Thirty six years after it blasted off from Florida, the Voyager spacecraft has officially entered interstellar space, the cold region where particles from our sun stop and the remnants of ancient exploded stars begin.
It’s hard to fathom how far Voyager has gone. It has travelled farther than any human-made object, 12 billion miles from home. It sent its last picture 13 years ago, just before shutting down its camera to conserve power. From its position, you can hardly see the Earth. (It's the dot on the right.)
Since that final photo, Voyager has been sending back pure data--measures of cosmic particles, magnetic fields, and plasma waves designed to give us a better understanding of the far reaches of our solar system. It was vibrations in the plasma surrounding Voyager that told Don Gurnett and the plasma-wave team at the University of Iowa that Voyager had entered interstellar space. It also gave us the most evocative message from Voyager in years, an eerie whistling that NASA played at a press conference announcing the discovery, calling it the “sound of interstellar space."
That’s the sound of interstellar plasma vibrating from explosions on the sun that happened a year before, and it gave Gurnett and his team the clue they needed to figure out when Voyager had entered interstellar space.
At some point in space well past Pluto--we didn’t know exactly where--the particles ejected from our sun begin to collide with particles from interstellar space, slowing down and eventually stopping, forming the relatively placid buffer between the solar buble, where we live, and the interstellar space beyond. (Contrary to some reports, Voyager is still in our solar system, which ends at the Oort cloud, a ring of icy chunks where comets originate, but it has left the heliosphere, the bubble of the sun's particles.) Voyager has been travelling through these doldrums for years, and we weren’t sure whether it had punched through to the interstellar space on the other side. The best indicator would be a sudden spike in the density of the plasma Voyager was travelling through, but Voyager’s plasma detector broke in 1980, shortly after it passed Jupiter.
That’s where the vibrations come in. Gurnett knew that he could use the vibrations in the plasma to determine its density. Solar flares send shockwaves through the plasma, causing particles to vibrate like a violin string, and those vibrations are picked up by Voyager’s antenna, recorded on an 8-track tape aboard the probe, and transmitted back to Earth. When the frequency goes up, it means the density of the plasma is increasing. The problem was that the plasma only vibrates when it’s disturbed by a large solar storm. “We were in a long dry spell,” says Gurnett. “The last time we detected a plasma oscillation was nine years ago.”
Then, on April 9, Gurnett went into his office and saw an oscillation on the data plot, one with a far higher frequency than the one nine years ago. “I looked at my colleagues,” he said, “and we knew we were in the interstellar medium.”
Going back through the data, Gurnett and his colleagues found another, far smaller plasma vibration (the first one you hear on the audio file), and used the two frequencies to extrapolate when Voyager passed from the heliopause to interstellar space. That date was August 25, 2012, around the same time Voyager had registered a sharp drop off in charged particles from the sun and a jump in galactic rays from outside the heliosphere. Contrary to what we used to think, the border of the solar bubble is sharp--Gurnett compares it to a cold front--and Voyager has now passed through it.
As for the “sound of interstellar space,” it’s not something you could hear if you were aboard Voyager. It’s the result of electrons oscillating back and forth, creating an electrostatic wave, not air particles colliding, like the pressure waves we hear. But the frequency is one that we can hear when it's played through a speaker here on Earth.
Gurnett keeps a list of his favorite space sounds on his site, including one of the howls detected when Voyager passed Jupiter, a case where listening to the sound helped confirm what Voyager was detecting. The plasma wave team detected a signal that looked like the electromagnetic waves created by Earth lightning and sometimes picked up by radio receivers, a phenomenon called a whistler. Sure enough, when played through speakers they sounded like Earth whistlers, confirming that Jupiter has lightning.
Bill Kurth, also on the plasma wave team, says the main reason they make audible translations of space sounds is that they like to listen to them, and because it’s a compelling way to illustrate the strangeness of space. “In the same way it’s easy to show your audience a new picture from Jupiter or Saturn, people can relate to the sounds, especially when the frequencies are the same as audible ones,” says Kurth.
As Voyager flies farther out into interstellar space, plasma density will increase as it piles up against the heliosphere, like snow in front of a snowplow. That’s why the pitch goes up toward the end of the audio recording. Should a solar flare vibrate the interstellar plasma in the next few years, the tone would be higher still. After that it will taper off, as Voyager goes beyond the sphere of solar influence, something Gurnett thinks will take another 50 years.
We won’t know when Voyager gets past the plasma pileup, though. Voyager’s signal is already faint. Twelve billion miles away, its 23-watt transmitter, about the strength of a refrigerator lightbulb, requires three massive super-cold antennas to detect its transmissions. But the real issue is power. Voyager’s camera was shut off in 1990 to save energy, and the heaters on its backup thrusters were shut off two years ago. In the next few years the gyros that maneuver the probe will be turned off, leaving it hurtling through space undirected. Starting in 2020, its instruments will be shut off one by one, and its nuclear reactors will give out sometime around 2025.
After that, Voyager will be sending sounds into space rather than transmitting it back to Earth. It's carrying a gold-plated phonograph record of Earth’s greatest hits, as determined by Carl Sagan in 1977, and with a needle to play it. There's the sound of crashing waves and thunder, greetings in 55 languages, along with Mozart, Bach, and Chuck Berry.