Cosmic rays are the only sample of matter from distant regions of the galaxy, and possibly elsewhere in the Universe, that can be directly observed by space experiments in the Solar System.  This high energy matter consists of atomic nuclei that travel at speeds very close to the speed of light.  In fact, the highest energy cosmic rays observed are more that 10,000,000 times more energetic than those that can be produced in the largest particle accelerators on Earth.  The cosmic ray matter includes electrons, the natural elements from Hydrogen and Helium to Iron and beyond as well as anti-matter in the form of positrons and anti-protons.  The cosmic rays play an important part in galactic dynamics and can be used to probe astrophysical conditions throughout our galaxy.  Cosmic rays appear to gain their very high energy as the result of supernova explosion shock waves traveling through interstellar gas.  The supernova shock wave acceleration model makes specific predictions about the cosmic ray energy spectrum as a function of elemental charge.  ATIC is designed to measure the energy spectrum of individual cosmic ray elements in order to study the validity of this model.

The Crab Nebula

The Crab Nebula is a supernova explosion remnant.  The nebula is 6000 light years from Earth and ancient Chinese astronomers observed the explosion in 1054 AD.  In the center of the nebula is what is left of the star that exploded.  This neutron star or pulsar is spinning so fast that it emits pulses of radiation 30 times a second.  We know that particles like electrons are being accelerated to high energies in supernova remnants like this one because we can see their effect in the form of x-rays, gamma-rays and synchrotron light.

Cosmic Rays are a part of galactic dynamics

Our galaxy is a dynamic, turbulent place; Gas clouds condense under the force of gravity and form stars;  During the life of these stars fusion reactions form complex, heavy elements; Massive stars generate strong stellar winds that, along with supernova explosions, distribute matter back to interstellar space and generate turbulence; Eventually massive star use up their fusion "fuel" and collapse leading to supernove explosions; The shock waves driven by these explosions accelerate matter to speeds very close to the speed of light resulting in what we call cosmic rays.

Astrophysics questions :

How do cosmic rays gain their enormous energies?
Are Supernova Remnants the site of particle acceleration?
Is there evidence for more than one cosmic accelerator?
How do high energy particles move through the galaxy?

Experiment Goals:

Obtain energy spectra for individual elements of a broad energy range with a single instrument.
Discover “breaks” or “bends” in the spectra.
Measure the energy dependence of the H/He ratio.
Determine the spectral differences between elements.
Obtain the composition of the cosmic ray matter.