Abstract A stack of CR-39 plastic track detectors with a time resolution system was exposed to the cosmic radiation aboard Spacelab-1. The impact time of the incoming particles was determined with an accuracy of about 2 minutes. For each time interval which is correlated to an orbit interval a minimum cutoff rigidity exists. In the energy range 50 – 150 MeV/nuc 22 “geomagnetically forbidden” particles were found with a rigidity less than the required cutoff value assuming fully stripped ions. These particles hit the stack mainly in the southern hemisphere at local night time. The charge resolution of the used CR-39 is 0.37 charge units for single cone measurements.