... Space Weather . ... Energetic Particles . ... Nuclear collisions produce gamma rays (after some intermediate steps) which, like light, move in straight lines, If only we could detect ultra-energetic gamma rays from the sky, we might pin-point sources of cosmic rays. ... They clearly indicate the presence of high-energy gamma rays, but for a long time there was no way of telling if those rays originated in distant space or (more likely) in nuclear collisions of cosmic ray ions in our own atmosphere. ...