What are cosmic rays? Why bother?

I have always thought that the name "cosmic rays" is a little unfortunate. It is somewhat reminiscent of little green men and ray guns.   But cosmic rays could not be further from alien life forms.  They are simply a stew of atomic nuclei and electrons, accelerated up to an appreciable fraction of the speed of light.  They are responsible for the majority of the background radiation we Earthlings experience.  If you have heard that airline pilots experience more radiation than groundlings, it's true - cosmic rays from space are the reason. The atmosphere absorbs them, so there are more at the top of the atmosphere than the bottom.  Some scientists study the ones that make it all the way through the atmosphere--I even used to work on an experiment that looked at particles created by cosmic rays that could penetrate all the way through a mountain!-- but my current experiment is focused on detection of the electron component above the atmosphere.

Tycho's supernova remnant

Where do they come from and how do they get here?  The space between the stars is not really empty (although it is a far better vacuum than we humans can create on Earth in the laboratory).  It has a gas of atoms floating in it.  Sometimes those atoms are ionized, meaning that just one or two electrons have been removed from the atom.  This provides a "handle" for electric fields and magnetic fields to use to move the atom around. When a violent event such as a supernova shock passes through the gas, it strips off any remaining electrons, and accelerates the bare nucleus and free electrons to very high speeds, within a wee fraction of the speed of light.

Now begins a long and arduous journey from the source, where the cosmic rays (for these nuclei and electrons are now promoted to full-fledged cosmic rays) originate, to Earth, where we can detect them.  And herein lies one of my favorite aspects of cosmic rays: I will never go to other parts of our galaxy, but I can study other parts of our galaxy as it comes to us. Astronomers look at ethereal photons with their telescopes; I look at small pieces of extrasolar matter with mine.  The galaxy comes to me!

The journey is fraught with danger.  The path they take is determined by the weak magnetic fields present throughout the galaxy, which guide the cosmic rays hither and yon in a random walk through the galaxy. During their galactic stroll, the cosmic rays can run into unaccelerated atoms (the atomic gas between the stars) and smash into pieces.  They can also slowly lose energy by scattering off those same atoms, and even the magnetic fields.  If they lose too much energy, they never make it to Earth.

Although they are invisible to the naked eye (these aren't meteorites!) we have instruments that can "see" them. Since we want to look at the them before they hit the atmosphere, we need to get our instrument up above the atmosphere. Ideally that means a space vehicle like a satellite, but a satellite-based payload requires over a decade of planning and waiting in the queue for a slot. A great alternative is an extremely large high altitude helium-filled balloon. These can go into "near space" (99.5% of the way there) for a lot less expense, and with a much shorter queue. NASA's Long Duration Ballooning (LDB) program provides these technological marvels, and you can basically get a slot any time. Plus, balloons are way cool.