We have been looking for alien life elsewhere in the universe for quite awhile now. Mars has been the main focus of attention and it looks like in the future Europa and Titan will also attract a lot of attention.
But how do we know that what we have found is alien life and that alien life hasn’t been transferred from the Earth to whichever planet or moon we are looking at? We are talking about bacteria and microbes that are obviously not visible but could cause quite a lot of damage to another planet that was sterile.
We do not want to risk bringing back uncontrolled alien life to Earth as we do not know the consequences. What is done to prevent the transfer of life from Earth to these bodies and from these bodies back to Earth?
It is impossible to remove 100% of contamination and something will always get through the net. If contamination is brought from Earth to, say, Mars this is called forward contamination. If contamination is brought back then this is logically called back contamination.
There are three factors that affect the amount of protection that is required. Which body in the solar system (in the future further afield) is a spacecraft travelling to? If it is just a rock or a planet that almost definitely will not have any interesting points about it (e.g. Mercury) decontamination can be carried out to a lower level.
The next factor is if the spacecraft will return to Earth or not, if it is, then obviously it will need a large amount of decontamination at a much higher level. The last factor involves whether a landing or just a flyby or an orbit will occur.
In the early days of exploration a formula was used to work out the probability of contamination.
Probability of contamination = probability of organisms survival X probability of growth and reproduction
This is really a very vague formula and open to a lot of interpretation and abuse so href="nofollow">COSPAR (committee on space research) set up some categories to make things a little simpler and easier.
Category one- there are no planetary protection requirements. This would be used for any mission to a planet that is interesting for the study of chemical evolution such as mercury.
Category two-planets that are of significant interest for understanding chemical evolution and have only a remote chance of contamination. This concerns orbiters and flybys and the concern is about an unintentional impact.
Category three-this also concerns flybys and orbits without landing at the planet. The planet or moon or even asteroid is important for understanding chemical evolution or life’s origins.
Category four-this concerns the probes and landers where there is a significant interest in chemical evolution and life and where any contamination would affect future biological experiments.
Category five-this one is to protect us and involves all Earth return missions. There is also the concern for protecting any sample that is being returned to the Earth.
The bioload is the amount of organisms that is carried on each spacecraft so for example on a category four mission 300,000 organisms are allowed as contamination. When you remember that a cold contains 1 million organisms that isn’t so bad.
Three things contribute to the bioload when a spacecraft is being constructed-
- The environment in which the spacecraft is being constructed.
- the materials that are being used.
- The people that are constructing it.
To get rid of contamination sterilising techniques are used. The components used can be cleaned using detergent, solvent and perhaps a hot helium purge. The workers can wear complete overalls with facemasks. If a spacecraft is going to be landing in a category four mission then moist or dry heat can be used, gas plasma sterilisation or a burst of gamma radiation. The heating and gamma radiation sterilisation techniques can cause damage but the gas plasma sterilisation technique is less harmful to electronic components even though the components are put in hydrogen peroxide plasma.
Decontamination is not particularly interesting but it is an essential part of spaceflight. I just hope that in the past we haven’t contaminated everything already in the solar system.