The most distant human-made object, Voyager 1, has made yet another amazing discovery in interstellar space.
This spacecraft from NASA detected a low-frequency hum droning away at approximately 3 kHz is constant, continuous, and long-lasting.
This is plasma, which is matter that has been heated to the point that electrons have been removed from their atoms, resulting in an ionized, or charged, gas. Plasma detection is trivial in and of itself.
After all, it's one of the universe's most abundant kinds of visible matter. The location of this plasma's detection, however, is significant: the Very Local Interstellar Medium.
"We're detecting the faint, persistent hum of interstellar gas," Stella Koch Ocker, Cornell doctoral student in astronomy and lead author of the study, said in a statement per Phys.org. "It's very faint and monotone, because it is in a narrow frequency bandwidth," Ocker added.
The fact that NASA's Voyager 1 could detect weak plasma movements in this far-flung area of space is even more important. The probe had previously observed intense perturbations in the plasma caused by coronal mass ejections from the Sun, known as plasma oscillation events.
In other words, without the Sun's interference, Voyager 1 registered the natural background, or ambient, levels of plasma that exist in deep space. Nature Communications released the details of this discovery. The study is titled "Persistent Plasma Waves in Interstellar Space Detected by Voyager 1."
NASA launched Voyager 1 in September 1977 and is now more than 14.1 billion miles away from Earth as of writing. It is the farthest human-made object (Voyager 2, its sister probe, is 11.8 billion miles away) yet.
At the farthest reaches of the solar system, Voyager 1 has passed through the heliopause, a region sandwiched between the hot solar plasma and the cooler interstellar medium. The probe is now flying through interstellar space, an extremely low matter densities area, at a speed of 38,000 miles per hour.
Thanks to its onboard Plasma Wave System, Voyager 1 can calculate plasma waves in the interstellar medium, unlike its sibling.
Since 2012, Voyager 1 has observed eight different plasma oscillation events that have lasted anywhere from a few days to a year. Instabilities in the electron foreshocks of advancing shockwaves generated by the Sun trigger these events.
Weak But Steady Plasma Signature Since 2017
Outside of these energetic events, Voyager 1 started to detect a weak, yet steady and persistent plasma signature in 2017.
The newly discovered signal dubbed a "plasma wave emission," is narrower than plasma oscillation cases, with a frequency of about 3 kHz and a bandwidth of 40 Hz.
Furthermore, the signal has lasted nearly three years, which the paper claims "corresponds to a distance traveled by the spacecraft of about 10 au," or about 930 million miles.
According to the astronomers, the plasma wave emission "appears to be distinct from the shock-generated [plasma oscillation events]" because of its small bandwidth, low amplitude, and multi-year persistence.
A recent discovery is a once-in-a-lifetime chance to investigate the density of interstellar space.
"We've never had a chance to evaluate [the density of interstellar space]," Shami Chatterjee, co-author and Cornell researcher, said in a Futurism report. "Now we know we don't need a fortuitous event related to the sun to measure interstellar plasma."
Although Voyager 1 is 14 billion miles away, scientists can still get tantalizing glimpses of what space is like outside of our own star system.
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