This question is of interest because there are terrestrial organisms, such as bacterial and fungal spores, and even complex multicellular organisms, such as tardigrades, which can survive deep space conditions indefinitely. Such organisms, if cooled to cryogenic temperatures (e.e., below -100 degrees C, exhibit profound radiation resistance and a practical arrest of all biochemical activity, potentially allowing for millions or even billions of years of survival. If such organisms were ejected into space via meteorite impact or ice volcanism, they might successfully traverse interplanetary and, more problematically, interstellar distances.

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    $\begingroup$ Interesting question. There are organisms (bacterial spores for example) which can survive temperatures well over 100C. Consider editing the question to reflect this? $\endgroup$
    – Woody
    Nov 13, 2023 at 14:25
  • $\begingroup$ There may be a sweet spot where a particle (large molecule, virus-like particle) can be heavy enough to not be blown away by the solar wind but light enough to be mostly suspended already at large altitudes (and therefore shed its potential energy very slowly). Nanometer scale particles can be found "throughout the atmosphere". $\endgroup$ Nov 13, 2023 at 19:13
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    $\begingroup$ whatifshow.com/what-if-you-dropped-a-steak-from-orbit says you can't even cook the stake with such drop... $\endgroup$ Nov 14, 2023 at 1:55
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    $\begingroup$ Are you looking into panspermia theory? If so, keep in mind that some simple organisms can survive frozen in ice, and that a large asteroid containing ice that enters Earth's atmosphere will shed its outer layers, but its core might arrive on the surface relatively cool. $\endgroup$
    – Philipp
    Nov 14, 2023 at 15:35

3 Answers 3


The largest viruses measure 0.5 μm. Bacterium typically measure up to 2.0 μm but their spores are a bit smaller.

Particles of 0.6 – 60 μm radius can remain cold enough to preserve organic matter during atmospheric entry to planets


  • $\begingroup$ Nice find, though worth noting this seemed to be for artificial capsules. The source paper seems to be 'Pre-biotic organic matter from comets and asteroids' is unfortunately paywalled, since it may have math that would support answering a number of other space SE questions on slow re-entry $\endgroup$ Nov 14, 2023 at 8:34

For anything where conventional physics is in play getting from orbital velocity of at least 7 kilometers per second to approximately zero on earths surface involves dissipating 30 kJ of energy per gram. This is easily enough to heat that gram from below zero past boiling. This ratio stays fixed as mass reduces, at least to the point where we start talking about individual atoms and molecules.

In addition individual cells or clusters of cells are unlikely to handle individual high energy strikes in the upper atmosphere well.

Increasing the area and optimal shape can help, though not enough to stay below boiling unless we cheat and start ablating mass.

So we are unlikely to see a single organism make a journey across space and re-entry alone unless it uses exotic chemistry.

Panspermia theory generally assumes cells hidden from the extremes of space and re-entry within larger rocks, small sections of which can sheltered by ablation and insulation of the parent body. So the concern from extra terrestrial contamination probably increases as object size goes up, at least until the body is large enough to make substantial craters, though even there smaller chunks might be shed during re-entry in a manner that is not immediately lethal.

This means that the technical upper limit for 'delivering a payload kept below 40 degrees C' is probably when the impactor is so large that even the chunks trailing behind the main impactor are landing in lava or boiling ocean.

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    $\begingroup$ The square/cube law permits small objects to radiate heat faster than large objects. I'm not sure about the relationship of size to drag coefficient, but I can throw a rock further than a grain of sand. I suspect there are small objects which could de-orbit without significant heating. Molecules for sure. Viruses? bacteria? $\endgroup$
    – Woody
    Nov 13, 2023 at 14:19
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    $\begingroup$ @Woody - This answer (briefly) addresses that. A small enough "lander" traveling at interplanetary speeds could be completely disrupted by impacting one molecule or atomic oxygen constituting the upper atmosphere. $\endgroup$
    – antlersoft
    Nov 13, 2023 at 15:19
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    $\begingroup$ Don't meteorites reach the surface of Earth with cold interiors? $\endgroup$ Nov 13, 2023 at 19:40
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    $\begingroup$ @WayneConrad yep, but most of the mass that hit the atmosphere is gone, melted and/or vaporised on the way, so more starting mass means more potential places that do not get hot enough to cook some form of life hiding there. The bits we find on Earth are by definition the bits that did not get hot enough to become vapor. $\endgroup$ Nov 14, 2023 at 8:24
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    $\begingroup$ Why would you assume orbital velocity as a starting point for something coming from outside Earth's orbit? $\endgroup$
    – MikeB
    Nov 14, 2023 at 15:37

There's an interesting passage in the Smithsonian Scientific Series "Minerals from Earth and Sky - THE STORY OF METEORITES" at page 22:

Of all the known iron meteorites, but seventeen were seen to fall, and of these only that of Mazapil, Mexico, need be given in detail. The account is that of one Eulojio Mijares, a ranchman living in Mazapil. This fall is of special interest, having taken place during one of the periodic meteor displays.


We returned after a little and found in the hole a hot stone, which we could barely handle, and which on the next day looked like a piece of iron.

There's no exact info on how much time passed between touchdown and handling, nor on the exact temperature, but it seems that we're not talking about molten iron lumps here.


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