Forty-four years after it rocketed off from Earth, the Voyager 1 spacecraft is detecting the background “hum” of interstellar space for the first time.

Voyager 1, launched in 1977, left the bounds of the solar system — known as the heliosphere — in 2012. The heliosphere is the bubble of space influenced by solar wind, the stream of charged particles that emanates from the sun. Since popping out of this bubble, Voyager 1 has been periodically sending back measurements of the interstellar medium. Occasionally, the sun sends off a burst of energy known as a coronal mass ejection that disturbs this medium, causing the plasma, or ionized gas, of interstellar space to vibrate. These vibrations are quite useful, as they allow astronomers to measure the density of the plasma — the frequency of the waves through the plasma can reveal how close together the ionized gas molecules are.