The performance of materials exposed to the radiations of space becomes important in the design of satellites and deep space probes. For practical purposes, space will be considered to begin at about 160 km (about 100 miles) above the Earth's surface. This is roughly the perigee of the lowest practical orbit for an Earth satellite. At several thousand kilometers above the Earth, the influence of the Earth's atmosphere becomes small, and the concentration of gas molecules approaches that of interplanetary space. Below these altitudes the perturbing influence of the Earth affects the characteristics of space.