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Comment by lizknope | Hacker News Reader

The article claims they are already converting CO2 to fuel on the ISS. This process has been TESTED but we aren't using it right now.

Article claims:

> Known as the “Sabatier reaction” from the late French chemist Paul Sabatier, it’s a process the International Space Station uses to scrub the carbon dioxide from air the astronauts breathe and generate rocket fuel to keep the station in high orbit.

Reality:

We just shipped a new CO2 scrubber a few months ago

https://www.nasa.gov/centers/marshall/news/releases/2021/mar...

For LONG space missions or colonies

https://www.eni.com/en-IT/scientific-research/space-free-co2...

The ISS is constantly falling back to earth. It maintains its orbit with thrusters on the Zvezda module or a visiting spacecraft boosts it higher

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The ISS uses rechargeable zeolite beds for CO2 scrubbing, they don't need a new one frequently, and the fact that we shipped a new one is coincidence.

Zeolite is a mineral, you get a bunch of small pellets of it in a bed, and run air over it while cooling it. The CO2 (and H2O) stick to it while everything else goes over it. Once you're at capacity you vent it to space and heat it, the CO2 (and H2O) leave the bed (into space) and recharge it for the next cycle. Repeat forever.

The consumables here are the small amount of gasses sent overboard, and energy, nothing else.

(And you use multiple of these beds because capturing CO2 is exothermic, and venting CO2 is endothermic, so you want to run a heat exchanger between the two beds in opposite cycles to minimize energy usage)

If both operations are exothermic, why the heat exchanger? I would assume one process would need to be endothermic.

There is a typo. The answer is in the second paragraph:

"Once you're at capacity you vent it to space and heat it, [...]"

Fixed the typo

If they’re not recycling CO2 into O2 and instead venting CO2 to space, does that mean ISS is constantly losing gas and it needs an ongoing resupply of oxygen all the time?

I thought oxygen was used fast enough to make resupply impractical, but perhaps there are in fact gigantic tanks of O2 under immense pressure somewhere aboard the ISS.

https://en.wikipedia.org/wiki/ISS_ECLSS#Oxygen_generating_sy...

Yes, and sort of.

The ISS is constantly losing gas, I believe that the primary source of new O2 is actually electrolysis of water, not pressurized tanks, but that they do also have pressurized tanks as a backup.

A person breathes out roughly 1kg of CO2/day, and it follows you breathe in about the same amount of O2, not an impossible amount of mass to resupply.

And, to complete the equation: that carbon comes up in the form of food.

> 1kg of CO2/day

which is crazy low if you put it in perspective and see that (gas powered) cars release about 0.2 kg of CO2 ... per kilometer.

That's insane, I didn't realize it was that much. If we could see cars drop carbon turds on the road instead of venting it invisibly in the air we'd all be freaking out right now.

Well, we used to have horses...

There's a reason all the old buildings in big cities are elevated a few steps above street level...

Where I used to live in the New Town of Edinburgh each house had a boot scraper fixed next to the step to the front door.

Yep ... that's about two chocolate bars every kilometer ... and that has not really changed too much with newer motor generations as it is mostly a function of how much gas is used.

My current car uses about half of what the previous did though. Old Toyota Avensis 1.6, new Toyota Corolla 1.8 HSD.

Yep, engines definitely have improved over time. Still the Corolla is producing about 80 grams of CO² per km (which in Germany is the benchmark to get a E-plate).

So... Only about one chocolate bar per km.

You could probably get a similar reaction if fossil fuel sales were given in the equivalent of trees or fuel crops. Someone might question running to the store just for donuts if they imagined burning half of a tree or 40 pounds of grain or something in the process, rather than just a bit of gasoline.

Honestly I was not even aware of that ... A quick search shows that a 100 yo birch tree stores only about 15 kg CO² ... So your half a tree seems to be in the right ball park.

Also ... CO² is bad ... but at least is non-toxic compared to some of the other side products.

Seems very low expressed like that, but actually it isn't because when you take your car you typically drive tens of kilometers.

You can easily walk tens of kilometers in a day. 10 km us basically nothing, 20 km would be somewhat taxing if you are unprepared ... But some (crazy) people are doing 100 km walks for "fun".

The average trip in the USA is 10 miles ~16km, so 3.2 kg/trip, at the average ~4 trips per day that's like 12-15x more than a person.

https://www.bts.gov/statistical-products/surveys/national-ho...

Takes me all damn day to run a 5K so that checks out.

> A person breathes out roughly 1kg of CO2/day

I've heard that you mostly love weight by breath, and wondered what was the actual limit. Thanks for that info.

Just remember that only the C part makes you lighter as the O2 comes from the air. So based on that number it's 375g/day (edit: it's 273g)

I get 273g/day? C is lighter than O.

Also... this is a typical number for a typical lifestyle, I believe it can vary substantially if you exercise a lot.

Also... I know nothing about wait loss, but surely a lot of the mass is in water?

Thanks for pointing out that mistake. I think people also breath out quite a bit of water. So total weight loss from breathing will be bigger than the 273g

As a sanity check: fat is around 9 kcal per gram. In a day you use around 2000 kcal of energy or 222 grams of fat. Protein and carbohydrates are around 4 kcal per gram.

The numbers are in the right ballpark.

> Also... I know nothing about wait loss, but surely a lot of the mass is in water?

Not really. Yes, you loose water (and its weight) when you e.g. sweat a lot when exercising, but it gets replaced when you drink. More generally your body keeps the amount of water more or less at the same level through intake (drinking, but also water in food), urinating and sweating. There are short-term variations, but besides exceptional circumstances on a day-to-day basis the amount of water in your body should be pretty constant.

Real weight loss is through loss of muscles (not preferable) or loss of fat.

The new scrubber sounds like it's not using the old zeolite bed system but one based on molecular sieves. But you're otherwise right that they're not really turning that CO2 into anything on the regular beyond the limited ISRU experiment(s).

Zeolite is a molecular sieve. That's just a fancy word for "material that selectively holds on to some types of gasses" (CO2 and H2O in the case of zeolite).

I haven't followed the new bed system though, I'd be very interested to know if it was using a different molecular sieve (and somewhat surprised if they are).

Ah yeah reading a bit closer further down they mention it's a 4 bed system and this is largely an iterative improvement instead of a new system entirely.

I don't disagree with anything you said but the article claims they are turning CO2 into fuel which the ISS is NOT doing.

Agreed. Reaction mass != Fuel

The Sabatier reaction is very energy intensive. It is pretty much the basis of Musk's demonstrations of cities on Mars, powered by lots of solar power. It has other advantages for human colonization as well, such as producing oxygen as a byproduct.

It does convert CO2 into "fuel" but the ISS just vents it into space. The utility for the ISS is getting the O2 back and being able to ship up water and use it for atmospheric regulation as well.

Space junk piling up as we speak.

Not sure what you mean. Anything in LEO like the ISS will burn up after a couple years at the latest if left unattended.

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