During almost 50 years of space activities approximately 27,000 tons of man-made material re-entered into the Earth atmosphere at a mean rate of ∼600 tons per year. This compares with an estimated 40,000 tons of natural meteoroid material which reaches the Earth atmosphere each year. If this is so, why are meteoroids only dealt with at the end of this book? The answer lies in the size spectrum of meteoroids, which is dominated by particles with diameters of ∼200 µm and corresponding masses of ∼1.5 ×10 −5 g. The resulting risk to operational spacecraft is generally low as compared with space debris, in spite of much higher impact velocities of up to 72 km/s (corresponding to a heliocentric escape orbit of 42 km/s intercepted by the Earth orbit at 30 km/s).
THE DIVINE-STAUBACH METEOROID MODELThe meteoroid environment of the Earth is comparatively well known from groundbased photographic and radar observations of meteors, from space-borne detector experiments, from retrieved space surfaces, from meteoritic material captured by high-flying airplanes, and from micro-craters on returned lunar rocks. In contrast with the highly dynamic space debris environment, the dominant part of the meteoroid environment can be assumed to be sporadic and invariant with time. There are, however, seasonally recurring meteoroid stream events, associated with material in cometary orbits, which can give noticeable contributions, particularly in the case of meteoroid storms. The topic of cosmic dust and meteoroid research has many interesting facets, and it deserves more attention than can be given in this brief summary. A comprehensive overview of the current state of research and its historic roots is provided in .During the Apollo Program, meteoroids were identified as a potential risk,