Please note that I have had some incredibly helpful and useful suggestions here that I have decided to make some major revisions as a result. I will reduce the overall atmospheric pressure and partial pressure of oxygen figures but will thus need to rethink my flying predators. I think that will go a long way to reducing the humidity levels to make them more tolerable to my crew. I want them uncomfortable, but not dead (apart from those attacked). Once I have rejigged things, I will come back here and correct. Thank you to all who have helped!

This will be my first post here.

For my novel-in-progress, I have a "hot 'n heavy" Earthlike world my crew are exploring.

I need to know whether the humans amongst my crew could breathe and generally survive medium-term (for say, up to 9 or 10 Earth months) and long-term (indefinitely) on this planet without technological support.

Genetic enhancements are available but not all crew have them.

Please assume all crew are extremely fit, that high standards of physical fitness are part of their job description. Assume also they do routinely use drugs to help manage hyperoxia and thermoregulation. They have also acclimatised as much as is humanly possible to the environmental conditions before leaving their landing craft.

If there are any issues, I will finagle technological solutions to them, but I need to know what medical issues my crew may face if I am to know what needs finagling.

The crew will need to be active if they are to evade my alien megapredators (some of them big and scary, but nothing too intelligent, the brightest would make a T. rex look like a Mensa candidate), so there will be some running, jumping, screaming, squelching and dying (not necessarily in that order).

This is the 2nd planet orbiting the primary sun of a binary system.

The primary sun is a K0V orange dwarf main sequence star appearing a fraction smaller in the sky than does our Sun when seen from Earth, but 3.18% brighter due to less distance (mean distance to planetary primary = 164 million km).

The secondary sun is a K1V orange dwarf main sequence star appearing in night skies (at the time of arrival in the local planetary year) as a "star" of apparent magnitude -17.5 (104 times brighter than a full Moon on Earth).

A small moon orbits so closely to the planet it is tidally locked and has a planetosynchronous orbit. This moon is 582 km in diameter and orbits in lock-step with the planetary rotation of a little over 31 hours. It is a red moon seen low on the horizon as they are exploring the cooler polar region, leaving equatorial areas to robots.

Both suns possess a small complement of planets.

Planet being explored: Mean equatorial diameter: 13,008 km

Orbital period: 219.93 Earth days / 168.86 Local days

Rotational period: 31h 15m 34s

Mean surface gravity: 11.30 m/s^2 (1.15G)

Atmospheric pressure: 274.6 kPa (2.7 Earth atmospheres -- I need this to help some of my local fauna fly) (UNDER REVISION, AS ARE THE MEANS BY WHICH MY ANIMALS WILL FLY)

Atmospheric composition: 74.1% N2, 24.7% O2, 0.5% H2O, 0.7% other (assume nothing toxic) (H2O AND O2 FIGURES UNDER REVISION)

Approx mean midday planetary temperature: 74°C / 347K / 165°F

Minimum nightly temperature at the landing party encampment (north polar circle late Autumn/Fall): 30°C/303K/86°F

Maximum midday temperature at the landing party encampment (north polar circle late Autumn/Fall): 46°C / 319K / 115°F

Local water supplies are plentiful and they have the tech to synthesise their own even if there are no local supplies. Essentially there are no limits to their supply.

Local plants and animals are toxic to eat due to incompatible protein chemistry. Zinc-based haemoproteins are the norm for animals on this planet. Crew have brought their own rations, but can synthesise foods using gravitic technology.

Any assistance with this would be greatly appreciated.

  • $\begingroup$ It would be useful to express the humidity in terms of en.wikipedia.org/wiki/Relative_humidity $\endgroup$
    – Tim B
    Mar 23, 2017 at 16:27
  • 2
    $\begingroup$ **BEEP** WARNING **BEEP** WARNING **BEEP** WARNING! You have way too much oxygen in the air! Humans won't really like more than 8-9% oxygen at 2.7 atm. At 0.67 atm partial pressure of oxygen many things will take fire at the slightest provocation and will burn hot and bright. $\endgroup$
    – AlexP
    Mar 23, 2017 at 23:31
  • 1
    $\begingroup$ You should not edit the question such that it invalidates the already-posted answers. $\endgroup$
    – JDługosz
    Mar 24, 2017 at 6:16
  • 1
    $\begingroup$ Yes, as @JDługosz says when you've modified the parameters ask a new question for the new setting and highlight what is different between that one and this :) $\endgroup$
    – Tim B
    Mar 25, 2017 at 13:45

5 Answers 5


Humidity presents a real problem to how long a human can live in open air. In dry air (like say, a sauna) over 120°C is possible, but uncomfortable. In wet air, though, at 100% humidity, over 50°C is acutely fatal. 40°C and 90% humidity is fatal in under 24 hours. So humidity is a huge issue here. Otherwise simply the air is too wet to breathe. Roughly calculating, since the air holds double the water every 10°C increase, even at 70°C, 11.25% RH is fatal in 24 hours.

