Image credit: NASA/JPL-Caltech

It's official: according to NASA, the probe Voyager 1 has become the first man-made object to leave the Solar System and fly through the space between the stars.

"Now that we have new, key data, we believe this is mankind's historic leap into interstellar space," NASA scientist Ed Stone said in a statement.

In fact, the data that NASA has gathered shows that Voyager 1 has probably been traveling in interstellar space for about a year - a claim that several different researchers have made. However, NASA has been conservative in making this claim, given its historic nature.

At this point, though, NASA believes that the evidence is conclusive. But what do NASA scientists mean by "out of the solar system," given that Voyager 1 left Pluto and the other small dwarf planets behind years ago?

The basic challenge has to do with the plasma in interstellar space. Plasma is highly energetic gas particles. Inside the solar system, the energy ionizing those particles comes primarily from our Sun, so the bulk of the energy is flowing outward from it. Outside our solar system, the energy for the plasma comes from stars, novae, black holes, and other sources of cosmic energy.

Voyager 1 is currently about 11 billion miles from the Sun - and things aren't entirely what scientists expected them to be. New discoveries about the nature of space so far from our Sun made it more difficult to figure out where the Solar System ended and interstellar space began.

The key, it turned out, was sound. It's a cliche that there's no sound in space - and to a certain extent that's when space is a vacuum. But sound is just a wave travelling through a medium - in this case, the plasma Voyager is travelling through. It's been predicted that the density of plasma in interstellar space would be different than the density in the solar system.

Unfortunately, Voyager 1's plasma density detector stopped working in 1980. So how do scientists figure out the plasma density? Through pitch! When you suck in helium and start talking, your voice has a higher pitch - that's because helium is less dense than the air we usually talk through, so it vibrates faster and has a higher pitch.

A little over a year ago, our Sun had a coronal mass ejection, which sent massive amounts of energy outward. That energy oscillated the plasma - like hitting the strings of a piano. And so by listening to the "sounds" of those oscillations, scientists determined that it was changing pitch, meaning the plasma was getting more dense. This proved to be the clue that Voyager had indeed left the solar system.

"We literally jumped out of our seats when we saw these oscillations in our data -- they showed us the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble," scientist Don Gurnett said in a press release. "Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma."

Now that Voyager 1 is in the space between the stars, it will help improve our knowledge of what conditions are like in interstellar space. And we should get a lot of information about it until about the year 2020, when Voyager's batteries will start to run out and shut down its systems.

But even without power, should another intelligent life form come across Voyager, they'll end up learning a great deal about human beings. Back in 1977 when the probe was launched, it included a "Golden Record" which contains music, photos, and other information about who we are.

In the meantime, while you can't travel to interstellar space yourself, you can hear what it "sounds like" in the video below provided by the Jet Propulsion Laboratory. These are the sounds that clued scientists in that Voyager 1 has probably left the Solar System.
