The Sounds of Space
You would think that interstellar space is silent. After all, in a vacuum, nobody can hear you scream... but interstellar space isn't a perfect vacuum. In fact, once you get out beyond the Solar System, and the Sun's protective field, there's actually more plasma than there is in the outer reaches of the Solar System: a whopping one electron per 13 cubic centimeters of space.
OK, that's not a lot, but it's enough that you can have sound waves moving through the plasma, and enough that the very delicate ears of Voyager I could actually hear it.
And that's what brings up this story today: astrophysicists have finally reached a consensus that on August 25th, 2012, Voyager I did indeed leave the Solar System, becoming the first man-made object ever to sail beyond our shores. (I've always had a soft spot for the two Voyagers: they were launched only months before I was born, and so I've been growing up with their mission. So now I can look up at the sky and think, it's spacecraft like these who make you realize how little you've accomplished. Why, if I had started travelling when I was born, I could have left the Solar System by now...
The reason it took over a year to confirm that this had happened is that there isn't a big sign at the edge saying "You Are Now Leaving The Solar System; Come Again Soon!" But there is an actual edge to it, called the heliopause. The Sun emits a steady stream of particles called the Solar Wind; the heliopause is the point at which this stream breaks up against the interstellar wind, the stream of particles which flows through our galaxy. Inside the heliopause, the Sun's magnetic field keeps out many of the particles of the interstellar wind, so when you cross that line, you start to feel the somewhat more powerful currents of interstellar space.
Which brings me to the video below: a recording of the sounds heard by the Voyager, transposed up quite a few octaves so that you can hear them. These are the sounds of individual plasma waves crashing against the bow of the ship; the graph shown in the video shows the density of the wave vertically, and the time horizontally.
We've never explored this area before. We can look at it via telescope, but a telescope won't reveal sounds, won't reveal what particles are there. Voyager I is somewhat limited in what it can reveal, since it was designed not for this, but to explore the planets of our Solar System; it sent back extraordinary photos of Jupiter and Saturn (as its sister-ship, Voyager II, did of Saturn, Jupiter, Uranus, and Neptune) and its technology is rather fascinatingly archaic, running on 22W of total power and using an 8-track tape for storage. But it continues to fly, 35 years into its mission, and is now bringing us our first images of the world beyond the solar shores.
For more information about the Voyagers:http://voyager.jpl.nasa.gov/http://voyager.jpl.nasa.gov/imagesvideo/imagesbyvoyager.html
The heliopause and the edge of the Solar System:http://en.wikipedia.org/wiki/Heliopause_(astronomy)#Heliopause
Article about Voyager's exit, with links to more:http://blogs.scientificamerican.com/life-unbounded/2013/09/12/voyager-has-entered-the-interstellar-medium/