Number 70 (Story #2), March 5, 1992 by Phillip F. Schewe and Ben Stein THE HADRON-ELECTRON RING ACCELERATOR (HERA) in Hamburg, Germany will be ready to do physics later this spring. At the 6.3-km-circumference HERA 30-GeV electrons will collide with 820-GeV protons. Because the electrons do not feel the strong nuclear force, they serve as excellent probes of the proton's inner structure; they are able to penetrate deep inside the proton, often so deep that they scatter not from the proton itself but from one of its constituent quarks. Two parameters, the total center-of-mass energy and the momentum transfer, will be at least a factor of ten higher at HERA than at other accelerators, which must depend on fixed-target experiments for exploring lepton-hadron collisions. Larger momentum transfers allow scientists to probe the distribution of matter inside protons down to very small distance scales, in HERA's case down to 10-17 cm, 10,000 times smaller than the size of the proton. (Physics Today, March 1992.)