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Suppose we have a planet whose atmosphere has roughly the same gaseous proportions as Earth, but helium replaces nitrogen, and has a strong hydrogen presence in the upper atmosphere; has a gravitational pull stronger than Earth; and has a thicker atmosphere than Earth (from the planet's surface to the boundary with space). What effect would these conditions have on the development of science and technology from a human perspective? Fields I am interested in seeing an impact to include, but are not limited to, the following:

  • Flight. Lighter-than-air vehicles would likely require heated helium or hydrogen to maintain flight, but what about fixed- and rotor-wing craft? Would it be easier or harder to fly, or would there not be a noticeable difference?
  • Communication. Waves propagate through different materials/densities at different rates. Would there be a noticeable effect on the transmission of, e.g., radio waves?
  • Diving. When scuba divers make deep dives, they swap out the nitrogen in their compressed air tanks for helium (this was the idea behind the atmospheric change). Would helium still be a viable option for deep dives?
  • Explosives. As with communication, waves have varying propagation factors. Would explosions be noticeably affected? For example, would they be more destructive, but in a smaller radius?
  • Firearms. In extension to explosives, would firearms be noticeably affected? Gravity would be likely to pull ammunition to the ground faster. Would projectiles face more or less resistance from the altered atmosphere, potentially affecting the effective range?
  • Spaceflight. Modern rocketry is built on the concept of a controlled explosion. Which factor would play the greatest role in deterring/encouraging spaceflight: the helium in the atmosphere, the strong gravity well, or the height of the atmosphere? Also, would modern rocket engines cause the high hydrogen content in the upper atmosphere to explode, effectively preventing such vehicles from reaching space?

Please note: I am not altogether interested in an exploration of the impact of these changes on the human physiology; humans would probably be shorter, denser, and sound like Alvin and the Chipmunks.

If there are any areas I have not already listed you believe would be noticeably affected by the conditions described above, feel free to list them.

Addendum: I've noticed a lot of the answers tend to be about the feasibility of life developing in a helium-based atmosphere rather than the impact the atmosphere would have on the development of scientific understanding. Just to be clear, this planet is not Earth, so any biological factors can be assumed to be properly developed for the indicated atmosphere. Answers to this question should only be about the impact on various fields of science and technology.

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    $\begingroup$ It would be highly improbable to have an atmosphere with both free Hydrogen and free Oxygen. You're more likely to have a Helium-dominant atmosphere, and either some Hydrogen or (less likely) some Oxygen, and a lot of water. Gases in planetary atmospheres tend not to stratify like that. $\endgroup$ – Monty Wild Feb 5 '15 at 23:25
  • $\begingroup$ @MontyWild I'm aware that an atmosphere tends not to form that way, but I was inspired by this question. While hydrogen and helium are close in terms of mass, I feel I can handwave enough of the discrepancy away to remain plausible. $\endgroup$ – Frostfyre Feb 5 '15 at 23:41
  • $\begingroup$ Oh no, not the oxygen-hydrogen Hindenburg atmosphere! $\endgroup$ – Serban Tanasa Feb 6 '15 at 4:31
  • $\begingroup$ For a good overview of what we would be missing (in relation to the chemical properties of nitrogen, most remarkable the strong chemical bond in the N2 molecule) listen to these 2 BBC's Business Daily podcasts in their subseries Elemental Business : Nitrogen Explosives and Nitrogen Fertiliser You would lose a lot of possible chemical reactions everywhere $\endgroup$ – Jan Doggen Feb 6 '15 at 14:51
  • $\begingroup$ @Frostfyre - In hindsight...something to be very careful with here is the mixing of your atmosphere. Nitrogen and Oxygen mix well..they are around the same weight and it doesn't really layer much. Helium and oxygen is a different story...helium will rise and oxygen will sink. Without constant winds or something to mix this, you might get the heavier oxygen pooling and getting to concentrations where anything could combust or it becomes toxic to human life. Caves and underground caverns might be 100% oxygen as the helium separates. $\endgroup$ – Twelfth Feb 17 '15 at 22:55
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One way to estimate the relative difficulty of flight is to compare the effect on Reynold's numbers. (tl;dr: flight is harder by a lot)

The formula we'll use is the cords Reynold's number:

$$ R = \frac{V c}{\nu} $$

where V is velocity, c is chord length of the airfoil and $\nu$ is kinematic viscoscity; it's $\nu$ that changes; with a larger value corresponding to a lower Reyndold's number, and more difficult flight.

