Introduction 11 Radiation on Earth

Planets and moons of our solar systems are exposed to a complex radiation field of galactic and solar origin (Figure 7-02). Galactic Cosmic Radiations (GCR) originate outside of our solar system in previous

This chapter describes the radiation field in space and basic radiobiological mechanisms, and then concentrates on the specific biological responses resulting from exposure of biological systems to the space radiation environment. These responses include the biological effects of the heavy ions of cosmic radiation, and interactions of radiation with the other parameters of spaceflight, above all weightlessness. Comparisons are made with terrestrial findings using particle accelerators on Earth. The cataclysmic events such as supernovae explosions. When they enter our solar system, their energies must be high enough to overcome the deflection by the magnetic fields of the solar wind. Solar Cosmic Radiations (SCR) consist of two components, the low energy solar wind particles that flow constantly from the sun, and the highly energetic Solar Particle Events (SPE) that are emitted from magnetically disturbed regions of the Sun in sporadic bursts.

The surface of the Earth is largely spared from this cosmic radiation due to the deflecting effect of the Earth's magnetic field and the huge shield of 1000 g/m2 provided by the atmosphere. The terrestrial average annual effective dose equivalent from cosmic rays amounts to 0.30 mSv (for definition of units, see Section 3.3 in this Chapter), which is about 100 times lower than that experienced in interplanetary space. Natural radiations from terrestrial radioactive elements and diagnostic medical exposures to radiation increase the total annual effective dose equivalent to about 2.4 mSv. In areas of high concentrations of natural radionuclides, such as Kerala in India, annual dose values up to 13 mSv are reached. The maximum allowed annual dose for radiation workers amounts to 20 mSv. Ordinarily, aggregate background and diagnostic medical levels of radiation as well as the limits for occupational radiation exposure pose little risk to human health. Under this clement level of background radiation our biosphere has flourished since its beginnings about 4 billion years ago.

Energetic

Neutral' particles

^Magnetic solar wind-

Atmosphere

Electromagnetic sSjadktimi

Planet

Galactic Cosmic Rays

Local interstellar medium

Galactic Cosmic Rays

Heliosphere

Figure 7-02. The radiation environment in our solar system.

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