<p>Even if a virus came to Earth from space, it would likely be harmless to life on Earth.</p>

<p>Let's look at some biology numbers:</p>

<p>Life on Earth uses 20 amino acids (out of about 400 considered chemically potential alternatives). The median length of a protein on Earth is about 300 amino acids. That's 20^300 combinations. And some proteins are 30,000 amino acids long. There are about 10^31 virus particles in Earth's global biosphere. They have such a large diversity that most of the (few hundred) virus "families" who had their genome mapped, are lone members of their families. Still, we all get along very well! That's because all life on Earth has the same origin and is very closely related to each other. Half of the human genome can also be found also in virus DNA.</p>

<p>For a virus to be dangerous, it needs to have evolved together with its victim and made evolutionary use out of hurting it. Given the enormous combinatorics involved, it seems most unlikely that an independently evolved virus could break our genetic code. It would take longer than the history of the universe for virus to evolve a crack to our genetic code from scratch. Earthly virus can do so because they originated and evolved here together with us. All life and virus on Earth share DNA that's why we can have relations with them for good and bad (from a humanitarian perspective, while evolutionarily virus are only long-term good for us!) I think our immune system and all kind of microbes would eat any space virus and only become stronger thanks to it.</p>

<p>An alien microbe, something which is in itself alive and active (as opposed to a virus which is dormant until it finds the specific host which it was evolved to infect), could be more dangerous. Instead of attacking unique Earth life organisms directly, they could maybe eat simple molecules which were present also where they once originated and evolved, such as oxygen or simple hydrocarbons, and thereby change the ecology on Earth. But they should be more complex and sensitive to radiation and other space threats, than virus. (Unless we do some how have common origin and evolution even with space virus which thus are fit to interact with us)</p>

CC BY-SA 3.0
2016-05-20T15:03:21.110Z
Many viruses actually use RNA, not DNA.
called2voyage
CC BY-SA 3.0
2018-01-21T19:33:45.687Z
Molecular biology is really complex and a never-ending source of surprises. No-one would have expected infectious proteins until they were discovered for example ([Prions](https://en.wikipedia.org/wiki/Prion)), or infectious cancers ([Clonally transmissible cancer](https://en.wikipedia.org/wiki/Clonally_transmissible_cancer)). I think the opposite is true here. The fact that they have evolved together *is the reason* there are so many different organisms that co-exist on Earth.
uhoh
CC BY-SA 3.0
2018-01-21T19:33:53.477Z
Introduce a new out-of-context species, and it's often a nightmare! ([Invasive species](https://en.wikipedia.org/wiki/Invasive_species))
uhoh
CC BY-SA 3.0
2016-01-10T13:42:27.197Z
2018-01-21T19:35:21.917Z
2018-01-21T19:35:21.917Z
LocalFluff
<p>In short, no. It is impossible for an infectious agent to come from another planet.</p>

<p>Comets formed in space, and while they may have some carbon based chemicals, nothing as complex as life could form on a comet.</p>

<p>Some meteoroids have been ejected from planets, and it is at least conceivable that something could survive "launch". It is not likely that something could live in space, but I suppose a virus might.</p>

<p>So I will assume the unlikely: An asteroid carrying an extra-terrestrial virus has landed on Earth.</p>

<p>What is it going to do? Well no infect humans - viruses are quite specific to their hosts. Most viruses infect a specific type of bacteria. An alien virus would be specific to an alien bacteria. And alien bacteria will be very alien. Most Movies assume that "DNA" is fundamental for life, and talk about "alien DNA merging with human DNA" or the like. However DNA is something that life on this planet has discovered: aliens won't have DNA. They will have something else that carries hereditary information, but it won't be DNA. An alien virus would be utterly incapable of infecting Earth cells, because it's fundamental biochemistry will be utterly dissimilar.</p>

