SPACECRAFT LIFE-SUPPORT SYSTEMS The earliest spaceflights of experimental animals were sufficiently short to require only
very simple life-support systems. In the 1950s, capsules with animals were launched on suborbital flights of 10 to 15 minutes at altitudes of about 150 to 400 km (about 90 to 250 mi). Aside from maintaining cabin pressure and supplying oxygen, such craft had no life-support requirements. In November 1957 the first animal in orbit, the dog Laika, was kept alive in the Soviet Union's Sputnik 2 for 10 days with intravenously provided nutrition, while carbon dioxide was removed from the cabin by chemicals. Early Human Orbital Flights The first human orbital flights required life-support systems that lasted for several days. For simplicity and lightness American spacecraft first used pure oxygen cabin atmospheres at 0.34 bar of pressure. This eased problems with space walks but also significantly raised the danger of fire. (After a fatal on-ground fire in an Apollo spacecraft, the decision was made to use an oxygen-
nitrogen atmosphere.) Soviet space vehicles duplicated the oxygen-nitrogen air mixture at Earth's sea level. Carbon dioxide was removed by passing air through canisters containing lithium hydroxide (LiOH), to which the gas molecules adhered. Eventually the canisters had to be replaced. Food supplies were primitive and generally unappetizing, including the infamous "toothpaste tubes" of meat paste and the dehydrated plastic bags that required injection of water and long periods of manual kneading. Disposal of body wastes posed a challenge. Urine was collected by means of a lower-pressure funnel and eventually dumped into space. Feces were collected in plastic bags and stored for later medical study. Accidents and spills were frequent, while opportunities to wash and shave were meager. Later Systems When missions lasting months or even years began to be planned, heavier recycling equipment became more economical than a continuous flow of consumables. Aboard the U.S. Skylab space station, for example, carbon dioxide was removed by a system of alternating chemical banks that absorbed the gas. The banks were then heated to expel it into space. The 0.34-bar atmosphere consisted of 70 parts oxygen to 30 parts nitrogen. Psychological requirements for missions of several months led to the use of fairly normal kinds of food, although many items still required the addition of water. Beginning with Skylab and the Soviet Salyut space travelers had genuine toilets in which air flow was used to direct body wastes into collection devices. In addition, occasional showers were possible inside large sealed plastic containersÑalthough collection of the waste water was an engineering challenge. In the mid-1970s, Soviet cosmonauts aboard Salyut 4 began using dehumidifying equipment to reclaim exhaled moisture for reuse as wash water and, later, as drinking water. By the mid-1980s, the Russian space station Mir was testing a "urine still" that produced water clean enough to be electrolyzed into oxygen. Early in the 1990s, the Russians were briefly able to stop sending fresh water to their space station, since the water was being recycled, but high operating costs and equipment breakdowns soon ended this practice and expendable resupply became necessary again. Problems of EVA (Extravehicular Activity) In order for humans to work in open space, they must wear protective clothing and carry portable life-support systems. Such systems are designed for only six to eight hours of use at a time and are under stringent weight and power constraints. At a minimum, the oxygen partial pressure must be at least 0.2 bar. Depressuring quickly from a full 1-bar cabin to 0.2 bar, however, causes dissolved nitrogen to bubble out of body tissue and fluids. This leads to the bends, an extremely painful and potentially fatal condition. Thus either higher spacesuit
pressures, lower cabin pressures (with less nitrogen), or longer transitional peods are required. Higher spacesuit pressures create "ballooning" forces, which must be overcome by the crew member's muscles or by intricate mechanisms in the suit's joints. Russian spacesuits now operate at about 0.4 bar, high enough to avoid medical problems but low enough to provide sufficient mobility. American spacesuits operate at pressures of 0.28 bar, which requires crew members in a 1-bar cabin to breathe pure oxygen for four to six hours prior to an EVA in order to purge nitrogen gas. When possible, the entire spacecraft cabin is lowered to 0.7 bar the day before a planned EVA. This makes possible a further depressurization for the EVA with only a short oxygen-breathing period. Future spacesuits may use sophisticated joints to allow pressures of up to 0.5 bar and thus a still simpler preparation routine. Maintaining proper thermal conditions can be difficult. On the one hand, metabolic heat during strenuous activity has to be removed by a plastic network of water tubes, or else the helmet faceplate may become fogged. On the other hand, heat may be needed for hands and feet in shadow or in contact with cold hardware. In general, food is not provided in a spacesuit, although a drinking tube is located within reach inside the helmet. Urine can be passed and stored, but no provision is made for feces beyond an emergency diaper. Degrees of Closure It might seem intuitive that the best closure level for a life-support system is 100%, but practical considerations argue against this.