Cosmic dust

On its journey around the sun, the earth constantly gathers extra-terrestrial material in the form of atoms, dust and larger bodies sometimes weighing in excess of 10,000,000 tonnes. Naturally, the number of objects decreases with increasing weight. Some 10 atoms of galactic cosmic radiation hit the earth per cm2 and second, the major part of which is attributable to the solar wind. Approximately 1 piece of fine, interplanetary dust settles per m2 per day. Meteors (from particles ranging in size from one millimetre to a centimetre) occur on average every 30 seconds and meteorites fall only a few times per year. Collisions of large meteoroids with the earth, creating craters such as the one in Arizona (where the projectile was 100 m in diameter and the dimension of the crater caused is 1,000 m), can be expected every 10,000 to 20,000 years. Global catastrophes however, which would be caused by objects exceeding a diameter of 5 km, can only be expected once every 20 to 30 million years. The mineralogical composition changes with the mass of the objects. The main mass of the daily infall (~ 100t/day, ~ 40 000 t/year) is composed of pieces with a mass of approximately 10-5 g (or ~ 0.2 mm diameter). These pieces are called "micrometeorites". They can be retrieved from the ice at both polar caps.