<p>This is a science fiction story and nothing more or less than than.</p>

CC BY-SA 3.0
2016-01-11T05:15:30.153Z
I think this is a good answer and it's correct for a virus.   A space bacteria or the exo-planet equivalent of a water-bear if one could survive the journey through radioactive space and actually find the Earth many light years away, it's possible that the new life would find life on Earth as bags of useful chemicals and water.   Its theoretically possible that an unfriendly exo-bacteria could do the life on Earth significant harm if it had an evolutionary advantage, but that kind of event actually happening is unlikely, or, at least, significantly infrequent for a number of reasons.
userLTK
CC BY-SA 3.0
2016-05-20T14:23:45.280Z
First of all, many viruses *on Earth* use RNA, not DNA. Second, it is entirely possible that RNA and DNA are the most likely nucleic acids for life to adopt (assuming life doesn't manage to form without nucleic acid). Even so, it does seem unlikely that alien RNA/DNA would be compatible with Earth RNA/DNA.
called2voyage
CC BY-SA 3.0
2016-01-10T12:15:17.890Z
2016-01-10T12:15:17.890Z
James K
<p>I think it's a fun question and I can kind of give a layman's answer.   (though reading the above answer, there is some overlap).</p>

<p>It seems likely that "tiny", life or viruses could survive/hibernate for a long time if blown into space on space rocks, so it follows that life could travel to earth that way, but I think we could apply a modified version of the <a href="https://en.wikipedia.org/wiki/Drake_equation" rel="nofollow">drake equation</a>.</p>

<p>The Earth gets hit by comets or meteors every day but most of those burn up in the atmosphere.   There is a temperature beyond which even bacteria and viruses have a hard time surviving.  So, many of those meteors or bits of comet, I suspect, would lose any life they might have.   Only a percentage could be the right size to both slow down enough while breaking apart to not impact at too high a speed and successfully deliver life to the surface or into the ocean.   Like the real drake equation, we get to plug in our own numbers, but I suspect most impacts wouldn't successfully deliver life to the planet they impact.   How many would?  1 in 10?  1 in 100?   I'm not sure.   </p>

<p>Now, what percentage of comets or meteors have life on them?   This is a tough one, I'm pretty sure life can only form on a planet because you need (I would think), liquid water, atmosphere to maintain surface water and reduce evaporation and some kind of heat to drive the mixing of chemicals, perhaps lightning or undersea volcanic vents.   Life likely cannot be formed in space or in a nebula.   It can be blown into space off a planet with life, but it can't form in space.   That likely reduces the number of meteors or objects in space that even have the possibility of carrying life.    I hate to guess this one, cause who knows, but 1 in 1000?   1 in 10,000?   Most of the meteors, asteroids and comets in space have probably formed without the possibility of carrying life.</p>

<p>There are some simple organic molecules in space, <a href="http://www.universetoday.com/8098/deep-space-alcohol/" rel="nofollow">alcohol</a> for example, but there's an enormous chasm between organic molecules and even the simplest life.   </p>

<p>Some planets are very difficult to blow debris off of.  Earth with an atmosphere and an escape velocity of some 25,000 miles per hour needs a very big impact to lose material into space.   Mars, with a very thin atmosphere and lower escape velocity is much easier.  Of some 60,000 recognized meteorites, 124 have been identified from Mars.  (<a href="http://www2.jpl.nasa.gov/snc/" rel="nofollow">Source</a>) and Mars is the easiest one.   Venus, with it's thick atmosphere and Mercury, being as close to the sun it is, adding the suns escape velocity to it's own, are much harder.   Material being blown off the gas giants is even less likely, so Mars is the easiest plant to potentially lose material where it could land on Earth or other planets.   Even if we use the Mars Meteor ratio of 400 to 1, which is, I think, too high, it's still a very low percentage of objects in space that potentially came off a planet or moon and carry life with them.</p>

<p>A 3rd point worth bringing up is repetition.   Lets say, just for fun, that <a href="https://en.wikipedia.org/wiki/Europa_%28moon%29" rel="nofollow">Europa</a> and <a href="https://en.wikipedia.org/wiki/Enceladus" rel="nofollow">Enceladus</a> both have life in their under the surface oceans and both planets regularly eject jets of life filled water into space, that freezes and carries basic life into orbits around Jupiter / Saturn and some of those life carrying bits of ice make their way on occasion to other planets.    If this happens it probably happens more than once and any bits of life that may come to earth from Europa or Enceladus, if it happens at all, probably happens more than once.   The first introduction of invasive species, even simple ones, can be problematic, but after the first, the planet should have adapted.  That's one of the mathematical quirks.   One Andromeda strain can in theory be a huge problem, but repeated Andromeda strains from the same source, the risk drops with each impact.   What you need, for a dangerous situation to happen is a new introduction of unfamiliar that's never happened before.   That's a pretty specific requirement.   Mars may have once had life, and if it had, life on Mars probably landed on earth many times over 4 plus billion years.</p>

