space travel Manned space flight commenced with the orbital flight by Gagarin in April 1961. By 1996 over six hundred human beings had flown in space for periods of time varying between a few hours and 438 days. Space travel is set to become more frequent in the twenty-first century, although the financial cost of the vehicles and their propulsion systems will limit the number of individuals who will experience the excitements and wonders of space flight. Flight in space is characterized by the absence of gravity, the absence of the Earth's atmosphere with the associated absence of protection against solar and cosmic radiation, and the vast distances which have to be traversed to reach other planets.
A spacecraft must be accelerated to a velocity of 8 km per second (17 900 mph) to enter into orbital flight around the Earth and to a velocity of 11.6 km per second (25 900 mph) to escape Earth's gravitational field and travel into outer space. Similar changes in velocity are required to return safely to the surface of the Earth. The velocity required to enter orbital flight can be attained by suitable combinations of acceleration (G) and time (828 G seconds are required to reach orbital velocity). In practice, the pattern of acceleration is determined by the design of the rocket motors. Development of the latter has resulted in a progressive reduction in the maximum acceleration from 5–7 G in early flights to 3 G in the Space Shuttle. The effects of the G on the circulation are minimized by placing the long axis of the body of the astronaut at right angles to the direction of travel of the space vehicle. The accelerative forces associated with returning to the Earth from orbital flight are about half those of launch into orbit, and the physiological effects are minimized by appropriate orientation of the spacecraft.
An artificial gaseous environment must be produced with the spacecraft, and its cabin must be sealed to a very high standard to avoid the loss of gases to the vacuum of space. Although 100%
oxygen at a pressure of 1/3 atmosphere was employed in early American space vehicles, air at 1 atmosphere is the environment of choice in order to avoid the deleterious physiological effects of prolonged exposure to 100% oxygen and the high risk of fire associated with an oxygen atmosphere. Air at a pressure of 1 atmosphere does, however, have the disadvantage that, with present designs of space suits, 100% oxygen must be breathed for several hours before undertaking extra-vehicular activity in order to avoid
decompression sickness. The atmosphere of the sealed cabin must be conditioned continuously by the removal of carbon dioxide and the addition of oxygen. The heat generated by occupants and some of the equipment within the cabin is removed by cooling the air and removing water vapour. The environmental control required in the cabin of a spacecraft has also to be provided in individual space suits. Inflation of a space suit to a pressure of 1 atmosphere is associated with an unacceptable loss of mobility due to the ‘stiffness’ of the suit. Suit pressures of the order of 0.3 to 0.5 atmosphere are generally employed, with an internal suit atmosphere of 100% oxygen. Heat produced by the body and by incident solar radiation is removed by means of a liquid conditioning garment worn beneath the suit. The heat gained by the fluid circulating through the garment is rejected to space by evaporation of a suitable refrigerant.
Microgravity
The principal effects of the
microgravity (also termed
weightlessness) of space travel arise in the vestibular, cardiovascular, and musculo-skeletal systems. Space sickness, which comprises nausea, a general feeling of malaise, and often actual vomiting, occurs early in a flight and is greatly aggravated by rapid movements of the head. Adaptation occurs over the first few days of flight and the condition seldom persists for longer than 5–6 days. Like other forms of
motion sickness, space sickness is due to a conflict of sensory information from the
vestibular system and eyes to the
central nervous system. The abnormal sensory input in space sickness is the absence of signals from the
otolith organs. The incidence and severity of the condition is reduced by avoiding head movements, especially rapid ones. The use of anti motion-sickness drugs during the first few days of flight is of considerable value.
In contrast to the effects of +G
z (see
G and G-suits), microgravity abolishes all the hydrostatic pressure gradients within the circulation so that blood and tissue fluids move from the lower parts of the body to the head and chest. It is estimated that the volume of blood and tissue fluid in the lower limbs is reduced by 1.0–1.5 litres within the first 24 hours of exposure to microgravity. The veins of the head and neck are distended, and there is puffiness of the face and congestion of the nasal passages. The distension of the heart caused by the central movement of blood stimulates an increase in the excretion of urine, which in turn produces a reduction in the total volume of blood in the body. The circulation is well maintained whilst in microgravity. The most important effect of the circulatory changes induced by microgravity, is, however, a reduced tolerance to the shift of blood to the feet which occurs on return to the 1 G environment on the surface of the earth. The astronaut may feel faint and indeed a few have fainted on standing upright after landing.
The muscular effort required to maintain posture and to carry out physical tasks is reduced in microgravity. This reduction in muscle activity gives rise to a progressive loss of muscular strength and wasting (atrophy) of muscles over days and weeks in much the same manner as a long period of rest in bed. These effects of microgravity can be reduced by special regimes which exercise the muscles, especially those of the trunk and lower limbs. Cycling on an ergometer has been widely used during prolonged space flights.
Of even greater concern in long duration spaceflight is the loss of
minerals from
bone produced by microgravity. The maintenance of normal bone structure and calcification of bone depend upon the physical stresses which living and working in a 1 G environment impose upon the bones. The greatly reduced forces in microgravity lead to
demineralization of the bones. There is an increase in the loss of
calcium from the body and a decrease in the absorption of calcium from food. Studies conducted to date suggest that the loss of calcium, bone structure, and bone strength continue for as long as the individual is exposed to microgravity. The concern is the risk of fracture of bones on return to normal gravity, or even in spaceflight when the duration of the flight extends into many months. Although exercises which place high stresses on bones may retard the demineralization process, the effect of microgravity upon bone is probably an important determinant of the time which an individual may spend in space.
Cosmic radiation
Very little of the ionizing radiation present in space reaches the surface of the Earth, owing to the shield provided by the atmosphere and the magnetic field of the Earth. Once outside this protection a space vehicle is exposed to galactic radiation and to radiation from the sun. Furthermore, the Van Allen belts of high energy particles which lie between 250 and 44 000 km above the surface of the Earth form a potential hazard on missions deep into space. Most manned space flights to date have occurred at altitudes well below the inner Van Allan belt. The damaging effect which ionizing radiation has upon living matter varies with the nature and dose of the radiation, from damage to blood-forming organs to prolonged vomiting and nausea, to incapacitation and death within a few hours. At present the cumulative dose of radiation to which astronauts can be exposed over their total career in space is limited by considerations of damage to the most sensitive tissues such as the bone marrow, the lens of the eye, and the skin. The dose of radiation received by individual astronauts is monitored routinely in space flights. The effect of cosmic radiation is probably the principal hazard for prolonged interplanetary space flight. There is a limit to the protection which can be afforded by either passive or active shielding against the ionizing radiation of space.
John Ernsting
Bibliography
Ernsting, J. and and King, P. (1988). Aviation medicine, (2nd edn). Butterworth-Heinemann, Oxford.
Nicogossian, A. E.,, Leach Huntoon, C.,, and and Pool, S. L. (1993). Space physiology and medicine, (3rd edn). Lea and Febiger, Malvern, Pennsylvania.
See also
G and G suits;
radiation, ionizing.
