The Space Environment
The vacuum conditions encountered in space required the encapsulation of apparatuses and passengers in a space vehicle or else the special and expensive design of equipment that could work without an air environment. The cooling of electronics systems became a problem, and moving parts required special lubricating systems because they otherwise tended to stick together when operating in space.
The free fall of satellites in low Earth orbits created the condition commonly called zero gravity. Technically, this term is a misnomer. The force of gravity in low Earth orbits is scarcely diminished from that experienced at the Earth's surface; it is the motion of the satellite that results in the effect of weightlessness. The term zero-G has passed into common usage, however, and is going to remain. Because slight accelerations actually do occur even on a satellite - due mainly to air drag and satellite motion - the more recent term microgravity has generally been adopted as well.
Unfiltered solar radiation can cause illuminated portions of a spacecraft to rise to high temperatures. Meanwhile, shaded portions of the craft will radiate their warmth into space and cool below the freezing point of common fluids such as water and storable rocket fuels. All such fluid containers and lines are commonly equipped with electrical heaters, while overall temperatures are moderated by rotating the spacecraft along an axis perpendicular to the spacecraft-Sun line. This is known as passive thermal control or, more colorfully, "barbecue mode." Unmanned spacecraft to the inner planets must be equipped with parasols to reflect away unwanted solar heat. Those sent to the outer solar system - or to the Moon's surface, with its two-week-long nights - have often used radioisotope heaters.
Radiation effects on spaceflights also took some time to appreciate. Satellites in LEO are protected by the magnetosphere from solar charged particles and from a large percentage of the cosmic rays arriving from outer space. Vehicles operating at GEO or on interplanetary missions, however, receive the full force of these radiations. Cosmic rays have been known to penetrate integrated circuits in spacecraft autopilots and to alter data and commands. A space version of static electricity has built up on other space vehicles during solar storms, resulting in electrical sparks that caused severe problems in onboard electronics. Experienced design of such systems has reduced the effects of these influences.
The danger from micrometeorites, on the other hand, has proved to be slight. Although numerous impacts have been recorded - and, on at least one occasion, actually heard by an orbiting crew - no spacecraft is known to have been seriously damaged by such particles. Debris from other artificial satellites appears to be increasing as a significant danger, however. By 1996 two satellites are believed to have been destroyed by collisions with "space junk," and from time to time Space Shuttle missions have to change course to avoid predicted near-misses with larger pieces of junk.