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Correct me if I'm wrong but a probe to Mars will take months, during which time it's exposed to a vacuum and irradiated by the sun. It sounds to me like this would kill any micro-organisms on the spacecraft.

Why then do we irradiate and sterilize our Mars probes? The journey would kill all micro-organisms anyway, right?

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    $\begingroup$ Just psychology and propaganda. Planetary protection is like SETI, no one knows what they are doing. I do wish they keep doing it, I have no better suggestion. Outer space conditions is certainly sterilizating for life as we know it. But there might be something else than an e coli bacteria out there. Or what do they expect? We humans share half of our genes with virus. There are 10^31 virii in the global biosphere. 700 virus particles have had their genome mapped. 700 is a bit less than 10^31. But we have survived for billions of years so I'm not worried, I'm just saying. $\endgroup$ – LocalFluff Aug 7 '15 at 17:09
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    $\begingroup$ @LocalFluff I strongly disagree with your premise. History on our own planet is replete with examples of what happens when bio contamination procedures aren't followed. The consequences are not generally positive. $\endgroup$ – Colyn1337 Aug 7 '15 at 18:42
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    $\begingroup$ This is an interesting question. In Aug 2014, sea plankton was identified on the exterior of the ISS, suggesting that life can withstand the extremes of temperature and cosmic radiation of space. $\endgroup$ – Octopus Aug 7 '15 at 21:34
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    $\begingroup$ @DrZ214 The first returning crews were quarantined for a week or two; the photo of Nixon meeting the Apollo 11 crew has them on the other side of a thick window. In practice, the value of this quarantine was limited - the recovery process probably would have exposed several recovery crew to anything infectious! - and it quickly became apparent that it wasn't needed in any case, so it was dropped after a few flights. $\endgroup$ – Andrew Aug 7 '15 at 23:00
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    $\begingroup$ A small point I haven't seen mentioned in the current answers is that not all areas of a probe are equally exposed to space. There is sensitive equipment inside the probe and that equipment sometimes needs protection from solar radiation. I'd imagine that anything which ended up in these protected areas would have a much higher chance of making the trip without getting it's atoms rearranged. $\endgroup$ – krowe Aug 8 '15 at 19:37
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Tardigrades can survive vacuum, low temperatures, and moderate radiation for quite a while. They're multicelled organisms. How tough is the toughest hypothetically viable single-celled extremophile? How sure are you that we've found all of them on Earth already?

It's a matter of caution. Contamination of another planet (or moon) is likely to be irreversible, may very easily make it difficult or impossible to determine the origin of any lifeforms there, or might outright take over and drive hypothetical native organisms to extinction (either before or after discovery), so all precautions are taken to avoid that.

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    $\begingroup$ A tiny von Neumann machine sent deliberately to us by some thing which is more clever than we are. Don't some of us already imagine doing exactly that!? Maybe we are alive because we are alone. $\endgroup$ – LocalFluff Aug 7 '15 at 17:18
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    $\begingroup$ @LocalFluff didn't you mean Maybe we're alive because we're not alone? $\endgroup$ – DrZ214 Aug 7 '15 at 17:56
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    $\begingroup$ @LocalFluff -- Panspermia, the idea that life originated elsewhere, is a highly hypothetical (and highly criticized) hypothesis. Disregarding that, the rationale for planetary protection is simple. Some humans have committed terrible genocidal crimes in the last century+. Committing planetocide would make those genocidal monsters look like rank amateurs. $\endgroup$ – David Hammen Aug 8 '15 at 9:52
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    $\begingroup$ @DavidHammen I certainly understand your concern for how terribly we humans behave against each other. But in a bigger perspective, life on Earth did actually not suffer from the last world war. Even the human population increased while the slaughter was going on. Your own immune system probably kills millions of microbes an hour even when you feel perfectly healthy. Insane wars are just part of the process that makes us. We can make subjective conscious value judgements about it, but no one even knows how that phenomena relates to physical reality. $\endgroup$ – LocalFluff Aug 8 '15 at 12:38
  • $\begingroup$ @LocalFluff Speaker for the Dead is an incredibly good book, if you haven't read it yet. $\endgroup$ – Rikki-Tikki-Tavi Mar 22 '16 at 14:23
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So back in the summer of 1935, some folks down in Australia were having problems with a beetle's larvae that were nomming on the sugar cane roots and harming crops. Since traditional methods of getting rid of the pest failed, they decided a good approach would be to introduce a few cane toads to go eat the beetles... a hundred or so of them in a couple of places in the north east. Seemed like a good solution.

Fast forward to today, there's now hundreds of millions of the things happily munching their way across the country, even evolving traits that help them move faster. They are spreading at a rate of 60km/year, trashing the ecosystem of everything but the dratted beetles they were introduced to combat. See the problem is that they're toxic and none of the native biosphere had evolved in the presence of that toxin, thus they had no natural predators and any potential predator deciding to eat one of these toads is likely to eat little else afterwards.

