Why is the breathing atmosphere of the ISS a standard atmosphere (at 1 atm containing nitrogen)?

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8

The Wikipedia page for the International Space Station says that it has a fairly Earth-like, sea-level atmosphere: 21% oxygen, balance nitrogen at 101.3 kPa. Supposedly it's because a pure-oxygen environment is dangerous as in the Apollo 1 disaster, but in that case "pure-oxygen" meant 1.15 atm of O2. It seems like a pure, 0.21 atm O2 atmosphere (or even lower) with no inert balance gas should be fine for people and would be ~ 80% less structurally demanding.

One strange aspect might be an effective drop in boiling point to ~ 60 °C, but I'm not sure if anyone would be boiling water for a tea up there. My presumption is that it's not for any safety or human reason but solely for the sake of making ISS experiments more similar (and directly comparable, save for the microgravity environment) to those on Earth. Am I not considering something?

Nick T

Posted 2014-10-24T04:56:17.930

Reputation: 1 455

Tangent: blog post and paper: An independent assessment of the technical feasibility of the mars one mission plan. Note the bit about the 'fire safety threshold'. Granted, ISS doesn't have to worry about growing their own food (and thus producing oxygen... still an interesting topical read.

– None – 2014-10-25T00:48:39.747

1

Lots of interesting information on this page particularly this image which partially refutes Rory. 100% O2 at about 3 - 9 psia is physiologically safe (assuming you've gotten the nitrogen out of your bloodstream). Other concerns still apply, though.

hobbs 2014-10-25T05:47:05.740

1

Here is an in-depth article on various constraints. http://spaceflightsystems.grc.nasa.gov/repository/NRA/cr-2005-213689.pdf I note that simple cooling constraints are called out as forcing a minimum of 7.35psi.

BowlOfRed 2014-10-28T00:47:41.427

While human body depends on absolute oxygen content, meaning pressure drop requires increase of oxygen content, ability of substances to burn depends on relative atmosphere composition: 100% oxygen at 0.3 bar creates risk of fire only a little smaller than at 1bar, and enormously higher than a 21% O2-N2 mix. In other words, increasing the oxygen content, even with the reduced pressure, drastically increases risk of fire.SF. 2016-05-13T21:06:06.830

1

Breathing pure oxygen at 1 or even 1.15 atm is not healthy when done for days and weeks, see https://en.wikipedia.org/wiki/Oxygen_toxicity#Lung_toxicity therefore the partial pressure of oxygen should be less than 0.5 bar.

Uwe 2017-08-30T20:15:44.270

Answers

54

Am I not considering something?

Yes. You are not considering Mir, Soyuz, and the Space Shuttle.

The International Space Station is a multinational program, jointly led by the US and Russia. While the US and Russia had to compromise on many design decisions, the makeup of the breathing atmosphere was not one of them. The decision to pressurize the ISS to one atmosphere with a standard mix of nitrogen and oxygen was probably one of the easiest design decisions agreed upon by those two countries. The Mir space station, the Soyuz capsules, and the Space Shuttle were all pressurized to one atmosphere. Making the ISS breathing atmosphere be anything but one standard atmosphere would have required extensive redesigns of the Soyuz capsule and the Shuttle, and would have precluded reuse of the Mir environmental control systems.

The real question then is why the breathing atmosphere in Mir, Soyuz, and the Space Shuttle is a standard atmosphere, both in terms of pressure and composition. There are significant advantages to a reduced pressure, pure oxygen environment. Such an environment reduces spacecraft mass, structural integrity issues, and complexity. A pure oxygen environment eliminates the need to carry nitrogen tanks, eliminates the need to carefully monitor the oxygen/nitrogen mix, and eliminates the possibility of the bends (decompression sickness). The reduced pressure means the spacecraft can be a bit less bulky as well. There are additional advantages, particularly with respect to EVAs. Both the Soviet Union and the US initially planned to use pure oxygen breathing atmospheres.

The Mercury, Gemini, and Apollo breathing atmospheres were pure oxygen. The Apollo 1 fire modified how that pure oxygen atmosphere was attained, but it did not change that the breathing atmosphere was transitioned to pure oxygen shortly after launch. The issues associated with a pure oxygen breathing atmosphere made NASA shift to having some nitrogen in the Skylab breathing atmosphere, but not much. The Skylab breathing air was 75% oxygen, 25% nitrogen. The use of a pure breathing atmosphere in the Apollo spacecraft continued to the very end, which created challenges for the Apollo-Soyuz test mission.

