Meteorites and space travel.

Meteorites represent a potential hazard to a pressurized space vessel. Of fundamental interest is the value of the probability that the skin of the vessel will be punctured by a meteorite. In case this probability is appreciable the problem of protection from meteorites becomes important. We shall assume: (a) That the space vessel travels in a part of the solar system where the meteoritic frequencies and velocities approximate those at the earth. (b) That 4.5 X ~~~ fifth-magnitude meteors strike the earth daily and that the number increases by a factor of 2.51 per magnitude fainter (Watson). (c) That (with Opik) the total kinetic energy of a telescopic meteor is I /o.ooo6 the energy observed in the wave length region from 4500 to 5700 angstroms. (d) That the penetrating distance of a meteorite into a solid is equal to (extreme assumption) the length of a right circular cone of 600 total apex angle the volume of which in the solid can be heated and melted by the total kinetic energy of the meteorite. It follows that a spherical space vessel of 12 feet diameter covered with a -41-inch steel skin will be penetrated by a meteorite corresponding to an eighth magnitude or brighter meteor at a rate of once in 50 years. Such a meteorite weighs approximately a milligram. For thinner coverings the probability increases rapidly. Although the probability of meteor penetration is small, a simple protection can be provided other than by the avoidance of known meteor streams. Considerations of the conservation of momentum and energy show that when a meteorite collides with a sheet of thickness comparable to the meteorite's diameter the result is an explosion in which both the meteorite and the corresponding material of the sheet are vaporized and ionized at very high temperatures. Hence a "meteor bumper" consisting perhaps of a millimeter-thick sheet of metal surrounding the -41-inch skin of the space vessel at a distance of an inch would dissipate the penetrating power of meteorites several times larger than one corresponding to an eighth-magnitude meteor. Harvard College Observatory, Cambridge, Mass.