Dynamic Viscocities at $20^\circ C$ in $ 10 ^ {-5} \frac{Kg}{m \cdot s} $

$$ \mu_{He} = 1.96$$ $$ \mu_{Air} = 1.82$$

Fluid Density at $20^\circ C$ in $\frac{Kg}{m^3}$

$$ \rho_{He} = 0.1664$$ $$ \rho_{Air} = 1.205$$

Giving us kinematic viscocities, by $\nu = \frac{\mu}{\rho} $ in $\frac{m^2}{s}$

$$ \nu_{He} = 11.8$$ $$ \nu_{Air} = 1.51$$

Of course, our actual atmosphere will probably be somewhere between these values, but helium's much higher viscosity should result in lower Reynold's Numbers for otherwise similar airfoils, combined with the higher gravity, should make flight more difficult on the world in question than it is here on earth.

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  • $\begingroup$ Thanks for this. It confirms my idea that flight would be significantly impacted. What would be the most efficient means to counter the effects of a thinner atmosphere? Or would that be a separate question? $\endgroup$ – Frostfyre Feb 12 '15 at 3:09
  • $\begingroup$ well, the two other parameters in the Reynold's number analysis are Velocity (fly faster), and length (use bigger wings.) $\endgroup$ – SingleNegationElimination Feb 12 '15 at 5:01
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There are some properties of Nitrogen vs Helium as far as specific heat and volumes go...So I'm answering the question you didn't post...climate!

Volume - Helium takes up around 7.5X the amount of volume as nitrogen does for the same weight. This ultimately means your atmosphere is going to extend upwards for a significantly larger distance to have the same mass...about 6 times the volume just to keep the same mass (and same sea level pressure). I honestly have no clue if that would effect the retention of your atmosphere compared to it leaching into space.

Diffusion - Water diffuses around 3 times faster in Helium than in Nitrogen. This would have a really interesting effect as far as weather goes as clouds would spread out significantly faster in a helium atmosphere and might even prevent large rain event. Instead you would get almost a hazy fog in the atmosphere instead of clouds. (mostly speculation on my part).

Heat. Helium has a crazy high heat capacity, almost 5 times that of nitrogen. This means it takes significantly more energy to warm, but also significantly more energy to cool. Your climate would be much more constant and vary less from day to day events.

I'd have to look into it more and edit the answer a few times...but I get the feeling you will have significantly more water vapor in the atmosphere that extends much further up than our current Earth. This will give the grey/white overcast look much more often than a clear sunny day.

I'd be curious what type of wind patterns this would initiate and what exactly would happen for a jet stream. More research...I know it's not what you were looking for in an answer, but most of the questions you have asked seem like there won't be too much change. The change I see is in weather, atmospheric heating, and water vapor.

I wonder if a helium atmosphere would have a different coloured sky or different sunset colours?

Adding

Helium has a much different spectrum asorbtion than nitrogen. Good chance the sky will have a different colouring to it

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  • $\begingroup$ So the altered atmosphere would have a significant impact on advances in astronomy, due to the increased cloud cover? Is the lack of heavy rain a definitive, science-based claim, or just a back-of-the-envelope speculation? There are a few regions on my world that depend on high rainfall. Regardless, +1 for discussing the climatological effects. $\endgroup$ – Frostfyre Feb 6 '15 at 1:53
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    $\begingroup$ @Frostfyre I've tried some additional research on Helium, but it is getting no where fast as it's semi rare on Earth. lack of rain is speculation...however it's decently scientific based. Water will spread out more in Helium and cause larger less dense clouds. Additionally, rainfall requires a particulate for the water to initially condense around...it would stand to reason in lighter helium there would be less particulates floating around. It's not to say the high rainfall can't happen as it eventually needs to fall, but larger more spread out clouds would be a consistent sight. $\endgroup$ – Twelfth Feb 6 '15 at 21:10
  • $\begingroup$ Doubt any real effects on Astronomy...climate varies and although some area's might see more cloud, other area's of your globe will be clear. $\endgroup$ – Twelfth Feb 6 '15 at 21:11
  • $\begingroup$ I know enough about physics to get to this point. I surmised there would be still be clouds, but hadn't considered they would be larger. Thanks for pointing that out. $\endgroup$ – Frostfyre Feb 6 '15 at 23:11
  • $\begingroup$ @Frostfyre - one issue to be aware of...nitrogen, oxygen, and carbon can all be sequestered away in the earth and reintroduced by a variety of processes. Helium is a bit different as it reacts with absolutely nothing (minus a HeH+ semi-stable state and some oxygen fluorine semi stable state). Helium can be created through radioactive decay in a planet but the rate of release is minuscule. Helium is light and is going to get knocked off our atmosphere and escape into space and there really is no method of it replenishing itself. This planet and it's atmosphere would have a limited lifespan. $\endgroup$ – Twelfth Feb 6 '15 at 23:48
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Nitrogen from atmosphere is also used by plants to build proteins. So with lack of gaseous nitrogen from atmosphere, life might not evolve, or life would be very different. Where it would come from?

