Life in Space
The environment in space seems to be very hostile to life. This is due to the high vacuum, intense radiation of galactic and solar origin, and extreme temperatures. In the endeavor to disentangle the network of potential interactions of the parameters of space, methods have been applied to investigate the impact of each parameter on biological integrity, separately, as well as in combination.
Space vacuum has been considered to be one of the factors that may prevent interplanetary transfer of life because of its extreme dehydrating effect. However, experiments in space have demonstrated that certain microorganisms survive exposure in space vacuum for extended periods of time, provided they are shielded against the intense deleterious solar UV radiation. Most results are available from spores (a dormant form) of the bacterium Bacillus subtilis.
Up to 70% of the bacterial spores survive short term (e.g. 10 days) exposure to space vacuum. The chances of survival in space are increased if the spores are embedded in chemical protectants such as sugars, or salt crystals, or if they are exposed in thick layers. For example, 30% of subtilis spores survived nearly 6 years in space when embedded in salt crystals and 80% survived in the presence of glucose.
Solar UV radiation has been found to be the most deleterious factor of space - as tested with dried preparations of viruses, bacterial and fungal spores - with DNA being the critical UV target for lethality. However, about 5% of a species of the extreme halophile Haloarcula (a salt-loving bacteria) survived a two week space environment during a Foton spaceflight.
The radiation field of the Solar System consists of components of galactic and solar origin. It is composed of electrons, alpha-particles and cosmic heavy ions, the latter being the most ionizing and therefore the most deleterious components. The heavy particles of cosmic radiation are conjectured as setting the ultimate limit on the spore survival in deep space, because they penetrate even heavy shielding. The maximum time for a spore to escape a hit by a heavy particle has been estimated to be 105-106 years.
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