The Soviet space program switched from a pure oxygen atmosphere to standard atmosphere very early on. Valentin Bondarenko died in a pure oxygen fire three weeks before Yuri Gagarin's historic flight. Having a standard atmosphere mix drastically reduces the likelihood and severity of fires, and also greatly simplifies the pre-launch process. A pure oxygen atmosphere requires extensive pre-breathing to purge nitrogen from the bloodstream. A standard atmosphere meant the cosmonauts could enter the capsule without wearing a helmet and they were physiologically ready to go.

NASA eventually learned these lessons, too. The Space Shuttle used a standard atmosphere. Having a standard atmosphere in the ISS was the only logical decision.

David Hammen

Posted 2014-10-24T04:56:17.930

Reputation: 25 280

1What are the nitrogen tanks for? Where is the nitrogen consumed? Are they just small tanks for maintaining a precise composition?Dave Nay 2014-10-25T13:53:44.040

7@DaveNay -- The ISS has leaks. The mechanisms used to remove CO2 from the breathing atmosphere concentrate CO2 and vent that concentrated gas to space. The vented gas is not pure CO2; it still contains some oxygen and nitrogen. The joints between modules leak breathing atmosphere. The tiny little gaps around windows leak breathing atmosphere. Oxygen is easily replaced; simply electrolyze some water and vent the hydrogen to space. Nitrogen? That's not so easy. It needs to be hauled up to the ISS as compressed nitrogen gas. Most Soyuz launches carry nitrogen to the ISS as part of their manifest.David Hammen 2014-10-25T14:01:13.563

1How much mass is saved with a reduced pressure, pure oxygen environment?smci 2014-10-27T07:14:37.823

2Wouldn't you be able to carry the nitrogen up as N2H4 (hydrazine)? Doesn't even need to be electrolyzed. In fact, a fuel cell can produce nitrogen and electricity from it. As a side benefit, it can also be used for thrusters. Also, why bother replacing the nitrogen? Even if the pre-launch benefits justify launching with a standard atmosphere, it doesn't seem to justify keeping it standard. And for the return trip you'd probably be able to just pressurize the Soyuz on its way down.MSalters 2014-10-27T13:15:53.593

@MSalters - Both the US and Russia carry it up as compressed nitrogen gas. Hydrazine and people don't mix. A leak in that fuel cell (there is none) would be extremely hazardous. There are no fuel cells on the ISS. Electrical power comes from the solar arrays and batteries. If the ISS was pure oxygen, the crew would need to undergo prolonged pre-breathing to transfer from the Soyuz (and formerly, the Shuttle) to the ISS. Keeping the ISS at Earth surface conditions simplifies crew transfer, but it complicates environmental control and life support and extravehicular activities.David Hammen 2014-10-27T14:07:47.907

2@smci - The nitrogen content of the ISS's breathing atmosphere is about 840 kg. That's the tip of the iceberg. There's added mass for storage tanks, plumbing, and life support equipment. The airlock also is more complex and hence more massive than it would need to be in a pure oxygen atmosphere. In fact, the airlock transitions to pure oxygen in advent of a spacewalk.David Hammen 2014-10-27T14:13:06.283

1@DavidHammen: You'd probably want to store the hydrazine on the outside. It's indeed fairly toxic but you can vent to space if needed. As for the pre-breathing, you have the entire trip. Looking at the speed at which divers go up and down, clearing 0.8 atm N2 appears doable while en-route to orbit.MSalters 2014-10-27T14:50:48.690

1@MSalters - You probably wouldn't want to do it, period. With regard to the makeup of the breathing atmosphere on the ISS, that agreement is close to an international treaty.David Hammen 2014-10-27T15:52:17.907

2"The Skylab breathing air was 75% oxygen, 25% nitrogen" this is very interesting. Presumably the total pressure was then about 1/3atm to keep the partial pressure of O2 at about its Earthly level of 0.2atm to avoid O2 toxicity, is that right? This suggests then that any long term effect of breathing a low pressure atmosphere of the right O2 concentration is very slow to show itself.WetSavannaAnimal aka Rod Vance 2015-03-22T09:39:31.250

1@WetSavannaAnimalakaRodVance - That is correct. Skylab 4 was an 84 day mission. NASA tested animals and people on that low pressure breathing mix before sending astronauts to that first US space station.David Hammen 2015-03-22T12:19:45.607

