Electron microscope photographs of extraterrestrial particles

With suitable collection methods it is possible to distinguish particles resident in the stratosphere from contaminants and to identify those of extraterrestrial origin. A catalogue of physical appearances under an electron microscope is presented which shows the most common types of such particles encountered in about 40 collections made from balloons in the course of three years. We conclude that small extraterrestrial particles are most commonly fragile low-density assemblages of still smaller particles, frequently composed of crystalline material and often associated with liquid or solid volatile components. It is suggested that these are of cometary origin.