Since this final photo, Voyager has been sending back pure data – measures of cosmic particles, magnetic fields, and plasma waves (in short, brainy stuff) designed to give far better understanding of our solar system. But since the camera was shut off, there was no way of knowing when the Voyager-I had reached interstellar space.
Here enters Don Gurnett and his plasma wave team at the University of Iowa. This intelligent being knew that he could use vibrations in the plasma to determine its density so he set off to work and waited for the day when he could finally see Voyager-I enter the interstellar space. But the problem was plasma only vibrates when it is disturbed by a large solar system. “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.”
As Voyager-I flies further out into the interstellar space, plasma density will increase as it piles up against the hemisphere, like snow in front of a snowplow. That’s why you would hear a high pitch sound if you could hear the audio recording of this famous space probe.
But to add a sad ending to the story, we won’t know when Voyager-I will get past the plasma pileup. Voyager’s signal is already faint. Twelve billion miles away, its 23-watt transmitter, about the strength of a refrigerator light-bulb, 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 hurling 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