Over 100 cosmic ray particles pass though our bodies every second. Some of these have enormous energies, way above energies accessible using man-made particle accelerators.

Many of them have travelled millions of light years across the Universe, and they bring with them clues about exotic astronomical systems like neutron stars, supernovae, quasars and black holes.

Roger Clay and Bruce Dawson are leading experts in the field of cosmic ray reseach and in Cosmic Bullets they point out that despite a century of intensive research, it is still far from certain where most cosmic rays come from or how they were created. Nevertheless, the little-known story of their discovery and subsequent study holds many intriguing twists and wonderful surprises.

The results from these observations have been so exciting and intriguing that they have challenged some cherished beliefs in particle physics and astronomy. Particular mystery surrounds the nature of the highest energy particles found recently - particles that by rights couldn't even get here. PAUL DAVIES

The titles in the series Frontiers of Science describe current research and developments in selected topics. The series is edited by Professor Paul Davies.