No one seems to have been shouting too loudly about NASA’s IBEX probe. In fairness I suppose, it isn’t going to be looking at any planets or surveying any galaxies. From an astrochemical perspective though, IBEX is far more exciting than satellites like Fermi — and that’s saying something!

Because everyone likes acronyms these days, IBEX stands for Interstellar Boundary Explorer, and it’s objective is to explore the very edge of the solar system, where interstellar space begins.

As any astrochemist will happily tell you, despite being technically a vacuum, interstellar space is far from empty. In fact, it’s awash with all kinds of molecules, ions, dust grains and other lovely things that we astrochemists try very hard to identify.

So to explain exactly what IBEX will be looking at, here’s a handy little picture…

The Sun, just like all other stars, has a stellar wind (or, when talking about the Sun, a Solar wind). It’s constantly flinging streams of charged particles in all directions. The Solar wind is powerful enough that it actually blows a bubble in the interstellar medium, called a heliosphere. Some parts of the heliosphere are pictured above. For a start, it’s big. Very big. Heading outwards, well past the Kuiper Belt, the first thing you reach is the Termination Shock. This is where the Solar wind begins to slow down significantly. It starts to compress and heat up. Incidentally, the Termination Shock varies. The more sunspots and flares the Sun is giving off, the stronger the Solar wind, so the further away the Termination Shock. Voyager II actually crossed the boundary 5 times!
(It recorded some data too…)

Cross the Termination Shock and you find yourself in the Heliosheath. This is where the Voyager probes now lie. Their current aim isto study the Heliosheath itself. It’s a turbulent region, full of swirling gasses and plasmas, heated by the collision between the Solar wind and the Interstellar Medium (ISM). Though much is still unknown, it’s thought that the heliosheath is drawn into a long teardrop shape, due to the Sun’s orbit around the galaxy. The ISM pushes harder against one side, causing an elongated shape.

Finally, as the Solar wind becomes too weak to push back the ISM, you reach the Heliopause, the actual boundary of the solar system, and the edge of true interstellar space. As the Sun plows it’s way around the galaxy though, it has one final effect. Stars create Bow Shocks as they plough their way through interstellar space, exactly the way a ship’s bow cuts through water. It’s not very obvious in The Sun’s case, but it’s been observed in several other stars. The effect’s a lot more obvious if the star happens to be travelling through a nebula (like LL Orionis here)…

IBEX launches on October 19th. Let’s hope it finds some interesting things!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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