The acceleration experienced by charged particles in space (such as occur in cosmic radiation), which causes them to gain energies much greater than thermal energies. There are two types of Fermi acceleration. In first-order Fermi acceleration, the acceleration is due to shock waves. In second-order Fermi acceleration, it is due to collisions with interstellar plasma clouds. Second-order Fermi acceleration was proposed by Enrico Fermi in 1949, with first-order Fermi acceleration being proposed by a number of physicists in the 1970s.