-1. Bondarenko did not die 3 months before Vostok 1. Closer to 3 weeks (20 days).DrZ214 2015-12-27T09:38:33.383

4We don't all live at sea level. The inhabitants of Denver Colorado live a mile above; jet aircraft transition to about a 7,000-8,000 foot cabin altitude (simply pressurizing the ambient air) as they climb to cruise. They don't maintain a sea-level pressure cabin because it reduces structural loads, making the airframe lighter. So why wouldn't spacecraft and/or ISS use a similar "high elevation" internal environment?Anthony X 2016-12-22T14:43:33.657

@DrZ214 -- Fixed that, about a year late.David Hammen 2016-12-22T15:08:27.613

@AnthonyX -- Having cabin pressure in an aircraft less than sea level pressure enables aircraft manufacturers to make the airframe a bit lighter than it would be if the cabin was pressurized at one atmosphere. This doesn't apply as much to manned spacecraft that undergo launch and landing, and in between are exposed to vacuum. The stresses of launch and landing dominate over the stresses of maintaining a one atmosphere cabin pressure.David Hammen 2016-12-22T15:21:23.260

1@DavidHammen How does that reconcile with the decision to use a ~3psi cabin in Apollo? Was it driven by the lunar module? As I understand it, the LM was relatively fragile due to efforts to minimize weight.Anthony X 2016-12-22T21:34:19.807

@DavidHammen Weight is much more important in spacecraft where they pay tens of thousands of dollars per pound. Aircraft cabin pressure is first and foremost chosen based on what is safely breathable air. The main reason they don't make it more pressurized than necessary is that aircraft go through thousands of cycles in their lifetime. The cycles of ascending/descending make the cabin expand and contract slightly, creating metal fatigue that limits lifetime. In comparison, spacecraft/airlocks need fewer cycles, and the Bondarenko/Apollo disasters have encouraged using normal air.DrZ214 2016-12-22T22:51:25.170

1You must remember that hydrazine doesn't cleanly decompose to H2 and N2 and nothing else. There's a lot of quite toxic byproducts.SF. 2017-09-07T13:37:38.223

Maybe it's worth a seperate question but was the ability to talk at a normal pitch part of the consideration?Christoph 2017-12-04T12:49:27.893

17

Rory mentions oxygenation rate which is an excellent point but there's additional reasons why not keep ISS atmosphere at a lower pressure - thermal convection and air cycling. Pressure at roughly one atmosphere means that the ventilation system on the station works better and no pockets of carbon dioxide or even carbon monoxide build up, which would be dangerous to astronauts. The air is easier recycled / replenished and mixed with oxygen (electrolysis of water) and carbon oxides removed from it (Sabatier reaction). The ventilation system also works more reliably at higher pressure and its parts lasts longer between failures. Astronauts / cosmonauts also exercise quite a bit on the station to combat adverse effects of prolonged stay in microgravity on human body, so air pressure also helps them shed excess body heat. Overheating is stressful to the body, lowers performance and can be deadly. And they use all kinds of equipment that requires air cooling too, and it would definitely complicate biology experiments or even render their results useless.

Nitrogen is also relatively cheap to deliver to the station since it's not really a consumable within the life support system and is only lost at a low rate to its inefficiencies, and is also used for all kinds of other things both on the station as well as visiting spacecraft (purging atmosphere, non-toxic fire suppressant, ullage gas in storage tanks to provide fluid / gas pressure,...). So it would be delivered to the station anyway. But in theory, if it made sense from the logistics standpoint, it could be replaced with some other inert and non-toxic gases like, say, Argon. Especially if they would for some reason decide to keep the atmosphere at a much increased pressure, where nitrogen narcosis might become a problem. But they won't do that, there's no good reason to and the station probably couldn't support it structurally without coming dangerously close its ability to maintain pressure and not lose it to space.