What does this have to do with space? Two things.

Firstly, this is an example of an introduced invasive species... in the case of the toads it was intentional, rabbits are another species that has had similar disastrous results in Australia's ecosystem. There are also examples of unintentionally introduced species, with much the same result. If space probes successfully transferred viable micro-organisms to another planet we have the risk that any micro-organisms already existing there will be overrun by our earth bred critters. That would be bad.

If we can mess up a relatively similar ecosystem with something as large and predictable as a rabbit, how much of a mess could we cause by introducing microscopic and unpredictable lifeforms to an ecosystem we know nothing about?

Secondly, the purpose of a lot of our planetary probes is to locate signs of life. Ideally currently live life, but signs that life once existed will also do in a pinch. This is kind of difficult if we're sending probes that are covered in lifeforms. It's equivalent to making a radiation detector, putting it in a uranium housing, and then releasing a study on the elevated background radiation in your location. You can't reliably detect something if your detection equipment is contaminated with the very thing you're trying to detect.

And even if the probe that is doing the detecting is clean, if another probe isn't then the life you detected might have piggybacked on that. If you want to say for certain "this life evolved on Mars" you need to ensure that it definitely didn't arrive on a probe.

Worst case, you introduce some lifeform to the planet and it promptly starts eradicating the life you're looking for. Red squirrels are the native species in the UK but with the way the introduced grey squirrel has been pushing them out for decades, if you were looking for signs of squirrels here then it's possible you might never encounter evidence of red squirrels.

The bottom line is that it's unlikely that such a contamination could happen but not nearly as unlikely as you'd think. If you send millions and millions of viable lifeforms or spores in to space, some of them are going to be able to survive, some of them are going to be in a spot that the solar radiation isn't hitting, some of them are going to be inside a shielded bit, some of them ... you get the idea. Accidental sterilisation isn't in the slightest bit reliable enough and the potential problems posed by the two reasons above are sufficient to justify the effort spent on trying to reduce the risk.

The argument that life could travel from Earth to Mars via meteorite is valid, but actually demonstrates why planetary protection is so important: If you find Earth micro-organisms on Mars, how do you know if they got there via meteorite and not via a probe? Knowing if life could make that trip would be very useful, in theories such as those involving the inverse, life being introduced to Earth via meteorite.

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    $\begingroup$ Another thing to consider is that a mission which might plausibly introduce contamination, whether or not any contamination actually occurs, may make it impossible to ever determine whether life forms which by chance strongly resemble those of Earth appeared on Mars as a consequence of something other than human exploration. $\endgroup$ – supercat Aug 7 '15 at 20:27
  • $\begingroup$ @supercat yeah, that's what I was aiming towards with the last paragraph. Science is all about reducing the variables in an experiment to measure only that which you actually care about. $\endgroup$ – Kaithar Aug 7 '15 at 20:35
  • $\begingroup$ Oh, how I love Australia's hypocritical policies on import bans while they themselves being the prime example how to do it wrong. Dingoes, toads, cats, what not... $\endgroup$ – Archimedix Aug 7 '15 at 20:37
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    $\begingroup$ @Archimedix they have import bans because they have the worst examples of this kind of invasive species. Australia is a weird isolated ecosystem and received a lot of imports during the colonial period that wouldn't have had such an impact on a Eurasian country where there's at least some species migration. Doesn't help that cane toads and rabbits are two of the worst invasive species around, up there with Japanese knotweed. They can have semi-effective national "biofilter" type policies because they're an isolated island, not much is going to just wander in across a border. $\endgroup$ – Kaithar Aug 7 '15 at 20:50
  • $\begingroup$ @Kaithar Australia won't ever learn. They have dozens of such fuckups. They counter invasive species with more invasive species, leading to the infamous Simpsons parody. See this: theverge.com/2016/5/1/11548946/australia-carp-herpes-budget $\endgroup$ – Archimedix May 2 '16 at 17:41
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In fact, this is a subject of much debate. The bottom line is, bacteria wouldn't survive, but their spores might. A few of these spores will make it to Mars, and even a few could potentially spawn life on Mars from Earth. This has been proven from samples sent to the International Space Station.

However, there are a lot of arguments against such protection. There have almost certainly been rocks which have survived from Earth to Mars that contain bacteria, so almost certainly life has already had a chance to reach Mars from Earth. Thus, it isn't particularly likely that a spacecraft will contaminate Mars with Earth life any more than has already been done.

The current thinking is to build Mars landers in a cleanroom, but not require strict contamination protection. The spacecraft surfaces are exposed to harsh light during the transit, and overall almost everything on them will die. The only things that are really subject to sterilization are those designed to detect life or organic chemicals, which it does still make sense to protect against. There is a lesser concern that such measures are still required for anything that digs into a surface, as it is more likely to reach fertile locations under the surface. Curiosity did not have as strict as measures regarding Planetary Protection, as one can see from this article.

