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We have all sorts of spacecraft orbiting just above a huge reservoir of nitrogen and oxygen. Isn't there some way one could be designed to reach down and pump that air into canisters, and store them until a ship came, picked them up, and left a new supply of empty canister?

Perhaps some type of tether system could make that feasible - electrodynamic or momentum-exchange tethers. The tether could extend down from the main body of the spacecraft in LEO far enough for there to be enough air that it can be pumped into air cylinders. Cylinders and pumps could be lowered down the tether for filling and then drawn back up.

This comes up due to fretting about volatiles on the Moon, which is becoming sort of a pastime. Could some kind of setup like this work? If it could, what hurdles would be involved in making it? Could it be cheaper than shipping from the surface of Earth (or Mars)?

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  • $\begingroup$ Among other issues that come to mind, you'd need a large amount of propellant to make up the drag losses. $\endgroup$ – Organic Marble Mar 19 '15 at 1:05
  • $\begingroup$ @OrganicMarble i'd hoped there might be a sweet spot, if the tether extends down just enough. It probably wouldn't matter much if it takes a long time to pump a useful amount, if it means the propellant is manageable. Electrodynamic tethers provide propulsion, as well. $\endgroup$ – kim holder Mar 19 '15 at 1:08
  • $\begingroup$ In the case of rotating tethers, at the lengths involved maybe the rotation could counteract a significant portion of the craft's orbital velocity? $\endgroup$ – kim holder Mar 19 '15 at 1:23
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    $\begingroup$ Scooping earth's air would be a huge drag on orbital momentum. You want to restore momementu by of solar cells running a current while passing through earth's magnetic field? Imparting 8 km/s means 32 million joules per kilogram. $\endgroup$ – HopDavid Mar 19 '15 at 3:31
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    $\begingroup$ I've thought of using a Phobos anchored tether to scoop Martian atmosphere. Phobos is such a huge momentum bank that momentum sapping drag isn't an issue. A Phobos tether foot would be moving about .6 km/s wrt Mars' surface. With Phobos' .015 orbital eccentricity, the tether foot would move up and down about 300 km. So the tether foot would rise above the atmosphere each orbit, giving it time to radiate heat. $\endgroup$ – HopDavid Mar 19 '15 at 4:16
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I'm going to say "no".

I thought of a lot of objections but realized I was overlooking a fundamental one, namely:

What's the most important difference between a kilogram of oxynitrogen in the upper atmosphere and that same kilogram in an air tank in low Earth orbit?

The difference is the speed that it is moving at. Eight km/sec!

No matter how your system works, it is going to have to accelerate that kg of gas to orbital speed, just like the kgs of aluminium-lithium in the tank that holds it had to be accelerated to orbital speed.

Other than diving your spacecraft through the atmosphere and ramming its tanks full, I can't think of a practical way to accelerate that gas.

Carrying it up in tanks seems like the best idea.

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There are some real hurdles here. They include:

  • Drag. Some portion of the "hose" will experience drag which will have to be compensated for by some sort of propulsion.

  • Angular momentum. Regardless of how you pump the gas up to a higher orbit, you will have to increase that mass' angular momentum. More propellant...

  • Rarefied gas. In order to be able to scoop up any reasonable quantity of gas, you are going to have to go more than just a little ways into the atmosphere.

  • Thermal dynamics. I suspect you will burn your tether up once you get deep enough to matter.

You might handle the first two with some fancy nuclear electric propulsion and a counterweight that would extend upwards. But gosh... So much complexity... There are easier ways to get these gases in the inner solar system.

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