Problems specific to spacecraft

The space environment is intrinsically hostile to organisms that respire or need an abundance of liquid water to survive. Exactly how hazardous this environment is depends on the organism being considered; a human will be killed in a few tens of seconds, but bacteria in the pores of the skin or in the gut will survive for much longer. This shielding principle also occurs in spacecraft, which tend to warm their electronic components to biologically benign temperatures. Several landers and probes have also had pressurized compartments and could have provided hospitable accommodation for microbial stowaways by slowing their desiccation.10

Once launched, a planetary spacecraft encounters environments that are generally inimical to terrestrial life. The spacecraft's bioburden will experience sterilization to some degree, with the aforementioned stresses of desiccation and radiation exposure being the most critical. In general, the bulk of a spacecraft's material does not experience prolonged temperature extremes, as spacecraft frequently need to keep electronic and mechanical systems at modest temperatures. Landers present special problems as it is possible that the craft may enter regions that raise the revival likelihood for spores. In the case of Martian landers, this could be a landing site at which water ice may be contacted (either directly by drills for buried ice, or traversing exposed ice deposits). Missions to distant ice-rich satellites of the outer planets face similar problems - potentially, a mission could require a remote device to be entirely immersed in liquid water. Clearly in such cases extreme measures to kill all microbes and then remove traces of their soluble biorelevant compounds should be considered.

14.4.1 The space environment - vacuum exposure

Exposing organisms to hard vacua leads to their desiccation through water evaporation or ice sublimation, and the irreversible polymerization of carbohydrates, lipids and nucleic proteins in spores and living bacteria. These Maillard reactions occur relatively slowly, with spores of Bacillus subtilis needing a D10 duration of the order of 10 days when exposed to low pressures in the range of 10~ Pa (Horneck, 1993). Such durations are comparable to recent planetary missions and so exposure to vacuum does not in itself cause a significant reduction in a

10 Early Soviet craft used the active circulation of partial atmospheres to even out thermal extremes in hermetic electronic subsystems.

spacecraft's bioload. As might be expected, the exposure of dead organisms and any biorelevant material such as digestion by-products to vacuum results in no substantial loss of material.

14.4.2 The space environment - ultra-violet radiation

The Sun's unfiltered spectrum contains shorter and more intense ultra-violet (UV) radiation than is seen at the Earth's surface. In Figure 14.5 the irradiance spectrum of solar light in space at 1 AU is shown along with that experienced at the surfaces of Mars and the Earth. The terrestrial ozone layer attenuates light below 300 nm, whereas the CO2 in Mars' atmosphere blocks light with wavelengths less than 200 nm. The grey line is a relative absorption curve for DNA; when compared to terrestrial illumination the Martian lighting environment deposits much more energy into this important molecule. Other organic molecules, such as amino acids associated with biological systems are also rapidly degraded under unfiltered solar light.

14.4.3 The space environment - penetrating radiation

Outside the protective barrier of the Earth's atmosphere and magnetic field, spacecraft hardware is exposed to energetic electromagnetic and corpuscular

200 300 400 500

200 300 400 500

Wavelength (nm)

Figure 14.5. A generic DNA sensitivity spectrum and comparative irradiation spectra for Earth orbital space (AM0), Mars' surface (Mars), and the Earth's surface (AM1.5).

Wavelength (nm)

Figure 14.5. A generic DNA sensitivity spectrum and comparative irradiation spectra for Earth orbital space (AM0), Mars' surface (Mars), and the Earth's surface (AM1.5).

radiations. Solar X-rays are the most commonly encountered example of the former, and of the latter, cosmic rays (CR) and solar protons form the greatest hazard. In low Earth orbits, trapped radiation is a common threat to spacecraft, but as interplanetary probes generally spend relatively little time in near-Earth space, damage from trapped radiation in the Van Allen belts and the South Atlantic Anomaly should be smaller than the dose gained en route to the target body.

The Sun's 11-year activity cycle is associated with increased flare activity and raised fluxes and energies of solar particles. During these events the dose rate of protons can rise by five orders of magnitude, and be sustained for tens of hours,11 delivering up to 2 grays per day at 1 AU. The intensity of such events falls off with distance from the Sun, thus for planetary missions longer than a few hundred days, a greater radiation dose is likely to arise from cosmic rays. These energetic and often multiply ionized particles yield dense ion-pairs when they penetrate materials, and generate secondary radiation which can be more hazardous than the initial radiation. Low atomic weight materials such as hydrogen-rich polymers absorb nucleonic radiation better than metals and so from a sterilization point of view, the move from traditional building materials such as aluminium towards composites in spacecraft structures causes a slight reduction in the killing efficacy of cosmic radiation (Wilson et al., 2001).

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