The search for extraterrestrial life with unmanned space probes requires
the total sterilization of the landing capsule and its contents.
Scientists agree that terrestrial organisms released on other planets
would interfere with exobiological explorations (refs.
[ref.36]-[ref.43]). Any flight that infects a planet with terrestrial
life will compromise a scientific opportunity of almost unequaled
proportions. Studies on microbiological survival in simulated deep-space
conditions (low temperature, high ultraviolet flux, and low dose levels
of ionizing radiation) indicate that these conditions will not sterilize
contaminated spacecraft (refs. [ref.44]-[ref.48]). Furthermore, many
terrestrial sporeformers and some vegetative bacteria, especially those
with anaerobic growth capabilities, readily survive in simulated Martian
environments (refs. [ref.49]-[ref.54]). It has been estimated that a
single micro-organism with a replication time of 30 days could, in 8
years of such replication, equal in number the bacterial population of
the Earth. This potential could result not only in competition with any
Martian life, but in drastic changes in the geochemical and atmospheric
characteristics of the planet. To avoid such a disaster, certainly the
first, and probably many succeeding landers on Mars, must be
sterile—devoid of terrestrial life ([ref.55]). Since the space
environment will not in itself kill all life aboard, the lander must
leave the Earth in a sterile condition.
The sterility of an object implies the complete absence of life. The
presence of life or the lack of sterility may be proven; but the absence
of life or sterility cannot be proven, for the one viable organism that
negates sterility may remain undetected. Many industrial products which
must be guaranteed as sterile cannot be tested for sterility in a
nondestructive manner. A similar situation exists in determining the
sterility of a spacecraft. Certification of sterility—based on
experience with the sterilizing process used, knowledge of the kinetics
of the death of micro-organisms, and computation of the probability of a
survivor from assays for sterility—is the only accurate approach to
defining the sterility of such treated items.
Macroscopic life can be readily detected and kept from or removed from
the spacecraft, but the detection and removal of microscopic and
submicroscopic life is an extremely difficult task. The destruction of
micro-organisms can be achieved by various chemical and physical
procedures. Sterilizing agents have been evaluated not only for their
ability to kill microbial life on surfaces and sealed inside components,
but also for the agents’ effects on spacecraft reliability as well
(refs. [ref.56]-[ref.59]). Of the available agents, only heat and
radiation will penetrate solid materials. Radiation is expensive,
hazardous, difficult to control, and apparently damages more materials
than does heat. Heat, therefore, has been selected as the primary method
of spacecraft sterilization and will be used, except in specific
instances where radiation may prove to be less detrimental to the
reliability of critical parts ([ref.60]).
The sterilization of spacecraft is a difficult problem if flight
reliability is not to be impaired. The development of heat-resistant
parts will enable the design and manufacture of a heat-sterilizable
spacecraft. Without careful microbiological monitoring of manufacture
and assembly procedures, many bacteria could be trapped in parts and
subassemblies. To permit sterilization at the lowest temperature-time
regimen that will insure kill of all organisms, the microbiological load
inside all parts and subassemblies must be held to a minimum.
The role of industrial clean rooms in reducing the biological load on
spacecraft is currently being defined. NASA-supported studies indicate
that biological contamination in industrial clean rooms for extended
time periods is about 1 logarithm less (tenfold reduction), compared
with conditions in a well-operated microbiological laboratory
([ref.61]). With the use of clean-room techniques and periodic
decontamination by low heat cycles or ethylene oxide treatment, it
should be possible to bring a spacecraft to the point of sterilization
with about 10⁶ organisms on board ([ref.60]).
The sterilization goal established for Mars landers is a probability of
less than 1 in 10 000 (10⁻⁴) that a single viable organism will be
present on the spacecraft. Laboratory studies of the kinetics of
dry-heat kill of resistant organisms show that at 135° C the number of
bacterial spores can be reduced 1 logarithm (90 percent) for every 2
hours of exposure (refs. [ref.58] and [ref.62]). The reduction in
microbial count needed is the logarithm of the maximum number on the
spacecraft (10⁶) plus the logarithm of the reciprocal of the probability
of a survivor (10⁴), or a total of 10 logarithms of reduction in
microbial count. Thus, with an additional 2 logarithms added as a safety
factor, a total of 12 logarithms of reduction in count has been accepted
as a safe value which can be achieved by a dry-heat treatment of 135° C
for 24 hours. This is the heat cycle that is currently under study and
being developed for use in spacecraft sterilization ([ref.60]). However,
other heat treatments at temperatures as low as 105° C for periods of
300 hours or longer are under study ([ref.63]).
Based on results to date, it is reasonable to believe that a full
complement of heat-sterilizable hardware will be available when needed
for planetary exploration. Every effort is being made to improve the
state of the art to a point where spacecraft can not only withstand
sterilization temperatures, but will be even more reliable than the
present state-of-the-art hardware that is not heated.
chapter 3
_Environmental Biology_