Earth’s Atmosphere Stretches Out to the Moon and Beyond (esa.int) 192 points by sohkamyung 29 days ago | hide | past | web | favorite | 77 comments

 Sure, it's kinda there... It'll count for free radical oxidation of spacecraft surfaces and cause minute amounts of drag at those altitudes, but it's worth reminding people that this is in the range of dozens of atoms per cubic meter. Water boils at body temperature at the Armstrong line, which is only 19.2 km above sea level.
 according to this article:> The denser dayside region of hydrogen is still rather sparse, with just 70 atoms per cubic centimeter at 60 000 kilometers above Earth’s surface, and about 0.2 atoms at the Moon’s distance.At the distance of the moon: 200000 per cubic meter so I think it's a bit higher than you think... of course you don't explicitly mention a height for the spacecraft surfaces...
 There’s around 10^25 atoms per cubic meter of air at sea level, so talking about a 10^5 density, a 10^20 reduction in density, is pushing the definition of “there”. (This also makes it closer to space, at 1 particle per cubic meter, than Earth atmosphere, by a dozen orders of magnitude.)For reference, lead in drinking water is actionable at a 10^8 factor, but beyond that — say 10^12 — we largely talk about it not being there.There’s basically no case we refer to something attenuated by twenty orders of magnitude as qualitatively the same.Edit:For fun, I looked up what it meant as energy differences — twenty orders of magnitude is the difference between you jumping and the total energy from the sun that strikes the face of the Earth each day.
 Your comment is consistent with why the parent replied to the grandparent: 2e5 is orders of magnitude greater than "dozens." That's still closer to space than to sea-level atmosphere, but it's not nearly as close as the grandparent comment stated.
 A density of 10^5 is eight orders of magnitude beyond “ultra high vacuum”, which is about 10^13 particles per cubic meter — or 10^-12 times normal. Poster is more correct to say “dozens” (four orders of magnitude) than that it’s not a vacuum (eight orders before the edge of ultra high).https://en.m.wikipedia.org/wiki/VacuumPoster was completely correct that this accounts for only minuscule interactions such as surface oxidation and teensy drag over prolonged interactions, and nothing like the “atmosphere” behaves.
 at what point exactly did I compare with sea level?
 It's hydrogen atoms as well, which is funny because the solar wind is mainly protons, which is hydrogen minus one electron.Leaves me wondering if it varies in size much, because electron flow is quite easy.
 You’d still be ok above the Armstrong line if you covered your head though. I think your skin would protect you?
 I'm surprised this would need to be verified experimentally. Why couldn't a density gradient like this be derived from fundamental laws?
 This is such a good question. Thinking about it reminds me of a quantum mechanics prof who once explained the way a physicist would model a person skiing through the forest. The person would be either a sphere or a box, depending on what's most convenient. The trees in the forest would be cylinders or boxes. The person skiing would then be something like a sphere bouncing around through all these cylinders.And then another time my statistical mechanics prof informed me that "physics is just drawing cartoons of reality with math." That came as a huge existential relief by the way.Popular science holds that physics is the study of "fundamental laws", but really it's just a special practice of articulating these very stable, ubiquitous patterns.When you look at things holistically, you will notice that they are often irreducably dynamic, complex and full of potential surprises, along with fully actualized surprises.So I'd imagine that all these relationships of atmospheric gases with things like the earth's magnetic field, weather patterns, chemical reactions in the atmosphere, interactions with extraterrestrial particles, the dynamics of other magnetic fields, etc. would all make this very difficult to model accurately.First principles apply very well to our nice toy models, but the sheer vastness of existence usually out-scales our toy models very quickly.
