Ballooning For Cosmic Rays

Astronomers have long thought that supernovas are the source of cosmic rays, but there's a troubling discrepancy between theory and measurements. Could a balloon flight over Antarctica have shed new light on the mystery?

Hold out your hand for 10 seconds. A dozen electrons and muons just zipped unfelt through your palm. The ghostly particles are what scientists call "secondary cosmic rays" - subatomic debris from collisions between molecules high in Earth's atmosphere and high-energy cosmic rays from outer space.

Cosmic rays are atomic nuclei and electrons that streak through the Galaxy at nearly the speed of light. The Milky Way is permeated with them. Fortunately, our planet's magnetosphere and atmosphere protects us from most cosmic rays. Even so, the most powerful ones, which can carry a billion times more energy than particles created inside atomic accelerators on Earth, produce large showers of secondary particles in the atmosphere that can reach our planet's surface.

Where do cosmic rays come from? Scientists have been trying to answer that question since 1912, when Victor Hess discovered the mysterious particles during a high altitude balloon flight over Europe. Galactic cosmic rays shower our planet from all directions. There's no definite source astronomers can pinpoint, although there is a popular candidate.

"Most researchers are betting that cosmic rays come from supernova explosions," says Jim Adams of the NASA/Marshall Space Flight Centre. When massive stars explode they blast their own atmospheres into space. The expanding shock waves can break apart interstellar atoms and accelerate the debris to cosmic ray energies. Cosmic rays are subsequently scattered by interstellar magnetic fields -- they wander through the Galaxy, losing their sense of direction as they go.

"It takes an awful lot of power to maintain the galactic population of cosmic rays," says Adams. "Cosmic rays that lose their energy or leak out of the Galaxy have to be replenished. Supernovae can do the job, but only if one goes off every 50 years or so." The actual supernova rate is unknown. Observers estimate that one supernova explodes somewhere in the Galaxy every 10 to 100 years - just enough to satisfy the energy needs of cosmic rays.

But there could be a problem with the supernova theory, says Adams.