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Cosmic Rays - Samples of matter

Crab Nebula Galactic cosmic rays (GCRs) are the high-energy particles that flow into our solar system from far away in the Galaxy. GCRs are mostly atomic nuclei from which all of the surrounding electrons have been stripped during their high-speed passage through the Galaxy. Cosmic rays provide one of our few direct samples of matter from outside the solar system. The magnetic fields of the Galaxy, the solar system, and the Earth have scrambled the flight paths of these particles so much that we can no longer point back to their source in the Galaxy. But the "composition" of GCRs can still tell us a lot about their sources and their passage through the Galaxy. They have probably been accelerated in the tremendous blast waves (shocks) that occur in supernova remnants such as the Crab nebula (pictured at left).

All of the natural elements in the periodic table are present in cosmic rays, in roughly the same proportion as they occur in the solar system. But detailed differences provide a fingerprint of the cosmic ray's source. Measuring the quantity of each different element is relatively easy, since the different charge of each element's nuclei give very different signatures. Harder to measure, but a better fingerprint, is the isotopic composition (nuclei of the same element but with different numbers of neutrons). To tell the isotopes apart involves in effect weighing each atomic nucleus that enters the detector.

How Do You Weigh a Particle Moving at Half the Speed of Light?

What Else Makes Observing Cosmic Rays Hard?

Tell Me About Flying Cosmic Ray Detectors On Balloons

Cosmic Ray Telescopes on ACE

The Advanced CompositionExplorer (ACE) ( was launched on a Delta II rocket in 1997. It is now orbiting the Earth-Sun libration point, L1, studying energetic particles coming from the Sun, interplanetary space, and the distant Galaxy. Of the nine instruments on board, the Cosmic Ray Isotope Spectrometer (CRIS) is specifically designed to look at Galactic Cosmic Rays.

Tell me more about the CRIS instrument on ACE

Future Projects

The CRIS instrument on ACE will measure the fingerprints of galactic cosmic rays very well up to iron and nickel (the 26th and 28th elements of the periodic table). The next steps will be to study the much rarer elements heavier than nickel, and to study cosmic rays at higher energies. An experiment, ACCESS (, is planned for the Space Station to study both these questions. Beyond that, an experiment, OWL (, is being designed that will have a pair of telescopes in orbit using the atmosphere to detect the very highest energy cosmic rays, individual particles with more kinetic energy than a Nolan Ryan fastball. An ambitious detector system like this is needed because fewer than a thousand of these particles hit the whole Earth per day.

Thank you to Eric Christian for contributing to this article.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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