You can wave this problem away by migrating your humanoid race could from other planet if you wish. But you should think what effect on local lifeforms would have atmosphere from inert gas.

Helium is inert, so it cannot replace nitrogen in reactions.

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  • $\begingroup$ The migration is actually a part of my timeline: technologically advanced humans from Earth decided to terraform/colonize the planet, experienced a catastrophe, and had to rebuild their society with only minimal traces of their advanced ancestors. $\endgroup$ – Frostfyre Feb 6 '15 at 0:20
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Some of your points:

  • Flight: Lighter than air would need huge hidrogen/heated helium spaces to generate lift(if it would be possible at all). Aviation would be more difficult (current airplanes have a flight envelope where the air is dense enough to produce enough lift, if they fly higher they stall).

  • Communication: your voice would be funny. Other than that, nothing (radio waves are electromagnetic waves, its speed would not be affected).

  • Diving: Helium is used because high pressures of nitrogen are toxic. Nothing would change.

  • Spaceflight: Hidrogen explossion. It would depend of what else is there to combine with. I guess most oxygen would be lower in the gravity well, and the upper layers probably won't be very dense anyway.

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  • $\begingroup$ Light is made of electromagnetic waves, are you suggesting light's speed and transmission are not affected by the substance it travels through? $\endgroup$ – Samuel Feb 5 '15 at 23:25
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    $\begingroup$ At the distances we use radio for, not noticeably. $\endgroup$ – SJuan76 Feb 5 '15 at 23:27
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    $\begingroup$ @Samuel. Speed of light through our air is very close to the vacuum value, about 90 km/s slower... $\endgroup$ – Serban Tanasa Feb 5 '15 at 23:31
  • $\begingroup$ @SerbanTanasa I was referring to Rayleigh fading, which you need to account for in high speed communication systems. It's an advanced digital communication detail that matters in the real world, but may not for a story. Point being some adjustments would need to be made, it's not "no affect". $\endgroup$ – Samuel Feb 5 '15 at 23:39
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We would weigh more

The largest effect you would have is that you need a much more massive planet to hold onto the Helium. Helium is light enough that it can be torn out of our atmosphere by solar wind. Helium makes up the tiniest fraction of our atmosphere because it is constantly being lost. Our planet creates it slowly, through radioactive decay.

Juptier can hold onto Helium much better than Earth, so its Helium percentage is higher.

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  • $\begingroup$ I had already considered that any species would weigh more on this planet than on, say, Earth. My interest in posing this question was how the environment would impact advances in science and technology. While this answer addresses the physiological changes in humans, it does not, as is, address the question. $\endgroup$ – Frostfyre Feb 6 '15 at 6:11
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To add to the existing answers. Both Helium and Hydrogen pose a threat to for the stability of your atmosphere over time (considering a geological time scale) as their molecular weight is that small to gain escape velocity by thermal motion alone. So these gases are pretty prone of diffusing into space hence being gone. So atmospheric depletion is a real issue here.

This needs careful balancing of the gravitational force (essentially "surface gravity") of your planet and the heat in the atmosphere (mainly governed by the distance to its central star). Jupiters pull is strong enough to keep H and He, while Mars' and Venus' are not. Earth itself has trouble keeping its Helium in the atmosphere which is why for all technical purposes He is extracted from natural gas.

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