 > physics is just drawing cartoons of reality with math
 This is a mean free path computation and everything is spheres (except the skier who is probably a point source).What you've missed out is that a physicist would need to derive a collision cross section for a typical tree-sphere. That may be as complex or as simple as you like. The simple limit would take into account the trunk diameter and tree density within the forest. But perhaps the skier is particularly agile and can avoid most trees, so the cross section is smaller.You could add other particles and cross sections, eg the bear in ski free.>First principles apply very well to our nice toy models, but the sheer vastness of existence usually out-scales our toy models very quickly.That said, this is how a lot of simulations in astronomy work. You take a bunch of physical laws and apply them at scale on millions of particles.
 the bear for anyone curious: https://imgur.com/r/nostalgia/qi8Is1q :)And to add a bit to how physicists simulate stuff. They build a model. Make it as simple as possible, while still approximating the natural phenomena under investigation. So the mean free path model would have a lot of parameters as the parent commenter noted (collisional cross-section, tree density, trunk diameter, etc.).And it's also important to keep in mind that there are theory builders in physics (theorists), "applied physicists" (phenomenologists, who try to use the fancy theories, match it with real data) and derive future predictions (like climatologists, cosmologists doing large scale universe evolution simulation from the big bang, and so on), the experiment designers, who basically try to weed out the useless models/theories through very targeted experiments.All of these are about the same natural world, but different aspects, different levels of details, different concerns. (Eg. experimental physicists place enormous care on quantifying the systemic and particular uncertainties of the experiment, whereas theoretical physicists instead focus on the fundamental workings of the theory - they test it through computation. All of those are math, math, math, just different.)
 > Thinking about it reminds me of a quantum mechanics prof who once explained the way a physicist would model a person skiing through the forest. The person would be either a sphere or a box, depending on what's most convenient. The trees in the forest would be cylinders or boxes. The person skiing would then be something like a sphere bouncing around through all these cylinders.This way of thinking is a bit flawed.Convenience is an important aspect, but it's not the entire reason. A physicist who wants to study how skiers might cause avalanches by modeling/simulation doesn't care about the details of the people they model because that's just not important. For what they want to study, it doesn't matter if they model the human as a sphere or as a real-life model with limbs and so on. It won't affect the grand picture.On the other hands, a physicist who wants to study how an avalanche would crush a person? Then yes, modeling the limbs and the person's physical properties become much more important.In modeling, you choose the model that best fits and represents what you aim to study. You start off with a complex system of equations and then you slowly go through and remove the parts that would be irrelevant. You don't need to account for turbulence if you know that you will never reach the high Reynolds numbers that require it. You don't need to model cellular biology if you're just trying to modeling how a piece of tissue stretches.
 > For what they want to study, it doesn't matter if they model the human as a sphere or as a real-life model with limbs and so on. It won't affect the grand picture.The exact claim is that such modeling, eg, fails to find avalanche dangers that truly exist in the world of skiers moving their body on physical skies, due to their shifting and localized weight, which is missing in the spherical skier model. Your post does nothing to address this.The simplifications you call out are all cases of decreased accuracy in exchange for easier computations: that is, for less accurate results that are “good enough”. But the intersection of many such simplified models may fail to accurately model reality.
 All great points.But yeah it was a joke! And basically I remember his joke having a tangential quip about choosing the shape based on the coordinate system you wanted to stay in.And the point I got from it was that in modeling we often use crass approximations that are oversimplified in order to get a rough picture of what's going on with respect to certain dynamics, and that one shouldn't assume that the model resembles the full complexities of real life.By the way, he was a cosmologist, if that helps you understand a little better why he was so flippant with the shapes.
 While it does seem silly trying to model real world physics by approximating everything as basic shapes like spheres/cubes, it turns out that these approximations really are accurate enough for many applications.You really can get a reasonable estimate of the drag on a cow in crossflow by approximating the cow as a cylinder.
 One thing that physics does well is to place upper and lower bounds around what that unpredictable thing that the actual reality can be or do
 > And then another time my statistical mechanics prof informed me that "physics is just drawing cartoons of reality with math." That came as a huge existential relief by the way.One of the most formative and sublimely influential 'aha' remarks I've ever encountered was a line in a Willam Goldman book about screenwriting which simply read, in the manner of a final summation to a chapter: "POETRY IS COMPRESSION." Caps in the original. I had the same sense of instant resolution, like wearing glasses for the first time, that you seem to have had with your prof's insight.
