NASA's Voyager 1 has captured a faint, persistent 'hum' from plasma in interstellar space—a groundbreaking first. Cornell University researchers, including lead author Stella Koch Ocker, detail the discovery in Nature Astronomy.
For nearly 45 years, Voyager 1 has ventured through interstellar space, far beyond the Kuiper Belt and over 22 billion kilometers from Earth. Engineers still receive its data transmissions today.
Analyzing the probe's data, Cornell astronomy PhD student Stella Koch Ocker isolated a constant emission from the tenuous interstellar plasma, sandwiched between disruptions from solar flares. "This is a very low, monotonous hum, confined to a narrow frequency band," Ocker explains. The signal peaks at 3 kHz—vibrations from plasma, where electrons are stripped from atoms by extreme heat.
While plasma is the universe's most common visible matter, Voyager 1 previously only detected intense oscillations from solar coronal mass ejections. Now, it has measured the undisturbed ambient plasma levels, free from solar influence.
Picture the interstellar medium as a gentle, steady rain. Occasional solar eruptions propagate outward like thunderclaps, but Voyager 1 has now 'heard' the quiet rain itself.
These findings illuminate interactions between interstellar plasma and the solar wind, revealing how the heliosphere—our solar system's protective bubble—is shaped by its surroundings.
Co-author Shami Chatterjee highlights the breakthrough: "We never had a way to measure it directly. Now we know we don't need a solar event to probe interstellar plasma density." Voyager 1 enables ongoing tracking of plasma distribution, undisturbed by flares.
Notably, neither Voyager 1 nor its twin, Voyager 2, were designed for deep interstellar study; their mission focused on outer planet data. NASA is now developing a dedicated probe to explore up to 1,000 AU from the Sun.
