The Ariel VI satellite carried, as a major part of its payload, an instrument designed by the Bristol University group to detect heavy primary cosmic rays. Owing to the extreme rarity of the heaviest of these particles, an unusual spherically symmetric design was chosen, thus maximising the aperture factor for a given size. This design, which included gas scintillator, plastic Cherenkov and plastic scintillator elements, is described, and the performance of the detector during 2 1 2 years in orbit is discussed. In this time no significant change was seen in the gas scintillation efficiency after correction to a standard gas temperature of −19°C. A decrease of 9% in both plastic Cherenkov signals and gas signals during the first 140 days in orbit is interpreted as due to a decrease in transmissivity of light guides and optical couplings. Consistent variations in gas scintillation efficiency of about −0.8% for each degree temperature increase in the range −19°C to −3°C were seen during all-sun periods. The signals obtained from the plastic scintillator element showed a saturating exponential increase, of total magnitude 19% and time constant 86 d. This is consistent with a scintillation efficiency which improves as the plastic loses adsorbed gases from its surface.