<p>a 4th point, space is full of radiation.  Even viruses will eventually die exposed to the radiation of space.   A space rock, carrying life from another solar-system light-years from earth has a very long journey and a very small target.   Bits of frozen ice or rock probably travel from other solar systems into ours quite rarely over very long periods of time, and, as I pointed out above, most of those probably won't have life.</p>

<p>This is just a kind of "shoot from the hip" sort of answer, but I think you would need a perfect storm of events for your scenario to actually happen.   statistically, I suspect it's a very rare occurrence and not something we need to worry about on a human time frame. </p>

CC BY-SA 3.0
2016-05-30T11:44:36.480Z
About your 3rd point, I saw this SETI talk that suggests that Enceladus is only a few hundred million years old. Its jetting panspermia, if it has yet originated life, would thus be pretty new. And evolved in a very different environment. I tend to think that incompatibility is safe, not dangerous. Most species on Earth completely  ignore each other. http://www.seti.org/weeky-lecture/orbital-dynamics-suggests-recent-formation-saturns-moons
LocalFluff
CC BY-SA 3.0
2016-05-30T22:59:52.653Z
@LocalFluff  Interesting.   So Enceladus formed, perhaps by some kind of collision or a breakup of a larger moon which entered Saturn's Roche Limit.   (I would think).  Interesting stuff.   A capture seems unlikely.   The article doesn't say much, so I'll take with a grain of salt, but it seems possible.
userLTK
CC BY-SA 3.0
2016-01-10T07:10:24.157Z
2016-01-10T07:10:24.157Z
userLTK
<p>We’re talking about stochastics, so it’s impossible to say <em>never</em>. But we can tackle the question two ways:</p>
<ol>
<li><p>Big picture: tons of dust fall on the Earth from space every day. TONS, every day. You can sweep your gutters, and there’s some possibility a micro-spherule is actually a very tiny meteorite. The issue is identification, because it would look like any other grit particle. And that’s spherical micrometeorites, not even the majority of the infall. One dust researcher estimates that, based on the dust flux, you walk over a cosmic dust particle with <em>every other step</em>. The implication is that, if even one percent of one percent of particles carries viruses, we’d be toast. That we’re still alive at all is a clear indication that cosmic infall is no threat.</p>
</li>
<li><p>Small picture: the flux of cosmic rays, which degrade and destroy DNA and other organics, is not low either. There are multiple classes and definitions of ‘cosmic ray’, but they’re measured in counts per square centimeter per second. Typical fluxes are one or two per cm2 per sec. Macroscopic organisms and even most microbes have DNA repair functions; viruses, I recall, do not (or they’re worse, perhaps). You can do the math: for a nontrivial DNA sequence (thousands of ‘bits’), crammed into a compact volume, what are the chances of surviving particle bombardment? This requires a time parameter: most meteorites have exposures (measured via cosmic-ray tracks, ha) of 10,000 to 10,000,000 years. Go on, tell me how much bombardment that is.</p>
</li>
</ol>

CC BY-SA 4.0
2022-08-30T13:58:11.400Z
2022-08-30T13:58:11.400Z
caInstrument
<p>There is, believe it or not, a scientific theory regarding this: <a href="https://en.wikipedia.org/wiki/Panspermia" rel="noreferrer"><em>panspermia</em></a>.</p>

<p>According to the panspermia hypothesis, microbes "hitch" a lift on bodies leaving a planet. They then travel through space and eventually land somewhere else.
Panspermia involves three difficult phases of travel:</p>

<ul>
<li>Launch (as well as the impact event)</li>
<li>Travel in the harsh environment of space</li>
<li>Atmospheric entry and landing, with high temperatures</li>
</ul>