TildalWave

Posted 2014-10-24T04:56:17.930

Reputation: 59 419

The article you link for blood oxygenation explicitly mentions partial pressures repeatedly, which, from a basic chemistry point-of-view is all that matters in any chemical reaction/equilibrium, including the scrubbing you mention. I'm skeptical of your "air mixing" claims. Cooling concerns (probably more-so of equipment versus people, who sweat, and reduced convection/conduction to the air would be mitigated by increased evaporation rates) seem like an interesting point though.Nick T 2014-10-24T17:19:01.550

How would the thermal mass of nitrogen compare with other gases? How about the cost per delivered kg (all gases would require some sort of container to keep them liquid, but some might require heavier containers than others).supercat 2014-10-24T19:40:23.310

@supercat You mean Nitrogen's specific heat capacity? Refer to this table http://www.engineeringtoolbox.com/specific-heat-capacity-gases-d_159.html It lists values that closely match ISS atmospheric pressure and temperature. Molecular Nitrogen (at 1 atm and 20°C) has a specific heat capacity slightly over Carbon Monoxide and nearly double that of Argon. And a bit over that of standard atmosphere air. Cost per kg delivered to the station would be much the same, both Nitrogen and Argon are easy to store as inert gases and are not difficult to handle. But you'd have to deliver Nitrogen anyway.

TildalWave 2014-10-25T16:51:55.017

@TildalWave: From a fire-suppression standpoint, I think what would be important would be the ratio of specific heat per unit density; I don't know if there's a term for that. Looking at the table (thanks for the link) the specific heat of helium is five times that of nitrogen, but if nitrogen were replaced with helium, each cubic meter of gas would have only 1/7 as much mass of inert gas.supercat 2014-10-27T00:29:55.907

@supercat Oh that's a science in its own right. For example, there's the disassociation temperature that, if too low, might render some gas or a liquid unsuitable as a fire suppressant as it might cause explosions. Water, for example, isn't any good to fight fires with in most circumstances exactly for this reason. And there's other properties such as viscosity, thermal inertia (related to specific heat capacity but in a volumetric sense, and describing thermal conductivity),... I'm afraid tho that it's not my area. All I know is that it's complicated and is simpler to use what's know to work.TildalWave 2014-10-27T00:46:11.340

@TildalWave: Fair enough. There aren't a huge number of gases which are biologically safe for extended durations; I know divers use helium for weeks on end, and would expect neon or argon would be comparably safe. Breathing an atmosphere with 0.8atm partial pressure of nitrogen, however, has long been considered safe even before anyone knew what nitrogen was; if its thermal inertia is better than the noble gases, so much the better.supercat 2014-10-27T01:00:50.583

@NickT That partial pressure refers to oxygen saturation, i.e. how much oxygen is already in the bloodstream compared to other gases and the ability of hemoglobin to transport oxygen through the body. Oxygenation rate is how much oxygen is absorbed into the body. Case in point are e.g. late stage lung carcinoma patients that would often die because of asphyxiation due to pulmonary edema as a consequence of respiratory difficulties, even when on pure oxygen. Similar would happen in 0.21 atm to healthy subjects. We're simply not built to breathe such tenuous atmosphere for long or under stress.TildalWave 2014-11-15T22:22:02.770

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Running a pure oxygen atmosphere at 0.21 is not going to be very healthy. Humans require atmospheric pressure within reasonable bounds of 'normal' in order to function correctly (gaseous transport across membranes etc) and a pure Oxygen atmosphere, even at lower pressures, is still going to be explosive.

Using nitrogen allows for normal atmospheric conditions and reduces that risk of explosion/fire.

As you pointed out, it does allow for experiments to be run in Earthlike conditions (if we exclude that whole gravity thing...) but that is almost certainly going to be less of an issue, after all, many experiments are run in deliberately non-Earthlike conditions, or in sealed micro-environments with their own atmosphere etc.

Rory Alsop

Posted 2014-10-24T04:56:17.930

Reputation: 8 889

5The Mercury, Gemini, and Apollo programs all used a reduced pressure pure oxygen atmosphere. The Apollo 1 fire made NASA modify how that pure oxygen atmosphere was achieved, but did not alter the fact that the breathing atmosphere became pure oxygen shortly after launch. Skylab's breathing atmosphere was 75% oxygen, 25% nitrogen. Astronauts survived for 84 days breathing this "unhealthy" mix.David Hammen 2014-10-24T15:54:51.903

5As a diver it seems like a dubious claim that 0.21 atm of O2 with or without any inert component would be any different. We're taught 1.4-1.6 atm of oxygen is acutely bad and that it's the partial oxygen pressure that matters, regardless of any other gas in the mix. Do you have any links for any of your claims?Nick T 2014-10-24T17:11:10.860

