Far from the protective embrace of the Sun, the edge of our Solar System would seem to be a cold, empty, and dark place. The yawning space between us and the nearest stars was for a long time thought to be a frighteningly vast expanse of nothingness.
Two spacecraft, built and launched in 1970s, have for the past few years been beaming back our first glimpses from this strange region we call interstellar space. As the first man-made objects to leave our Solar System, they are venturing into uncharted territory, billions of miles from home. No other spacecraft have travelled as far.
And they have revealed that beyond the boundaries of our solar system lies an invisible region of chaotic, frothing activity.
“When you look at different parts of the electromagnetic spectrum, that area of space is very different from the blackness we perceive with our eyes,” says Michelle Bannister, an astronomer at the University of Canterbury in Christchurch, New Zealand, who studies the outer reaches of the Solar System.
“Magnetic fields are fighting and pushing and tied up with each other. The image you should have is like the plunge pool under Niagara Falls.”
Instead of tumbling water, however, the turbulence is the result of the solar wind – a constant, powerful stream of charged particles, or plasma, spraying out in every direction from the Sun – as it crashes into a cocktail of gas, dust, and cosmic rays that blows between star systems, known as the “interstellar medium”.
The heliosphere is also unexpectedly large, which suggests that the interstellar medium in this part of the galaxy is less dense than people thought. The Sun cuts a path through interstellar space like a boat moving through water, creating a “bow wave” and stretching a wake out behind it, possibly with a tail (or tails) in shapes similar to those of comets. Both Voyagers exited through the “nose” of the heliosphere, and so provided no information about the tail.