<p>It's difficult to survive these. However, materials related to organic matter have been found that might be evidence of panspermia; see <a href="http://www.pnas.org/content/early/2015/10/14/1517557112.full.pdf" rel="noreferrer">Bell et al. (2015)</a> for one example.</p>

<p>Panspermia is traditionally applied to bacteria known as <em>extremophiles</em>, which are accustomed to living in harsh environments. Space contains temperatures ranging from hundreds of degrees Fahrenheit to hundreds of degrees below zero, and any organism would have to survive those. This may be possible - <a href="http://www.nature.com/news/high-flying-bacteria-spark-interest-in-possible-climate-effects-1.12310" rel="noreferrer">bacteria have survived in conditions like there</a> - but difficult.</p>

<p>Viruses pose an additional challenge because they cannot survive long without a host organism on which to live. This means they would have to be carried along by an extremophilic bacteria. This also means that they might not be a huge harm to creatures on Earth. Viruses only attack certain bacteria because of proteins on their surfaces; extraterrestrial biology most likely differs from that on Earth, and these viruses might not be equipped to harm terrestrial organisms.</p>

CC BY-SA 3.0
2016-01-10T01:37:10.263Z
what about the water bear there can survire a lot of wild temperatues and ems radations
Henry WH Hack v3.0
CC BY-SA 3.0
2016-01-10T01:38:33.677Z
@HenryWHHack Yes, [they can survive](https://www.ncbi.nlm.nih.gov/pubmed/18786368).
HDE 226868
CC BY-SA 3.0
2016-01-10T01:41:58.977Z
@HenryWHHack No; the two are completely different. That doesn't mean that viruses can't survive in space, though.
HDE 226868
CC BY-SA 3.0
2016-01-10T01:44:11.990Z
So if a virus came form space will it be look at as an aliens because there space beings
Henry WH Hack v3.0
CC BY-SA 3.0
2016-01-10T01:44:56.843Z
@HenryWHHack I don't understand what you mean, partly because of your grammar.
HDE 226868
CC BY-SA 3.0
2016-05-20T03:01:41.827Z
Racemization of amino acids is a problem over  the time it would take a rock to travel light years: http://www.lpl.arizona.edu/~bcohen/publications/Cohen_Amino.pdf Cosmic rays are also likely to hit and damage DNA. However, we really don't have much in the way of good numbers as to how fast these processes act in space.
Wayfaring Stranger
CC BY-SA 3.0
2016-01-10T01:31:00.857Z
2016-01-10T22:45:38.197Z
2016-01-10T22:45:38.197Z
HDE 226868
<p>So I am not a true scientist and I was just wondering if an extraterrestrial rock from space can have some dangerous unidentified virus or bacteria that can be deadly and wipe out the Earth if it not quarantined. Is this science fiction?.</p>

<p>P.S.: Note I said extraterrestrial <em>rocks</em> not extraterrestrial as in aliens.</p>

CC BY-SA 3.0
2018-01-20T09:42:05.023Z
Is it possible to get hit in the balls by a meteorite when you were scoring the last goal of the world cup? sure it's possible.
bandybabboon
CC BY-SA 4.0
2021-05-04T22:46:22.933Z
Love this question!
James Ervin
<space><comets><meteor><extra-terrestrial>
Is it possible for a virus to come from a meteorite?
Henry WH Hack v3.0
# Is it possible for a virus to come from a meteorite?

(Asked by: username_0 on 2016-01-10)

So I am not a true scientist and I was just wondering if an extraterrestrial rock from space can have some dangerous unidentified virus or bacteria that can be deadly and wipe out the Earth if it not quarantined. Is this science fiction?.

P.S.: Note I said extraterrestrial *rocks* not extraterrestrial as in aliens.

**Tags:** space, comets, meteor, extra-terrestrial

> **Comments:**
> **username_1**: Is it possible to get hit in the balls by a meteorite when you were scoring the last goal of the world cup? sure it's possible.  
> **username_2**: Love this question!  

---

## 5 answers

### Answer by username_9 on 2016-01-10. Score: 10 | Accepted answer

There is, believe it or not, a scientific theory regarding this: [*panspermia*](https://en.wikipedia.org/wiki/Panspermia).