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    $\begingroup$ @MasonWheeler: giant trebuchet. No, wait, not that. Debris from asteroid impacts on Earth, spends forever bodding about the inner solar system and eventually hits Mars. I don't want to speculate whether the bacteria would survive all that, but since Mars rocks have "survived" to hit Earth it's not all that surprising that we can model Earth rocks hitting Mars. Ofc Earth-Mars is uphill whereas Mars-Earth is downhill, I don't know how much difference that makes to the expected quantity going in each direction. $\endgroup$ – Steve Jessop Aug 7 '15 at 17:39
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    $\begingroup$ Mars rocks have survived, in such a condition that if they had life, the life would have remained alive. The same has likely happened in reverse. $\endgroup$ – PearsonArtPhoto Aug 7 '15 at 17:43
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    $\begingroup$ Now that's interesting, although I'm immediately skeptical on two points. First, how do we know they're Mars rocks? And second, Earth has a much heavier atmosphere than Mars, which makes it harder for something to get out, and harder for anything that does get out to keep from sterilizing itself in the process--any bacteria on the rock should burn up from all the friction, right? $\endgroup$ – Mason Wheeler Aug 7 '15 at 17:45
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    $\begingroup$ That's a lot of good questions, see the Wikipedia article, or ask another question. en.wikipedia.org/wiki/Martian_meteorite $\endgroup$ – PearsonArtPhoto Aug 7 '15 at 17:47
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    $\begingroup$ @MasonWheeler and SteveJessop I have to agree with Mason. The heavier gravity, plus thick atmosphere of Earth, would make it much harder for an Earth rock (impact ejecta) to escape to Mars than vice versa. The uphill/downhill part does't apply to orbital trajectories; it's the same delta-v going from one orbit to the other. But the uphill part definitely applies to the escape from Earth because Earth's surface gravity is 3x stronger. $\endgroup$ – DrZ214 Aug 7 '15 at 18:01
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Leaving aside the very good reasons why you don't want any organisms on a probe trying to detect Martian organisms... we already have fairly good evidence that life can survive irradiation and vacuum.

The Apollo 12 crew landed near an early unmanned lander, Surveyor 3, and recovered parts of it for examination. Bacteria were successfully cultivated after samples were taken from the lander's camera, which had spent two or three years on the lunar surface. Whether these bacterial samples had actually survived that long, or whether they were contamination in the lab, is debatable; the results are contested, and we don't have firm evidence to prove or disprove reliable sterile procedure, but they're certainly seen as plausible.

More recently, and with substantially more rigorous sterile procedures, bacterial spores and lichen have been regrown after spending a year and a half on an exposed platform on the ISS - albeit they seem to have had some rock as protection. (The main problem for microbial hitchhikers in long-duration spaceflight seems to be UV, rather than vacuum.)

Most planetary probes don't haul along bits of sandstone... but they do have sheltered corners that might easily escape UV throughout the flight.

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  • $\begingroup$ I am not a biologist so this is hard to understand. How can those little guys go so long without food, water, and air! What else is there to live on? Do they eat each other? Maybe with a multi-million cell colony, cannabilization can last a long time? $\endgroup$ – DrZ214 Aug 7 '15 at 23:58
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    $\begingroup$ @DrZ214: I'm not a biologist either, but I've spent enough time around people who study this kind of stuff. Two thoughts: 1) consider how seeds can survive after decades and even millennia without nutrients; and 2) scientists are discovering that there are a ton of microorganisms called "extremophiles" that can survive really really inhospitable environments (high heat, high acidity/alkalinity, high pressures, no light, little/no nutrients, etc.). Some space debris could have inner hollow spots—pockets that could have air or moisture. $\endgroup$ – jvriesem Aug 8 '15 at 3:18
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    $\begingroup$ A sufficiently simple organism can sometimes survive things a more complex organism can't. Some small creatures can essentially put life on "pause", drastically slowing or stopping chemical reactions, recovering when water at reasonable temperatures becomes available. Small groups of cells, or small animals, may survive being quick-frozen to liquid-nitrogen temperatures and re-thawed; that's done on a regular basis in seed banks and fertility clinics, and I've seen it done with a goldfish. One more:, viruses when not in a cell are essentially just encapsulated RNA, needing nothing. $\endgroup$ – keshlam Aug 8 '15 at 3:54
  • $\begingroup$ @DrZ214 The "pause" concept that keshlam mentions is key here. Note that the studies were on bacterial spores - these are effectively dormant 'seeds' which will only become 'alive' again when in favourable conditions. (That sentence is mostly wrong, but it gets the idea across). You might also be interested to read about cryptobiosis (en.wikipedia.org/wiki/Cryptobiosis), the ability of organisms to basically shut down in order to survive abysmally bad conditions - it's more common than you might think. (Moss has been scraped out of Antarctic permafrost & regrown after 1600 years!) $\endgroup$ – Andrew Aug 8 '15 at 23:21

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