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Answers and comments to this question about the advantages of a Mars colony over a Moon colony refer to the fact that aerobraking can be used on Mars but not the Moon. So the pressure on Mars, $\approx 6 \times 10^{-3}$ bar (source), although weak by Earth standards, is sufficient for aerobraking. On the other hand, the much lower pressure on the Moon, $\approx 3 \times 10^{-15}$ bar is not sufficient for aerobraking. This answer claims that Pluto's atmosphere ($\approx 10^{-5}$ bar) is sufficient for aerobraking. So somewhere between $10^{-5}$ and $10^{-15}$ bar, aerobraking must become infeasible.

I realize that there are probably several factors involved (the mass of the craft, the thickness of the atmosphere), but around what order of magnitude would aerobraking become infeasible for a craft of similar size to Mars InSight (a 608 kg craft entering the atmosphere)?

Is atmospheric pressure even the right quantity to consider? Is mass density more important?

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  • $\begingroup$ Are you asking about aerobraking or aerocapture specifically? $\endgroup$ Jun 1 '20 at 19:04
  • $\begingroup$ @RussellBorogove I'm asking about aerobraking, although now that I understand the distinction, I'm interested in aerocapture as well. It seems like even in tenuous atmospheres, aerobraking can be done over many orbits, but still, it doesn't seem that aerobraking was used at all for the lunar missions. So it seems like there is still some kind of practical cutoff where aerobraking would take many years to have any effect. $\endgroup$ Jun 1 '20 at 19:15
  • $\begingroup$ NASA's documentary "7 Minutes of Terror youtube.com/watch?v=Ki_Af_o9Q9s explicitly stattes that the Martian atmosphere is not enough to stop an incoming probe by aerobraking... $\endgroup$
    – DJohnM
    Jun 2 '20 at 1:32
  • $\begingroup$ So can we start with someone posting the "rules," i.e. how much deceleration in what vector direction is required while making sure that the craft hasn't hit the surface yet? $\endgroup$ Jun 2 '20 at 11:30
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    $\begingroup$ @DJohnM neither is Earth's of course. In both cases though, the atmosphere will slow a probe massing a few tons to terminal velocity from escape velocity, provided you get the entry angle right. It's just that on Mars terminal velocity is annoyingly much higher. $\endgroup$ Jun 2 '20 at 12:09

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