Mars’ underground brine could be a good source of oxygen

Re:Mars’ underground brine could be a good source of oxygen by chongli on Wednesday December 02, 2020 @08:49AM (#25276285)

That’s a double-edged sword. Yes, Mars’s atmosphere can protect you from micrometeoroids, but it also makes it absolute hell to land anything there. The atmosphere is thick enough to burn up any spacecraft you try to land there but nowhere near thick enough to use a parachute. The moon, on the other hand, is much easier to land on using only retro rockets as brakes.

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Re:Mars’ underground brine could be a good source of oxygen by m4rtink on Wednesday December 02, 2020 @11:16AM (#25277783)

Sure, but you need to break all your speed at arrival with rocket engines. Not a big difference for regular trips from Earth - no need to make these more quickly than the regular few days.

But say you are coming back from Mars and want to land on a Moon base or even just dock to a lunar space station. Now you have to brake all the excess interplanetary speed with your engines! And that might be quite a lot, especially if you opted for going faster to reduce the transit time.

But if you are arriving with the same speed at Mars, you can use the atmosphere for aerocapture first without having to brake all that speed with engines. And people might do that for Moon bound trips as well - first do an aerocapture to an excetric Earth orbit using Earths atmosphere and only then adjust the resulting orbit for Moon encounter. That should reduce the raking requirements quite a lot.

For that reason I can imagine future interplanetary ships always having a degree of aerodynamic capability, so they can make use of the atmosphere to slow down when needed. Even if they are not really made to land on a body with atmosphere.

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Re:Mars’ underground brine could be a good source of oxygen by chongli on Thursday December 03, 2020 @12:08AM (#25285611)

Aerocapture with the thin Martian atmosphere is much riskier than aerobraking [1]. Since you’re trying to bleed off all of your delta-v in one pass, your margin for error is much lower due to the high energies involved. Aerobraking is much less risky but way slower, taking months to decelerate and so much less feasible with human passengers.

For landing on earth you only need aerobraking and parachutes. For the moon all you need are the retro rockets (we’ve already done it). To send humans to Mars you need retro rockets, a heat shield, and a parachute. The combination of systems makes the whole procedure a lot more complicated and error prone, increasing risk.

[1] http://sicsa.egr.uh.edu/sites/sicsa/files/files/projects/dec...

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Re:Mars’ underground brine could be a good source of oxygen by m4rtink on Thursday December 03, 2020 @02:35PM (#25293492)

Fair enough - and indeed, the Lunar delta-v needed is usually quite small. Apparently Chang'e 5 sample module needed just 1,67 km/s to reach lunar orbit for example.

Still, I think there are definitely cases where atmosphere helps (even if it causes issues elsewhere).

So even if you used propulsive breaking to get captured into orbit of a body with atmosphere for safety reasons, you can still capture into a very eccentric orbit & then carefully use aerobraking (as demonstrated many times at Mars already) to lower and circularize your orbit before landing. And with a bit better aerosurfaces you could even use the atmosphere to tune your orbits inclination as needed.

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