PBS NEWSHOUR EXTRA passes along NASA’s announcement that the Voyager 1 spacecraft has reached interstellar space.
The scientists working on the Voyager 1 mission say the spacecraft entered interstellar space on or around August 25, 2012. They estimate that Voyager 1 is 11 billion miles from Earth, which puts it beyond all of the planets and Pluto. Voyager 2, which was actually launched about two weeks before Voyager 1, is also traveling towards interstellar space, but is 2 billion miles behind its identical twin. This development will provide an opportunity for a new type of space exploration, in which scientists will be able to better study space outside of the sun’s influence. What does it mean to go “interstellar”? Interstellar space refers to the space in the galaxy that is not occupied by stars, their magnetic fields or their planetary systems. To get to interstellar space, Voyager 1 had to pass through the heliosphere, a vast bullet-shaped bubble surrounding the sun (see above illustration) that is created by the sun’s own magnetic field, according to NASA. The heliosphere is filled with charged particles from the sun’s solar winds, a medium that scientists refer to as plasma. Before Voyager 1 broke into interstellar space, it and Voyager 2 were both travelling through the narrowest and outermost part of the heliosphere known as the “heliosheath.” Less than two months ago, NASA scientists expressed doubt that Voyager had reached interstellar space. “If you looked at the cosmic ray and energetic particle data in isolation, you might think Voyager had reached interstellar space,” Voyager project scientist Ed Stone told “Science” in late July. “But the team feels Voyager 1 has not yet gotten there because we are still within the domain of the sun’s magnetic field.” The cosmic ray and energetic particle data refers to readings that Voyagers 1 and 2 send back to Earth. The plasma in the heliosphere and in interstellar space have different densities. When solar flares send ripples through the plasma, causing the particles to vibrate, two antennae on Voyager pick up the frequency and transmit it back to Earth as an audible radio transmission. Every six months, scientists listen to the tone, played back to them on a tape. When the tone in the recording changed, the scientists knew that Voyager was now traveling through plasma of a different density, indicating that it had broken free of the heliosphere and into interstellar space.