Snorkel in space

[SkepticLance]

I am currently reading a scifi book called Gradisil. Intersting theme - it is about a community of people who live in space habitats in Earth orbit. However, the author just introduced an idea I think would not work, and I would like to hear what others think the implication is.

 

The author suggests a person living in one of the orbiting habitats lowers a 50 km long pipe to the Earth's atmosphere to pump air up.

 

My immediate thought is that this will act as a brake, and slow the movement of the habitat, hence causing it to move into lower and lower orbit, till it finally falls to Earth, burning up.

 

I also wonder about the difficulty in actually pumping air up a 50 km long thin tube.

[Cap'n Refsmmat]

You wouldn't be able to pump air up that far just by sucking with a pump at the top -- you'd need to push the air up from the bottom and probably at several stations along the way.

 

A space shuttle in orbit less than 200 miles usually has its orbit decay (assuming no boosters) within a month. Dropping a tube down to suck up extra air would no doubt make it happen much, much faster. And it would probably spin the spacecraft.

[swansont]

You wouldn't be able to pump air up that far just by sucking with a pump at the top -- you'd need to push the air up from the bottom and probably at several stations along the way.

 

Indeed — you already have a pretty good vacuum up there. There's essentially no additional pressure difference you could generate.

[Sisyphus]

Also, what's the point? It's not like air actually disappears when you breathe it in. It's recyclable. Have some plants onboard.

 

And finally, all other factors aside, if you're sucking up a significant quantity of air, you're directly pulling yourself downwards, via Newton's Third Law.

[Mokele]

Also, what's the point? It's not like air actually disappears when you breathe it in. It's recyclable. Have some plants onboard.

 

That's actually part of the problem - plants actually have very low metabolic rates in most cases, so it would take a LOT of plants to sustain one human.

 

And finally, all other factors aside, if you're sucking up a significant quantity of air, you're directly pulling yourself downwards, via Newton's Third Law.

 

Couldn't you counter this by blowing out exactly the same quantity of air in the opposite direction?

[Sisyphus]

That's actually part of the problem - plants actually have very low metabolic rates in most cases, so it would take a LOT of plants to sustain one human.

 

Ok, but that was just an example. The ISS doesn't rely on shipments of oxygen or spew CO2, because their life support system mechanically recycles the air they have. The point being that that's clearly a lot easier.

 

Couldn't you counter this by blowing out exactly the same quantity of air in the opposite direction?

 

Well, you'd shoot it downwards, but yeah, I guess.

[SkepticLance]

I have read a bit further in the book, and it is clear that the author has an abysmal lack of knowledge of the science he writes about. For example : He writes about a habitat in space, in which the designers want to introduce gravity. They do it by blowing air down to hold people to the floor!!! Obviously has not heard about pseudogravity from spinning.

[D H]

Ok, but that was just an example. The ISS doesn't rely on shipments of oxygen or spew CO2, because their life support system mechanically recycles the air they have. The point being that that's clearly a lot easier.

The ISS most certainly does rely on shipments of oxygen, in the form of water. The water is electrolyzed, with the hydrogen vented to vacuum. People (and biological specimens) burn the oxygen to form CO₂ and water. The CO₂ is scrubbed and vented to vacuum. The water is recycled.

 

[CaptainPanic]

I am currently reading a scifi book called Gradisil. Intersting theme - it is about a community of people who live in space habitats in Earth orbit. However, the author just introduced an idea I think would not work, and I would like to hear what others think the implication is.

 

The author suggests a person living in one of the orbiting habitats lowers a 50 km long pipe to the Earth's atmosphere to pump air up.

 

My immediate thought is that this will act as a brake, and slow the movement of the habitat, hence causing it to move into lower and lower orbit, till it finally falls to Earth, burning up.

 

I also wonder about the difficulty in actually pumping air up a 50 km long thin tube.

 

At that altitude, there is a vacuum. If you put a pipe down into the atmosphere, then there is still a vacuum at the top of the pipe.

Then you can turn on the pump, but you're pumping a vacuum.

 

Now, we know that there is in fact always a molecule around, so you'll pump something. But it will be so horribly inefficient to compress the gas to something breathable that it's simply a bad idea.

 

Secondly, the upper atmosphere (ionosphere) contains different gases than the lower atmosphere, so you might have to reconsider if you even want to breathe it. I believe that there is only 9% O2. The rest of the oxygen is bound in some other components.

 

Third, such a long pipe into the atmosphere will certainly make the spacecraft crash into the atmosphere.

[Sisyphus]

The ISS most certainly does rely on shipments of oxygen, in the form of water. The water is electrolyzed, with the hydrogen vented to vacuum. People (and biological specimens) burn the oxygen to form CO₂ and water. The CO₂ is scrubbed and vented to vacuum. The water is recycled.

 

Alright, my bad. That's really interesting, actually. So does that mean there's no artificial way (as opposed to bringing a large ecosystem with you) to get a cycle that has an input of energy and nothing else?

[insane_alien]

well, you could have a closed cycle but it would need a lot more space than is available on the ISS. and you'd still need regular shipments of fuel(unless you can capture the gases present up there and throw them through an ion drive to maintain orbit).

 

as for pumping gasses up to the space station the obvious solution is to have the pump at the bottom of the hose, then it is perfectly possible. still need a hell of a pump but you'd get some flow.

[CaptainPanic]

Alright, my bad. That's really interesting, actually. So does that mean there's no artificial way (as opposed to bringing a large ecosystem with you) to get a cycle that has an input of energy and nothing else?

 

You can probably make a closed cycle for the main components: water, oxygen, CO2, food. This should be easy. Sunlight will be needed.

 

You just have to add a purge to get rid of all the components which will build up (which aren't broken down). This might include all kinds of components which evolve from experiments, food processing and also from humans themselves (skin cells, hairs, farts (methane)). These will not be treated easily in the main cycle, but cannot be allowed to build up.

 

The easy solution is to simply dump this and be done... you need a relatively small amount of air for such a purge. I guess that this is what the ISS is doing... they definitely create their own clean water and oxygen... but occasionally (or continuously?) dump some air to get rid of contaminants.

 

A purge flow and make-up (clean) flow are standard parts of a recycle in chemical industry.