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It's expensive to launch things out of a gravity well. So in the future, as we seek to manufacture things in space, it's worth exploring the possibility of harvesting them there as well.

In the interest of some degree of practicality, it seems to make sense to constrain the discussion to nitrogen that might be found within our solar system... maybe even especially close to Earth's orbit.

EDIT: To provide a little more practical context: one author (paper linked below) recently noted that Kevlar (which contains nitrogen) could maybe be used to construct the cable of a terrestrial space elevator if not for the prohibitive launch cost. So, I’m wondering how we might get around launching it out of earth’s gravity well. This was the train of thought that directly led to me asking the question. But the general discussion has been productive as well, IMO.

"[...] Additionally, although Kevlar was found to be strong enough to maintain reliability, its density remains prohibitively large to make it practical, given the massive volume of material which would need to be transported. On the other hand, carbon nanotubes already have the necessary strength, provided a repair mechanism can be incorporated to operate at higher working stress ratios.

Estimating the repair rates for carbon nanotubes remains an open question, contingent on the availability of data regarding their creep-rupture lifetime distribution, which has not yet been thoroughly studied to our knowledge"

From page 17 of https://arxiv.org/pdf/1804.06453.pdf (Building the Space Elevator: Lessons from Biological Design, Popescu/Sun, 2018)

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    $\begingroup$ This question is absolutely clear to me. Is “unclear what you're asking” close reason just a catch-all for your own close reason, or does it mean what it says? $\endgroup$ – uhoh Jul 5 '19 at 23:15
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    $\begingroup$ The Venusian atmosphere has 4 times more N2 than ours. There is also a lot on the Titan. I suggest to make your question much more specific. $\endgroup$ – peterh - Reinstate Monica Jul 5 '19 at 23:19
  • $\begingroup$ At a pressure lower than about 0.5 bar we don't need nitrogen for breathing. $\endgroup$ – Uwe Jul 6 '19 at 10:12
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    $\begingroup$ The comment: "its density remains prohibitively large to make it practical" does not refer to the launch mass problem. I refers to the problem of the tether snapping under its own weight, even if you could 'magic' it into place. They explicitly state Kevlar is only an example to study durability: "We are not suggesting the space elevator be built out of Kevlar, but wanted to show concretely that even a material 10 times weaker than carbon nanotubes leads to reliable segments, given a reasonable repair mechanism" $\endgroup$ – ANone Jul 19 '19 at 14:23
  • $\begingroup$ Thank you for pointing out that they were not suggesting it. I would be interested in seeing a more thorough discussion or explanation of the idea that one made out of Kevlar would snap under its own weight. Where does that conclusion come from? $\endgroup$ – DJG Jul 21 '19 at 20:39
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Nitrogen in space can come from several sources. Once we reach the stage of actually extracting resources from other planets and moons as the question seems to imply, we really should not have a problem with this element.

To name a few prominent sources:

As peterh mentions in the comments, there is four times as much nitrogen in the atmosphere of Venus as in the atmosphere of Earth. One possible reason for this is the fact that nitrogen is fixed in Earth's thunderstorms, forming nitrogen oxides from oxygen which then react with water to make nitric acid, then with rock to make nitrate salts.

Mars has nitrogen fixed into the soil as nitrates, found by Curiosity, as well as in the atmosphere. Unlike Venus or Earth, Mars has a much smaller gravity well making this nitrogen source easier to extract.

Ammonia and ammonium compounds are also known. This abstract reports ammonium salts on Ceres. Saturn's moon Enceladus is reported to contain nitrogen compounds, labeled simply "biologically available nitrogen" in this abstract but identified as ammonia in Wikipedia. Ammonia as a solute lowers the freezing point of a possible submerged ocean in colder parts of the Solar System, and the presence of it's salts as minerals is indirect evidence for such an ocean as well as a nitrogen source.

Elemental nitrogen can also be found in the outer Solar System. Again peterh notes the case of Titan. Not only is nitrogen in the atmosphere of Titan, it may enter the hydrocarbon lakes as well. Nitrogen-rich ice is also present on most of the surface of the Neptunian moon Triton and in Pluto's famous "heart", although these are relatively far away and Pluto is more difficult to reach since a nearby giant planet is not available for gravity-assisted braking.

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  • $\begingroup$ Nitrogen is fixed in Earth's thunderstorms. It would not happen in a storm without lightning. $\endgroup$ – Uwe Jul 6 '19 at 17:09
  • $\begingroup$ Corrected. Thanks. $\endgroup$ – Oscar Lanzi Jul 6 '19 at 18:33
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    $\begingroup$ The atmosphere of Mars is 2.6% nitrogen, it's going to be a waste product of propellant manufacture there. Pluto has "oceans" of nitrogen ice (but is harder to reach than Triton due to the lack of a big gas giant nearby for aerobraking and a capture gravity well). Ceres has ammonia salts that could be harvested as a source of nitrogen, and some other icy asteroids and gas giant moons may have the same. $\endgroup$ – Christopher James Huff Jul 7 '19 at 11:27
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    $\begingroup$ @Chris feel free to add those in. List says "a few" sources, not exhaustive. $\endgroup$ – Oscar Lanzi Jul 7 '19 at 11:40

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