The period of time during which a meteoroid has been exposed to cosmic radiation in space; also known as cosmic-ray exposure age, or radiation age. This is usually the time between a meteoroid leaving its parent body (such as an asteroid) and arriving on Earth. Bombardment of the meteoroid by cosmic rays in space produces radioactive isotopes (such as helium-3, neon-21, and argon-38), or phenomena such as fission tracks, whose abundances can be used to estimate the exposure age. Typical exposure ages range from a few million years to a few hundred million years.