According to the panspermia hypothesis, microbes "hitch" a lift on bodies leaving a planet. They then travel through space and eventually land somewhere else.
Panspermia involves three difficult phases of travel:

* Launch (as well as the impact event)
* Travel in the harsh environment of space
* Atmospheric entry and landing, with high temperatures

It's difficult to survive these. However, materials related to organic matter have been found that might be evidence of panspermia; see [Bell et al. (2015)](http://www.pnas.org/content/early/2015/10/14/1517557112.full.pdf) for one example.

Panspermia is traditionally applied to bacteria known as *extremophiles*, which are accustomed to living in harsh environments. Space contains temperatures ranging from hundreds of degrees Fahrenheit to hundreds of degrees below zero, and any organism would have to survive those. This may be possible - [bacteria have survived in conditions like there](http://www.nature.com/news/high-flying-bacteria-spark-interest-in-possible-climate-effects-1.12310) - but difficult.

Viruses pose an additional challenge because they cannot survive long without a host organism on which to live. This means they would have to be carried along by an extremophilic bacteria. This also means that they might not be a huge harm to creatures on Earth. Viruses only attack certain bacteria because of proteins on their surfaces; extraterrestrial biology most likely differs from that on Earth, and these viruses might not be equipped to harm terrestrial organisms.

> **Comments:**
> **username_0**: what about the water bear there can survire a lot of wild temperatues and ems radations  
> **username_9**: @HenryWHHack Yes, [they can survive](https://www.ncbi.nlm.nih.gov/pubmed/18786368).  
> **username_9**: @HenryWHHack No; the two are completely different. That doesn't mean that viruses can't survive in space, though.  
> **username_0**: So if a virus came form space will it be look at as an aliens because there space beings  
> **username_9**: @HenryWHHack I don't understand what you mean, partly because of your grammar.  
> **username_10**: Racemization of amino acids is a problem over  the time it would take a rock to travel light years: http://www.lpl.arizona.edu/~bcohen/publications/Cohen_Amino.pdf Cosmic rays are also likely to hit and damage DNA. However, we really don't have much in the way of good numbers as to how fast these processes act in space.  

---

### Answer by username_3 on 2016-01-10. Score: 9

Even if a virus came to Earth from space, it would likely be harmless to life on Earth.

Let's look at some biology numbers:

Life on Earth uses 20 amino acids (out of about 400 considered chemically potential alternatives). The median length of a protein on Earth is about 300 amino acids. That's 20^300 combinations. And some proteins are 30,000 amino acids long. There are about 10^31 virus particles in Earth's global biosphere. They have such a large diversity that most of the (few hundred) virus "families" who had their genome mapped, are lone members of their families. Still, we all get along very well! That's because all life on Earth has the same origin and is very closely related to each other. Half of the human genome can also be found also in virus DNA.

For a virus to be dangerous, it needs to have evolved together with its victim and made evolutionary use out of hurting it. Given the enormous combinatorics involved, it seems most unlikely that an independently evolved virus could break our genetic code. It would take longer than the history of the universe for virus to evolve a crack to our genetic code from scratch. Earthly virus can do so because they originated and evolved here together with us. All life and virus on Earth share DNA that's why we can have relations with them for good and bad (from a humanitarian perspective, while evolutionarily virus are only long-term good for us!) I think our immune system and all kind of microbes would eat any space virus and only become stronger thanks to it.

An alien microbe, something which is in itself alive and active (as opposed to a virus which is dormant until it finds the specific host which it was evolved to infect), could be more dangerous. Instead of attacking unique Earth life organisms directly, they could maybe eat simple molecules which were present also where they once originated and evolved, such as oxygen or simple hydrocarbons, and thereby change the ecology on Earth. But they should be more complex and sensitive to radiation and other space threats, than virus. (Unless we do some how have common origin and evolution even with space virus which thus are fit to interact with us)

> **Comments:**
> **username_4**: Many viruses actually use RNA, not DNA.  
> **username_5**: Molecular biology is really complex and a never-ending source of surprises. No-one would have expected infectious proteins until they were discovered for example ([Prions](https://en.wikipedia.org/wiki/Prion)), or infectious cancers ([Clonally transmissible cancer](https://en.wikipedia.org/wiki/Clonally_transmissible_cancer)). I think the opposite is true here. The fact that they have evolved together *is the reason* there are so many different organisms that co-exist on Earth.  
> **username_5**: Introduce a new out-of-context species, and it's often a nightmare! ([Invasive species](https://en.wikipedia.org/wiki/Invasive_species))  

---

### Answer by username_6 on 2016-01-10. Score: 5

In short, no. It is impossible for an infectious agent to come from another planet.

