ENERGETIC COSMIC RAYS MAY BE IRON NUCLEI. Cosmic rays are particles that come from far away (many likely to be of extra-galactic origin) and strike our atmosphere, where they engender a shower of new particles that show up in detectors on the ground. One of the biggest puzzles in cosmic ray physics is why there should be so many events with total energies above 100 EeV (10²⁰ eV). The main reason for this was the notion that if the primary particle were journeying from a distant galaxy, the particle's energy would be sapped by interactions with cosmic microwave background photons or with infrared background radiation (IBR). There is the additional problem of how cosmic rays with energies as high as 300 EeV could have been accelerated to such a degree if the first place (see Update 243). Floyd Stecker of the Goddard Space Flight Center has determined that a 200 EeV nucleus (starting out as an iron nucleus) could negotiate a 300-million-light- year journey through the IBR. The nucleus would partially disintegrate en route, but would still arrive at Earth with a potent energy. (Physical Review Letters, 2 March.)