  • $\begingroup$ So I would either need to cut the humidity or give the crew some tech to surmount that? $\endgroup$ Mar 23, 2017 at 16:28
  • $\begingroup$ Humidity can easily be reduced. I think perhaps I was trying to torture my crew a little too much! $\endgroup$ Mar 23, 2017 at 16:30
  • $\begingroup$ About to edit question to incorporate comments. $\endgroup$ Mar 23, 2017 at 16:31
  • $\begingroup$ over 120°C is possible...? Uh, that's hotter than boiling water...Earth's record high is only 43°C $\endgroup$ Mar 24, 2017 at 2:49
  • $\begingroup$ @Draco18s He was talking about inside a sauna, not about having a climate that hot. $\endgroup$ Mar 24, 2017 at 6:07

I would be concerned about oxygen toxicity.


The increased atmospheric pressure (2.7x) and higher percentage oxygen (24.7% vs Earth 21%) would increase the partial pressure of oxygen in the air to around 0.66 bar (vs Earth of ~0.2 bar).

In divers who can experience higher pressures and up to 100% oxygen gas mixtures this can cause lung and eye damage and seizures relatively quickly (hours). On your world it isn't quite that bad but is still 3x Earth normal, I imagine this would help your flying creatures, but it would cause long term damage to the humans living there. Expect decreased lung capacity, which wouldn't necessarily be noticed while on the planet, but would become obvious if trying to return to an Earth normal atmosphere. This can also cause retinal issues resulting in vision problems up to blindness in some people.

The only real treatment we have developed for oxygen toxicity is to remove the person from the high oxygen environment before the damage occurs. Use of a breathing apparatus or controlled gas mixtures in sealed habitats (not necessarily pressurized) would be needed for any long term stay on this planet.

  • 1
    $\begingroup$ I thought about the same, but it seems that it would be on edge of safe region: worldbuilding.stackexchange.com/questions/21079/… $\endgroup$
    – Shadow1024
    Mar 23, 2017 at 19:02
  • $\begingroup$ Thank you @Josh_King and Shadow1024. Shadow1024 is right according to the chart linked. 274.6 kPa = 39.8 psi which at 24.7% O2 is on the edges of the safe region. Still, it could do with a little tinkering. $\endgroup$ Mar 24, 2017 at 2:48

Ok, I aint the brightest scientific mind, but about questions of survivability, I have some ideas.

Enemy poisoning: If you don't know the composition of the poison, or at the very least so e basic stuff about it, it can prove deadly very fast without any immediate solution.

What you could say is that the poison on this planet, hasn't had much of a time to develop as in the earth, and organisms there don't rely on it, so it's even more undeveloped.

Viruses-infections-uknown pathogenic diseases ( you can think of the kind, doesn't have to be pathogenic ): Almost the same as the above, but in this case time to develop has little meaning, as something deadly, doesn't mean it needed had time to develop more.

Though this could be a very serious problem ( if you get injured, you are more vulnerable to this kind of stuff ), you can make the exploration team survive, by simple medicines. Even if the effects and composition of the infections and viruses are vastly different, I doubt they won't have any similarities to ours, proving that medicines will still be somewhat effective, till they find a more permanent solution, like concoct their own medicine ( would be pretty hard, but if they come prepared it should be doable ),or something better like abandon that 'infected' person, isolate him from the rest so as not to let the disease spread and so on ( may gain even valuable information about it )

Camping, safety during the night and available resources : it's pretty much a death wish if you go unprepared to explore an unexplored planet without a basic preparation ( which is why I assume that they will at least have a devise, or someone that is knowledgeable about medicines and treatments of various of injuries and so on... )

They could prepare the smell of a strong beast to mark their territory as their own during the night ( that's what dogs do... I think. Anyways the important part is the marking of their territory so as to keep other wild animals or beasts away from them ). If that doesn't suit you, maybe constant patrolling like usual and sleeping on turns would be more applicable ( or both )

Unexpected dangers : provided that this is an unexplored planet something like out of plan and bad surprises stuff will happen is almost possibly a certainty.

A scouting animal could solve this, or if your world allows slaves, a bunch of slaves in front of you every time should make things safer ( but slaves need food, so not as hopeful of a solution... besides the apparent problem of moral issues ).

Generally I doubt much will happen without technological support, since it will be like sending today's humans back to the past to fight with mammoths... those guys back then were tempered by the environment...

A community should solve most problems, as it will be all for all. A strong base, a safe place to be able to return and rest.

