nevertheless means the loss of adaptation to earth gravity. The more complete the adaptation to weightlessness, the more difficult the return to earth. How dangerous the loss of adaptation to gravity would be to the astronauts remains to be determined with the help of more information gathered during long flights. 2. Radiation Exposure Another aspect of life in space which is not related to microgravity is exposure to radiation: cosmic rays, heavy ions and solar flare particles pose a significant hazard and might be an important constraint on long-term manned space activity [19, 20, 22]. The radiation environment encountered by a spacecraft in earth orbit is composed of galactic cosmic rays, trapped solar flare particles, charged particles in the radiation belts and secondaries such as proton recoils, neutrons and pions. The heavy nuclei (HZE particles) like iron provide the dominant contribution to the dose equivalent during exposures in space. The resulting injury from external radiation exposure depends on the total dose, the dose rate, the per cent of the body and the region of the body exposed. In the case of astronauts the entire body is exposed, the dose rate is variable with the environment and the total dose will increase with the increased length of flights, in particular flights going to Mars [18, 19]. The total dose delivered to the spacecraft depends on the altitude of the orbit (the higher the orbit the higher the dose) and the inclination of the spacecraft on its orbit (the higher the inclination the lower the dose) [22]. The basic unit of radiation exposure is the roentgen, which measures the ability of X-or gamma rays to bring about ionization. Additional units have been developed to express radiation dose. The rad is the unit of absorbed dose, (1 rad =100 ergs of energy absorbed per gram of tissue) and the rem is the unit of dose equivalent and is the product of the absorbed dose (in rads) by the quality factor (QF) and the modifying factor N. The quality factor is intended to allow for the effect of microscopic differences in the distribution of the absorbed energy; the modifying factor is available to take into account differences in the absorbed dose rate, in any fractionation in the receipt of the dose, or in the manner, uniform or non uniform, in which a radionuclide is deposited within a given body tissue. For external sources of X and gamma radiation, the factors Q and N are equal to one. The general approach to exposure control is to assume that radiation damage has no threshold, or in other words that any dose of radiation results in some damage to the body [21]. The recommendations for radiation safety on earth are to limit radiation exoposure by artificial sources to a maximum of 350 mrems per year for the general population: this rule does not apply to radiation workers who usually have exposures 5 -10 times higher, but is meant to protect the genetic quality of the world's population. We know the effects on the organism of high doses of ionizing radiation. Several studies have been conducted to evaluate the effect of HZE particles, which cause the dominating dose from cosmic rays, and which have by far the highest relative biological effectiveness for radiation damage. Solar flares are relatively rate events which may occur two or three times during a three-year flight. The absorbed dose for solar flare high energy protons can approach 1000 rem over the duration of the flare (2-3 days). A dose over 800 rem is always fatal, even with the best available treatment; doses between 400 and 800 rem cause severe illness (500 rem is lethal in 50% of cases). Measures performed during the Spacelab-1 flight showed an overall absorbed dose of about 150 mrems for a flight of 10 days at an altitude of 241 km. The total dose of
RkJQdWJsaXNoZXIy MTU5NjU0Mg==