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I'm reading that NASA plans to use a Deep Space Gateway, a space station in some kind of Lunar or Lagrange orbit, in order to send astronauts to Mars. How does such a "gateway" assist human spaceflight to Mars? Wouldn't it be easier to do assembly and tests in low Earth orbit instead? How does the Moon get involved here?

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    $\begingroup$ I was going to do an answer then I read the article and .... well That article is dealing with toys to justify the SLS. They reference refueling and refurbish but fail to mention where the Fuel is coming from. E.Musk has it right this is just a rewording of the Same Old Crap handing money to the Good Old Boys One and Done/destroy group. Bigelow's B330 en.wikipedia.org/wiki/B330 offers bigger, better and more adaptable Yet NASA needs to give more money for less to the Good Old Boys. $\endgroup$ – Enigma Maitreya Apr 9 '17 at 20:13
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    $\begingroup$ @EnigmaMaitreya Wouldn't it be better if they just stole the money and ran away? Creating a useless space architecture as an excuse for the theft just makes more damage. $\endgroup$ – LocalFluff Apr 9 '17 at 20:21
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    $\begingroup$ LocalFluff, there has been a long term battle (since 1972 when Congress with NASA support ended all Humans in space efforts. What happened is a Token effort of launch humans to LEO let them do .... stuff bring them back and talk about how dangerous space is. Of course the habitats were designed to marginal standards for Humans. Even with plans for habitats available to them NASA and partners chose to expose Humans to Weightlessness, Radiation, not enough to kill them but enough to say SEE Space is Dangerous. Ergo we build brain dead 1 and done rovers that answer nothing for 40+ years. $\endgroup$ – Enigma Maitreya Apr 9 '17 at 20:39
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    $\begingroup$ See these answers. $\endgroup$ – Mark Adler Apr 9 '17 at 22:47
  • $\begingroup$ Why do we even need the SLS? $\endgroup$ – Chris B. Behrens Mar 6 '18 at 4:18
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I believe every answer here is concerning itself with trying to justify it in terms of fuel and delta-v when there really is no such justification at present.

Some day when we are better established in space we will surely be mining the Moon for resources and it would make sense to have a point near the Moon to transfer these resources and use them or ship them further out.

At present our technology is not there, and we have no immediate plans to take advantage of the DSG as a fueling station or waypoint for Lunar resources.

With our current level of technology, the DSG is to serve as a shakedown/proving ground, a sort of deep space(ish) ISS to prepare for a long term voyage outside of Earth's magnetic fields and relatively benign LEO conditions. We have very little experience with long term operations beyond LEO, and the DSG provides that opportunity.

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It may help to visualise the task.

Earth is at the bottom of a deep pit in space-time, the moon is further up the side of the pit, rolling around it with its own smaller pit.

To construct a spacecraft in earth-orbit, you're still more or less at the bottom of hole and have to use huge amounts of fuel to climb out of it.

The problem is that you use nearly all your fuel in that climb and have very little left when you reach the "surface".

Building a way-station at the moon or out at a lagrange point means that halfway up (or more) you'll be able to stop, replenish supplies and have more to be going on with for missions to other planets.

The big advantage of building in orbit of the moon rather than a lagrange point is the abundance of raw materials there, it's comparatively easy to get mass off the surface of the moon compared to earth, so you could potentially mine Helium3 or ice-water for fuel and create a large fuel-dump in lunar-orbit.

From the moon it's far far easier to escape the earth/moon system and reach other planets simply because you're further up the gravity-pit.

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  • $\begingroup$ obligatory xkcd $\endgroup$ – coblr Mar 10 '18 at 0:44
  • $\begingroup$ Huh, I had completely forgotten about that one. It's a very good visualisation of what I was describing! $\endgroup$ – Ruadhan2300 Mar 12 '18 at 9:05
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The chart below shows us something of what is going on. To get from Low Earth Orbit (LEO) to Mars transfer orbit takes 2.5+0.7+0.6 = 3.8 km/s of delta-V. Starting from Lunar orbit, it's only 1.3 km/s. What this means is that you need massively less fuel to ship a ton of anything from Lunar orbit to Mars than you do from Low Earth Orbit. So far, so good, but of course you need to get stuff to Lunar Orbit, which costs 4.1 km/s if you start from LEO and use the most obvious trajectories. So as a way to move stuff from Earth, this looks like a really bad idea. So why might you want such a Gateway? (ignoring politics).

Well, one reason is if the stuff you want to send to Mars doesn't come from Earth but from the Moon. So If you have can find ice in the craters at the Moon's poles (or somewhere else), you could ship that up to lunar orbit and pick it up to use at, or on the way tom Mars. Similarly if you bring in ice from comets or water from Ceres, you may want it close to Earth, so you can work on it, but you probably don't want to bring it all the way down to LEO. The other possibility is that you can take advantage of the interactions between the Moon's, the Sun's and Earth's gravity to get to Lunar orbit much more cheaply (if relatively slowly) as described in research paper. This might be a useful way to shift long-lived cargo from LEO up to Lunar orbit using relatively little fuel, even if it is too slow for astronauts.

Delta-V Chart

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Well, NASA would make a space station in the orbit of the moon to basically restock and resupply on any needed materials. But the big reason why NASA would do this is because of gravity. Since Our Moon has way less gravity, it would make a launch from there very easy on the craft itself. It would save them fuel and it would make their trip a lot more safe.

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  • $\begingroup$ Thanks for your opinion, but Stack Exchange answers are not supposed to be opinions. When you get a little more reputation (points) you'll be able to post proper comments below other people's posts. Welcome to Stack Exchange! Consider taking the tour and visiting the help center. $\endgroup$ – uhoh Mar 6 '18 at 4:09
  • $\begingroup$ If you gain 50 rep, you will be able to comment everywhere so you will be able to say if you think it is important. $\endgroup$ – peterh says reinstate Monica Mar 6 '18 at 6:12

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