In this answer to Why are there no robotic missions on Europa or Enceladus I wrote:

  1. Missions going through the ice and into the ocean are currently hypothetical and problematic. You need a lot of energy to get through 10 km of ice no matter how you do it, and unless your spacecraft is absolutely completely 100% certain sterile you may contaminate a lifeless ocean or disastrously infect it with Earth organisms. To my knowledge there is not yet documented capability that spacecraft can be absolutely sterilized and yet still functional.

Some life is easy to sterilize, but some isn't. We can say that Earth viruses and bacterial spores probably won't pose a threat to alien life and would be inert, but that's just wishful thinking and hand-waving.

There are even single protein molecules that can infect and kill animals and humans, something that was dismissed out of hand as impossible until enough people died.

So I'd like to ask:

Question: Is there any demonstrated or even proposed technology that can sterilize a spacecraft with 100% certainty and yet leave it electronically functional?

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    $\begingroup$ 100% certainty is a high bar. $\endgroup$ Commented Jul 25, 2021 at 1:05
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    $\begingroup$ What you wrote in that answer was incorrect. The only certainties in life are death and taxes. The goal for the planned Europa lander is a less than 1/10000 chance of infecting Europa with Earth life. Source: europa.nasa.gov/system/downloadable_items/… . $\endgroup$ Commented Jul 25, 2021 at 8:42
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    $\begingroup$ The premise that motivates the question incorrect; the goal is not 100%. The question remains valid, but the answer is a simple no. $\endgroup$ Commented Jul 25, 2021 at 9:52
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    $\begingroup$ @DavidHammen thanks for the link. Only a crazy engineering team would sign up for a 100% goal. To misquote Kirk: I don't believe in the no-lose scenario. $\endgroup$ Commented Jul 25, 2021 at 18:53
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    $\begingroup$ @Joshua NASA has similar (but reduced) forms of planetary protection rules for vehicles that land on Mars. The Perseverance rover probably has over 40000 spores spread across the body of the rover, and the entire lander system, less than 300000 spores. $\endgroup$ Commented Jul 25, 2021 at 21:25

2 Answers 2


Absolute 100% sterilization is impossible. There's always a tiny chance that some microbe lands on a component during manufacturing, gets entombed inside the packaging, and therefore is unreachable by sterilization. Getting such a microbe out to the environment would require breaking the spacecraft in a very peculiar way, without incinerating or damaging the microbe itself. Most crash landings will either not break open the electronics, or will incinerate the electronics.

The gold standard for sterilizing electronics is ethylene oxide gas. It is used to sterilize implanted electronic medical devices, such as pacemakers, which also require strict sterilization to prevent putting microbes into the human body. (Autoclaving is also used for non-electronic implants, but the heat will destroy electronics.)

The NASA Contamination Control Handbook, pp. VI-21 to 23, describes the properties of ethylene oxide:

  • "Most versatile gas for sterilizing purposes"
  • Strong, rapid penetrating ability.
  • Easily penetrates a variety of materials which can be used as "sterile packaging", maintaining a device's sterility until it is used.
  • Moderately microbicidal. Effectiveness is increased by longer exposure times; 24 hours at high concentrations is as good as any other form of sterilization.
  • Absorbed into rubber and plastics; blisters rubber and attacks plasticizers; considerable aeration time required after sterilization for these materials.
  • Aeration after sterilization is effective at dispersing the gas. No residue.

Other sterilization methods listed in the Contamination Control Handbook (p. VI-17) are not suitable for electronics:

  • Wet heat (autoclaving) and dry heat will damage electronics by excessive heat.
  • Formaldehyde (steam or dunkbath) and beta propiolactone are in water solutions that can short out electronics. They are surface disinfectants with poor penetration, and do not penetrate sterile packaging. They will also outgas. Formaldehyde can also polymerize, leaving a residue.
  • Peracetic acid and sodium hypochlorite (bleach) corrode metals, are in water solutions, damage sterile packaging, and can outgas.
  • Ultraviolet radiation has low penetration.
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    $\begingroup$ Instrumentation for UHV is routinely baked but doesn't have much electronics, but hight temperature semiconductors being developed for Venus missions will be able to handle quite high temperature. It's possible that baking will be the new gold standard for spacecraft that reach biologically viable environments. I suppose it would have to happen in the fairing in LEO to be edfective. $\endgroup$
    – uhoh
    Commented Jul 25, 2021 at 7:35
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    $\begingroup$ You always trade something in and something out. High temperature semiconductors will always be limited in one way or another - how about reducing the transistor density 1000-fold? And, baking is not a panacea - some of our lifeforms are pretty hard to kill. $\endgroup$
    – fraxinus
    Commented Jul 25, 2021 at 9:33
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    $\begingroup$ @Peter-ReinstateMonica High energy radiation is why we can't have nice electronic things in space. The kind of radiation needed to kill various bacteria in spore form would fry the radiation hardened electronics we have to use in space. $\endgroup$ Commented Jul 25, 2021 at 20:01
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    $\begingroup$ @fraxinus well, but all that is assuming standard silicone-transistor and electrolytic-capacitor technology. Diamond-based operates in a completely different domain, and I'm pretty sure you could also have capacitors whose electrolyte is liquid at 300°C or so (but probably would be solid at room temperature then). Cooling would actually work better at those temperatures as far as space applications are concerned, because radiation becomes an effective mechanism, no need for conductance and/or air convection. $\endgroup$ Commented Jul 26, 2021 at 10:40
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    $\begingroup$ @leftaroundabout no high-value capacitors, no batteries and high thermal noise are not small factors. On the other hand, we use diamond electronics even right now (UV LEDs anyone) that don't require high temperatures in order to operate. The progress in the field undobtfully will affect our everyday life as well. $\endgroup$
    – fraxinus
    Commented Jul 26, 2021 at 10:49