 Fundamental laws are "just" models, and models are only as good as we can validate them. My day-to-day involves running simulations of physical systems and while things like the Navier-Stokes equations are well known and understood (to the best we can), the difficult part comes in the constants we use in the models to describe specific aspects. The Navier-Stokes equations themselves can describe and tell us how fluids behave, but we still need constitutive models to describe the fluid. We get those models by proposing ideas and testing them.
 I guess it's because of the interactions between gravity of Earth, Moon and other bodies in the Solar System, solar radiation pressure, and magnetic fields. It's hard to account for all the possible influences, and verifying things experimentally lets you both check your theoretical work, and possibly discover sources of influence you've missed.
 As I recall you can't even do this for Earth's atmosphere below the Karman line—it's experimentally derived. https://en.wikipedia.org/wiki/International_Standard_Atmosph...
 You could just assume constant temperature (using e.g. blackbody temperature based on solar radiation) and get a first order approximation of the behavior of the atmosphere. Won't be as good as the standard atmosphere model which uses empirical temperature gradients, but it will provide a ballpark model of the atmosphere.
 I'm surprised you're surprised. Why wouldn't you want to verify your theory? Look at all the missions to comets we did, they all returned totally unexpected results. Results we wouldn't have if we had never gone out there to verify our theories.
 Imagine how exciting it would be if you built a model, with every factor you could think of, and then it turned out to be even just a little bit wrong.
 This is a typical exercise in thermodynamics courses, but simple models may not be very accurate.
 The magnetosphere and solar wind
 I would imagine therefore that Earth's micro-organims occasionally land on the surface of the moon.Not a particularly hospital place the moon. But if it were made of cheese, it'd probably be green by now. (:
 Would any be theoretically viable? I imagine a cyst gets blown into space from time to time but wouldn't last long in a radiation filled vacuum. Solar UV radiation is intense without the atmosphere. It's why space suit visors look reflective and why spaceships tend to always be white or shiny.Bonus: myth: space is dark. You don't get the omnipresent lit sky effect since there is no atmosphere to scatter the light but if you are anywhere near a star it is blindingly bright... in the star's direction.
 I do not know. But, there have been experiments, and some microbes are more durable than expected.https://en.wikipedia.org/wiki/List_of_microorganisms_tested_...https://motherboard.vice.com/en_us/article/qkvk33/bacteria-a...and... well... the moon has been there a while.
 > I would imagine therefore that Earth's micro-organims occasionally land on the surface of the moon.Only on the moon? They might be hit by space debris and travel through space to other planet's atmospheres.
 I'd think that's highly unlikely as the geocorona is mostly sparse hydrogen atoms; I doubt even viruses can float in a hydrogen atmosphere, let alone micro-organisms.
 The smallest virus [1] is to a hydrogen atom what a T-34 tank is to a ping pong ball. Ping pong balls tend to not lift tanks off the ground, especially if it’s not a lot of them ...
 hospitable* ?
 Naturally. But I thought we had em beat in terms of rocket propulsion.
 Hilarious fact!
 So humans did not leave earth’s atmosphere after all.
 If we want to just play with words to make them meaningless: humans have never left the bonds of Earth's gravity.
 And literally never will.
 Fun fact! The Earth is inside the sun's atmosphere.
 i have been wondering about just that...
 Why doesn't the gravity of the moon cause atmosphere to collect around it?
 Because the radiation from the sun pushes it away faster than the moon can hold onto it. The moon has no, or a lot less of a, magnetuc field to shield itself.This happens on earth too, but only for the light gasses like hydrogen and helium. That's why there is no helium layer at the top of our atmosphere. It gets pushed out into space by the sun. The process on earth is called called thermal escape. On the moon it just happens much more quickly and to heavier gasses.
 Neat! Thanks for taking the time to explain!So if no heliosphere (but warm?) a moon theoretically could strip atmosphere?
 Do you mean: could the moon strip away our atmosphere with its gravity in the absence of solar radiation?Well, Earth has gravity, too. It's much stronger than the moon's gravity. In fact, Earth's gravity is the only thing that's holding the moon where it is! The moon might manage to steal a little gas from Earth and hold onto it, but the transfer of mass will be nowhere near what I would call "stripping".
