Introduction to Space Radiation Biophysics
 

Manned space exploration started about three decades ago, and space radiation was soon recognized as a hazard for the crew. Monitoring of U.S. astronauts radiation exposure by NASA started during Project Mercury, and has continued through the current Shuttle Program. Radiation monitoring for Russian cosmonauts has been also performed routinely on Cosmos, Salyut and MIR.
The space radiation environment is significantly different from that experienced on Earth. Terrestrial radionuclides contribute to the natural radiation burden either by external exposure or by internal exposure following incorporation of the nuclides. Space radiation consists primarily of high-energy charged particles, such as protons, helium, and heavier ions, originating from several sources, including galactic cosmic radiation, solar flares and Van Allen belts. High-energy electromagnetic radiation and neutrons have been measured on spacecrafts, too. Space radiation is very penetrating, and shielding can reduce, but not eliminate, crew exposure to ionizing radiation.
Due to the unavoidable presence of ionizing radiation in space, astronauts are classified as radiation workers. In fact, dose rate in space is considerably higher than on Earth. Radiation dose absorbed after one day in space is close to the dose received by all natural sources, excluding radon, in one year on Earth. Large solar particle events can considerably increase this dose, and could even be life threatening for an inadequately protected crew.
For short-duration mission under the protection of Earth' geomagnetic field, such as Shuttle flights, accumulated dose for crew is still very low. So far, radiation exposures from medical procedures account for the majority of cumulative dose in astronauts when compared to space radiation exposures. However, concern about space radiation is rapidly increasing because duration of astronaut's sojourns in space is becoming considerably longer. The International Space Station is now under construction.

ISS will be the largest human base outside Earth ever, and its construction involves NASA, ESA, as well as Russia, Japan, and Canada Space Agencies. Astronauts are supposed to live and work on ISS for longer and longer periods. In addition, manned interplanetary missions are planned in the next years. During interplanetary missions, crews do not benefit from protection by cosmic rays and solar flares afforded by Earth's geomagnetic field. A mission to Mars with current technology for spacecraft propulsion would lead to exceed astronaut exposure guidelines.

Monitoring of crew exposure is performed with different dosimeters. Before each flight, estimated doses calculated by mathematical models of the space radiation environment are provided for mission planning. Crew dosimeters and active radiation monitors are available during the flight. After the mission, crew dosimeters are analyzed and used for health risk assessment. Physical dosimeters and models of radiation exposures are fairly accurate. Nonetheless, it is generally believed that this information is still insufficient to estimate health risk to the crews. In fact, significant uncertainty about the biological effectiveness of the measured physical dose are present in space. Sources of uncertainty are outlined below.
a) Biological effects of swift heavy ions are not completely understood. HZE particles (particles with Z>2 and energy high enough to penetrate at least 1 mm of spacecraft) represent less than 1% of the flux of space particles, but up to 50% of the radiation dose. They induce extremely high ionization densities in cylindrical volumes, and can damage a number of contiguous cells along their tracks.
b) Astronauts are exposed to mixed radiation fields (photons, neutrons, light and heavy charged particles of different energies) in space. Interactions between different radiations can produce synergistic effects on the biological damage.
c) Variation in individual radiation response could cause a different risk for astronauts exposed to the same dose. Persons who are homozygous for ataxia-telangectasia are up to three times more sensitive to radiation than normal individuals. Analysis of groups of patients treated with radiation therapy suggest the existence of other groups of persons who are slightly more sensitive or resistant to ionizing radiation.
d) Impact of spaceflight environment on radiation response is still largely unknown. Work environment factors, in particular chemical contaminants, are of concern because of their possible interaction with radiation in the induction of somatic or genetic damage. Most important is the interaction between radiation and microgravity. It is well known that microgravity produce a large number of physiological and cellular alterations. However, influence of microgravity on organism response to radiation burden is still unclear.
The above-mentioned uncertainties on the biological effects of recorded radiation doses make necessary more research in the field of biological effects of space radiation.

These webpages will collect information concerning Space Radiation Health Projects in Italy. To learn more about NASA projects in Space Radiation Health, please download the NASA Space Radiation Health Strategic Plan below.