The DIAS-ESTEC Ultra-Heavy Cosmic Ray Experiment (UHCRE) flew on board the Long Duration Exposure Facility (LDEF) spacecraft in low earth orbit for 69 months. The experiment consisted of a large number (~2 0 0 ) of solid-state nuclear track detector stacks and its objective was to determine the charge abundance spectrum o f ultra-heavy (Z >70) galactic cosmic rays at energies of a few GeV/nucleon. Many hundreds of such ultra-heavy cosmic rays were detected, including 35 actinides (Z > 8 8 ), making this the largest sample of such data to date. An overview of our current knowledge of the ultra-heavy cosmic-ray flux is presented in Chapter 1. Charge, energy and isotopic spectra are examined and the clues they present to cosmic-ray origin discussed. Current theories of fractionation of the cosmic-ray source matter during transport and suggested sources o f such matter are outlined. The possible insights provided by ultra-heavy cosmicray measurements are then suggested. Chapter 2 discusses the configuration o f the UHCRE and the LDEF spacecraft. Solid-state nuclear track detectors, detector processing and data extraction are discussed. Chapter 3 contains information on detector calibration and error analysis and discusses a Monte-Carlo simulation o f the apparatus used to corroborate the error estimates. Chapter 4 describes the corrections performed on the raw data for detector bias, detector-processing anomalies and fragmentation within the apparatus. The final charge-abundance spectra are presented. Chapter 5 outlines the conclusions which can be drawn from the data, comparing the results with theoretical predictions and other spacebased observations. The experiment’s impact on theories o f the origin of the cosmic rays is discussed.