The "tsunami wave" encountered by NASA's Voyager 1 in interstellar space earlier this year is believed to still be propagating outwards.
The finding marks the longest-lasting shockwave that has been observed in interstellar space, NASA's Jet Propulsion Laboratory reported.
"Most people would have thought the interstellar medium would have been smooth and quiet. But these shock waves seem to be more common than we thought," said Don Gurnett, professor of physics at the University of Iowa in Iowa City.
These types of "tsunami waves" occur when the sun releases a coronal mass ejection and emits a magnetic cloud of plasma. This phenomenon generates a "wave" of pressure that turns into a shockwave when it runs into interstellar plasma (the charged particles between stars).
"The tsunami causes the ionized gas that is out there to resonate - 'sing' or vibrate like a bell," said Ed Stone, project scientist for the Voyager mission based at California Institute of Technology in Pasadena.
This is the third shockwave experienced by the Voyager 1, but is by far the longest having been first detected as far back as February.
"This remarkable event raises questions that will stimulate new studies of the nature of shocks in the interstellar medium," said Leonard Burlaga, astrophysicist emeritus at NASA Goddard Spaceflight Center.
Researchers are still unsure how large and fast-moving the wave is, or why it is so long-lived.
The Voyager 1 reached interstellar space back in 2013. Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977. Voyager 2, which was launched before Voyager 1, is the "longest continuously operated spacecraft," and researchers hope it will reach interstellar space as soon as within the next few years.
The findings were presented Dec. 15 at the American Geophysical Union meeting in San Francisco.
JPL, a division of Caltech, built the twin Voyager spacecrafts and operates them for the Heliophysics Division within NASA's Science Mission Directorate in Washington.
