One way to determine the number of larger impactors and the potential impact threat to Earth is to observe the Moon. Using a system developed under an ESA contract, the Greek NELIOTA project has begun to detect flashes of light caused by small pieces of rock striking the Moon’s surface. NELIOTA is the first system that can determine the temperature of these impact flashes. The size and mass of the impacting object can be estimated from the brightness of the flash. See http://sci.esa.int/solar-system/59165-new-neliota-project-detects-flashes-from-lunar-impacts/
