Curiosity itself doesn't mind radiation all that much. But the human explorers we plan to one day send to Mars might be a little more picky about the stuff. So as one of the few tools sent to Mars to prepare for human exploration, the Radiation Assessment Detector (RAD) has an important job. About the size of a small toaster, the device will look into the Martian atmosphere and use a stack of silicon detectors and a crystal of cesium iodide to measure cosmic rays and solar particles. As high-energy charged particles from the atmosphere head through the detectors, they produce electron or light pulses, allowing the RAD to determine their energy. The process could also tell us more about how radiation might have once hindered the development of life on Mars.