David - yes those missions did, and they stopped doing that because of the issues that arose, both pre-flight and safety.Rory Alsop 2014-10-24T18:31:24.940

3There are adverse safety and operational issues with using diluent gases as well. Regarding a pure oxygen atmosphere at 1/5 atmospheric pressure being "unhealthy", citation needed. I looked. No such luck. I also looked for the trade studies that made the Shuttle lean toward using a standard atmosphere. I couldn't find that, either. I found lots of trade studies that claimed to justify the pure oxygen atmosphere on Mercury, Gemini, and Apollo.David Hammen 2014-10-24T19:38:45.797

With regard to the Shuttle, I suspect that it's pre-launch and post-landing concerns that drove the decision rather than on-orbit concerns. That a pure oxygen atmosphere (or even a Skylab-style mix) is "unhealthy" seems rather bogus.David Hammen 2014-10-24T19:41:45.303

I never said a pure oxygen atmosphere was unhealthy. See sentence 2 :-)Rory Alsop 2014-10-24T21:01:05.797

2-1 pure oxygen is not explosive.Mehrdad 2014-10-25T10:02:15.707

1Note: I am not one of the downvoters. I don't agree with this answer, but not enough to downvote it. If you downvote an answer, the only polite thing to do is to post a comment that says why.David Hammen 2014-10-25T14:49:22.983

@Mehrdad: Of course pure oxygen by itself is not explosive if it has nothing to react with; is that what you meant? The question is, given a fuel source, does pure oxygen at 0.21 atmosphere pressure give you more danger of explosion than 21% oxygen and 79% nitrogen at 1.0 atmosphere pressure (same partial pressure of O2 in both cases). I don't know the answer to that.Keith Thompson 2015-06-25T22:11:16.657

@KeithThompson: I literally meant that oxygen is not explosive. Hydrogen, on the other hand, is.Mehrdad 2015-06-25T22:42:01.197

@Mehrdad: Neither oxygen nor hydrogen is explosive by itself (ignoring fusion). Unless the meaning of "explosive" is limited to "may explode in the presence of oxygen".Keith Thompson 2015-06-25T22:50:57.847

@KeithThompson: How about something more practical, like "may explode in the presence of oxygen with common elements"? After all, you don't call a substance an "explosive" if there's nothing around you for it to react with. I'm trying to make a very practical point here, not trying to split hairs with chemistry. Compare what would happen if they filled the rest of the 1atm of pressure with (1) oxygen, (2) nitrogen, and (3) hydrogen. The first two would not result in an "explosive" atmosphere. The third one would. It should be pretty easy to understand what I'm saying, I think.Mehrdad 2015-06-25T23:06:38.797

2@Mehrdad: The point, I think, is that a full atmosphere of pure O2 creates the risk of an explosion (as proven by Apollo 1) as long as there are flammable substances in the area. Pure O2 at 0.21 atmosphere creates a lesser risk of explosion, but still some.Keith Thompson 2015-06-25T23:09:34.303

@NickT 1.4-1.6 atm of oxygen is acutely bad, that is only a part of the truth, more than 0.5 atm of pure oxygen may be unhealthy too, see https://en.wikipedia.org/wiki/Oxygen_toxicity#Lung_toxicity . But divers don't breathe pure oxygen for many hours.

Uwe 2017-08-30T20:22:07.020

@Uwe by "acutely bad" I mean you get seizures immediately. From that I was vaguely implying that you need to calculate at what depth (pressure) your chosen gas mix (Air, EAN, Trimix) you might hit certain partial pressures of certain gases: the percentage is irrelevant.Nick T 2017-08-30T20:26:53.917

7

When an object is burning in an atmosphere which is 80% nitrogen and 20% oxygen, the nitrogen will absorb a lot of the generated heat while doing nothing to assist in combustion. While it's entirely possible that some other gas would be better than nitrogen (e.g. have a greater higher thermal mass per mole for better fire-suppression characteristics, or have comparable thermal mass at lower density for lower payload weight), nitrogen has the advantage that human beings can breathe an 80% concentration of it (at standard atmospheric pressure) for extended periods of time without ill effect.

supercat

Posted 2014-10-24T04:56:17.930

Reputation: 171

6

Low pressure can imply decompression sickness. Maintaining enough O2 is not sufficient; when overall pressure drops, gases dissolved in the blood (especially nitrogen) regain their freedom, and bubbles form.
Given the price involved in sending a live human being up in orbit, it would not be very rational to first keep him in a decompression chamber for a few days; maintaining such a chamber in the ISS would also use substantial space, which is a scarce resource up there. Alternatively, decompression would be done prior to the flight, which would be technologically challenging (the vessel would have to be kept at low pressure at all times in the pre-flight phase).

