Voyager 1, the farthest human-made object in space, is still sending scientific information about its surroundings.
NASA launched Voyager 1 about 44 years ago. The probe has since zipped past the heliosphere and made the historic entry into interstellar space, the region between stars, filled with material ejected by the death of nearby stars millions of years ago.
The heliosphere extends far past the orbits of the planets until it reaches the heliopause—the boundary between itself and interstellar space. There, the interstellar medium stops the effect of our sun’s solar wind. The distance from the sun to the heliopause is thought to be around 11 billion miles.
Traveling at about 38,000 mph, Voyager 1 crossed the heliopause in August 2012.
Even though Voyager 1 is now over 14 billion miles away from Earth, the probe continues to send scientific information through the Deep Space Network.
Now, its instruments have detected a faint, persistent hum.
On Monday, scientists published new research in the journal Nature Astronomy analyzing the data that Voyager 1’s Plasma Wave System (PWS) sent back to Earth after it passed through the heliopause. The instrument has detected a low, constant pattering against its detector, space raindrops gently falling on a window. Those drops signify plasma waves or interstellar gas.
“It’s very faint and monotone, because it is in a narrow frequency bandwidth,” said Stella Koch Ocker, a Cornell doctoral student in astronomy. “We’re detecting the faint, persistent hum of interstellar gas.”
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Researchers think there is more low-level activity in the interstellar gas than they previously thought.
Picking up this signal was trickier than it may seem. Typically, the solar winds don’t reach beyond the edge of the heliosphere. However, the Sun’s influence can sometimes extend into interstellar space when it throws off coronal mass ejections (CMEs). Voyager 1’s Plasma Wave Instrument detected these energetic outbursts for the first time back in 2014. The probe registered them as “tsunamis” and made it hard to pick up anything else. But given enough time, the instrument has been able to listen closely in between these events.
“The interstellar medium is like a quiet or gentle rain,” says James Cordes, senior author of the study. “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.”
The new detection can help scientists understand how the interstellar medium interacts with the solar wind. It could also boost our understanding of how the interstellar environment impacts the shape and modification of the protective bubble of the solar system’s heliosphere.
Voyager 1′s odyssey began in 1977 when NASA launched the spacecraft and its twin, Voyager 2, on a tour of the gas giant planets of the solar system. The probes have since exceeded official expectations. Their two-planet mission became four and their five-year lifetimes stretched to 12 and is now about 44 years.
Cornell research scientist Shami Chatterjee explained how continuous tracking of the density of interstellar space is important. “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.”
The new study helps us learn more about the universe beyond the influence of the Sun and could also influence the design of upcoming missions headed for interstellar space.