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In what main ways would a human return to the Moon facilitate human travel to Mars?

It is often argued that the Moon could work as a stepping stone to Mars. I want to know what the merits are of that argument. The differences seem huge. Travel time is really the main problem with going to Mars, while a Moon trip is done in a week. People have to survive away from Earth during an entire Mars opposition period of about 2 years. Atmosphere and gravity differ so that the two missions do best in having separate lander/ascent vehicle designs, I would suppose. Space suites have very different challenges too with different kinds of dust and temperature variations. The day/night cycle is a tough problem on the Moon, while Mars on the other hand has less than half the insolation, requiring different energy supply and heating/cooling systems. The Apollo lunar rover would hardly hold up in the double Martian gravity. Communication to Earth from a rotating Mars 400 000 000 km away is quite different than from the front side of the Moon. The resources which could potentially be made use of are very different too, such as water ice in eternally shadowed craters at Lunar poles and CO2 in the Martian atmosphere.

If humans returned to the Moon, what could they bring from that to also go to Mars? For example, what Apollo equipment would be useful on Mars?

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    $\begingroup$ You means beside launching from the Moon to Mars, with resources lifted out a much shallower gravity well? $\endgroup$ – Hennes Nov 1 '14 at 16:06
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    $\begingroup$ @Hennes Lunar sourced fuel would need a fairly large scale space flight activity in order to be profitable. But I was thinking more about the first few steps. $\endgroup$ – LocalFluff Nov 1 '14 at 18:01
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Bob Zubrin, famed for "The Case for Mars" argues that there is no direct synergy.

That is:

  • EDL (Entry, Descent, Landing) needs are totally different. No atmosphere on Moon, so parachutes/heat shields/deceleration systems are completely different for Mars vs Moon.
  • ISRU (In-Situ Resource Utilization) which makes Mars much more affordable, is totally different on Mars vs Moon. (No CO2 in the atmosphere on the moon, but tons on Mars).

Now there is a direct synergy, if going the SLS route as NASA seems determined. The current SLS plan is to fly maybe once every 4 years if lucky. That just plain doesn't work. Even in best case, only a Mars launch window around every 2 years. So what do you do with your large booster in between those launch windows?

Well the Moon is a great destination.

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    $\begingroup$ Is it clear ISRU makes Mars more affordable, considering all the other things that make Mars more challenging - distance, time, heat shields, gravity? $\endgroup$ – kim holder Nov 3 '14 at 14:46
  • $\begingroup$ I read that point mainly as "ISRU is much more practical on Mars vs Moon". Maybe the phrasing needs to be improved? @briligg $\endgroup$ – a CVn Jan 12 '15 at 10:28
  • $\begingroup$ Late response, but I read it as literally what it says - there is no synergy between the two. ISRU is possible on the Moon, but it's different processes collecting different resources, with little overlap (other than both places possibly having water ice). The more affordable claim might require more study, but a simple differentiating factor is that Martian ice appears to be available in locations that experience "standard" day/night cycles, where the Moon has 2 week on/off spells (or deep polar craters) to deal with. $\endgroup$ – Saiboogu Feb 5 '18 at 19:01
  • $\begingroup$ @geoffc Are there alternatives to SLS? It seems like launching into space is definitely a precursor to reaching Mars. $\endgroup$ – jpaugh Jun 11 '18 at 19:20
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    $\begingroup$ @jpaugh I guess that is what you get when you call it a Space Launch System. Seems like that would be more generic than a specific launcher. Good point. $\endgroup$ – geoffc Jun 11 '18 at 19:54
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From what I've read, the main goal is to learn how to operate a long term base that far from Earth.

The Moon's main advantage in that is that it's much closer to Earth. You can have real time communications with the Moon base and provide material support fairly quickly if something goes wrong.

That also makes it much more convenient for science. Not having any atmosphere makes the Moon perfect for optical telescopes and the far side being shielded from RF interference from Earth is ideal for radio telescopes.

While this is not directly related to Mars, that extra value makes the Moon much more attractive destination.

Finally, some of the technology will be transferable - habitats, life support systems, etc.

Further down the road, the Moon can be mined and fuel produced that can then be deposited in LEO for in orbit refueling thus significantly lowering the cost of spaceflight.

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