of the solar system’s heliosphere is shaped and modified by the interstellar environment.

The Voyager 1 spacecraft, launched in September 1977, flew by Jupiter in 1979 and Saturn in late 1980. Traveling at a speed of about 38,000 miles per hour, Voyager 1 passed through the heliopause in August 2012. After it entered interstellar space, the probe’s plasma wave system detected disturbances in the gas. But in between those eruptions – caused by our own sun – researchers found a steady, persistent signature created by the near-vacuum of space.

“The interstellar medium is like a silent or gentle rain,” says astronomer James Cordes, lead author of the study. “In the case of a solar outburst, it’s like detecting lightning in a thunderstorm, and then it’s soft rain again.” Ochre believes there is more low-level activity in the interstellar gas than scientists previously thought, which allows researchers to track the spatial distribution of the plasma – that is, when it is not disturbed by solar flares.