Why don't they run out of air in space?

Forgive me if this is a stupid question, but if one of my kids asked me I'm not sure what the right answer is. For example, why doesn't any given space station run out of air? As background, exhaled air is much the same in inhaled air, but some of the oxygen is converted to carbon dioxide.

Do they just carry additional air to replace the lost air, and somehow filter out the carbon dioxide? Or is there a chemical process to convert the carbon dioxide back to oxygen? Would such a process be possible on a mass scale, to address concerns of climate change?

I remember reading this article as a part of one of my high school science classes. Enjoy!

Quote: The Article

The primary source of oxygen will be water electrolysis, followed by O2 in a pressurized storage tank," said Jay Perry, an aerospace engineer at NASA's Marshall Space Flight Center working on the Environmental Control and Life Support Systems (ECLSS) project. ECLSS engineers at Marshall, at the Johnson Space Center and elsewhere are developing, improving and testing primary life support systems for the ISS.
Most of the station's oxygen will come from a process called "electrolysis," which uses electricity from the ISS solar panels to split water into hydrogen gas and oxygen gas.

NASA ECLSS

Quote: Wizard

Would such a process be possible on a mass scale, to address concerns of climate change?

What did all the climate changes in the past do
without man to be concerned about them? They
just happened and life went on.

They use systems similar to a rebreather in principle, but more sophisticated and with less expendable volumes.

Carbon dioxide is recycled via an absorption scrubber. The scrubber is then heated to high temperature to remove CO2 from it. CO2 is dumped overboard. There are two modules, one US and one Russian, doing it.
Oxygen is not carried in tanks, but in the form of water - it is 16 parts oxygen and only 2 parts hydrogen. Electrolysis is used to split it, H2 is dumped overboard.

Both processes are very energy-expensive. It's theoretically possible, but prohibitively energy-expensive, to split CO2 back into carbon and oxygen, by cracking it at several thousand Kelvins.

Quote: Wizard

Forgive me if this is a stupid question, but if one of my kids asked me I'm not sure what the right answer is. For example, why doesn't any given space station run out of air? As background, exhaled air is much the same in inhaled air, but some of the oxygen is converted to carbon dioxide.

Do they just carry additional air to replace the lost air, and somehow filter out the carbon dioxide? Or is there a chemical process to convert the carbon dioxide back to oxygen? Would such a process be possible on a mass scale, to address concerns of climate change?

Generally, any device that allows you to breathe your air more than once is called a "rebreather". The technology is widely used underwater, where you have pressure to contend with as well.

You have three principal steps in rebreathing.

1) Remove your exhaled carbon dioxide. This is accomplished through the use of a cannister of sodium hydroxide (Sofnolime). The carbon dioxide (gas) reacts with sodium hydroxide and calcium hydroxide to form calcium carbonate (solid).

2) Replace oxygen that you have consumed. Small tanks of pure oxygen or mixed gases (nitrogen-oxygen or helium-oxygen) inject fresh oxygen into the breathing loop.

3) Control the oxygen concentration in the breathing loop. Solid-state oxygen sensors monitor the partial pressure of oxygen in the breathing loop.


Wikipedia article on rebreather technology.
Company that specializes in rebreathers.

I think on a mass scale it would be very very expensive. They tried it in Arizona, and the volunteers were supposed to be sealed up for a month, but they quit because of safety concerns and decided to let some outside air in.

On a mass scale you would make liberal use of good old fashioned plants.

For SCUBA gear, only the military uses pure oxygen. Civilians cannot legally go over 50% oxygen, and for a practical matter most dive masters require far less (usually less than 30%). The danger of going into convulsions from oxygen toxicity at pressure are too severe. Even the special operations guys sometimes die on their mission.

Tell your kid that It's a very good question. I think that most space systems use liquid oxygen as a back up at the very least.

Quote: pacomartin

I think that most space systems use liquid oxygen as a back up at the very least.

Just gas bottles. Liquid oxygen would require energy to heat it up to safe temperatures, and if you have energy, you can as well use water.
But CO2 poisoning is a more difficult concern to address long-term than running out of O2.

Quote: pacomartin

For SCUBA gear, only the military uses pure oxygen. Civilians cannot legally go over 50% oxygen, and for a practical matter most dive masters require far less (usually less than 30%).

That's not strictly the case. Every closed circuit rebreather I've seen use pure oxygen (mind you, you don't see a lot of them). The diluent mixture is carried separately and added on the fly to maintain maximum safe concentration.
Nitrox or Trimix diving is not the same as a rebreather, they can't use pure O2.

Semi-closed circuits do use mixtures, you're probably talking about these. Semi-CCR are much cheaper and safer than CCR, and require minimal training, but they still do produce bubbles (but in less ridiculous quantities than aqualungs) and don't provide the dive time and depth limits of CCR.

Pure O2 rebreathers are used by the military for their cheapness and simplicity. However, they can't be used at significant depths, you can only go about 10m without running into issues, as little as 6m without special procedures.
Since tech and tec-rec divers are primarily interested in exploring greater depths than possible on compressed air, they have limited interest in pure O2 rebreathers. When they do, O2 rebreathers are easily homebuilt and the gas is easily and legally available.

The military uses rebreathers for 1) lack of bubbles, 2) compact size for basic models. Depth isn't sought in operations where pure O2 rebreathers are used.
There exist specialized military rebreathers for serious diving that are on par with civilian CCR in quality if not better, such as CDLSE. I've never seen one, though they are not illegal to own, but most of the cost goes towards silencing and strictly non-magnetic materials.

Quote: EvenBob

What did all the climate changes in the past do without man to be concerned about them? They just happened and life went on.

Well life went on, but most of the species died. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.

This particular species does not care to go extinct.

Quote: EvenBob

What did all the climate changes in the past do
without man to be concerned about them? They
just happened and life went on.

Removal of a large number of the causes of excess CO2 production, to such a level that the CO2 -> O2+C converter were able to reduce the levels of carbon dioxide in the atomosphere again. It's possible that such a feedback mechanism may not always work (in either direction).

Quote: pacomartin

Well life went on, but most of the species died.

You act like that's a bad thing. We won't go extinct, we're
the last major species this planet will ever produce. The
earth finally got it right.

Quote: thecesspit

Removal of a large number of the causes of excess CO2 production,

You do realize that the vast majority of the effort
to combat climate change it to pay shit bag 3rd
world countries reparations for all 'harm' we've done
them. Do you know what a 'shakedown' is?