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Just what the title states.

Free hydrogen accumulates in the exosphere. At high altitudes there are large volumes of open space between molecules - which would be a hindrance to collection.

Would it be feasible for spacecraft returning to the surface (just thinking aloud) to skim through the exosphere collecting the necessary quantities of hydrogen into, say an ionized, container to ease the way for the next launch?

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It's probably easier to use some of the energy of re-entry to zap some water vapor instead. –  Ignacio Vazquez-Abrams Jun 15 '12 at 7:10
I didn't think of that ... –  Everyone Jun 15 '12 at 7:20
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According to Wolfram Alpha the gas density in the exosphere is $10^{-13}$ to $10^{-15}$ kg per cubic metre. Suppose your spaceship has a collector with a 10 square metre area, then it would have to travel at least $10^{12}$ metres to collect 1kg of hydrogen.

For comparison, $10^{12}$ metres is about 2,500 times the distance to the moon. I suspect that makes hydrogen collection in the exosphere uneconomic :-)

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Would it be easier to collect by an order or more if the gas molecules could be 'attracted' towards the collector? Say, by ionizing the collection surface. –  Everyone Jun 15 '12 at 8:54
No. The hydrogen atoms are probably charged so they could be attracted by negatively charging your collector. However the speed of the spaceship is likely to be much greater than the speed due to electrostatic attraction, so the electrostatic attraction would make little differene to the rate you could collect hydrogen. –  John Rennie Jun 15 '12 at 9:01
Oh well, it was a thought. I'll wait a few hours though ; see if anybody might be able to build on this a little more –  Everyone Jun 15 '12 at 9:04
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