In the early vessels like Apollo, DCS was solved by "pre-breathing", i.e. having astronauts breathe pure O2 for half an hour before launch; and, more generally, by assuming that the astronauts were tough guys who could suck it up. Some problems remained which can explain why "normal" air was used in the Space Shuttle and the ISS.

Less seriously, some experiments on the ISS involve live subjects (e.g. small animals) that would not necessarily accommodate a low pressure; results would be biased. Unless, again, compression chambers are used.

Thomas Pornin

Posted 2014-10-24T04:56:17.930

Reputation: 1 372

1I would think nitrogen purging could be accomplished earth-side by having astronauts in an atmosphere with a mix of O2 and He and then having them breathe such an atmosphere in launch vehicles. While a pre-breathing requirement would mean that astronauts couldn't be sent up without short notice, I wouldn't think it would have to shorten the duration of astronauts' trips to space.supercat 2014-10-24T19:43:49.493

You should not only read https://en.wikipedia.org/wiki/Decompression_sickness but also https://en.wikipedia.org/wiki/Altitude_sickness . DCS is not a problem for Everest climbers but altitude sickness really is. Not low pressure causes DCS, but a sudden pressure drop can do when to much nitrogen was solved in the blood and the tissues. I delete the wrong sentence about DCS and the Everest.

Uwe 2017-08-30T20:38:06.650

3

It is simpler to design because things behave like on Earth, and less chance to have things go horribly wrong.

To add to what was already mentioned - less problems with overheating, and fires are less dangerous... also lower air pressure lowers the boiling point of water. Also simpler to not have to transition from earth's surface to different atmosphere.

Down sides - structural of several times the air pressure, no need for nitrogen and balancing nitrogen, harder and more time consuming to do space walks - in space suit they use low pressure pure oxygen type atmosphere, more risk of decompression sickness (nitrogen boiling in blood), it may actually be easier to get rid of co2 if only oxygen and co2 in atmosphere, etc...

A permanent space colony might use atmosphere like you suggest for such reasons... there are ways to adapt, eg if overheating is a problem then you lower temp of habitat, eg 5 degrees Celsius rather than 20 degrees. At times you want to lose less heat - means you need less food and burn less oxygen, metabolism can slow down. A colony might have constant wind/airflow coming from roof and going into holes in floor as one way to help you stay on ground in zero gravity and take care of problems like spilling a liquid, would also help take care of you overheating.

But people are not so serious about thinking long term "colony"... if they were they would have a system that uses plants or similar to recycle the co2 into o2 and food rather than expensively have to ship up tonnes of consumables to keep astronauts alive... current system works for a few astronauts but would be unsustainable/too expensive for a colony of 100 or 1000 people. Typical people, especially in government do not take risks/do stuff different because you suffer much more for failure than you can hope to gain from success in trying to do things newer/better.

David Kay

Posted 2014-10-24T04:56:17.930

Reputation: 31

-4

Great question and some very informative answers.

The problem for anything human rated in "space" remains one of pressurization no matter the composition of the "air." This might sound simple but since there is breathing and "rebreathing" (gases exiting) going on before any other concern is addressed the structural integrity of the "vessel" itself must be considered. In other words there will be issues of expansion and contraction by maintaining "1 atmosphere" not only in the vacuum of "space" but more importantly in the "traverse" from the Actual 1 Atmosphere to the "contrived" 1 Atmosphere. During ascent this doesn't appear to complicated since I can control my velocity with a high degree of certainty going up.

Coming down can be a serious problem however since soon as I leave the "Space Station" and descend "through the atmospheres" so to speak I am descending at an incredible rate of speed almost instantly (6000 mph I believe)

I am traveling "with" Earth's rotation so that does "buy time" in the sense that I am not falling "straight down" but at an angle to these "atmospheres"... In short not mere minutes but hours.

But the speed of descent is still near constant making maintaining that low air pressure while at the same time avoiding a " sudden impact" a real challenge. None did it better than the Shuttle Orbiter...and none worse than a "space Capsule" IMHO.

user14394

Posted 2014-10-24T04:56:17.930

Reputation: 99