And since we are talking about evolution, and the fact that they are already accustomed to that planet, why not add the fact that they developed resistance to most of the infections, diseases and poisons on the planet, if you don't want to use much of a technology part...

Other than that, as I haven't read your world, I can only offer suggestions. Maybe more information about the world would help.

  • 1
    $\begingroup$ While this is relevant information I think the main thrust of the question is about the environmental factors (i.e. gravity, atmosphere, temperature, etc) and whether humans can survive that. $\endgroup$
    – Tim B
    Mar 23, 2017 at 16:56
  • $\begingroup$ I pay more attention on the ecosystem part, as it is mentioned they have adapted to a part of the world's climate. $\endgroup$ Mar 23, 2017 at 16:59
  • $\begingroup$ Yes, my main concern is environmental. I am studying up on relative humidity as per Tim B's suggestion. May leave it until tomorrow as it is after 4am here and I need sleep! Thank you also Marios. $\endgroup$ Mar 23, 2017 at 17:12

I'll use the rules of 3 to see when they will die. (guesstimates, your mileage may very.)

3 Minutes if the atmosphere isn't breathable. Shorter if toxic.

3 Hours if the temperature is not within human tolerances.

3 Days if you can't drink / replenish fluids.

300 Hours without sleep.

3 Weeks without food and physical activity.

3 Months without food and very little movement.

3 Seasons without mineral replenishment. (no resources easily found. Potatoes)

3 Years without social contact is very hard.

3 Decades without kids is the end of your line.

3 Centuries without outside contact and you have your own language and culture.

3 Pages or less if the writer wants you dead. Maybe longer if you can surprise him / her.


I need to know whether the humans amongst my crew could breathe and generally survive medium-term (for say, up to 9 or 10 Earth months) and long-term (indefinitely) on this planet without technological support.

Breath? Okay, see this:

Partial Pressure

Note that I don't know the $\text{gr/mol}$ of $\text{Others}$ so I tried to make an average with your actual gases. $$ \left| \begin{array}{cc|ccc|c|c} \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}\\ \text{N}_{2}&\text{74.10%}&28.01&20.7&\text{71.69%}&196.87\\ \text{O}_{2}&\text{24.70%}&32.00&7.90&\text{27.30%}&74.96\\ \text{H}_{2}\text{O}&\text{0.5%}&18.02&0.09&\text{0.31%}&0.85\\ \text{Others}&\text{0.7%}&28.95&0.2&\text{0.70%}&1.92\\ \text{Total}&\text{100%}&106.98&28.95&\text{100%}&274.6 \end{array} \right| $$

  • Nitrogen (N2): 196.87 kPa
    • Nitrogen Narcosis: No, don't worry, you don't have nitrogen narcosis because it's developed under pressures above 240 kPa and 354 kPa, and you only have 196.87 kPa of N2.
  • Oxygen (O2): 74.96 kPa:
    • Oxygen toxicity: When O2 partial pressure is above 50 kPa oxygen become toxic. Also you would suffer hyperoxia.
      • Disorientation, breathing problems, vision changes such as myopia.
      • Prolonged exposures of higher O2 PP or shorter exposure but very higher, can cause oxidative damage to cell membranes, the collapse of the alveoli in the lungs, retinal detachment, and seizures.
      • A lot more, click on the link for more info.
      • In this question you can get more information, also you can see symptoms diagram.
  • Water (H2O): 0.85 kPa: Sorry, I don't anything about it, but you have too much water, I can't say you the effect but I know that it's a lot.
  • Other: 1.92 kPa: I don't know what it's others.
  • Carbon dioxide (CO2): None
    • Hypocapnia: You don't have carbon dioxide in the air.
      Hypocapnia is normally well tolerated. However, hypocapnia causes cerebral vasoconstriction, leading to cerebral hypoxia and this can cause transient dizziness, visual disturbances, and anxiety. A low partial pressure of carbon dioxide in the blood also causes alkalosis (because CO2 is acidic in solution), leading to lowered plasma calcium ions and increased nerve and muscle excitability. This explains the other common symptoms of hyperventilation: pins and needles, muscle cramps and tetany in the extremities, especially hands and feet.
      Because the brain stem regulates breathing by monitoring the level of blood CO2, hypocapnia can suppress breathing to the point of blackout from cerebral hypoxia (you have 3 time more O2 in the air so I don't think you would get hypoxia).

You should:

  • Decrease oxygen levels: Normally we have 21 kPa but we can survive at 49 kPa.
  • Increase CO2: But very little, less than 1%. With this answer (see the table) you can use the CO2 level to determine the "level" of lethality of your planet: from nothing to some minutes (you can use months, weeks, days, hours, minutes and even seconds!!!)

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