Comets formed in space, and while they may have some carbon based chemicals, nothing as complex as life could form on a comet.

Some meteoroids have been ejected from planets, and it is at least conceivable that something could survive "launch". It is not likely that something could live in space, but I suppose a virus might.

So I will assume the unlikely: An asteroid carrying an extra-terrestrial virus has landed on Earth.

What is it going to do? Well no infect humans - viruses are quite specific to their hosts. Most viruses infect a specific type of bacteria. An alien virus would be specific to an alien bacteria. And alien bacteria will be very alien. Most Movies assume that "DNA" is fundamental for life, and talk about "alien DNA merging with human DNA" or the like. However DNA is something that life on this planet has discovered: aliens won't have DNA. They will have something else that carries hereditary information, but it won't be DNA. An alien virus would be utterly incapable of infecting Earth cells, because it's fundamental biochemistry will be utterly dissimilar.

This is a science fiction story and nothing more or less than than.

> **Comments:**
> **username_7**: I think this is a good answer and it's correct for a virus.   A space bacteria or the exo-planet equivalent of a water-bear if one could survive the journey through radioactive space and actually find the Earth many light years away, it's possible that the new life would find life on Earth as bags of useful chemicals and water.   Its theoretically possible that an unfriendly exo-bacteria could do the life on Earth significant harm if it had an evolutionary advantage, but that kind of event actually happening is unlikely, or, at least, significantly infrequent for a number of reasons.  
> **username_4**: First of all, many viruses *on Earth* use RNA, not DNA. Second, it is entirely possible that RNA and DNA are the most likely nucleic acids for life to adopt (assuming life doesn't manage to form without nucleic acid). Even so, it does seem unlikely that alien RNA/DNA would be compatible with Earth RNA/DNA.  

---

### Answer by username_7 on 2016-01-10. Score: 4

I think it's a fun question and I can kind of give a layman's answer. (though reading the above answer, there is some overlap).

It seems likely that "tiny", life or viruses could survive/hibernate for a long time if blown into space on space rocks, so it follows that life could travel to earth that way, but I think we could apply a modified version of the [drake equation](https://en.wikipedia.org/wiki/Drake_equation).

The Earth gets hit by comets or meteors every day but most of those burn up in the atmosphere. There is a temperature beyond which even bacteria and viruses have a hard time surviving. So, many of those meteors or bits of comet, I suspect, would lose any life they might have. Only a percentage could be the right size to both slow down enough while breaking apart to not impact at too high a speed and successfully deliver life to the surface or into the ocean. Like the real drake equation, we get to plug in our own numbers, but I suspect most impacts wouldn't successfully deliver life to the planet they impact. How many would? 1 in 10? 1 in 100? I'm not sure.

Now, what percentage of comets or meteors have life on them? This is a tough one, I'm pretty sure life can only form on a planet because you need (I would think), liquid water, atmosphere to maintain surface water and reduce evaporation and some kind of heat to drive the mixing of chemicals, perhaps lightning or undersea volcanic vents. Life likely cannot be formed in space or in a nebula. It can be blown into space off a planet with life, but it can't form in space. That likely reduces the number of meteors or objects in space that even have the possibility of carrying life. I hate to guess this one, cause who knows, but 1 in 1000? 1 in 10,000? Most of the meteors, asteroids and comets in space have probably formed without the possibility of carrying life.

There are some simple organic molecules in space, [alcohol](http://www.universetoday.com/8098/deep-space-alcohol/) for example, but there's an enormous chasm between organic molecules and even the simplest life.

