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Could we use the atmospheres of planets like Mars or Jupiter to separate xenon from them to replenish the engines? I mean, touch the atmosphere of Mars and separate Argon and Xenon for ionic engines, would this be possible? Or the atmosphere would lower us?

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  • $\begingroup$ Touching the atmosphere from an orbit would result in loosing orbital velocity by drag and entering to the denser lower parts of the atmosphere. Final a crash to the surface of Mars or a destruction by the high pressure of the gas giant Jupiter. $\endgroup$ – Uwe Mar 3 at 23:35
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    $\begingroup$ Related, possible dupe: space.stackexchange.com/questions/19771/… $\endgroup$ – Organic Marble Mar 4 at 0:21
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    $\begingroup$ @OrganicMarble Not a duplicate. The only thing there that addresses ion engines is HopDavid's "But it's difficult to imagine..." Since the exhaust velocity of an ion engine is so much faster than the incident velocity of the atoms collected, you can conceivably gain more momentum than the resulting drag removes. That's not really covered well in answers there (except for that quote) so I think this needs a different answer. $\endgroup$ – uhoh Mar 4 at 4:07
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    $\begingroup$ "planets like mars or jupiter"... those places are Quite Different, and the problems with resource extraction from one are likely to be somewhat different to resource extraction from the other. You might consider focussing your question a little more. $\endgroup$ – Starfish Prime Mar 4 at 11:27
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    $\begingroup$ Shouldn't you verify the concentration of Xe before worrying about trying to retrieve it? Which question are you asking? $\endgroup$ – Carl Witthoft Mar 4 at 14:42
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Let's see if the physics of collecting ion propellant from atmospheres makes sense first. Using the vis-viva equation and the standard gravitational parameters for the two extreme cases that you've asked about, we can see what those orbital velocities might be for a circular orbit near the atmosphere (I'll use 5% larger than the planet's radius, it doesn't matter much for the speed calculation):

$$v^2 = GM \left( \frac{2}{r} - \frac{1}{a}\right)$$

planet     1.05 R        GM        v_circ      v_esc   
             m         m^3/s^2       m/s         m/s
Mars      3.56E+06    4.28E+13      3,500       4,900
Jupiter   7.34E+07    1.26E+17     42,000      59,000

The escape velocity is the speed at 1.05 $$ when you are just transitioning in a capture or release.

Now let's look at the velocities of ion engine exhaust. Assuming 100 kV acceleration voltage producing a kinetic energy E of 1E-04 GeV for a charge of +1, and using 1 GeV per nucleon for the masses and using $v/c = 2E/mc^2$

atom      mass      v/c          v
           GeV       -          m/s
helium       4     0.0071    2,100,000
argon       40     0.0022      670,000
xenon      131     0.0012      370,000

So even for the largest atom and the largest planet, the exhaust velocity at 100 kV is six times the orbital velocity.

This means that the available thrust can potentially be much larger than the drag you incur by dropping into the atmosphere and collecting it.

However that's if you use all the gas and don't try to separate a tiny amount of useful gas from the rest. If you are doing that you could certainly lose this advantage. For example if the concentration is only 1% then you'll need an exhaust velocity 100x larger than the orbital velocity just to break even!

So it's better to choose an ion engine design that can somehow work with the planet's native atmospheric mixture that occurs at the altitude where you're collecting it.

I have a hunch that this fraction problem has been explained in another answer on this site before, but I can't remember where.

Now, if you are dropping into orbit, then drag is your friend and it will work together with your thrust. For more on that see the tag.

To lower your orbit after dropping into it, or to raise your orbit when it's time to go, or to just do a make-up burn to counteract drag and keep your current orbit, you'll do your burn somewhere near the lowest point.

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