A crewmember of typical size requires approximately of
food,
water, and
oxygen per day to perform standard activities on a space mission, and outputs a similar amount in the form of waste solids, waste liquids, and
carbon dioxide. The mass breakdown of these metabolic parameters is as follows: of oxygen, of food, and of water consumed, converted through the body's physiological processes to of solid wastes, of liquid wastes, and of carbon dioxide produced. These levels can vary due to activity level of a specific mission assignment, but must obey the principle of
mass balance. Actual water use during space missions is typically double the given value, mainly due to non-biological use (e.g. showering). Additionally, the volume and variety of waste products varies with mission duration to include hair, finger nails, skin flaking, and other biological wastes in missions exceeding one week in length. Other environmental considerations such as radiation, gravity, noise, vibration, and lighting also factor into human physiological response in outer space, though not with the more immediate effect that the metabolic parameters have.
Atmosphere Outer space life-support systems maintain atmospheres composed, at a minimum, of oxygen, water vapor and carbon dioxide. The
partial pressure of each component gas adds to the overall
barometric pressure. However, the elimination of diluent gases substantially increases fire risks, especially in ground operations when for structural reasons the total cabin pressure must exceed the external atmospheric pressure; see
Apollo 1. Furthermore,
oxygen toxicity becomes a factor at high oxygen concentrations. For this reason, most modern crewed spacecraft use conventional air (nitrogen/oxygen) atmospheres and use pure oxygen only in
pressure suits during
extravehicular activity where acceptable suit flexibility mandates the lowest inflation pressure possible.
Water Water is consumed by crew members for drinking, cleaning activities, EVA thermal control, and emergency uses. It must be stored, used, and reclaimed (from waste water and exhaled water vapor) efficiently since no on-site sources currently exist for the environments reached in the course of human space exploration. Future lunar missions may utilize water sourced from polar ices; Mars missions may utilize water from the atmosphere or ice deposits.
Food All space missions to date have used supplied food. Life-support systems could include a plant cultivation system which allows food to be grown within buildings or vessels. This would also regenerate water and oxygen. However, no such system has flown in outer space as yet. Such a system could be designed so that it reuses most (otherwise lost) nutrients. This is done, for example, by
composting toilets which reintegrate waste material (excrement) back into the system, allowing the nutrients to be taken up by the food crops. The food coming from the crops is then consumed again by the system's users and the cycle continues. The logistics and area requirements involved however have been prohibitive in implementing such a system to date.
Gravity Depending on the length of the mission, astronauts may need artificial gravity to reduce the effects of
space adaptation syndrome, body fluid redistribution, and loss of bone and muscle mass. Two methods of generating artificial weight in outer space exist.
Linear acceleration If a spacecraft's engines could produce thrust continuously on the outbound trip with a thrust level equal to the mass of the ship, it would continuously accelerate at the rate of per second, and the crew would experience a pull toward the ship's aft
bulkhead at normal Earth gravity (one g). The effect is proportional to the rate of acceleration. When the ship reaches the halfway point, it would turn around and produce thrust in the retrograde direction to slow down.
Rotation Alternatively, if the ship's cabin is designed with a large cylindrical wall, or with a long beam extending another cabin section or counterweight, spinning it at an appropriate speed will cause
centrifugal force to simulate the effect of gravity. If
ω is the
angular velocity of the ship's spin, then the acceleration at a radius
r is: Notice the magnitude of this effect varies with the radius of rotation, which crewmembers might find inconvenient depending on the cabin design. Also, the effects of
Coriolis force (a force imparted at right angles to motion within the cabin) must be dealt with. And there is concern that rotation could aggravate the effects of vestibular disruption.
Radiation protection Life support systems for long-term space missions will need to protect the crew from
space radiation, to reduce the risk of cancer development and safeguard astronaut health. Future missions may use radiation-resistant fungi-derived paints, to coat the walls of spacecraft. == Space vehicle systems ==