NASA's Voyager I spacecraft has been steadily journeying away from the sun to the outer reaches of the solar system since its 1977 launch. As it travels farther out and enters a different region of the solar system, it's occasionally affected by coronal mass ejections -- shock waves caused from massive violent eruptions from our sun.
There have been three of these space "tsunamis" since 2012, and the third one -- described by NASA on Monday -- has helped the space agency confirm something it posited in late 2013: that Voyager is the first Earth craft to travel into interstellar space.
Interstellar space is the area just beyond the reach of what's known as our heliosphere: an area where the solar wind pushes back the dense plasma of space in a sort of protective bubble. This plasma was ejected into the universe by the death of stars millions of years ago.
The plasma outside the heliosphere is about 40 times denser than the plasma that lies inside it. By using its 37-year-old cosmic ray and plasma wave instruments, Voyager has sent back signals to Earth that prove it has popped through our sun's protective bubble and is now moving through the thicker plasma. Scientists can tell this is the case because the thicker plasma in interstellar space oscillates at a faster rate than less dense plasma and produces a different frequency when hit by the sun's shock waves.
"The tsunami wave rings the plasma like a bell," Ed Stone of the California Institute of Technology , the mission's project scientist since 1972, said in NASA's statement. "While the plasma wave instrument lets us measure the frequency of this ringing, the cosmic ray instrument reveals what struck the bell -- the shock wave from the sun."
"Normally, interstellar space is like a quiet lake," Stone added. "But when our sun has a burst, it sends a shock wave outward that reaches Voyager about a year later. The wave causes the plasma surrounding the spacecraft to sing."
And just what does that singing sound like? Funny you should ask. Here's a video that lets you hear the eerie sounds of plasma ringing in deep space.
Voyager 1, whose mission was to study Jupiter and Saturn, is currently located about 11 billion miles from Earth and over 5 billion miles past Pluto. Still, Voyager is not quite out of our solar system, as it has one final ring of comets to penetrate before it can claim that distinction. It has Voyager 2 close on its heels, and scientists predict that it too will soon burst out of the heliosphere into interstellar space and continue sending data back about this heretofore-unexplored area.
Correction at 5:35 a.m. PT Wednesday: Voyager has traveled more than 5 billion miles past Pluto. The story had listed the wrong planet.