Voyager 1 records mysterious ‘hum’ in interstellar space
44 years after it rocketed off from Earth, Voyager 1 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 the 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. The Sun sends off bursts of energy known as coronal mass ejections that disturb this medium, causing the plasma, or ionised 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 ionised gas molecules are.
Now, however, researchers have realised that Voyager 1 is also sending back a far more subtle signal: the constant ‘hum’ of the interstellar plasma. This low-level vibration is fainter, but much longer lasting than the oscillations that occur after coronal mass ejections. The hum lasts at least three years. “Now we don’t have to wait for a fortuitous event to get a density measurement,” said Stella Ocker, a doctoral student in astronomy at Cornell University.
Voyager 1 is currently over 152 astronomical units from the Sun. An astronomical unit is the distance between the Sun and Earth, so that means that the antennae-studded spacecraft is now 152 times as far away as Earth is from the Sun. The far-flung craft is one of a pair originally designed to fly by Jupiter, Saturn, Uranus and Neptune, taking advantage of a rare planetary alignment that would allow Voyager 1 and 2 to use the gravity of each planet to propel themselves to the next.
Both Voyager 1 and 2 are still transmitting from interstellar space. Voyager 2 made it past the heliosphere in 2018. Ocker and her colleagues combed through data from the last five years of Voyager 1’s transmissions to find the subtle hum of interstellar space. They were surprised to find that the vibrations occur on a narrow set of frequencies, unlike the vibrations from the coronal mass events, which tend to show up more broadly. The researchers don’t yet know exactly what causes the low-key plasma vibration, but it probably has to do with the ‘jitter’ of electrons in the medium due to their basic thermal properties.