A NASA space probe has observed an intriguing steady hum deep in interstellar space.

The Voyager 1 spacecraft, one of two sibling NASA spacecraft launched 44 years ago and now the most distant human-made object in space, is still operational and zooming toward infinity.

The ship has long since zipped past the solar system’s edge and through the interstellar medium, passing through the heliopause—the solar system’s boundary with interstellar space.

According to Cornell University-led study, it has been detecting the steady drone of interstellar gas (plasma waves) there.

Stella Koch Ocker, a Cornell doctoral student in astronomy, discovered the emission by analysing data slowly sent back from a distance of more than 14 billion miles. “It’s very faint and monotone, because it is in a narrow frequency bandwidth,” Ocker said. “We’re detecting the faint, persistent hum of interstellar gas.”

This research will help scientists better understand how the interstellar medium interacts with the solar wind, as well as how the interstellar atmosphere shapes and modifies the protective bubble of the solar system’s heliosphere, according to Ocker.

The Voyager 1 spacecraft was launched in September 1977 and flew by Jupiter in 1979, then Saturn in late 1980. In August 2012, Voyager 1 passed through the heliopause at a speed of around 38,000 miles per hour.

The spacecraft’s Plasma Wave System observed perturbations in the gas after it entered interstellar space. According to new research published in Nature Astronomy, in between those eruptions—caused by our own roiling sun—researchers have discovered a steady, persistent signature created by the tenuous near-vacuum of space.

“The interstellar medium is like a quiet or gentle rain,” said senior author James Cordes, the George Feldstein Professor of Astronomy. “In the case of a solar outburst, it’s like detecting a lightning burst in a thunderstorm and then it’s back to a gentle rain.”

Ocker assumes that there is more low-level activity in interstellar gas than previously believed, allowing researchers to monitor the spatial distribution of plasma when it isn’t disrupted by solar flares.

Shami Chatterjee, a Cornell research scientist, demonstrated why it’s important to keep track of the density of interstellar space. “We’ve never had a chance to evaluate it. Now we know we don’t need a fortuitous event related to the sun to measure interstellar plasma,” Chatterjee said.

“Regardless of what the sun is doing, Voyager is sending back detail. The craft is saying, ‘Here’s the density I’m swimming through right now. And here it is now. And here it is now. And here it is now.’ Voyager is quite distant and will be doing this continuously.”

Voyager 1 carried a golden record made by a committee chaired by late Cornell professor Carl Sagan, as well as technology from the mid-1970s. According to NASA’s Jet Propulsion Laboratory, it took 22 watts to transmit a signal to Earth. The craft has nearly 70 kilobytes of computer memory and a data rate of 21 kilobits per second at the start of the flight.

The contact rate has since slowed to 160 bits per second, or around half of a 300-baud rate, due to the 14-billion-mile gap.

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