Some planets are very difficult to blow debris off of. Earth with an atmosphere and an escape velocity of some 25,000 miles per hour needs a very big impact to lose material into space. Mars, with a very thin atmosphere and lower escape velocity is much easier. Of some 60,000 recognized meteorites, 124 have been identified from Mars. ([Source](http://www2.jpl.nasa.gov/snc/)) and Mars is the easiest one. Venus, with it's thick atmosphere and Mercury, being as close to the sun it is, adding the suns escape velocity to it's own, are much harder. Material being blown off the gas giants is even less likely, so Mars is the easiest plant to potentially lose material where it could land on Earth or other planets. Even if we use the Mars Meteor ratio of 400 to 1, which is, I think, too high, it's still a very low percentage of objects in space that potentially came off a planet or moon and carry life with them.

A 3rd point worth bringing up is repetition. Lets say, just for fun, that [Europa](https://en.wikipedia.org/wiki/Europa_%28moon%29) and [Enceladus](https://en.wikipedia.org/wiki/Enceladus) both have life in their under the surface oceans and both planets regularly eject jets of life filled water into space, that freezes and carries basic life into orbits around Jupiter / Saturn and some of those life carrying bits of ice make their way on occasion to other planets. If this happens it probably happens more than once and any bits of life that may come to earth from Europa or Enceladus, if it happens at all, probably happens more than once. The first introduction of invasive species, even simple ones, can be problematic, but after the first, the planet should have adapted. That's one of the mathematical quirks. One Andromeda strain can in theory be a huge problem, but repeated Andromeda strains from the same source, the risk drops with each impact. What you need, for a dangerous situation to happen is a new introduction of unfamiliar that's never happened before. That's a pretty specific requirement. Mars may have once had life, and if it had, life on Mars probably landed on earth many times over 4 plus billion years.

a 4th point, space is full of radiation. Even viruses will eventually die exposed to the radiation of space. A space rock, carrying life from another solar-system light-years from earth has a very long journey and a very small target. Bits of frozen ice or rock probably travel from other solar systems into ours quite rarely over very long periods of time, and, as I pointed out above, most of those probably won't have life.

This is just a kind of "shoot from the hip" sort of answer, but I think you would need a perfect storm of events for your scenario to actually happen. statistically, I suspect it's a very rare occurrence and not something we need to worry about on a human time frame.

> **Comments:**
> **username_3**: About your 3rd point, I saw this SETI talk that suggests that Enceladus is only a few hundred million years old. Its jetting panspermia, if it has yet originated life, would thus be pretty new. And evolved in a very different environment. I tend to think that incompatibility is safe, not dangerous. Most species on Earth completely  ignore each other. http://www.seti.org/weeky-lecture/orbital-dynamics-suggests-recent-formation-saturns-moons  
> **username_7**: @username_3  Interesting.   So Enceladus formed, perhaps by some kind of collision or a breakup of a larger moon which entered Saturn's Roche Limit.   (I would think).  Interesting stuff.   A capture seems unlikely.   The article doesn't say much, so I'll take with a grain of salt, but it seems possible.  

---

### Answer by username_8 on 2022-08-30. Score: 1

We’re talking about stochastics, so it’s impossible to say *never*. But we can tackle the question two ways:

1. Big picture: tons of dust fall on the Earth from space every day. TONS, every day. You can sweep your gutters, and there’s some possibility a micro-spherule is actually a very tiny meteorite. The issue is identification, because it would look like any other grit particle. And that’s spherical micrometeorites, not even the majority of the infall. One dust researcher estimates that, based on the dust flux, you walk over a cosmic dust particle with *every other step*. The implication is that, if even one percent of one percent of particles carries viruses, we’d be toast. That we’re still alive at all is a clear indication that cosmic infall is no threat.
2. Small picture: the flux of cosmic rays, which degrade and destroy DNA and other organics, is not low either. There are multiple classes and definitions of ‘cosmic ray’, but they’re measured in counts per square centimeter per second. Typical fluxes are one or two per cm2 per sec. Macroscopic organisms and even most microbes have DNA repair functions; viruses, I recall, do not (or they’re worse, perhaps). You can do the math: for a nontrivial DNA sequence (thousands of ‘bits’), crammed into a compact volume, what are the chances of surviving particle bombardment? This requires a time parameter: most meteorites have exposures (measured via cosmic-ray tracks, ha) of 10,000 to 10,000,000 years. Go on, tell me how much bombardment that is.


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