A customized lightweight and low-power radiation micro-tracker has been constructed for the microsatellite RISESAT developed by Tohoku University for scientific experiments in space. The instrument is based on hybrid semiconductor pixel detector technology developed by the Medipix Collaboration. These Timepix detectors consist of an array of 256 × 256 (= 65k) pixels with 55μm pitch distributed over area of 2cm2. Each cell is connected to the corresponding pixel on the 300μm thick siliconsensor. Each pixel integrates electronics which enables counting of the number of particles, to measure the energy deposited or to register the time of interaction of ionizing particles per-pixel. Two independent detector layers operate as a radiation micro-telescope and provide high resolution and high sensitivity tracking and directional detection of energetic charged particles (electrons, muons, pions, protons, light ions up to heavy and relativistic ions). Also X-rays and even neutrons can be detected. The pixel detectors provide noiseless detection of single radiation quanta, measure their energy, position, direction of trajectory and time of interaction.,Different types of particles and interactions can be identified and distinguished thanks to the detector’s high spatial granularity.