Each of the nine passive dosimetry packets worn by the crew of Apollo VIII contained two 190 μ layers of Lexan polycarbonate plastic. The plastics served as threshold type, heavy charged particle track detectors. Upon recovery the detectors were fmt sensitized using the recently discovered photo-oxidation technique, and then etched to reveal tracks of particles whose restricted energy loss rate exceeded 2 × 102MeV cm2/g. A flight average of about 1 tr/cm2was observed. In tracing tracks through from one layer to the next it is possible to say whether a particle stops in the plastic, the sense and direction of travel, and in many cases make estimates ofzto ± 1 unit of charge. All the particles observed had a charge in excess of 10, 30 per cent came to rest in the plastic. The measurements demonstrate that plastic track detectors can be conveniently and easily utilized in accurately monitoring the astronauts' high LET radiation exposure on extended space missions lasting up to several years.