 Shouldn't the dark side of the moon have a small atmosphere then?
 There is no dark side of the moon, it's a misnomer - there's a far side from earth's point of view, but it isn't dark or blocked from the sun https://en.wikipedia.org/wiki/Far_side_of_the_Moon
 Actually do mean "the side of the moon not in light", like night on the moon itself.
 The same face of the moon is always towards the Earth, but the sun hits all points of the moon at different times, so no part is perpetually dark. The "dark side" is just the side facing away from Earth.
 There’s no permanently dark side of the moon. The moon rotates with respect to the sun. The rotation is synchronised —due to tidal forces— with its orbital period around the earth. Therefore we see the moon as non-rotating.
 Yes. I think people are too quick to point this factoid out and miss the point.He is wondering if the actual dark side of the moon at any given moment (which there is one) would retain parts of an atmosphere since it's getting blasted less by the sun.
 And, if there is a wind or gradient, that side's putative atmosphere may persist its thin atmosphere regardless of the rotation of Moon relative to Sun.
 That is exactly what I am asking.The bigger revelation is the cognitive blind spot folks are exhibiting from having a raft of knowledge.
 True, but gasses are fuilds. Any gas on the night side will flow towards the day side eventually.
 The pressures are different between the two sides, my hypothesis is that given the information in the article, that there could be a permanent higher concentration of “atmosphere” on the, let’s call it ‘shade side’ to not trigger the weilders of shallow facts.
 The what now?
 It probably does, giving a small contribution to the "atmosphere" of the moon. A cubic centimeter of lunar atmosphere contains about 10^6 particles (compared to 10^19 on Earth), and according to the article, the Earth's geocorona at that point only has 0.2 particles per cubic centimeter.Of course, the lunar atmosphere is constantly being stripped away as well.
 So, when will the Moon fall on us?
 This is a terrific question that should not have been downvoted. The article literally says "the moon flies through the Earth's atmosphere", which implies drag.
 A downvote without a (short, simple) statement of reason can be quite impolite and disrespectful. This is a perfect example.Not only is it discouraging (not everyone knows they've made an error...and so it punishes an innocent question ), it leaves the recipient without knowing what the problem is.
 It won't. This negligible smattering of gases would have less of an impact on the moon's orbit than the tidal forces pushing it further away.
 Yeah, it's actually moving away from the Earth at something like 4cm per year.
 IIRC, it's believed that the Moon would eventually leave the Earth's orbit, if not for the fact that both the Earth and Moon will be destroyed by the Sun going red giant long before that would happen.
 The moon is actually getting farther and farther away over time and increasing its orbital period as it does.
 Super interesting. The article says the SWAN instrument measured/detected hydrogen atoms, is it possible there's other gases/elements there too?
 Mostly ionized oxygen, sulfur, iron, various random metals and light elements.
 The article notes that the study used twenty-year-old archival data from the ESA/NASA Solar and Heliospheric Observatory (SOHO). It's good to keep that stuff around, organized so that you can find and use it.
 Wow. Apparently, those sensors generate so much data that even 20 years later one can still make discoveries by evaluating it.
 So, in a way, the Moon has an adopted/hijacked atmosphere?
 Does this technically make the Earth a gas giant?
 Bussard ramscoop here we come!
 now i need to find a very very very big balloon
 "... The denser dayside region of hydrogen is still rather sparse, with just 70 atoms per cubic centimeter at 60 000 kilometers above Earth’s surface, and about 0.2 atoms at the Moon’s distance..."tldr: Well that's hardly anything, nothing to see here
 Not quite true. It might affect astronomical observations from the moon:On the down side, the Earth’s geocorona could interfere with future astronomical observations performed in the vicinity of the Moon.“Space telescopes observing the sky in ultraviolet wavelengths to study the chemical composition of stars and galaxies would need to take this into account,” adds Jean-Loup.
 imagine a gradiant of uv scattering atoms going from a density of 70 atoms to 0.2 attoms ... it should not be quite a interference, for the average astronomer out there, this is not an issue, simply dealing with the lower atmosphere is problematic enough.edit for missing not

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