No, it is not possible to make it 100% sterile. Especially if you consider self-replication as only requirement to classify something as life (It's complicated. See obligatory xkcd reference)

Few issues to consider:

  • In addition to other answer, one would likely use chips and PCBs in your spacecraft. Those are usually manufactured outside Space agencies, and while they're certainly not manufactured at dirty premises, there is absolutely no guarantee that a virus or two won't have landed on chips and then being covered by epoxy, anti-corrosive paint or other sealant which would protect microbes inside during sterilization processeses (to be released into environment some thousand years later when sealant degrades).

  • also, even if one could make it completely sterile when manufactured, there is still an issue of launching your ship through atmosphere, which is full of microbes of various kinds, some of which have a non-zero chance of sticking to it and surviving. And even if you built your spaceship completely from scratch in orbit by mining asteroids and doing all manufacturing completely by robots, there is still non-zero chance that your space-factory was not 100% (but only for example 99.999999% clean). Or one of "infected" sub-millimeter pieces of space debris lands on it.

  • even then, there is no 100% guarantee that there isn't other (non-Earth originated) life in space that your spaceship might not collect in space, and then deposit in usually-more-protected destination (like boring or melting through 19km ice barriers)

So yes, chances of contamination are very low and space agencies (at least some of them) seem to try to make it as sterile as possible, but it is not going to be 100% free of life.

But then again, in colloquial speech, 100% almost never really means 100% (eg. taking a placebo pill is not 100% safe, as one could for example choke on it and die, or emotional response to it could put cardiovascular system at elevated - and thus more likely to fail - levels, etc.)

  • $\begingroup$ The "obligatory xkcd reference" is neither obligatory nor scientifically factual. It sacrifices any value that it could potentially have in the context of being a reliable reference in favour of being "funny", and quite ironically does a bad job at being even remotely amusing, too, at least in my impression. I understand the "humour" is not to be taken literally, but in the case where actual and genuine humour is hardly even present, one might as well look at the scientific content instead to attempt finding at least some value, and that's where it begins to matter. $\endgroup$ Commented May 1, 2022 at 0:15
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    $\begingroup$ @user47149 failure to understand why some people find something funny, is not a proof that something is not funny. You cannot invalidate someone's feelings by claims to the contrary. Also, that is how the jokes often work - by building from accepted, normal and probable to unexpected, unlikely and ridiculous. The linked xkcd (besides being funny to me and many others) is quite educational, as it highlights that there is currently no consensus regarding the definition of life itself, which is why it is relevant to thing being asked here. $\endgroup$ Commented May 2, 2022 at 9:44
  • $\begingroup$ "Failure"? That's quite accusative. Surprise, I actually understand the "jokes" and still regard them as poor quality humour. The only failure I could see is the author's failure to amuse me, I guess. SE network does not operate like a forum, but more like Wikipedia; you don't see random links to xkcd comics in Wikipedia articles, do you? And for a good reason. Lack of consensus regarding the definition of life is not equivalent to worldly entities existing on a spectrum where fungi are supposedly "more alive" than plants, or inanimate objects/phenomena, like fire and clouds, are {continues} $\endgroup$ Commented May 3, 2022 at 0:28
  • $\begingroup$ supposedly "more" or "less" alive than one another. There is absolutely no value in linking to such comic, there is not even a punchline: only the author's neurotic turmoil presented as some sort of god-given truth. And especially given that you are, maybe unknowingly, subtly insulting your readers: the link in your answer does not even link to the original xkcd page, but to explainxkcd.com, which puts extremely condescending and pretentious phrase "Explain xkcd: It's 'cause you're dumb." at top before each article. Yikes. {continues} $\endgroup$ Commented May 3, 2022 at 0:28
  • $\begingroup$ And also, voicing one's opinion is not an attempt of invalidating anyone's feelings -- please get away from me with such overreactively defensive woke nonsense. My original comment in this thread explicitly disclaims that the described observations are just my impression (implicitly: subjective impression). By the way, content on SE should be based on merit, not on validation of "feelings" -- the fact that, in your response, you brought said validation of "feelings" into the equation is yet another suggestion that this xkcd link has no business being there. $\endgroup$ Commented May 3, 2022 at 0:33

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