Thursday, November 25, 2010

Honk If You Love the Heliosphere

I’ve heard that there’s a point, marked by turbulence, that marks the edge of the solar system as a satellite passes out of it. What causes the turbulence? What is out there to be turbulent? ~Matt

Racing from the Sun in all directions at about a million miles per hour, the solar wind is a stream of electrically charged particles. The solar wind, carrying with it the Sun’s magnetic field, forms an enormous magnetic “bubble” called the heliosphere that encases the solar system. This protective bubble shields our solar system and planet from (some but not all) harmful cosmic rays, high-energy particles traveling through space. We heart the heliosphere!

The transition zone where the solar wind meets the interstellar medium, the thin gas and dust that exists between stars, is called the heliopause. Here at the outer edges of the heliosphere, the solar wind mixes with the interstellar medium; this interaction between two different densities and pressures creates turbulence.

The Heliosphere and the Voyager Spacecrafts
Courtesy NASA/JPL-Caltech

When the solar wind approaches the heliopause, it slows very abruptly, causing a shock wave to form. The shock wave is called the termination shock, and this boundary is marked by dramatic changes in magnetism and temperature.

NASA’s Voyager 2 spacecraft crossed the termination shock in 2007; Voyager 1 had crossed it three years earlier in 2004. And here’s a funny twist: Voyager 1 only had to cross the termination shock once; Voyager 2 had to cross it five times! This occurred because the heliosphere bubble flexes in response to solar flares and other ejections of material from the Sun. In this case, the termination shock became a moving finish line.

Voyager 1, launched in 1977 and now ten-billion-plus miles from Earth, is the most distant of all active spacecraft. Voyager 2, which launched a few weeks earlier than Voyager 1, is only eight-billion-plus miles from Earth. This lag exists because Voyager 2 took a little planetary side tour. It began heading out of the solar system nine years later than Voyager 1, after becoming the first spacecraft to observe Uranus and Neptune.

In about five years, Voyager 1 will leave the heliosphere behind and enter interstellar space; Voyager 2 a few years after that, each spacecraft heading in a different direction. What new wonders will these stalwart space voyagers encounter, in the realm beyond the Sun? Stay tuned.