Because the spacecraft systems are rather delicate. And because you have to sterilize it far better than any form of sterilization normally used on the Earth. Because you have to sterilize them of all life. Including life that is totally harmless to humans or our animals or our food or crops but could be a nuisance on another planet.
The Viking spacecraft were heat-treated for 30 hours at 125 °C. Ideally you'd heat it to higher temperatures and for longer - it still had an estimated 30 cultivable spores, so probably 30,000 or so microbes in total, at the end of that process.
But if you did that to modern spacecraft, you have problems with nanoscale features only a few atoms thick, and plastic packaging, also instruments would go out of alignment and many sensors can't be exposed to high temperatures, and the conductive epoxy (used in place of solder) that make electrical connections might be damaged at those high temperatures also.
There are many new methods being explored for spacecraft sterilization. But right now it is still somewhat of a challenge. We don't really have a suitable sterilization method to use to send to a habitat on another planet if we think there is a decent chance that Earth life could flourish there - such as e.g. the so called special regions on Mars. Viking was our best attempt at that, but it's not easy to sterilize a modern spacecraft to the same standards.
One of the most promising right now is low vapour pressure hydrogen peroxide. And ESA are also evaluating use of high pressure, moderate temperature super critical CO2. Hopefully by the time we need to send spacecraft to do in situ investigation of very sensitive areas, we will have found a way to do this.
It's also important to realize, I think, that so far this is all based on theory and calculations, we haven't yet examined any of our spacecraft on another planet to check to make sure what bioload it has by way of viable life, which would help us to have confidence that the methods we are using are working as expected to protect other planets from Earth life.
Luckily Mars is thought to be (mostly) rather hostile to Earth life. So, most would say, we have almost certainly managed to keep it free of Earth life (but you can't say 100% sure we have).
And so far we haven't sent any spacecraft to anywhere else that would need similar levels of sterilization to Viking.
See also: Planetary protection
The Viking spacecraft were heat-treated for 30 hours at 125 °C. Ideally you'd heat it to higher temperatures and for longer - it still had an estimated 30 cultivable spores, so probably 30,000 or so microbes in total, at the end of that process.
One of the Viking spacecraft before it was put into the oven for heat sterilization.
But if you did that to modern spacecraft, you have problems with nanoscale features only a few atoms thick, and plastic packaging, also instruments would go out of alignment and many sensors can't be exposed to high temperatures, and the conductive epoxy (used in place of solder) that make electrical connections might be damaged at those high temperatures also.
There are many new methods being explored for spacecraft sterilization. But right now it is still somewhat of a challenge. We don't really have a suitable sterilization method to use to send to a habitat on another planet if we think there is a decent chance that Earth life could flourish there - such as e.g. the so called special regions on Mars. Viking was our best attempt at that, but it's not easy to sterilize a modern spacecraft to the same standards.
One of the most promising right now is low vapour pressure hydrogen peroxide. And ESA are also evaluating use of high pressure, moderate temperature super critical CO2. Hopefully by the time we need to send spacecraft to do in situ investigation of very sensitive areas, we will have found a way to do this.
It's also important to realize, I think, that so far this is all based on theory and calculations, we haven't yet examined any of our spacecraft on another planet to check to make sure what bioload it has by way of viable life, which would help us to have confidence that the methods we are using are working as expected to protect other planets from Earth life.
Luckily Mars is thought to be (mostly) rather hostile to Earth life. So, most would say, we have almost certainly managed to keep it free of Earth life (but you can't say 100% sure we have).
And so far we haven't sent any spacecraft to anywhere else that would need similar levels of sterilization to Viking.
See also: Planetary protection
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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